Material Modelling of Adipose Tissue for Traffic Injury Prevention

Doctoral thesis, 2020

Traffic injury is one of the main reasons for traumatic injuries. Obese occupants are among the vulnerable populations with a higher risk of death and severe injuries. Notably, obesity is associated with a thick layer of subcutaneous adipose (fat) tissue. In case of a crash, this may influence how the lap belt engages with the pelvis resulting in submarining, i.e., the lap belt slipping over the iliac crest of the pelvis, causing severe injuries. A popular numerical method to study occupant injuries in motor-vehicle collisions involves using Finite Element Human Body Models (FEHBMs). However, current FEHBMs, such as the THUMS or GHBMC models, do not represent the obese population in body shape or material properties, and are unable to represent the submarining phenomenon. In particular, there is no appropriate constitutive model for adipose tissue in the FEHBMs, while the mechanical property of adipose tissue is important in the simulation of interaction between the human body and restraint systems or the impact with interior objects.

The first aim of this research was to establish a biofidelic constitutive model for adipose tissue mechanical response, at high strain rates and large deformations. For this purpose, a nonlinear viscoelastic constitutive model was formulated. Global sensitivity analysis was used as a tool to learn what mechanical properties of adipose tissue are identifiable from different test setups. Thus, a frequency-sweep test and a ramp loading-unloading shear test were applied to account for the adipose tissue behaviour at high strain rates and large deformations, respectively. The second aim was to identify which parameters influence submarining the most. It was found that the incompressibility (Poisson's ratio) of adipose tissue is the most important material parameter.
With regard to safety design, important parameters include lap belt angle and pelvis rotation. Due to a thicker layer of adipose tissue, the effect of these parameters becomes more important for obese occupants, resulting in a higher risk of submarining. These findings support the development of biofidelic FEHBMs, as well as suitable restraint system designs in order to reduce the risk of submarining.