Altered mechanical behavior of epicardium due to isothermal heating under biaxial isotonic loads

Recent isothermal biaxial isotonic tests suggest that increasing the temperature hastens the rate of denaturation of epicardium whereas increasing the mechanical load during heating delays this process, findings that are consistent with prior uniaxial tests on tendons. Yet, contrary to uniaxial reports, a clear time-temperature-load equivalency was not found in this multiaxial setting. There is, therefore, a need to delineate multiaxial thermomechanical behavior in greater detail, and ultimately, to correlate changes therein with the underlying microstructure. Toward this end, we describe a new experimental approach for quantifying heating-induced changes in the multiaxial mechanical response of thin sheet-like specimens. Illustrative results are presented for bovine epicardium subjected to nine different thermomechanical loading protocols. Among other results, it is shown that thermal damage tends to increase the stiffness at low strains and that overall changes in extensibility correlate well with the degree of thermal damage independent of the specific thermomechanical protocol. Multiaxial changes in behavior are nevertheless complex, and there is a need for significantly more testing before constitutive relations can be formulated.