CONNECTIONS IN STRUCTURAL FRP MEMBERS Master of Science Thesis in the Master’s Programme Infrastructure and Environmental Engineering MOHAMMAD SADEGH KHANI Department of Civil and Environmental Engineering Division of Structural Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2015 Master’s Thesis 2015:67 MASTER’S THESIS 2015:67 CONNECTIONS IN STRUCTURAL FRP MEMBERS Master of Science Thesis in the Master’s Programme Infrastructure and Environmental Engineering MOHAMMAD SADEGH KHANI Department of Civil and Environmental Engineering Division of Structural Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2015 Connections in structural FRP members Master of Science Thesis in the Master’s Programme Infrastructure and Environmental Engineering MOHAMMAD SADEGH KHANI © MOHAMMAD SADEGH KHANI 2015 Examensarbete/Institutionen för bygg- och miljöteknik, Chalmers tekniska högskola 2015:67 Department of Civil and Environmental Engineering Division of Structural Engineering Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone: + 46 (0)31-772 1000 Cover: Common failure in FRP bolted connection ,Figure source: Mosallam, A. S, & Knovel (e-book collection). (2011). ‘’Design guide for FRP composite connections’’. Reston, Va: American Society of Civil Engineers. Chalmers reportservice/ Göteborg, Sweden 2015 I Connection in Structural FRP members Master of Science Thesis in the Master’s Programme Infrastructure and Environmental Engineering MOHAMMAD SADEGH KHANI Department of Civil and Environmental Engineering Division of Infrastructure and Environmental engineering Chalmers University of Technology ABSTRACT At the present, there is an increasing demand for alternatives to conventional construction materials in infrastructural applications. In this regards, application of new construction materials such as FRPs has attracted a freat deal of attention. One of the obstacles towards widespread use of FRP materials in infrastructural applications is connection of FRP members. Due to orthotropic nature of FRP materials and their failure susceptibility with regard to stresses perpendicular to the plane of the FRP member, the design of connections in such members is a rather complicated task and some behavioral aspects of these connections are still not known. In order to use the FRP material in structural applications a better understanding of FRP connections and setting proper design methods is very important. This thesis is a part of an on-going project in the field of FRP composite connections at Chalmers University of technology. The thesis provides short review on different types of connection use to join FRP structural members such as mechanical, bonded and hybrid connections with focus on mechanical connections. Different aspects such load transfer mechanism and different failure modes have been presented. Design parameters affecting the strength of the joint are introduced and discussed. The results of an experimental testing program which was conducted in the framework of the research project is presented. In the first stage, experiments were conducted in order to determine the material properties and in the next stage, double lap shear joints were performed under quasi static loading to determine the characteristics of bolted and bonded joints. Due to importance of the fatigue, especially in the infrastructural applications, this phenomenon has been investigated in the thesis. Key words: GFRP, CFRP, FRP bolted Connection, adhesive Connection,Hybrid connection. II CHALMERS Civil and Environmental Engineering, Master’s Thesis 2015:67 III Contents 1 INTRODUCTION ................................................................................................. 1 1.1 Background ........................................................................................................ 1 1.2 Aim, objectives and methodology ..................................................................... 2 2 FRP STRUCTURES .............................................................................................. 3 2.1 Characteristics of FRP materials ........................................................................ 3 2.1.1 Fiber ............................................................................................................... 3 2.1.2 Polymer matrix ............................................................................................... 7 2.2 General properties of FRP ................................................................................. 8 2.3 Applications ..................................................................................................... 11 2.4 Design approaches ........................................................................................... 13 3 CONNECTIONS ................................................................................................. 15 3.1 Joint categories and joint technique ................................................................. 15 4 BEHAVIOR OF CONNECTIONS ...................................................................... 20 4.1 Mechanical connections ................................................................................... 20 4.1.1 Failure in mechanical connections ............................................................... 22 4.1.2 Effective parameter in mechanical connection ............................................ 28 4.1.3 Creep and relaxation in the mechanical connection .................................... 36 4.2 Bonded connections ......................................................................................... 38 4.2.1 Adhesive ...................................................................................................... 39 4.2.2 Bonding mechanism ..................................................................................... 42 4.2.3 Surface pre-treatment ................................................................................... 43 4.2.4 Failure modes ............................................................................................... 43 4.2.5 Bonded joint configuration .......................................................................... 45 4.2.6 Shear stress of bonded connection ............................................................... 47 4.2.7 Peal stress of bonded connection ................................................................. 48 4.2.8 Design methodology of bonded connection ................................................ 49 4.3 Combined joints ............................................................................................... 49 5 FATIGUE ............................................................................................................ 51 5.1 Fatigue in mechanical connection .................................................................... 51 5.1.1 Tension –tension connection ........................................................................ 55 5.1.2 Compression-compression Connection ....................................................... 55 5.2 Fatigue in the bonded connection .................................................................... 55 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 IV 5.3 Fatigue in combined connection ...................................................................... 57 6 EXPERIMENTAL TEST .................................................................................... 58 6.1 Tensile test ....................................................................................................... 58 6.1.1 Tension test result ........................................................................................ 59 6.2 Compressive test .............................................................................................. 61 6.2.1 Compressive test results ............................................................................... 62 6.3 Shear test .......................................................................................................... 63 6.4 Connection test ................................................................................................. 65 6.4.1 Connection test results ................................................................................. 67 7 REFERENCES .................................................................................................... 69 CHALMERS Civil and Environmental Engineering, Master’s Thesis 2015:67 V Preface First of all, I would like to express my gratitude and cordial thanks to my examiner Professor, Reza Haghani for his worth-while and constructive key-guidance to successfully complete this project. I am indeed happy to see his distinguished comments and notation that has helped me to upgrade the level and quality of this project. I would like to give special thanks to Valbona Mara for her support, time and shared- thought. I also like to give a warm thanks to Sebastian Almfeldt for his support for our experimental tests. Finally, I am really grateful to my wife for her inspiration and patience to see my project completed. Mohammad Sadegh Khani Spring 2015 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 VI Notations 𝐸𝑎 Modulus of elasticity of adherent 𝐸𝐶 Adhesive film tensile modulus in peel 𝑊 Width of the joint 𝑊𝑋 Spacing between the fastener hole in the x-direction 𝑊𝑦 Spacing between the fastener hole in the y-direction TS Applied axial force, (N) 𝑁𝑠𝑑 Pulling through failure force 𝑃0 Initial load, (kN) 𝑀𝑒 Bending moment in the adherents at the end of the overlap B Relaxation exponent constant 𝑃𝐾 Characteristic load per unit width Roman lower case letters 𝑐 Half the bonded overlap length in the direction of applied load 𝑒 Edge distance (distance from hole centre to short edge) 𝑠 Side distance (distance from the hole centre to the long edge) 𝑡 Thickness of laminate considered 𝑑𝑏 Fastener Shank diameter 𝑟 Radius of bolt bet area, (mm) 𝛼 Angle of inclination of bolt threads 𝜃 Angle of static friction 𝜎 Average baring stress P Applied load D Bending stiffness of the adherents 𝑛 Number of bolts 𝑑 𝑜𝑟𝑑𝑏 Bolt diameter or fastnar shank diameter 𝑘𝑡𝑒 Elastic stress factor, in the net tension failure mode C Stress concentration coefficient 𝜏 Shear stress 𝛼 Angle of inclination of bolt threads 𝑓𝑉,𝑅𝑑 Pulling through strength 𝑑𝑟 Washer diameter 𝑑ℎ Hole diameter 𝜎𝑐𝑚𝑎𝑥 Peak adhesive peel stress CHALMERS Civil and Environmental Engineering, Master’s Thesis 2015:67 VII 𝜏𝑝 Peak adhesive shear stress 𝑡0 Adherent thickness 𝑡𝑎 Adhesive thickness 𝜗 Poisson ratio of adherent 𝜏0 𝑎𝑙𝑙𝑜𝑤𝑎𝑏𝑙𝑒 Allowable adhesive shear stress 𝜎0 𝑚𝑎𝑥 Allowable adhesive peel (tensile) stress 𝜎0 𝑎𝑙𝑙𝑜𝑤𝑎𝑏𝑙𝑒 Allowable adhesive peel (tensile) stress 𝜎𝑧 𝑎𝑙𝑙𝑜𝑤𝑎𝑏𝑙𝑒 Allowable adherent through-the-thickness tensile stress CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 1 1 Introduction Historically, four traditional construction materials including steel, concrete, timber and masonry have dominated the construction industry. Masonry and timber were the main materials for building structures and also infrastructures up until a few hundred years ago. (Friberg and Olsson(2014).Today, timber, steel and concrete are the most common used materials in the construction. However, these materials suffer from a number of drawbacks such as susceptibility to corrosion, degradation and other aging problems. There is therefore an increasing demand at the present for alternatives to the conventional construction materials. In this regard, fiber reinforced polymer (FRP) composite materials have emerged as a new construction material in the past 30 years due to their superior mechanical, chemical and physical properties. The FRP composites were first developed during 1940s for military and aerospace applications, where their high strength and light weight used to optimum benefit. The Fiber reinforced polymer composites have high strength-to-weight ratios with outstanding durability qualities in a variety of environments and are easy and quick to install. Setting the mechanical properties of polymer composites by appropriate choice and direction of fibers is a distinct advantage of using FRP materials. The FRPs are resistant to corrosion and they are dimensionally stable ( Friberg and Olsson(2014). These materials are desirable in many applications such as repairing of existing structures and construction of new ones (Tang & Podolny, 1998; Muniz & Bansal, 2009). Figure 1 FRP bridge and strengthening and repair of bridges. Figure sources: GEF Company,Tenroc Company. At the present, FRP materials are used for construction of whole and hybrid FRP bridges for both vehicular and pedestrian purposes. At the present, the connection of FRP members is an obstacle in front of FRP bridge industry. FRP bridges cannot be manufacture off site, and there is demand to produce them in segments and connect them on-site. Then there is demand for some reliable methods to connect the segments to each other such as mechanical connections, bonding, etc. 1.1 Background The FRP was first used in building industry during the 1970s as non-load bearing infill panels manufactured by hand lay-up process; using the polyester polymer. Afterwards using the FRP as a material for structure was increasing steady and at the present FRP is known as a construction material. