Calibration and Modelling of Adipose Tissue Under Impact Loading

Licentiate thesis, 2017

Vehicular injury is one of the main reasons for traumatic injuries. Finite Element Human Body Models (FEHBM) have become very popular to assess car crashes and the subsequent injuries. It provides the possibility to predict stress and strain values in tissue level by representing anatomical structures in details. An essential requirement for the FEHBM is to exhibit human-life response, i.e. being biofidelic. Obese occupants are one of the vulnerable populations at higher risk of death and severe injuries in car crashes. However, the developed FEHBMs do not, neither in body shape nor the material properties, represent obese population. In particular, there is no appropriate constitutive model for adipose tissue (fat tissue) in FEHBMs. In the interaction between obese occupants and restraint systems both the body shape and the material property of adipose tissue plays an important role. Therefore the first goal of this research (and the main goal of this thesis) is finding and calibrating a biofidelic constitutive model for adipose tissue. To this end a nonlinear viscoelastic constitutive model was formulated. To have a reliable model calibration at large deformation and a wide range of strain rates (similar to car crash situations), test data of two experiments were used; the frequency-sweep test and the ramp loading-unloading shear test. Prescribing the power-law relation for shear stiffness, which is suggested in the frequency-sweep test, as a constraint in the ramp loading-unloading shear test considerably improved the model prediction for large deformations and high strain rates. To investigate the effect of uncertainties in model parameters and identify important parameters in different experiments, commonly used mechanical testing setups were analyzed. Global sensitivity analysis was used for this purpose. It was found that the amount of compressibility highly affects the behavior of adipose samples in high rates. It is important specially when studying how adipose tissue behavior affects the dynamics of obese occupant responses during crash situations.