Correlation of Fatigue with the Dynamic Modulus of Polymers

Abstract

An experimental investigation of the relationship between fatigue and dynamic modulus of two polymers, using wave propagation techniques, has shown that the deter mination of dynamie modulus during constant-rate elongation of partially fatigued samples yields Information about the extent of fatigue, which is unobtainable from measurements of modulus during fatigue or from a conventional stress-strain test of the fatigued samples. It is possible to differentiate between normal and fatigued samples by comparing values of dynamic modulus at an arbitrary elongation subsequent to fatiguing. For polyethylene samples, a "fatigue severity index," based on frequency and stroke of axial cycling, is defined. The dynamic modulus measured subsequent to fatiguing is dependent on this index. The creep behavior of the two polymers tested. as reflected in measurement of dynamic modulus, is essentially different from the fatigue behavior disclosed by the same type of measurements. The dynamic modulus, measured subsequent to creep testing and during constant-rate elongation, provides information not obtainable by measurements during creep or from the stress-strain test by itself. The results support the theory that fatigue failure is due to fracture caused by the' formation and growth of micro-flaws under the conditions of frequency, strain, and temperature employed in the experiments.