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 2 The investigations on characterization of FRP connections (especially mechanical connections) started in aerospace industry in the US in the mid 1960 th as there was a demand at that time to achieve a proper design for connections. The conventional plastic design method which had been used in bolted connection in steel and aluminum and other ductile materials is not applicable to FRP connections with the brittle behavior. Moreover, using linear elastic analysis is not appropriate due to great strength increase resulted from fine micro failure in the intermediate vicinity of small bolts holes (Hart-Smith 1989).Nowadays based on research work which has been conducted, different recommendations and standards are prepared for FRPs including the design criteria for FRP connections. 1.2 Aim, objectives and methodology The present knowledge about the FRP connections is pretty advanced knowledge. However, this sophisticated knowledge is not good enough among structural Engineers. There are a number of differences which distinguishes the connections in structural applications from those in other engineering areas such as, the thickness of the adherents, mechanical properties of adherents, the size of the fasteners, the type and thickness of the adhesive layer in bonded joints, etc. which influence the design assumptions and make many design theories in other fields (e.g. aerospace) not working in this field. Therefore it is important to study the behaviour of such FRP joints and get a good understanding about the behaviour of connections and joints. At the present, there is no unified approach for design of the FRP connections, hence designing the connections and predicting their real behavior involves a great deal of uncertainty. The aim of this project is to provide a better understanding of the behavior of FRP connections. In order to reach this aim two objectives were set as following.  To carry out a literature review on FRP connections. Available codes, design recommendations and other available documents and materials have been studied to get a clear picture about the behavior and identify important design parameters in FRP connection.  To perform experimental tests to study the behavior of bonded and mechanical joints including stiffness, failure loads and modes of failure. . CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 3 2 FRP structures 2.1 Characteristics of FRP materials In this section a brief introduction of FRP materials and structures is presented. It is crystal clear that in connections both the connected and the connector’s material properties play a significant role in the design; thereby, a short review of the FRP material can be useful. FRPs are composite materials consisting a polymeric matrix reinforced with fibers. Like any other composite material every component (i.e. the fibers and the matrix) affects the final property of the FRP material. Moreover, size, shape, volume, orientation and distribution of fibers have influence on the final properties. The FRP material in essence is anisotropic and the fiber orientation and arrangement of them play an important role in the FRP properties. The figure below illustrates the FRP material both in micro and macro scale. Figure 1 FRP material in micro and macro scale_ Source: The International Handbook of FRP Composites in Civil Engineering 2.1.1 Fiber A fiber, which is a long filament, is made out of a single material such as ballast fiber, glass, aramid or carbon. The main purpose of the fiber is to provide sufficient strength and stiffness (Clarke 1996; Bisby 2006). Due to the nature of the fibers as long filaments, the fibers provide strength and stiffness mainly in the direction of the fibers (longitudinal direction). However, the fibers should provide a sufficient aspect ratio in more than one direction in order to provide a reinforcing function in the transversal direction as well (Halpin, Hastak & Hong 2004). The figure 2 shows the differences CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 4 between fibers, matrix and FRP stress-strain characteristic. It is clear that the FRP as final material is completely dependent on the fiber characteristics. Figure 2 FRP component characteristic- Source: The International Handbook of FRP Composites in Civil Engineering The fibers generally represent about 30-70% of the volume and 30% to 70% of the weight depending on the manufacturing technique and the form of the reinforcement in FRP (Potyrala, 2011;Clarke, 1996; ZRMK et al., 2013). The three types of fibers that are the most common in civil engineering applications are: glass, aramid and, to a lesser extent, carbon fibers (Potyrala, 2011; Muniz & Bansal, 2009). They are suitable for different types of applications depending on requirements such as strength, stiffness, cost, durability and availability (Bisby, 2006). The Glass fiber is the most common reinforcement in FRP composites, where it stands for approximately 90% of the market (Chlosta, 2012). Glass fibers are typically used in low-cost applications of polymer matrix composites. The Glass fibers are a processed form of glass, and consist of a number of oxides (largest extent of silica oxide) and raw materials (such as limestone, fluorspar, boric acid, clay etc.) (Potyrala, 2011). Due to that, glass fibers that containing silica they are in general very sensitive to alkaline environments (Friberg and Olson 2014). Some disadvantages of glass fibers are reported to be: low modulus of elasticity, poor abrasion resistance and poor adhesion to polymer matrix resin in presence of moisture. (Friberg and Olson 2014). The most common glass fibers are E-glass fibers , which is the best choice of fiber type if the highest strength as well as electrical and thermal resistance is strived for (Clarke 1996). The “E” in E-glass stands for electrical since it was initially developed for electrical application (Kaw, 2005,Friberg and Olson, 2014).The other different types of glass fibers available for various applications are: “A” or “AR” glass for alkaline resistance , “C” for corrosion resistance , “S” and “R” for high strength and stiffness (Zoghi, 2013). The table 1 shows the typical properties of glass fibers. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 5 Table 1 Typical properties of glass Fiber Source: The International Handbook of FRP Composites in Civil Engineering Properties E- Glass A- Glass S-Glass C- Glass R- Glass D- Glass High Modulus Density,ρ(g/cm 3 ) 2.54 2.45 2.49 2.45 2.58 2.14 2.89 Tensile strength,σult(GPa) 3.54 3.30 4.58 3.31 4.40 2.50 3.40 Elastic modulus,E(GPa) 72.4 69 85.5 69 84.8 55 110.4 Diameter,d(µm) 3-20 - 8-13 - - - - Thermal expansion αL(10 - 6/˚C) 5.0 - 2.9 6.3 - 3.1 - Max operation temp.(˚C) 550 - 650 600 - 477 - Carbon fiber is one of the two types of high-performance fibers used in civil engineering applications (Potyrala, 2011). The use of carbon fibers was increasing and between the years 1998 and 2010. The worldwide usage of carbon fibers was doubled to an amount of 30.000 tons annually (Chlosta, 2012). It has been used in many civil infrastructure applications, including seismic rehabilitation, retrofitting and repair of the structural system such as concrete beams and columns. The Carbon fibers are generally used when high strength and stiffness is required and where a lower weight justifies the higher cost (Cytec n.d, 2014). The Carbon fibers are furthermore considered to be the most durable fiber type (Friberg and Olson, 2014). Based on the fabrication method carbon fibers are categorized to three main groups as PAN, rayon and pitch. The table below shows the typical properties of Carbon fibers. Table 2 Typical properties of carbon fibers. Table source: The International Handbook of FRP Composites in Civil Engineering Properties PAN(Type Ι) PAN(Type ΙΙ) Pitch Rayon Density,ρ(g/cm3) 1.95 1.75 2.0 1.7 Tensile strength,σult(GPa) 2.4-2.7 3.4-4.5 1.55 2.50 Long.Elastic modulus,E1(GPa) 380 230 380 500 Transverse .elastic modulus,ET (GPa) 12 20 - - Diameter,d(µm) 7-10 8-9 10-11 6.5 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 6 Long thermal expansion αL(10 -6/˚C) -0.5 -0.5 -1.0 -0.9 Transverse thermal expansion αL(10 -6/˚C) 7-12 7-12 8 7.8 Aramid fibers are the second of the two types of high-performance fibers used in civil engineering (Potyrala 2011). The fiber is known as manmade and is used in many civil engineering applications. Aramid fibers are mostly named as organic fibers and Kelvar ® (produced by Dupont) is the most commercially recognized brand (Zoghi, 2013). The organic fibers have high toughness and this characteristic make them useful in the impact and ballistic applications. The disadvantages of aramid fibers are known as low compressive strength, susceptibility to creep, moisture absorption, sensitivity to UV light, temperature-dependent mechanical properties and high cost. Table 3 The typical properties of organic fibers. Table source: The International Handbook of FRP Composites in Civil Engineering Properties K29 K49 K68 K119 K129 K149 Density,ρ(g/cm3) 1.44 1.44 1.44 1.44 1.45 1.47 Tensile strength,σult(GPa) 2.9 3.0 2.8 3.0 3.4 2.4 Long.Elastic modulus,E1(GPa) 70.5 112.4 101 55 100 147 Diameter,d(µm) 12 12 12 12 12 12 Long,thermal expansion αL(10 -6/˚C) -4.0 -4.9 - - - - The graph in the figure 3 illustrates stress-strain curves of the different kinds of fibers. The tensile strength are also has compared with steel wires. