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Macroscopic observation of healing process in stress‐whitened polypropylene under hydrostatic pressure

Stress-whitening and its healing phenomena in polypropylene under tensile load superposed on hydrostatic pressure up to 89.7 MPa have been investigated. Tensile samples of polypropylene were first cold-drawn beyond the yielding at atmospheric pressure to develop stress-whitening, and the stress-whitened samples were further drawn under various intensities of hydrostatic pressure (P = 7.6 - 89.7 MPa). As a result of the experiment, morphological changes in stress-whitening occurred in the necked gauge sections (R I and R II ) and in the shoulders of the neck (R III ). The macroscopic observations were made by optical microscopy on the entire specimen and on the sliced sections. The stress-whitening in the R I region was found to be either partially or nearly completely healed depending on the level of hydrostatic pressure at which the specimens were drawn. The cross sections of polypropylene specimens exhibited two concentric ring structures upon stress-whitening. The stress-whitened core region is surrounded by clear shell region. The size and degree of healing of stress-whitening in the core region were affected by the applied hydrostatic pressure.