The High Strain Rate Response of Adipose Tissue

Microscopy suggests that adipose tissue can be idealised as an oil-filled closed-cell foam. Collagenous viscoelastic basement membrane forms the solid walls of the foam and the cavities of the foam are filled with lipid. The lipid has sufficiently low viscosity that it can be treated as an incompressible inviscid fluid. Measurements of the uniaxial compressive stress versus strain behaviour of the tissue have been made for strain rates from quasi-static to 6,000s(-1). Screw driven tensile test machines were used to collect data at strain rates less than 200s(-1). A split Hopkinson pressure bar constructed from polycarbonate was used for strain rates greater than 1,000s(-1). The measured stress versus strain curves are non-linear with stiffening at increasing strains. The response at low strain appears to be strongly rate sensitive. It is instructive to fit a standard linear solid (or Kelvin model) comprising three elements: a spring (stiffness E(1)) and dashpot (viscosity eta) in series, which are both in parallel with a second spring (stiffness E(2)) to the data. The stiffness modulus E2 is taken as the average stress at 10% strain and low strain rates and is found to be 1.15 kPa. E(1) is taken to be 0.5 GPa which corresponds to the assumed bulk modulus of the tissue. A least squares regression fit of the experimental data gives a time constant of 97 ns.