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 7 Figure 3 Stress strain in different FRP material. Graph source: The International Handbook of FRP Composites in Civil Engineering. As stated before, regardless of fiber kind and mechanical properties of fiber, the fiber arrangement is also important in FRP material properties. The fiber arrangement can be assigned into three main categories which are unidirectional, bidirectional and random (Clarke, 1996). The unidirectional is referred to the case when all fibers are arranged in one direction (Clarke, 1996).The bidirectional is formed when fibers are arranged in two directions either as 0° and 90° or ±45° (Clarke, 1996). This is performed with woven or non- woven fabrics (Clarke, 1996). For the arrangement termed random, which also is referred to as multidirectional fiber direction, the fibers are randomly distributed (Clarke, 1996). In general better mechanical performance is achieved of the composite if continuous fibers are used instead of chopped fibers (Clarke, 1996). 2.1.2 Polymer matrix A matrix is composed of resins, fillers and additives (Potyrala, 2011) as well as catalysts and in some cases accelerator, release agents, inhibitors, monomer etc. (Clarke, 1996). A matrix has different rules in composite materials like the following: - Holds the fibers together and arranges them as intended - Transfers the load between fibers through shear - Separates the fibers within the composite material - Gives stiffness and shape to the member - Reduces or stops the crack propagation rate by letting the fibers to act independently - Protects the fibers from environmental impacts such as chemical and mechanical - Increases ductility and impact strength - Shields from abrasion for notch sensitive fibers - Provides shear, transverse tensile and compression properties CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 8 The governing parameters regarding the mechanical properties of the bond between fiber and polymer are both the compatibility between matrix and fiber, called the “interface”, in addition with the loading direction and angle between fibers (Potyrala, 2011; Hollaway, 2010). The characteristic of the bond between the fibers and the matrix is of important since it controls the load transfer in the structure (Bisby,2006). The polymer matrix is highly viscoelastic and the viscoelastic behavior of the FRP material mostly comes from the viscoelastic behavior of matrix. The matrix material is also temperature dependent. At increased temperature, at low loading rates or long- term loading, the response tends to be more ductile, while at low temperature and high loading are result in a rigid and more brittle behavior (Zoghi, 2013). The mechanical properties of the polymer material drop drastically when reaching the glass transition temperature (Zoghi, 2013). The glass transition temperature is the temperature, when the polymer softens, meaning that it changes from hard (often brittle) solid state to more rubber–like (soft and tough) solid state (Zoghi, 2013). Polyester, vinyl ester and epoxy are the most common matrices used in civil engineering applications. The table below shows some properties of resin according to (Zoghi, 2013) and (Malek, 1996). Table 4 Typical properties of resin and matrices. Table source: The International Handbook of FRP Composites in Civil Engineering Resin Type Tensile strength (MPa) Modulus of Elasticity (GPa) Density (kg/m3) Cure Shirinkage(%) Polyester 35-104 2.1-3.5 1100-1400 5-12 Vinyl ester 73-81 3-3.5 1100-1300 5-10 Epoxy 55-130 2.8-4.1 1200-1300 1-5 2.2 General properties of FRP The FRP material properties are presenting in the table 5 and also in order to have a better understanding the material properties of conventional material is also stated. The table below compares typical properties of FRP, fiber, epoxy, concrete and steel. Table 5 FRP and the material properties. Table source: The International Handbook of FRP Composites in Civil Engineering. Properties FRP a Fiber Epoxy Concrete Steel Compressive Strength (MPa) 125-2400 - 55-100 25-150 200- 2000 Tensile strength(MPa) 400-3000 1800-5000 9-20 1-6 200- 2000 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 9 E-Moulus(GPa) 35-500 60-800 0.5-20 25-50 200 CTE(10 -6/˚C) ̰̰ 0(longitudinal) 25- 30(Transverse) -6 to 5.0 (Longitudinal) ̰ 45 b (Transverse) 25-30 8-12 10 Density,ρ(kg/m 3 ) 1300-2200 1400-2600 1200- 1300 2400 7800 Poisson ratio 0.25-0.35 0.25-0.35 0.3 0.15-0.2 0.3 CTE, Coefficient of thermal expansion a FRP based on glass, carbon or aramid fiber with epoxy matrix b For aramid fiber (for carbon and glass, the transfer CTE is similar to the longitudinal (CTE) Some important engineering properties both in compression and tension of the three different kinds of FRP material more separately are also stated in the tables 6. Table 6 Poperties of different kinds of FRP. Table’s source: The International Handbook of FRP Composites in Civil Engineering FRP type Tensile strength (MPa) Modulus of Elasticity (GPa) Ultimate Strain, ɛfu(%) CFRP 600-3000 80-500 0.5-1.8 GFRP 400-1600 30-60 1.2-3.7 AFRP 600-2500 30-125 1.8-4.0 FRP Type Compressive to tension Ratio Ff,comp/ff,tens Modulus of Elasticity ,Ef (GPa) CFRP 0.78 0.85 GFRP 0.55 0.80 AFRP 0.2 1.00 Like all materials, the environmental conditions affect the properties of FRP material. The most affective environmental parameters, which have been the subject of several researches, are: temperature, thermal stresses like freeze and thaw, moisture, acidic or alkane environment and ultraviolet radiation. The structures are usually subjected to a combination of these environmental conditions, in which two common forms are a combination of temperature and moisture. In the FRP composite material, fibers CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 10 exhibit relatively high thermal stability, whereas polymers are strongly affected by the temperature (Zoghi, 2013). One of the influential parameters in FRP properties is moisture. Experimental that has been done on the effect of moisture reported that there was not any reduction in tensile strength in CFRP material whereas GFRP and AFRP. (Mosallam, A. S., & Knovel, 2011). The same reference has also reported that 30% and 35% tensile reduction in GFRP and AFRP in the wet-dry experimental test. The experimental was set such that can be able to model the condition for 50 years period (Mosallam, A. S., & Knovel, 2011). In addition to the environmental effects, the effect of relaxation and creep and also cyclic loading condition (fatigue) are important in FRP materials. The Creep and relaxation are two parameters which are almost similar in essence. Table 7 shows the degradation level under fatigue (cyclic) loading condition. The table 7 shows the degradation of initial strength of different kinds of FRP materials per decade of logarithmic fatigue lifetime in tension-tension loading. In the following chapters the fatigue will be discuss more in detail. Table 7 Degradation of FRP under fatigue, Tension-tension. Table’s source: The International Handbook of FRP Composites in Civil Engineering. FRP type Degradation of initial static strength per decade of logarithmic fatigue lifetime CFRP ≈ 5% GFRP ≈ 10% AFRP ≈ 5%-6% Table 8 also shows the fatigue in the two million cycles for different kinds of FRP and also prestressing steel. Table 8 Fatigue strength after two million cycles. Table’s source: The International Handbook of FRP Composites in Civil Engineering. Type of material σ max /ftk a ⧍σ b (MPa) Prestressing steel ≈ 0.6 ≈200 E-glass/polyester (rod) ≈ 0.5 ≈60 E-glass/epoxy (rod) ≈ 0.5 ≈ 75 Aramid/vinylester (rod) ≈ 0.6 ≈235 Carbon/vinylester (rod) ≈ 0.6 ˃350 Carbon/epoxy(strand) ≈ 0.6 ≈310 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 11 a Applied maximum stress as a function of characteristic tensile strength of the reinforcement b Stress range yielding fatigue failure at two million cycles In the cycling load condition growth of internal or surface flaws may occur, resulting in a reduced mechanical strength compare to the short term static strength. (Zoghi, 2013) . 2.3 Applications The FRP is widely used in industries such as aerospace, construction, offshore and marine, appliance and business equipment, corrosion resistance equipment, electronic, consumer products and transportation. In the civil engineering industry, like in the other industries, there are on-going efforts to improve design and construction technologies to get better economic solutions for engineering problems. The FRP industry is a billion dollars industry worldwide. The FRP materials have potentials that fit them well for different industries. The FRP as a construction material offers advantages such as high specific strength, light weight, high performance speed, good durability, fitting well with structural retrofitting demands and also environmental resistance. The main characteristics of FRP materials that made them favorable in different technologies is the high strength to weight ratio and also high stiffness to weight ratio. For the structural engineering applications Halloyway ( Halloway2010) has explained and categorized FRP applications in civil Engineering using the flowchart shown in Figure 4. Figure 4 FRP applications in civil Engineering-Holloway 2012. FRP materials can be used as the main construction material or can be used in combination of conventional materials. At the present, using FRPs as the main material it is expensive; however it can be economical for special purposes. For instance, it can be used in manufacturing bridge decks. The weight of the bridge deck is quite important in design especially in long span bridges. The maintenance cost and time of maintenance and performance can also be diminished by using the FRP CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 12 materials. The fast assembly of FRP panels is one of the advantages of using FRP materials for this purpose. By using FRP panels both construction time and energy consumption will be decrease due to the weight of the material. The FRP structures are lighter in comparison to the structure with the conventional material. The reason is attributed to the difference between the densities of FRP materials in comparison to that of conventional materials, whereas for instance the unit weight of FRP is about 15kN/m 3 in compare to 23.6 kN/m 3 for concrete. Another example of using FRP materials as construction material is the road culverts. The FRP is corrosion resistant and can be a great choice in underground applications where there is a risk of corrosion due to harsh environment. The culverts which constructed by FRP material can be beneficial in conditions where both steel and concrete culverts are vulnerable. Using FRP cables in suspension bridges instead of steel cables is another application area where FRPs are used as main construction material. This application requires materials that incorporate high tensile strength, low weight and corrosion resistance. In this context, the FRP materials are fitting well with demands related to offshore structures and oil industry. FRP materials can also be used for strengthening and repair purposes. For instance, the FRP material are used for retrofitting concrete structural elements. Retrofitting of concrete columns to withstand earthquakes is a common and widespread practice. The repair and strengthening of deteriorated and damaged infrastructure has become one of the most important challenges that authorities face. For instance, in many countries the need for rehabilitation of existing structures is growing, especially in developed countries with their infrastructure built in the first half of the last century. Many of the structures that were built after World War II had little attention to durability issues. These structures are now in need of urgent repair and retrofit. (L.C.Halloway 2010). The FRP materials can also be used instead of steel rebars as reinforcement in reinforced concrete structures and pre-stressed concrete structures. FRP rebars can resist oxidation and corrosion from sea-water, de-icing salts