Usage of the adaptive quasi-linear viscoelastic model to predict load-unload, stress-relaxation, and sine load of porcine liver

High impact load and corresponding large strains of soft collagenous tissues, such as liver, require a proper characterization of its viscoelastic behavior. Hereby, the adaptive quasi linear viscoelastic (AQLV) model is a reasonable choice. However, as the model is calibrated at a slow strain rate it remains unclear if obtained parameters are valid for different, large strain rates. In the current study, AQLV model parameters were used from previous tensile tests on porcine liver to predict the stresses of fast load-hold, load-unload and sine load tests performed in a previous study. None of the load cases could be predicted properly. However, re-calibration of the model parameters with a least-squares algorithm yielded reasonable results for all load cases. In an additional evaluation, re-calibrated AQLV model parameters of the fast-ramp hold experiments were used to predict the stress curves for the sine loads, which were performed at a similar mean strain rate. Here, storage modulus was predicted properly, whereas loss tangent was significantly different from reference experimental values. Taken together, AQLV model parameters are not universal material constants, but are highly dependent on strain rate.