Cavitation during tensile deformation of high-density polyethylene

Cavitation process of high-density polyethylene during tensile deformation was studied. It has been shown that the crystallinity and perfection of HDPE crystals govern whether plastic deformation of the polymer is associated with cavitation or deformation occurs without cavitation. The strength of crystals may be controlled by crystallization conditions during preparation of a polymer. If the crystals are thin and the degree of crystallinity is low then the plastic deformation of crystals occurs before reaching the level of stress that initiates cavitation. On the other hand, if the crystals are thick, more perfect, and crystallinity is high, then the cavitation in an amorphous phase is initiated first, and later followed by the deformation of crystals. The cavitation process is usually initiated at the stress level close to the yield point. However, the level of stress necessary for cavitation may be decreased substantially by the orientation of crystalline lamellae, as it was observed in the skin layers of injection-molded material. Voids formed in the skin layer do not influence the yielding process. Typically, cavitation was initiated in volume at the stress of 29-30 MPa, but in the skin of injected samples voids were observed even at the macroscopic stresses of 2 MPa only. The development of voids with deformation was studied both for skin and volume of injection-molded HDPE sample. The shape of voids is strictly connected with deformation of crystalline phase around them.