and the abrasive environment. They have a better strength to weight ratio than steel. The world’s first highway bridge which was constructed by using FRP instead of steel rebar was in Germany in 1986. Nowadays there are hundreds of constructed or in construction projects over the world mostly in North America and Japan with FRP rebars instead of steel. As it was mentioned, composite cable applications in the infrastructure are used in the construction of suspension and stay cables for bridges, pre-stressed tendons for various concrete structures and external reinforcements for structural beams. All these applications require materials that incorporate high tensile strength and, in addition, require characteristics such as corrosion resistance and light weight. (L.C.Halloway 2010). As it was mentioned, one of the applications of FRP martial is in bridge construction. FRP material can be used in different parts of bridges. FRPs can also be used in construction of whole FRP bridges. . One of the first examples is Tech 21 Bridge in Ohio United State. This bridge was the first fully composite vehicular bridge in the USA. Due to light weight of structure, compared to its concrete counterpart, the structure was installed in just 6 weeks (Friberg and Olson, 2014).This two lane bridge has a length of 10 meters and is 7.3 meters wide. The first all composite bridge in Europe was constructed 5 years after the first one in USA and in England. Mill Bridge was constructed in 2002.The Mille Bridge has the CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 13 10 meter span and 7 meters width. A series of load tests were performed on the bridge during 8 year period (Friberg and Olson 2014).The first test was performed after the construction, the second one was conducted three years after construction and the last test was done eight years after construction (Canning, 2007).The load tests were carried out by passing a military tank over the bridge in order to control the static deflections and the dynamic performance and creep behavior (Canning, 2007) .The results of all tests were all positive and reasonable. Another project was conducted in 2004 in Spain. This bridge was the first highway composite bridge in Europe with the 46 meters length and 8 meters width. In the bridge trapezoidal cross sections was made by fully FRP composites (Friberg and Olson 2014). Manufacturing of FRP using glass and carbon fibers in combination with epoxy and vinylester resins is very common construction. In most of the applications, glass fiber reinforced polymers (GFRP) are used due to their comparatively low cost. The Glass fibers however have relatively low elastic modulus and exhibit some durability concerns in alkaline environments. The Carbon FRPs have relatively high elastic modulus, comparable to steel, and are more attractive but more expensive. The Aramid FRPs are sensitive to creep and also display poor durability characteristics and more susceptible to moisture absorptions. The Aramid FRPs also perform poorly at high temperature. Figure 5 Bridge with FRP deck during construction and after compilation. 2.4 Design approaches During the recent years different committees have prepared design guidelines for FRP construction. However, at present there is no accepted official international codes available to design FRP composites for infrastructure applications (Friberg and Olson, 2014). One of the guidelines is the European approach from the European standard polymeric Composites Group (EUROCOMP). The first version of EUROCOMP was written in 1996 based on scientific researches which were conducted before 1996. In EUROCOMP compilation there was a try to depict it like Euro code and in order to make it more useful for designer and engineers that have been working with Euro code. In the EUROCOMP connections are considered separately and not as an entity (Cholosta 2012).The other guidelines that contain the valuable recommendation and useful information on connections are BD 90/05 standard, Fiberline design manual CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 14 and also Design guide for FRP connection written by Mosslam. (Mosallam, A. S., & Knovel, 2011). American society of civil engineering realizes two design approaches for connection namely (1) allowable stress design approach and (2) load resistance factor design (LRFD). The Eurocomp (Structural design of polymer composites deign code and handbook) presents four different approaches for designing connections as (1) simplified procedure (2) rigorous procedure (3) design by testing and finally (4) numerical method. Allowable stress design which was popular and was used for decades in civil engineering aspects has been recently replaced by LRFD approach. In the allowable design approach only one safety factor (SF) is in use and it is not sophisticated enough to weight all effective parameters. Whereas in LRFD method a better estimation of applied loads and also expected strength of particular member is weighted and is under consideration. The EUROCOMP design code and handbook published in 1996 adopted the LRFD approach. Base on the EUROCOMP design code simplified and rigorous procedures can only be used to a limited number of joint whereas, the testing approach can be used and applied for all connection. In general, based on EUROCOMP design code or other codes a connection should be designed for resistance and should be effective enough to work properly in its intended life. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 15 3 Connections It is expected that by improving the knowledge on FRP connections, the application and use of FRP as a new material with anisotropic properties for different purposes become more facile. Most of the time FRP elements and especially structural pultruded seems similar to steel profiles and the connection between these elements seems to be similar as steel connections. In the review it will be seen that FRP connections are similar to steel elements connections in general, although there are important differences between them. As it was mentioned earlier one of the unsolved questions for extended usage of FRP materials is the lack of the knowledge about connection’s behavior. For instance, a recent feasibility study at Chalmers University of Technology indicates that the FRP panels can be used in underground culvert bridges and they can work properly. But the study does not cover the assembling issues and also information related to connections. In the following parts ,more general information about different kinds of connection in FRP will be discussed. EUROCOMP has classified FRP connections into three main groups based on importance as - Primary structural connection - Secondary structural connection - Non-structural connections And also has divided joint categories base on joining techniques in three different categories as - Mechanical connection - Bonded or adhesive connection - Combined connection 3.1 Joint categories and joint technique In this part a general overview about different kinds of connection will be discussed and in the following parts each kind will be discussed more in detail. As it was mentioned before the EUROCOMP code has classified FRP connection into three groups as - Mechanical connection - Bonded or adhesive connection - Combined connection The Mechanical connections are defined as shear loaded fasteners or axially loaded fasteners like bolts, rivets, contact strap and embedded fasteners (EUROCOMP). The research work on using mechanical joints especially bolted joints for FRP began in aerospace industry in the mid 1960s (Mosallam , & Knovel 2011) . The EUROCOMP guideline categorizes bonded connection like adhesive bonded joints, laminated joints, and moulded joints, bonded insert joints and cast in joints in the bonded or adhesive connection group. The Adhesive connection is realized as the most common method of connection in FRP material (Mosallam, & Knovel 2011). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 16 The adhesive joints in essence transfer loads as shear forces while holding the geometry of the shape (Mosallam, & Knovel 2011).The Stress concentration in adhesive connection is less compared to bolted and other mechanical connections. The catastrophic failure or brittle failure is expected when brittle adhesives are used. Another problem which is expected in these kinds of connections is expected when using ductile adhesive. In this case creep is a problem and may occur. Usually in these kind of connections adhesive is known as the weak link in the joint (Mosallam, & Knovel 2011) and it is not fit with the philosophy of designing the connections. However, efforts have been made to ensure that failure in the adherent precedes failure in the adhesive (Mosallam, & Knovel 2011).The adhesive strength is a function of percentage of void, thickness variation, environmental effects, processing variation, deficiency in surface preparation and other factors that are not always properly controlled (Mosallam, & Knovel 2011). The adhesive connection needs special treatment and also is sensitive to environmental conditions. The structure cannot be loaded directly after using the adhesive and the adhesive material needs a certain time for chemical reaction. Regardless of time these kinds of connections are also sensitive to the temperature and humidity. For instance, a rigid epoxy provides high strength at room temperature, but essentially no strength at -23 C or 177C (Mosallam, A. S., & Knovel 2011). Based on the EUROCOMP categorization the third group of connection is combination of the first two kinds and it was named as combine connections. The combined joints are known as bonded-bolted and bonded-riveted joints. It is expected that by combination of these two methods a better mechanical performance would be obtained. In this kind of connection a better strength and fatigue life is expected .on the other words the superposition of the stiffness and strength and also fatigue life is bigger in these kinds of connections. The analysis and design of combined joint is very complex and requires using nonlinear techniques (Mosallam& Knovel 2011). The table 9 shows a comparison of three joining methods. Table 9 General characterization of different connections. (EUROCOMP). Characteristic Mechanical Adhesive Welded Stress concentration at joint High Medium Medium Stress to weight ratio Low Medium Medium Use with non-rigid polymer Inserts required Yes Yes Seals assembly (water tightness) No Yes Yes Thermal or electrical insulation No Yes Yes CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 17 Attractiveness Bad Good Good Fatigue endurance Bad Good Good Sensitive to peel loading No Yes No Disassembly Possible Impossible Impossible Inspection Easy Difficult Difficult Skill required fabricator Low High High Heat or pressured required No Yes Yes Tooling cost Low High High Time to attain ultimate strain Instantaneously Long Short Typical joint configurations are shown in the figure 6. