Detailed FE rib modelling for fracture prediction

Examensarbete för masterexamen

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Type: Examensarbete för masterexamen
Master Thesis
Title: Detailed FE rib modelling for fracture prediction
Authors: Lundin, Linus
Storm, Simon
Abstract: The purpose of this thesis was to investigate if rib fracture can be predicted in dynamic analysis using the first principal strain estimate on subject specific finite element (FE) models of human ribs. The investigation was first to be conducted on models represented by an all hexahedral (all-hex) element based mesh. If the rib fracture was captured with a detailed all-hex modeling approach, the aim was to determine what would be the maximum level of simplification that could be used in the FE model, without losing the capability to estimate the fracture location. Subject specific FE models were developed to reproduce dynamic end-to-end rib displacement tests, conducted prior to this thesis, on twelve number sixth rib specimens. Pre-test, high resolution CT images of the rib geometries were taken to be individually processed with a cortical bone mapping algorithm to provide subject specific cortical bone thickness. The thickness distributions enabled a manual Hexa-Block meshing procedure of the cortical and trabecular bone of the ribs. The all-hex meshed cortical bone was in a later step converted to, eight node thick shell elements, followed by quadrilateral shell elements with nodal thicknesses. Material tension tests were conducted on coupons of the cortical bone to provide subject specific isotropic material properties. Isotropic linear elastic heterogeneous material properties for the rib specimens’ trabecular bone were obtained based on density estimates from the same CT data, though processed in an earlier Master’s thesis on the same set of rib specimens. In a later step, the heterogeneous material data was also homogenized to a linear elastic isotropic material representation. Four different modeling approaches were analyzed. The most detailed model used an all-hex mesh with heterogeneous trabecular material properties. The first simplification step used an allhex mesh with a subject specific homogenized trabecular material property. In the second and third simplification steps, the cortical bone was represented with thick and thin shell elements, respectively. Both employing heterogeneous trabecular material properties. Because of numerical instabilities unable to be resolved within the timeframe of the project, the thick shell modeling approach failed the energy balance assessment. Hence, the pertaining results were deemed unfeasible to evaluate. Overall, the model validations showed that seven out of twelve rib models had a non optimized reaction force-displacement response which agreed well with the original experiment. In addition, other metrics measured in the experiment were also accurately captured for the seven ribs, independent of modeling approach (excluding thick shell). However, only the all-hex models accurately captured the correct fracture locations. The five ribs that did not capture the force-displacement response also did not predict the correct fracture location. The disagreements for the five ribs are hypothesized to be owed to cortical porosity effects. These effects influence the quality of the coupon tension tests as well as the cortical bone mapping algorithm, which lack the ability to correctly represent changes in tissuelevel properties due to cortical porosity. The results imply that the human rib fracture is strain controlled and can be successfully captured in a subject specific finite element model which uses elements supporting a 3D stress state to model the cortical bone. Consequently, the most simplified model to capture the fracture location was an all-hex model with homogeneous material properties. The results provide guidelines for further development of the thorax used in impact biomechanic human body models.
Keywords: Teknisk mekanik;Applied Mechanics
Issue Date: 2018
Publisher: Chalmers tekniska högskola / Institutionen för mekanik och maritima vetenskaper
Chalmers University of Technology / Department of Mechanics and Maritime Sciences
Series/Report no.: Master's thesis - Department of Mechanics and Maritime Sciences : 2018:97
Collection:Examensarbeten för masterexamen // Master Theses

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