EFFECTIVE VOLUME CHANGES DURING FATIGUE AND FRACTURE OF POLYACETAL

Conventional tensile dilatometry techniques are extended to cyclic fat igue applications to study volume changes that occur during controlled -load cyclic fatigue of polyacetal. During fatigue, in-situ measures o f the irreversible and elastic volume change are monitored together wi th dynamic viscoelastic parameters (E', E '', and Tan delta], and chan ges in the energy densities (strain energy, potential energy, and irre versible work). The results show that the effective irreversible volum e of the polyacetal gradually increases over a wide range of applied c yclic stress. However, at high stress levels and/of frequencies (i.e., low-cycle, thermally dominated regime), the effective Poisson's ratio of the polyacetal increases as it softens (evidenced by the dynamic v iscoelastic data). Conversely, at lower stress levels, the Poisson's r atio continually decreases coincident with decreases in the loss modul us (E '') and the irreversible work density. These results are indicat ive of entirely different mechanisms governing the low-cycle (high str ess level] and high-cycle (low stress) regimes, Also, comparisons betw een tensile and fatigue dilatometry studies show that the dilational-s train response of samples fatigued at high stress levels are similar t o data obtained from monotonic tensile dilatometry. However, the dilat ion-strain response of samples fatigued at lower stress levels are dis tinctly different from low-cycle fatigue and tensile dilatometry.