Effect of Particle Size on the Plastic Zone Formation in Front of Mode I Crack in ABS Polymers
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EFFECT OF PARTICLE SIZE ON THE PLASTIC ZONE FORMATION IN FRONT OF MODE I CRACK IN ABS POLYMERS Beta Y. Ni and J. C. M. Li Materials Science Program, Depaxtment of Mechanical Engineering, University of Rochcstcr, Rochester, NY 14627.
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Abstract
Two different sizes (0.4 and 0.1 pim) of rubber particles (polybutadiene) are usually used to toughen the glassy SAN (styrene acrylonitrile) matrix in ABS copolymers. Experiments on a batch of specimens without the large particles confirmed our previous contention that cavitation of large particles within a zone of cardioid shape is an important part of crack tip plasticity which protects the crack tip from the applied stress field. Large rubber particles require a less strong apl)plicd stress field than small rubber particles to initiate crazes so that the large rubber particles help avoid brittle fracture which occurs without crazes.
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Introduction
Defects in polymeric materials such as crazes, shear bands, and cracks produced by deformation often can enhance the toughness of glassy polymers. To monitor the creation and propagation of these defects, rubber particles are introduced into the glassy matrix. Then the size and its distribution of the rubber particles are importanit factors affecting the mechanical properties of rubber-toughened plastics. Donald and Kramer [1] concluded that particles less than 1 /Lm were ineffective in inducing crazes in polystyrene. It was also reported [2] that 0.8 pim was the minimumn critical particle size for toughening polystyrene. Small rubber particles fail to initiate crazes since the stress concentration around the small particle extends too short a distance to produce a region of desired length scale to support the crazes t3]. On the other hand, the role of very small rubber particles in toughening SAN copolymers is assumed to be associated with their ability to initiate shear bands [4], but we believe that the small particles help to branch the crazes resulting in high toughness in ABS polymers [5]. Recently, the bimodal particle size distribution has been proven to be effective to increase the fracture toughness of ABS polymers. But the toughening mechanisms of both the large particles (0.4 ani) and small particles (,- 0.1 jim) remain to be further elucidated.
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Experiment
Two kinds of ABS polymers from Dow Chemical company, one with bimodal distribution (0.4 pim and 0.1 nimrubber particles) and the other having only a
Mat. Res. Soc. Symp. Proc. Vol. 209. @1991 Materials Research Society
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uniinodal distribution (0.1 jim) are investigated. Both kinds have the same total 17.3% rubber content. For the bimodal sample, the large particles have only 0.2% out of the 17.5% total rubber content. The samples are in the form of 1/4 inch plates and are cut into Compact Tension (CT) specimens with either a sharp notch (razer blade cut) or a blunt notch (diamond saw cut). The CT specimens are loaded in an Instron machine with a cross head speed of 0.01 mm/sec to produce the plastic zone in front of the Mode I crack in order to stu
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