THE TEMPERATURE AND STRAIN-RATE DEPENDENCE OF MECHANICAL-PROPERTIES IN POLYOXYMETHYLENE

The computer aided design approach used in current applications of sem icrystalline polyoxymethylene (POM) requires high strain-rate mechanic al data. The primary aim of this work has been to measure the room tem perature modulus and tensile strength of injection molded samples of P OM of different molecular weights at cross-head speeds of between 10(- 5) ms(-1) and 10 ms(-1). We observe no major transition in bulk mechan ical behavior in this range of test speeds, the Young's modulus E, in particular, showing little strain rate dependence. This is rationalize d on the basis of tensile tests over a range of temperatures, these in dicating room temperature to correspond to the plateau in the E(T) cur ves (T-g for these materials is taken to be - 70 degrees C, and the DS C melting onset occurs at similar to 170 degrees C). The tensile stren gth increases as similar to log(d epsilon/dt) and the behavior is foun d to be highly nonlinear, strains to fail of the order of 1 being obse rved even at the highest strain rates, depending on the molecular weig ht;. It is believed that the yield stress of the crystalline regions d etermines the tensile strength above T-g, the higher degree of crystal linity associated with lower molecular weights resulting in a slightly higher tensile strength. Nevertheless, failure is qualitatively britt le, with no necking and relatively little permanent deformation. This behavior is discussed in terms of morphological investigations of the fractured samples by optical and scanning electron microscopy (SEM). I n attempting to relate ultimate failure to the molecular/crystalline s tructure of the samples, measurements of the critical stress intensity for crack initiation in mode I opening, K-IC, as a function of crysta llization temperature T-c have been carried out using compact tension specimens machined from injection molded and compression molded plaque s. K-IC increases with molecular weight and decreases with T-c at low test speeds (in spite of an increase in crystallinity with T-c). This is accounted for in terms of a crack shielding model for crack initiat ion and of molecular rearrangements occurring during crystallization w hich lead to a decrease in the effective entanglement density with T-c . The implications of this model are then compared with K-IC results o ver a range of cross-head speeds and temperatures.