THE INFLUENCE OF TESTING METHODOLOGY ON THE FATIGUE-CRACK PROPAGATION BEHAVIOR OF CELLULOSE ESTERS

In the fatigue crack propagation behavior of cellulose esters, a crazing mechanism controls growth at low stress intensity (Delta K) levels while a plane-strain-shear yielding mechanism becomes dominant at higher Delta K values. The growth rate associated with the transition between these two mechanisms has been found to be a function of the material's yield strength. Most recent data have shown that the onset of this transition is also related to the test methodology. Constant-load-amplitude, K-increasing tests produce the most conservative (highest fatigue rate) values for the transition, whereas with decreasing stress intensity tests, the transition occurs at much lower growth rate values. It is important to note, however, that test methodology does not affect the relation between crack growth rate and Delta K when growth is dominated by only a single mechanism.