EFFECTIVE AND RECOVERY STRESSES IN DEFORMATION STUDIES OF POLYVINYL-CHLORIDE AND POLYPROPYLENE USING THE MODIFIED STRAIN TRANSIENT DIP TEST

Partitioning the applied stress into internal stress components (effec tive and recovery) using the modified strain transient dip test is a u seful approach towards a better understanding of the viscoelastic natu re of polymers. The internal stresses of polyvinyl chloride (PVC) and polypropylene (PP) were measured successfully using this test on a com puter-controlled electro-servo hydraulic tensile testing machine which was designed for rapid step unloading in less than 1 s to avoid memor y effects of the polymers. A power-law relationship can be used to des cribe the variation of the internal stress components with strain. Act ual yield strains occurred at smaller values (less than 2%) than those obtained from a conventional stress-strain diagram (which for PVC and PP exceed 3.5% and 7%, respectively). This observation indicated that plastic yielding occurred much earlier and yield strains from convent ional stress-strain diagrams may be overestimates. For very ductile ma terial (PP) the activation volumes were comparable in magnitude to tha t obtained conventionally; whilst for less ductile material (PVC), the activation volume was four times higher. One of the main advantages o f stress partitioning is for the detailed definition of the extrapolat ed yield point which otherwise will be missed out in a conventional pl ot of applied stress and strain.