Modelling of the mechanical behaviour of clips for automotive interior panels
Examensarbete för masterexamen
Applied mechanics (MPAME), MSc
In the automotive industry many different types of clips are used to attach components. Clips enable an easy assembly of components and gives a clean finish since they provide attachments invisible to the customer. The design of the clips affect how attached components move relatively as they are exposed to external loads, for example when the temperature in the vehicle rises. The design also determine the amount of manual force that is required for assembly and disassembly of attached components and therefore affects the ergonomic aspects of these processes. The purpose of this work is to study the mechanical behaviour of a metal clip used to attach interior plastic panels at Volvo Car Corporation (Volvo Cars). This type of clip is widely used to attach different interior panels including the front sill moulding to the car body. The clip together with the front sill moulding is numerically studied by use of the finite element method (FEM). A method to numerically predict the behaviour of the clip in terms of sliding capability and stiffness is developed. The results show that when the clip slides there are two sliding-phases with different force levels. First there is sliding between clip and panel and then sliding between clip and bracket. The effects of various friction coefficients between parts are studied and a combination of friction coefficients that correlate well to measured sliding capability is presented. The behaviour in terms of torsional stiffness in the clip joint was discovered to be non-linear for rotations around all axes. Furthermore a parameter study on torsional sti ness is carried out to establish the in uence of various parameters. Also a method to numerically evaluate the force required for assembly of the front sill moulding is developed. The method includes geometries and boundary conditions based on previously performed mechanical testing where a part of the front sill moulding together with one clip were included. Results show that the force contribution from the clip during assembly have two peak values, the first one due to sticking friction and the second one due to compression of the clip waist. The effects of varying hole sizes are evaluated and the results correlate well to previously performed physical tests. A parameter study is carried out on varied friction coefficients and velocity of assembly. The results show that when using the friction coefficients found from correlation of sliding capability, the assembly force is best captured as compared to physical tests. Furthermore, the effect on assembly force when including the complete front sill moulding and three clips was numerically investigated. For the first clip connection the behaviour during assembly is similar to the behaviour when only including a part of the panel. This result indicate that a simplified method is sucient in order to numerically capture the contribution to assembly force from the clip. Numerical evaluation of force required for disassembly was proven difficult without modelling residual stresses and hardening due to manufacturing of the clip. Using elastoplastic material data for the clip the disassembly force is underestimated and when using linear elastic steel the disassembly force is overestimated.
Maskinteknik , Mechanical Engineering