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 18 Figure 6 Joint configurations, (EUROCOMP). The Pros and cons of different kinds of connection are presented in the tables 10. The most challenging issue in mechanical connection can is the stress concentration problem while the easy assembly without need of preparation is the advantage of such joints. Table 10 Pros and cons of mechanical connection. Mechanical Connections Advantages Disadvantages Requires no special surface preparation Low stress or stress concentration Can be dissembled Special practice required in assembly; results in time consuming assembly CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 19 Ease to inspection Fluid and water tightness In the bonded connections, the high strength is an advantage while surface preparation and curing time is considered as a negative aspect. Table 11 shows the most important pros and cons of bonded connection. Table 11 Pros and cons of bonded connection. Bonded Connections Advantages Disadvantages High joint strength can be achieved Cannot be disassemble Low part count Required special surface preparation Fluid and weather tightness Difficulty to inspection Potential corrosion problems are minimized Temperature and high humidity can effect joint strength Smooth external surfaces The prediction for the behaviour of combined connections is hard ,moreover the avalible information about the combined connection is not too much available for structural enginnering.Howevr, it is expected to have a better performance from the structural point of view. The advantages of combined connection is shown in the table 11. Table 11 Pros and cons of mechanical connection. Combined Connections Advantages Disadvantages Bolts provide support and pressure during the assembly and curing Structurally bolts work as back up in an intact joint, bolts carry no load Growth of bond line effect is hindered by bolts CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 20 4 Behavior of connections 4.1 Mechanical connections Self-tapping screw, rivets and also bolts connection are the common forms of mechanical connectors. The Self- tapping screw is known as the simplest form of mechanical connectors and is very efficient when the access to the other side of FRP plate is impossible (Mosallam& Knovel, 2011) .The rivets are known mostly as good connectors for plates less than 3 mm thick however; in the thicker plates using rivets it is not advisable. The clamping pressure resulted from rivets is not ideally controllable and the process of riveting can damage the FRP internal structure. The bolted connection is one of the best forms of mechanical joining for FRP materials and is the most common type used in civil engineering applications. This kind of connection has been used in FRP trusses, beams, and braced frame structures, as well as multicellular FRP bridge deck structures. In the connection both metallic (usually galvanized or stainless steel) and FRP bolts can be used as fasteners (Zoghi, 2013). Unlike adhesive connection brittle failure is not likely in this kind of connection. The stress concentration is great deal in this kind of connection. Since the FRP materials are anisotropic, the stress concentration at a circular hole can be as 8 times bigger than the average stress, compared to 3 times of average stress for isotropic materials like steel (Collings, 1987). In the FRP bolted connection the failure modes are similar to steel mechanical connections, however, the mechanism of the damage and failure initiation and propagation are basically different. The nominations of different parameters in a mechanical connection are shown in figure 7. Figure 7 Nomination of different parameters, EUROCOMP. The mechanical joints are classified according the way that it transfers loads. One connection can transfer loads in shear or tension or combination of shear and tension. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 21 The Shear loaded fasteners are also dividing into two groups base on whether the applied load is acting eccentrically or concentrically. The figure 8 shows the three load transfer mechanisms in mechanical connections. Figure 8 The Load transferring in a connection, (Mosallam & Knovel 2011). The EUROCOMP design code has suggested that several parameters should be considered in mechanical connections including (a) Design parameters  Geometry (width (w), spacing (wy or py) , edge distance(e), side distance(s), hole pattern, etc.)  Hole diameter (dh) and bolt size(db)  Joint type (single lap, double lap, etc.)  Plate thickness  Loading condition (tensile, compressive, shear, etc.) (b) Material parameters  Fiber form and type  Resin  Fiber orientation  Form of construction (e.g. solid laminate, sandwich construction, etc.)  Stacking sequences CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 22  Fiber volume fraction  Fastener material (c) Fastener parameters  Fastener type (screw, fastener, rivet, etc.)  Clamping force  Washers  Fastener and hole size (d) Long term and environmental exposure factors  Creep and creep rupture  Humidity  Temperature  Load cycling  Chemical attacks  Stress corrosion and etc. 4.1.1 Failure in mechanical connections The failure in the mechanical connection even in the FRP material is not sudden and this is one of the priorities and benefits of the using the mechanical connection, in other words, the warning before the failure is known as a great benefit of mechanical connections. The Composite joints subjected to compression are less sensitive to joint geometry (edge, distance, width and thickness) and are generally stronger than joints subjected to tensile forces (Mosallam& Knovel, 2011). For this reason the bolted connections in tension are in more focus than joints in compression mode. In the FRP mechanical connections there are two important limitations compared to steel mechanical connections. The conventional fully plastic method of designing metallic bolted joint is not valid in FRP connections. It is common in FRP connection that failure takes place at 2% strain however; in the ductile metal the expected strain is much larger than 2 percent and this ductility cause drastic load distribution between different fasteners. In the connections subjected to tension seven probable failures is expected as following 1- Bearing failure 2- Shear out failure 3- Net tension failure 4- Cleavage/tension failure (tension and shear out failure) 5- Bolt failure 6- Punching failure 7- Combination of the modes 1-6 Figure 9 shows the different modes of failure in the bolted connection. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 23 Figure 9 Common failures in bolted connection, (Mosallam, & Knovel 2011) EUROCOMP suggests that failure analysis shall be based on detailed of stress distribution around the fastener holes. Moreover, the EUROCOMP design suggests that the failure analysis shall evaluate in four modes as net-section, bearing, and shear-out and fastener shear failure in the mechanical connections. In the following sections the most common forms of failure is going to be presented more in detail. 4.1.1.1 Bearing failure Among different modes of failure the bearing failure mode is less catastrophic and the least possible mode of failure. However, in order to ensure an efficient bolted design, the bearing strength should be kept as low as possible on the most critical fastener in the composite structures (Mosallam & Knovel, 2011) .Reaching the ultimate Bearing compression strength on the loaded on the half of the bolt, is the reason for this kind of failure, hence compressive strength of the zero degree fibers as well as the clamping pressure are important parameters in determining bearing strength (Collings, 1977). Several factors affect the joint bearing strength. For instance, increasing the clamping torque and using the adequate washer diameter will increase the joint bearing strength (Mosallam, A. S., & Knovel 2011) .The fiber direction and shape of orientation of them is also affecting the bearing strength. In order to prevent the bearing failure mode it is recommended to use weaker bolts which are more compatible with limited bearing strength of lamented FRP material. Using the stronger fastener is not fitted well with FRP composite material. It is also recommended to use more numbers of small diameter bolts rather than a few numbers of larger bolts (Mosallam, A. S., & Knovel 2011) .The ASCE plastic design manual also suggests bearing strength as the average bearing stress at the deformation of 4% CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 24 of the bolt diameter. Regardless of what was mentioned above, some joint geometrical parameter have great impact on the joint bearing strength ,while others have minimal or do not have effect.(Mosallam, A. S., & Knovel 2011) .For instance, the increase in the hole diameter to thickness ratio ( 𝑑 𝑡 ) will results on an appreciable decrease of bearing strength of connection (Mosallam, A. S., & Knovel 2011), while the other geometrical parameters, such as edge distance to hole diameter ratio ( 𝑒 𝑑 ) and joint width to diameter ratio ( 𝑤 𝑑 ) control the ultimate failure mode of the joint and they have little influence on joint bearing strength (Mosallam, A. S., & Knovel 2011) . An average bearing stress assumed to act uniformly on the cross sectional area of the hole in the uniaxial loading can be calculated from. 𝜎 = 𝑃 𝑛 ∗ 𝑑 ∗ 𝑡 (4.1) σ, average baring stress P, load n, Number of bolts t, thickness of the section d, bolt diameter 4.1.1.2 Net section failure The net section tension strength in bolted connection is dependent to the geometry of a connection and also the tension strength of material. This kind of failure is more probable when hole diameter to width ratio is large and bypass to bearing load ratio is high. Moreover, in essence this mode of failure is brittle. Net section failure is transverse to the direction of the connection force. It causes primarily by tangential tensile or compressive stresses at the hole edge and for joint subjected to the axial load, occurs when the ratio of by-pass load to bearing load is high, or when the ratio of the hole diameter to plate width ( 𝑒 𝑑 ) is high.(EUROCOMP) The average net stress 𝜎𝑛 or net stress across a section can be define easily as 𝜎𝑛 = 𝑃 (𝑤 − 𝑛𝑑) ∗ 𝑡 (4 − 2) Where P, tension force w, width n, bolts d, diameter within the section. However to the basic formula above, it is more logical not to ignore the effect of stress concentration 𝜎 𝑛 = 𝑘𝑡𝑒 𝑃 (𝑤 − 𝑛𝑑) ∗ 𝑡 (4 − 3) CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 25 The 𝑘𝑡𝑒, is defined as elastic concentration factor which is related to maximum stress adjacent to the hole. This factor is known as elastic isotropic stress concentration factor also. This factor only is recommended in the net tension failure mode. The behavior of bolted connection in tension is not like steel bolted connection. In the steel bolted connection is expected that by deformation and yielding some stress concentration diminishes, whereas in the FRP bolted connection the behaviour of connection is quite linear elastic until failure and high stress concentration is expected on the connection. The Stress concentration factor or 𝑘𝑡𝑒, was defined by Hart-Smith (1994). The Stress concentration factor for net tension failure was defined on that research in two kinds of single bolted joint and multiple bolted joint both in loaded and unloaded cases. For Single bolted joint,  Loaded hole:(most of the time θ can be ignored in this case) 1 𝑘𝑡𝑒 = 𝑤 𝑑 + 𝑑 𝑤 + 0.5 (1 − 𝑑 𝑤 ) 𝜃 ≈ 𝑤 𝑑 + 𝑑 𝑤 (4 − 4) Where 𝜃 = ( 𝑤 𝑒 − 1) for 𝑒 𝑤 ≪ 1, 𝜃 = 0 for 𝑒 𝑤 ≫ 1 (4 − 5)  Unloaded hole: 𝑘𝑡𝑒 = 2 + 𝑑 𝑤 + (1 − 𝑑 𝑤 ) 3 (4 − 6) For multiple bolted joints with identical holes:  Loaded hole: 𝑘𝑡𝑒 = 𝑝 𝑑 + 0.5 (1 − 𝑑 𝑝 ) 𝜃 ≈ 𝑝 𝑑 (4 − 7) 𝜃 = ( 𝑝 𝑒 − 1) for 𝑒 𝑝 ≪ 1 = 0 𝑓𝑜𝑟 𝑒 𝑝 ≫ 1 (4 − 8)  Unloaded hole: 𝑘𝑡𝑒 = 1 + 2 (1 − 𝑑 𝑤 ) 3 2 (4 − 9) Where θ ,a non-dimensional factor which is a function of e/w in the case of single bolted joints and a function of p/e for multi bolted joints p, is defined as hole pitch or on the other words the distance between the bolt holes centerlines. As stated before, the tensile strength is highly dependent to fiber orientation. The fiber parallel to the load carry most of the load, and for CFRP, failure is initiated at the stress concentration at the edge of the hole which is perpendicular to loading axis. (Potter 1978). The behavior of GFRP is expected to be more complex under tension. 