Micronecking Operative during Crazing in Polymer Glasses

Abstract

The topography of crazes in polystyrene films was studied by scanning force microscopy (SFM), and evidence of micronecking operative during crazing was observed. It was found that crazing not only generates fibrillar structures but also creates an unexpectedly large surface depression in the crazed regions. The depression was found to initially increase linearly with the craze width but leveled off to approximately 35% of the original film thickness when the craze width became larger than a critical width ωc. This behavior is independent of strain rate, aging time and the total applied deformation. The micronecking process during crazing indicates that the newly drawn fibrils were continuously deformed even after the fibrillation. Due to the large depression, the surface-to-surface distance between the craze fibrils is much smaller than that previously realized. This discovery may help to explain the fast fibril coalescence at room temperature and to examine the process of solvent-induced crazing. The mechanics of craze initiation and breakdown were then studied on the basis of necking theories of Considere construction and Bridgman's plasticity theory. The results from the stress analysis are in excellent agreement with the fibril breakdown behavior observed experimentally. © 1993, American Chemical Society. All rights reserved.