Critical strains determining the yield behavior of s-PP

Video-controlled tensile deformation experiments giving true stress-strain curves were carried out on samples of s-PP. Dividing the total tensile defo rmation into elastic and plastic parts shows three critical strains (A, B, C) for which the differential compliance and the recovery properties change . It was found that all critical strains remain constant on varying the cry stallite thickness or the testing temperature. Point A marks the end of the linear-elastic range. The yield point, as given by the maximum on the engi neering stress-strain curve for a necking sample, essentially corresponds t o the second critical point, although the yield point is shifted relative t o B to higher strains and varies with temperature. It is associated with th e collective onset of inter- and intralamellar slip processes. At the third critical strain (C), the lamellae become destroyed, and fibrils are formed . The texture changes accompanying the drawing determined by a simultaneous measurement of wide-angle X-ray scattering (WAXS) patterns indicate a comm on activity of intra- and interlamellar slip processes setting in at A and a dominance of intralamellar block slips at moderate deformations above B. The Young's modulus and the yield stress show a large decrease with tempera ture, whereas there are only minor changes in the crystallinity. We underst and it as resulting from a change of coupling, both between the crystalline lamellae and of the blocks within the lamellae.