1 Formula from Hart-Smith(1994) CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 26 It is expected that the complex combination of shear and tension causes failure. The concentration factor that defined previously can also affected by fiber orientation in ultimate failure of a composite. The Hart-Smith report defined stress concentration factor for ultimate failure which is fiber orientation dependent as 𝑘𝑡𝑐 = 𝐶(𝑘𝑡𝑒 − 1)+1 (4-10) Where C as stress concentration coefficient factor is defined in table below and for brittle material C = 1 was recommended. For ductile material the C coefficient is more close to zero. For instance, for aluminum C = 0 was recommended by Hart-Smith report. Table 12 Stress concentration coefficient Fiber orientation degree C factor 0 0.33 45 0.21 90 0.25 It should be noted that, in the case of having the Aramid fibers, the Aramid fiber has low lateral adhesion strength in compare to high tensile strength and tends to split along the fiber axis. This relieves the stress concentration at the hole edge and contributes to the tensile strength of the connection (Colling 1987). 4.1.1.3 Shear out of failure The Shear out of failure is happening when the shear stress is affecting the connection and take places along out of plane on the hole boundary in the principal fastener. Most of the time shear out failure is a consequence of bearing failure because of short edge distance, however, it also is probable in highly orthotic laminated and it is independent of the end distance. The shear out of failure is a combination of in plane and interlinear shear failure. It should be noted that the bolt shear out failure which is caused by high shear failure in the fastener is quite rare in FRP material. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 27 Figure 10 Shear out failure, (Mosallam, A. S., & Knovel 2011) In order to prevent the shear out failure in the FRP composites optimization of fiber direction is indispensable. This optimization is necessary especially in the parts at the connection zone. The shear-out failure is more common for fiber pattern that are both rich in 0 degree piles and deficient in 90 degree piles .In should be noted that the most pultruded commercially manufactured are unidirectional and the bidirectional ones are rare. Figure 11 Shear out failure, (Mosallam, A. S., & Knovel 2011). It is clear that in order to prevent shear out failure the shear strength of material should be more than shear force. The shear stress or τ produce by force P is calculated from the same way that it is calculated for isotropic material. 𝜏 = 𝑃 2𝑒𝑡 (4 − 11) 4.1.1.4 Cleavage tension failure Cleavage failures is another form of low strength failure resulting from the too-close proximity of the end specimen (Mosallam, A. S., & Knovel 2011).This type of failure CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 28 is started at the joint end rather than adjacent to the bolt hole (Mosallam, A. S., & Knovel 2011).In this kind of failure the combination of tension and shear is the reason of the failure. Most of the times the cleavage is starting from by incomplete net tension failure. In summary, the cleavage failure can be avoided by selecting the appropriate edge distance and by optimizing the joint member laminate architecture which should contain an adequate percentage of transverse 90-degree piles (Mosallam, A. S., & Knovel 2011). 4.1.1.5 Bolt pulling through laminated failure mode This kind of failure takes places for connections with sufficiency large thickness to bolt hole diameter( 𝑡 𝑑 ). In this case the bolt head is pulled through after bolt bending (Mosallam, A. S., & Knovel 2011). The rivet in FRP material mostly is using in the single shear mode and bolt pulling through laminated failure is common on them. The bolt pulling through is a function dependent to the washer diameter. The figure below shows this kind of failure effective parameter and mechanism more clear. 𝑁𝑠𝑑 ≪ 𝑓𝑉,𝑅𝑑 ∗ 𝜋 ∗ 𝑑𝑟 ∗ 𝑡 Figure 12 Bolt pulling through failure, EUROCOMP 4.1.2 Effective parameter in mechanical connection As it was mentioned in the in the introduction of mechanical connection several parameters are effective in connections and joint strength. The effective parameters were divided there in five main groups. In the following part of this report influential parameters are explaining and discussing more in detail in mechanical joints. 4.1.2.1 Thickness The effect of joined member thickness ,t, is usually expresses as a ratio of the hole diameter to thickness (Mosallam, A. S., & Knovel 2011).The ratio of (d/t) is s suggested to be greater than one in order to decrease the possibility of bolt failure. The joint ultimate bearing strength increases with decreasing the (d/t) ratio (Mosallam, A. S., & Knovel 2011).Based on this fact, the designer should try to use CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 29 smaller (d/t) values, nothing that a lower limit exists below which fasteners would fail in shear (Mosallam, A. S., & Knovel 2011). The amount of this value varies in each reference or recommendation. The design guide for FRP connection has recommended not using this ratio less than 1,2. The EUROCOMP DESIGN code has limited this value between 1, 2 to 1,5. Then number of publications and researches including the effect of thickness and have focused on thickness effect are not too many. Between them Colling and Mathews have mentioned about the effect of thickness better than others. 4.1.2.2 Width Regardless of the (d/t) ratio which has influence in FRP connection, the hole diameter to joint width ratio is important and effective. The bearing failure takes place in composites joints with small hole diameter to joint width ratio (Mosallam, A. S., & Knovel 2011). In general, it is expected that by increasing the value of the w/d the joint stiffness increases also. By changing the member width and when it is increasing the mode of failure also will change from bearing to the net tension failure. Some experimental results suggest that there is not clear point for changing the failure mode from one to the other failure mode in this case and as it was mentioned instead of a point of transmission considering a region of transmission is more acceptable. (Mosallam, A. S., & Knovel 2011). Between the different kinds of failure net tension failure is highly dependent to the width. Hence, the FRP material cannot reduce stress concentration by help of plasticity like what happens in metal connections. The effect of width is most marked for laminated with high proportion of zero fibers and least for 45 fibers laminates. (Collings 1977). Figure 13 Bearing failure versus Net-Tension failure, Mosallam, A. S., & Knovel 2011) 4.1.2.3 Edge distance The 𝑒 𝑑 ratio is the other effective parameter for composite joint strength. The edge distance or, e, is the distance from the corner hole centre to the edge. In the case that e/d ratio decreases the failure mode changes from bearing mode to the shear out failure mode. There is a difference between this kind of situation in the FRP composite with other material like steel in the similar case, hence, the shear out failure CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 30 results from unsatisfactory fiber pattern and is not because of shorter edge dimension. The design guide of FRP connection (Mosallam, A. S., & Knovel 2011) has mentioned that by increasing the edge distance until it equals to the bolt pitch, an appreciable increase in both bearing and tensile strength is expected ,but it does not mean the enough resistance to shear out failure is happening. One of the researches about the effect of edge distance was conducted by Hart-Smith. They have reported that a minimum value of the e/w is required to develop full net section strength. In order to that research, edge distance must be greater than or equal to the side distance (EUROCOMP). 4.1.2.4 Pitch or bolt spacing In order to use the full strength of all bolts minimum distance between the connector should be defined properly. The distance between the connectors also is defined as a ratio of fastener diameter in most of the materials. The Design guide for FRP connection (Mosallam, A. S., & Knovel 2011) suggests five for (p/d) ratio as the optimum value. The back pitch ratio is affecting by fiber orientation and also was one of the objectives of some different researches. The researches which were conducted by Godwin and Matthews 1980, Kretsis and Matthews 1985, Wong and Mathews 1981 proved that unlike the metal connections the multiple rows fastener are not effective in FRP material. The reason of this event might come from using the forcing fiber at the bolt hole and inability to re-establish untouched capacities between the rows. The figure 14 show the limitation of center to center spacing for one or two rows bolts. The Part A in the figure below indicates geometry for tension failure and the part B in the figure 14 indicates single row joint geometry for bearing failure. The Part C in the figure 14 is showing the geometry condition in the two row joints for bearing failure. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 31 Figure 14 Bolts center to center recommendation, Mosallam, A. S., & Knovel 2011. A. Single row joint geometry for tension failure B. Single row joint geometry for bearing failure C. Two row joint geometry for bearing failure 4.1.2.5 Fiber orientation effects As it was mentioned before FRP are fiber oriented materials and anisotropic, moreover, the fiber orientation is affecting the connection joint strength and failure mechanism as well. It is expected that the maximum strength of bolted joint achieves when the direction of applied load coincides with the direction of maximum load. Or on the other words, the most efficient lay up for mechanical fasteners are those that are nearly quasi-isotropic, as opposed to those that are highly orthotropic (Mosallam, A. S., & Knovel 2011). The highest bearing strength is attained with lay-ups containing about 50% of degree piles. When 45 degree piles are added to this fiber architecture, an improvement in the compressive strength of the concentrated 0 degree piles is expected (Mosallam, A. S., & Knovel 2011). In 1977 Colling showed that, the best joint performance for CFRP is possible when the combination of 0 and 45 degree applied. The figure 15 shows that CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 32 the place for shear failure dominant at 45 degree (Dat2000). When the degree increases the shear strength increases also and the bearing failure becomes the critical mode of failure (Dat 2000). Figure 15 Influence of CFRP fiber orientation on failure mode, Dat Duthinh 2000 The figure 16 shows the fiber effect in the different clamping mode. It is obvious from the figure that the pre stressed bolted connection are less susceptible to fiber directions, whereas the pin bolted connection are more susceptible. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 33 Figure 16 Effect of clamping pressure on the bearing strength from Colling 1982. One of the best experimental researches on the effect of the orientation was conducted in 1996 by Prabhakaran. The further researches which were done for instance by Turvey also confirmed the result of Prabhakaran try. The Prabhakaran research was conducted on 105 numbers of specimens from E glass. Three pined loaded joints were tested for angles between 0 to 90, the changing steps has been set with the 15 degree. The following conclusions were drawn from that study. 