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 deformation 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 crystallite 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 engineering stress-strain curve for a necking sample, essentially corresponds to the second critical point, although the yield point is shifted relative to B to higher strains and varies with temperature. It is associated with the 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 common 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 temperature, whereas there are only minor changes in the crystallinity. We understand it as resulting from a change of coupling, both between the crystalline lamellae and of the blocks within the lamellae.