Sternal Fracture Prediction for Human Body Models : Establishment and Evaluation of Fracture Criteria Using Finite Element Crash Simulations

Sammanfattning

Thoracic skeletal injuries, including sternal fractures, are common among occupants in vehicle crashes. Injury risk can be predicted in virtual crash simulations using human body models (HBMs) to represent the occupants. The aim of this thesis was to establish a fracture criterion for a new sternum model, calibrated against the results from a three point bending study. The fracture criterion was evaluated by performing crash simulations and comparing with crash data. Initially, the current sternum model used in the SAFER HBM (SHBM) was positioned into a reconstructed model of the bending test setup. The sensitivity of the setup and material parameters of the sternum were then investigated before a new sternum model was created, positioned, and calibrated against the results of the reference tests. Three fracture criteria candidates were established for the calibrated model before it was implemented into the SHBM. The HBM with the new sternum model was then positioned into a sled model, which was used to simulate frontal crashes at different velocities and to reconstruct crashes with and without sternal fractures. The analysis of the current SHBM sternum model showed that it was unable to accurately replicate the behaviour found in the reference tests. The new sternum model added complexity by including modelling of the manubriosternal joint and a variable thickness of the cortical bone. The joint was modelled using the same technique as for the intervertebral discs, which have a similar structure. Calibration of the material models for the new sternum model gave a cortical Young's modulus of 6.8 GPa and a yield stress of 48 MPa, which more closely replicated the behaviour found in the reference tests. The fracture criteria candidates found had averages around 2\% first principal strain in the cortical bone of the sternal body. These compared well with studies on fracture criteria for other cortical bones. Frontal crash simulations at different speeds (delta-v) showed strains ranging between 1.33\% and 3.30\% for delta-v ranging from 30 km/h to 100 km/h. According to the fracture criteria, sternal fracture was predicted to occur in frontal crashes with a delta-v greater than 65 km/h to 75 km/h depending on the criteria, which was slightly lower than what sternal fracture risk curves from accident statistics have predicted. In the crash reconstructions, all three fracture criteria predicted sternal fractures in 2/10 cases with sternal fractures, and in 1/10 cases without sternal fractures. The results of this thesis serve to provide some indication of the material and failure properties of the sternum. Further testing of the sternum is considered necessary to advance the prediction of sternal fractures. This includes material testing of the sternum cortical bone to quantify material properties for modelling, as well as failure strains to develop fracture risk functions. Structural tests of the sternal body and the manubriosternal joint are also vital, as these are needed to validate the presented sternum model.