1- The tensile strength is increasing from 0 until 45 degree and the maximum strength will be getting on 45 degree. The tensile strength after 45 degree is not too much sensitive to the angle however it decreases. 2- Dual fracture modes were observed because of the dual laminate structure 3- When the angle of fiber orientation is increasing from 0 to 90 the joint bearing strength decreases. Thirty percent of degradation is expected on the 60 degree in compare to 𝜃 = 0. This part was fitted well with the results from Mallick and Little report (1985). 4- Shear out strength was observed the minimum amount at the 𝜃 = 75 ,with the 28% reduction. 5- Initiated cracks at the hole and propagated along the roving is accelerator for the ultimate failure of the joints. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 34 6- A sharp drop with 60% degradation was observed in the joint net-tension at = 30 . Although it was observed that the net tension strength was not sensitive to angle orientation up to 90 degrees. The ductile or brittle failure is quite important parameter in designing of connection. One of the researches which was conducted on connections and reported about the effect of the fiber orientation on ductility of a connection was conducted in 1996 by Yuan. It was reported that fiber orientation less than 45 is susceptible to ductile failure, while the brittle failure was observed for joints with larger than 45 degree joint orientation. 4.1.2.6 Loading The best strength of a connection are expecting when the loading direction fits well with the direction of maximum strength. In reality and in the construction it is really hard to achieve this kind of situation. In 1996 the study about effect of loading direction was conducted by Prabhakaran. The results of that research were stated in the previous part in the fiber orientation. In the normal condition, normal loading rate does not affect the strength of mechanical joints in FRP ,however, the FRP material are in essence viscoelastic materials and in the constant loading condition lower strength is excepted. The studies which were done by Schitz in 1996 and also Mosallam in 1999 reported the decrease up to 46% after 420 hours in strength. The fast rate loading or impact loading is not studied a lot for FRP connection, besides the reliable information is not available now. However, similar to creep, it is expected that impact loading will contribute to a reduction of bolted composite joint strength (Mosallam, A. S., & Knovel 2011). 4.1.2.7 Washer In the all kind bolted connection washer is reported as an essential part that has a role in generating and sustaining the integrity of bolted connection. The rule of the washer in mechanical connections can be change due to kinds of loading. For instance, when a connection is designed in order to transform the shear, washer can has two substantial rules. It prevents the damage to the bearing surface and also holds up the bolt from embedment. In the case that a clamping force is in purpose the washer also is quite important element in a connection. In this case the washer prevents the surface damage near to the fastener, hold up the bolt from head embedment, hold the hole geometry and finally maintains the tension in the connection. By using a washer torque force will be transferred to the specimen in the form of lateral pressure. In the FRP connection like other laminated washer prevents the composite from splitting through the thickness on the hole loaded side. By using a washer delimitation area will be spread to a larger area even outside of the washer boundary under compression. The mechanism of the washer rule is simple but useful. If the laminate is strained laterally the part of the laminate under the washer develop shear cracks but is not allowed to expand under compression, thus the lateral expansion, hence, the delamination is spread into a wider area outside the washer boundary (Mosallam, A. S., & Knovel 2011). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 35 Using the washer cannot change the form of failure in the connection. However, the size and also the thickness of washer is important on the fatigue life of a mechanical bolted connection. The EUROCOMP design code in the design requirement of bolted connection (part 5.2.2.3) has given several recommendations and obligations about the washer in the FRP bolted connection. For instance, the thickness of the washer shall be sufficient to provide even surface pressure on the outer laminate. The thickness of washer should be not less than 20% of the thickness of the outermost laminate through which the fastener passes. (EUROCOMP). The washer external diameter is quite important parameter in bolt pulling through failure mode. The least external diameter for FRP mechanical connection was suggested as last twice as bolt thickness in FRP design recommendation like CNR recommendation. The oversized washer is not expected to have lots of effects. Report from experimental study which was done by Klett indicated that using the oversize washer can increases static strength approximately 10% (Mosallam, A. S., & Knovel 2011) . As was mentioned earlier the Washer is quite important in fatigue behaviour and fatigue life also. Hence, the rule of washer in fatigue will be discussed more in detail in the fatigue chapter. 4.1.2.8 Environmental effects The mechanical properties of FRP material and FRP connection can be degraded under environmental condition especially under the harsh environmental condition. Between large number of scientific researches which were done in environmental effects in composite materials only a few number of them where specifically have been done on connections and specially bolted connections. One of the primary studies which was done in 1976 by Kim and Whitney reported that 40% of ultimate strength is decreasing on the effect of temperature and moisture. That report also reported that temperature has more influences than moisture on bearing strength. That research was one of the best researches on this subject and also mentioned that the combination of temperature and moisture will be 10% more effective in compare to temperature effect only and can be more destructive. 4.1.2.9 Bolt fit One of the other determinant parameters in the FRP connection is the bolt fit or on the other word hole clearance. In practice the best connection strength is expected when hole is fitted well with the bolt. The allowing clearance for the hole and the washer can reduce the joint bearing strength by up to 25% over that for a complete fit (Mosallam, A. S., & Knovel 2011). 4.1.2.10 Bolt torque By doing clamping on the bolt failure strength of the connection will be increased by preventing delimitation, however, the bolt in FRP material should not be over tightened. If it happens, it causes the damage to the composite laminated by pressing the washer to the material. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 36 Doyle in 1991 reported that the an increase of up to 31% in the PFRP bolted joint strength is achieving by the tightening the steel bolts by the maximum permissible tension .The applied load axial forces can be calculated from the following expression (Mosallam, A. S., & Knovel 2011) . 𝑊𝑓 = 12 ∗ 𝑇𝑠 𝑟. tan (𝛼 + 𝜃𝑠) (4.12) 𝑊𝑓=Applied axial force, Ib(N) Ts=applied torque, ft-Ib(N.m) r=radius of bolt bet area, in. (mm) α=angle of inclination of bolt threads θ=angle of static friction 4.1.3 Creep and relaxation in the mechanical connection Mechanical connections in FRP composites are prone to relaxation or/and creep like many different materials in construction, however the creep and relaxation in FRP is severe than conventional material like steel. The relaxation in the mechanical connections is a great concern in design and causes loss or reloading the bolted connections especially in pre stressed bolts, for instance, it was reported that for steel bolts (A325 AND A354 grade bolts) immediately after completion of the torqueing, a drop of 2% to 11% of the load had occurred (Mosallam, A. S., & Knovel 2011) .The information and researches about creep are limited and further investigation is needed in this subject. One of the proper researches which was done is this field was conducted by Mosallam and Schmitz (1996) .From that research the degradation up to 46% was reported in 420 hour test in the room temperature condition. The FRP material in essence has time dependent or viscoelastic behavior that manifests itself in creep, stress relaxation and dependence of strain stress diagram on the rate of loading (Kaw A.K2005). The creep in FRP will be mainly due to creep of the matrix and time dependent growth of fiber matrix deboning and resin micro cracking (Zoghi 2013). The relaxation initiates from high stress level in the threaded part in a connection. Because of that, design guide for FRP connection recommends to use the special type of epoxy for both bolts and nuts in order to decrease the relaxation effects. The material in the connector and the surface condition of the connector is quite important in relaxation. For instance, galvanized bolted which is recommended is harsh and corrosive environment has the double creep and relaxation as stainless steel bolted connection. The reason of this difference is coming from large creep on the zinc coat around the bolt. The Creep behavior of FRP material can be checked in two different levels based on level of loading. First, at the initial level of loading the creep effect is small and is recoverable, whereas in the higher level of loading the internal damages and deboning is irreversible and will cause permanent deformation. The EUROCOMP handbook like the other references has divided the creep effect and has explained the creep in the three stages as CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 37  Stage one, in the initial stage on loading and when the strain is less than 0.2%.In this stage the creep is recoverable by reloading however some shake down effects should be recognized.  Stage two, the next stage is constant but not fast growth of creep. In this phase because of finite damage take place the creep behavior is linear and can be partly recoverable.  Stage three, this stage is the final stage and causes the failure. In the high loading rate stage two may not occur and instead stage three is expected immediately after stage one. Most creep is expected in the glass and epoxy material whereas the least is related to carbon fibers. Time dependency of different kind of composites is shown in the figures 17. It should be noted that the first diagram (stress versus time) is showing more the relaxation effect and the second diagram (strain versus time) is showing more the creep effects. However, both creep and relaxation are close to each other and can be explain by the same mechanism (Vasiliev and Morozov 2001). Figure 17 Creep and Relaxation in FRP material, (1) Carbon (2) aramid (3) glass (4) epoxy composites, Vasiliev and Morozov(2001). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 38 Time dependency and also nonlinear behavior of composites under time can be clearer under the diagram which is named as isochoric stress strain diagram. Figure 18 Isochoric stress strain diagram, Vasiliev and Morozov(2001). 