Analysis of the internal crazes in notched SAN

Abstract

SYNOPSISCrazing in styrene-acrylonitrile copolymer ( SAN) under a triaxial stress state was examined. The damage zone that formed ahead of a semicircular notch under slow tensile loading consisted of two kinds of crazes: internal notch crazes that grew out from the notch root and discontinuous surface crazes. Examination of the damage zone in the optical and scanning electron microscopes revealed that the internal notch crazes sometimes extended through most of the thickness but never penetrated the free surfaces, whereas the surface crazes penetrated about Fm inward from the surface. Initially, the internal craze tips defined a crescent-shaped zone. The plane strain elastic stress distribution at the zone boundary satisfied a constant mean stress condition, and the critical mean stress for craze tip growth was determined to be about 35 MPa. This value varied slightly for different resins but was independent of thickness. Propagation of the internal notch crazes occurred in a straight line parallel to the minor principal stress vector a t the point of origin on the notch surface, whereas the surface crazes followed the minor principal stress trajectories. The presence of the internal notch crazes resulted in significant stress redistribution, the stress redistribution was described quantitatively using both the deviation of the craze trajectory from the minor principal stress trajectory, and the deviation of the zone shape from the critical elastic mean stress condition. I NTRODU CTlO NIrreversible prefracture deformation and damage of polystyrene ( PS ) and styrene-acrylonitrile copolymer (SAN) , as with many brittle glassy polymers, is characterized by crazing. Several major reviews describe the complex nature of crazing, including such aspects as craze initiation and growth, craze structure and morphology, stress and environmental effects, temperature dependence, kinetics and viscoelastic behavior, and crazing in heterogeneous copolymers and blends.'-4 The present understanding of the crazing phenomenon is based primarily on observations of crazing in thin films or of surface crazes in thicker sheet.Crazing in a triaxial stress state is facilitated by use of a notched geometry. Deformation of some glassy amorphous polymers at a round notch has * To whom correspondence should be addressed. been comprehensively described by the initial formation of a plastic zone, followed by nucleation of an internal craze at the tip of the local plastic zone when a critical condition is The craze initiation condition can be quantitatively described by slip line analysis of the plastic zone and related directly to fracture toughness. Under conditions where the craze initiation stress is smaller than the shear yield stress, a situation that might be achieved by varying the temperature, for example, crazes initiate at the notch root before shear yielding occurs and grow radially from the notch root." This type of damage zone has not been as extensively studied.The goal of this study was to characterize the nature of crazing in S...

Damage zone in PVC and PVC/MBS blends. II. Analysis of the stress‐whitened zone

“…At low enough temperatures, the shape and growth pattern of the ensuing zones resembled the crescent-shaped damage zone, previously observed by Tse et al in PVC/CPE blends 2 and in poly (4-methylpentene-1) 11 in the ambient and subambient temperature range. The same type of contours were also reported by Shin et al 3 in SAN, where the damage occurred in the form of internal multiple notch crazes.…”

“…This is in agreedition. [2][3][4][5] The method to evaluate the stress distriment with the sequence of failure events in PVC, bution around the notch is dictated by the sein which the SWZ was always observed to initiate quence of yielding modes at the notch root, which away from the notch root, as seen in Table I. In is dependent on the amount of rubber in the blend, contrast with PVC, the temperature dependence as well as the temperature.…”

“…The same approach was also applied to describe the initiation and growth of internal multiple notch crazes in styrene-acrylonitrile (SAN). 3 In several other polymeric systems, such as PVC, 4 and rubber modified polycarbonate, 5 stress whitening did not initiate at the notch but formed at a distance away from the notch root at the tip of a shear yielding zone emanating from the critical mean stress was obtained by a plastic analysis that gives the triaxial stress intensification at the tip of the shear yielding zone.…”

“…Stress whitening in the used to describe the onset of cavitation in the triblends was caused by rubber cavitation; while in axial stress state ahead of the semicircular unmodified PVC, it was attributed to the growth notch. [2][3][4][5] Blends of PVC with chlorinated polyethof preexisting microvoids. The onset of whitening, ylene (CPE) rubber 2 exhibited a crescent-shaped damage zone, with contours corresponding to those of the elastic mean stress distribution around the notch.…”

Mechanical properties of poly(ethylene terephthalate) modified with functionalized polymers

“…5(a)]. 4,9 The size and shape of the preexisting craze were revealed by the boundary of the smooth region; in this example, the length of the craze was about 0.9 mm at fracture. At higher magnification, the region of craze fracture did not exhibit the fibrillated texture that is characteristic of fracture through the craze fibrils.…”