Cavitation during Tensile Deformation of Polypropylene

Abstract

The process of cavitation during tensile deformation of polypropylene was studied. It was shown that in injection-molded polypropylene samples cavities appear in the center of a sample shortly before yielding. With increasing deformation the cavities change their size, number, and orientation from elongated perpendicular to parallel to deformation. The cavitation process is visible also as a rapid increase of volume of deformed material. The cavitation could be suppressed by changing internal morphology of polypropylene by fast cooling. The samples prepared by compression molding followed by quenching, with less perfect crystals, were able to deform by plastic deformation of crystals without cavitation. However, for faster draw rate the cavitation in amorphous phase was preferred again due to stronger response of crystals.

Time and temperature effects on Poisson’s ratio of poly(butylene terephthalate)

“…In the author's opinion these high Poisson's ratio values have to be interpreted as scattered around 0.5, and affected by inaccuracies related to the correction of transverse deformation for long times and high temperatures. At the same time, it can not be excluded that the upper plateau could be higher than 0.5, due to non-homogeneity on a microstructural scale, already present in the materials (such as crystalline regions below the melting temperature [1] or porosity [28]) or developed during the deformational process (as cavitational effects [29][30][31][32], although these should have major importance at deformations higher than those investigated here). The decrease of stress was simultaneously monitored, permitting an evaluation of the time evolution of the relaxation modulus E REL , which is reported in Figure 11b in terms of isothermal curves for an axial deformation !…”

Cavitation and morphological changes in polypropylene deformed at elevated temperatures

“…G'Sell et al 22 , Pawlak et al 10 and Na and Lv 19 observed that the volume of deformed polypropylene substantially increases with increasing strain. According to Bucknall's proposition for similar behavior of amorphous polymers 24 the overall volume variation is the sum of three components: elastic, plastic and cavitational.…”

“…Plastic deformation of crystalline and amorphous phases in polypropylene is often accompanied by the formation of cavities [1][2][3][4][5][6][7][8][9][10] . Cavitation process is then initiated in tensile drawing shortly before or at the yield point 11 if the resilience of the amorphous phase is lower than the strength of crystals 3,9,10 .…”

“…Cavitation process is then initiated in tensile drawing shortly before or at the yield point 11 if the resilience of the amorphous phase is lower than the strength of crystals 3,9,10 . Formation of a cavity changes rapidly the local distribution of stresses in surrounding crystals and initiates their plastic deformation, mainly by crystallographic slips 3 .…”

Cavitation Behavior of Semi-Crystalline Polymers during Uniaxial Stretching Studied by Synchrotron Small-Angle X-Ray Scattering

“…Pawlak and Galeski [23] compared the cavitation behavior of PP with similar crystallinity and crystal thickness but different molecular masses of 400 and 250 kg/mol. They found that the samples having lower molecular weight showed stronger cavitation as a result of reduced number of entanglements in the amorphous phase.…”