4.1.3.1 Effective parameter in creep Three main parameter as stress, temperature and moisture have more influence on FRP structure and FRP connections. As it was shown in the figure above (strain versus time figure 16 second part) creep is completely dependent to stress level and it was mentioned the level of the loading can omit the intermediate stage of creep. 4.1.3.2 Relaxation of bolted connection Relaxation on the bolted mechanical connection is quite important parameter; hence it can determine the rigidity of a connection after a loading. If the effect of relaxation is ignored in the designing process it can have harmful consequence in stability and workability of a connection, for instance after relaxation a presumed rigid connection can be changed to a hinge connection. In 1997 the study which was done by Prabhakaran on steel bolted in pultruded composite joints has suggested the following equation for bolt relaxation. 𝑃 = 𝑃0𝑇−𝐵 (4 − 13) Where P = load at time t, (kN) 𝑃0=Initial load, (kN) B = relaxation exponent constant 4.2 Bonded connections In the introduction for different connection types, the advantages and disadvantages of using bonded connection was explained. In the following, more parameters and CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 39 characterization related to bonded connection will be discussed in details. The Bonded connections in civil engineering application are not practical a lot. The technic for joining the elements by adhesive require time, warming condition and clamping or sometimes autoclave condition. That is why using these kind of connection is rare and also difficult in civil infrastructures. In total, in order to reach a proper bonded connection the different parameters like selecting the proper adhesive, surface treatment, adhesive kind, adhesive thickness, proper condition for clamping and fitting the adhesive with adherent should be under consideration. Mostly a proper bonded connection is known as a connection that can be fulfils the following conditions  The failure of joint should occur either in the adhesive or in its adherents but not in the interfaces.  Allowable shear stress and peeling stress of are not exceeded than the limit.  Allowable through thickness tensile stress and out of plane shear stress should not be exceeded (Zoghi 2013). The Adhesive joints can be effective in the condition that large surface are available and the members are thin. The Load transferring mechanism in the bonded connection complies with the nature of laminated material as transferring the load as inter laminar shear forces. The adhesive connections are mostly known between designers as connections with brittle behavior and brittle failure. Usually but not always adhesive connection has brittle behavior and this believe between designers is correct. Most of the time brittle adhesive that causes brittle failure has priority than ductile adhesive which are susceptible to creep. Several parameters have effect on the ductility and brittleness of a bonded connection. The parameters in adhesive and adherent like shear modulus and strength, maximum shear strength, tensile modulus and strength are effective parameter and important parameters in the bonded connections and should be under consideration. It should be noted that in the following the co-curing technique for bonded connection will not be discuss since, this method is very difficult to be used and to be performed in infrastructural applications. 4.2.1 Adhesive In the bonded connections adhesive has a substantial rule. Adhesion is associated with intermolecular forces acting across an interface and involves a consideration of surface energies and interfacial tensions (Mosallam, A. S., & Knovel 2011). Adhesive are categorizing based on different parameters like chemical properties, curing process activation time , curing process requirement and also form of them. In general it is recommended to use more ductile adhesive than brittle one especially when the material has the same stiffness. The most common adhesives that are used widely can be named as epoxies, polyurethanes, cyanoacrylates, methacrylate and also solvent cements. (EUROCOMP& Mosallam, A. S., & Knovel 2011). Epoxies are the most commonly employed adhesive for joining rigid structural composites (Mosallam, A. S., & Knovel 2011) .Epoxies provide strong joints and their excellent creep properties make them particularly suitable for structural CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 40 application, but unmodified epoxies have only moderate peel and low impact strength. (EUROCOMP handbook).The Design guide of FRP connection named epoxy as the most employed adhesive for getting the rigid structures and also mentioned that it is environmentally resistance (Mosallam, A. S., & Knovel 2011).To the vast benefits of epoxy the corrosion resistance and not or less affecting by water and heat should be also added (Zoghi 2013). Polyurethanes are the oldest adhesives and first were employed during World War II. (Mosallam, A. S., & Knovel 2011) . Polyurethanes display high resistance to impact load strong resistance to swelling in the wet and humid environments has ability to remain flexible in temperate environment (Mosallam, A. S., & Knovel 2011) .However, the polyurethanes has weak characteristics to be worked as proper adhesive. Unlike the epoxies it does not have proper creep characteristics .Moreover; it is quite temperature dependent as it is not recommended to use it in the temperature more than 66˚ (Mosallam, A. S., & Knovel 2011). The EUROCOMP hand book defined the polyurethanes as durable, load bearing adhesives, with adequate water resistance and high tolerant to oil and fluid. Typical service temperatures range for this kind of adhesive was defined from -75 to +80. (EUROCOMP). Cyanoacrylates are the other kind of popular adhesive in structural application but the application of them is less than the other form of adhesives in civil engineering applications. The Cyanoacrylates adherent can be used with different adherent. The bond strength of the material is quite high and because of that in some commercial paper is named it as super glue. The Super glue is most of the time watery and clear with low viscosity. Against the advantages of the connection which made by cyanoacrylates, the bonded connection by cyanoacrylates are brittle and it is more sensitive to heat and moisture and ultraviolet wave. In the rough and improper surfaces using cyanoacrylates may result of inadequate coverage because of the low viscosity of glue (Mosallam, A. S., & Knovel 2011). One of three kinds of adhesives which is recently has been in use in structural applications is methacrylate’s. These methacrylate’s offer some distinct advantages over the other types of adhesives, including the significant improvement in strength and toughness, fast cure rates and ability to bond to wide variety of adherents (Mosallam, A. S., & Knovel 2011) .Tables 13 and 14 which were extracted from EUROCOMP are showing more in details the advantages and limitation of each of adhesive and show some characterizations of several common adhesives. Table 13 Typical properties of common structural adhesives. EUROCOMP Property Epoxy Phenolic Polyurethane Acrylic Service temperature range ,˚C -55 to 120 -55 to 90 -155 to 80 -70 to 120 Impact resistance Poor Fair Excellent Good Tensile shear 15 15 15 26 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 41 strength ,N/mm 2 T-peel strength ,N/m < 525 2500 14,000 5250 Creep resistance Excellent Excellent Poor Poor Heat cure or mixing required Yes Yes Yes No Solvent resistance Excellent Excellent Good Good Moisture resistance Excellent Excellent Fair Good Gap lamination, mm None None None 0.75 Odour Mild Mild Mild Strong Toxicity Moderate Moderate Moderate Moderate Flammability Low High(liquid),Low(film) Low High Table 14 Advantages and limitation of common adhesive in structural application. EUROCOMP Epoxy Phenolic Polyurethane Acrylic Advantages High strength High strength Varying curing times Good flexibility Good solvent resistance Good hot water Tough Good peel Good gap filling capacities Good weathering resistance Good flexibility at low temperature Good shear strength Good elevated temperature resistance Good elevated temperature resistance Cure at RT or at elevated temperature Bonding the oil dirty surfaces is possible Relatively low cost Relatively low cost Moderate cost Moderate cost Room temperature cure Limitations Exothermic reaction High-pressure Both uncured and cured are Low hot- temperature CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 42 moisture sensitive strength Exact proportion needed for optimum properties High temperature cure Both uncured and cured are moisture sensitive Low hot- temperature strength Two- component formulations require exact measuring Two-component formulation Poor elevated temperature resistance Toxic One- component formulation often require refrigerated storage and elevate temperature cure May revert with heat and moisture Flammable Short pot life Limited shelf life Dispensing requirement needed Odour The EUROCOMP has defined shear modulus, shear strength, shear strain, tensile modulus and tensile strength as the most important mechanical properties of adhesives which should be under-consideration for design. Material properties of adhesive is quite different base on manufacturer, hence it was recommended that get the mechanical properties of adhesive from manufacturer or from standard testing. 4.2.2 Bonding mechanism Adhesive bonding occurs as a result of three type of interaction between the adhesive and adherent at their interface (Agarwal and Broutman 2006). Three kinds of bonding as chemical bonding, mechanically interlocking and secondary or electrostatic bonding are known as the forms of bonding. In the chemical bonding the adherent and adhesive will be chemically connected to each other after the chemical mechanism and reaction. This kind of mechanism is known as the most effective form of connection between adherent and adhesive (Mosllam,A.S 2011).The mechanical interlocking is taking place when the adhesive fill into micro pores of adherent. It is enhanced by surface roughness and provides a micro mechanism to mechanically inhibit separation of adherent and adhesive (Agarwal and Broutman 2006). The electrostatic force between adhesive and adherent is the mechanism for secondary bonding. These forces resist shear deformation at the interface and thus provide some shear strength to the bonded line (Agarwal and Broutman 2006). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2015:67 43 4.2.3 Surface pre-treatment The surface pre-treatment is considered as a quite important in the bonding process. The need for careful attention to remove the dirt’s and other contamination like chemical, wax, grease is quite essential in the bonded process and inadequate surface pre-treatment is the main causes of the bonded joint failure (Taib et al.2006).In the surface treatment it is recommended to remove both contamination and the weak layer from the surface. And it is expected that the pre-treatment provides surface conditions that can be more readily bonded (Mosallam, A. S., & Knovel 2011). The proper surface treatment in the FRP material can be achieved by help of the minor abrasion or solvent cleaning. However, the pretreatment of composites are typically more complicated that those employed for the boning of material such as wood, metal or concrete (Mosallam, A. S., & Knovel 2011). For instance, any abrasion by help of sandpaper should be followed by proper solvent wash like alcoholic solvents. 4.2.4 Failure modes Researches on bonded strength and also failure mode of bonded connection has been started a quite long time ago and the first use