Crazing plastics mechanical behavior

PMMA specimens immersed in methanol. The time-dependent craze behavior was interpreted in terms of a plasticization mechanism incorporating the effect of the fluid Due to its porous nature the craze has a very high area to volume ratio so that penetration of the fluid by only a small distance leads to a complete plasticization of the fibrils and a subsequent drop in the load carrying capacity cr of the fibrils the material effectively behaves as one with a lower craze stress aa/a < 1). 

It is well known that the mechanical behavior of glassy amorphous polymers is strongly influenced by hydrostatic pressure. A pronounced change is that polymers, which fracture in a brittle manner, can be made to yield by the application of hydrostatic pressure Additional experimental evidence for the role of a dilatational stress component in crazing in semicrystalline thermoplastics is obtainai by the tests in which hydrostatic pressure suppresses craze nucleation as a result, above a certain critical hydrostatic pressure the material can be plastically deformed. 

Similar stress-strain curves have been obtained for polystyrene crazes. However, these results do not necessarily reveal the real mechanical behavior of the craze. The removal of the solvent from samples will cause shrinkage and have a significant plasticizing effect on the craze fibrils. This has to... 

The core of the book is devoted to subjects starting with anelastic behavior of polymers and rubber elasticity, but proceeds with greater emphasis in following chapters to mechanisms of plastic relaxations in glassy polymers and semicrystalline polymers with initial spherulitic morphology. Other chapters concentrate on craze plasticity in homo-polymers and block copolymers, culminating with a chapter on toughening mechanisms in brittle polymers. To make the... 

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

Amorphous polymers exhibit two mechanisms of localized plasticity crazing and shear yielding. These are generally thought of separately, with crazing corresponding to a brittle response while shear yielding is associated with ductile behavior and the development of noticeable plastic deformation prior... 

As with tensile and impact behavior of rubber-toughened plastics, a major energy-absorbing mechanism appears to be crazing. Thus, at least qualitatively, low-frequency fatigue behavior of rubber-modified plastics appears to involve the same phenomena as are seen in tensile and impact loadings. 

The obtainable increase in the tensile strength depends on both the plastic and the reinforcing fiber. Craze formation is caused by shear stress peaks at the fiber-plastic interface. Consequently, plastics with ductile deformation behavior lead to better mechanical properties than brittle plastics glass-fiber-reinforced polyamides exhibit the larger increase in tensile strength when compared with glass-fiber-reinforced epoxides. 

In these types of polymers the micromechanical behavior depends on the interrelation between cavitation and micronecking. Two examples of deformed SBM diblock copolymers with 76% PS and hexagonal-packed poly(butyl methacrylate) (PBMA) cylinders embedded in the PS matrix are shown in Fig. 3.9. Figure 3.9(a) shows craze-like deformation zones running perpendicular to the main strain direction (shown by an arrow). Inside the crazes, both the PBMA cylinders and PS matrix show large plastic deformation. In Fig. 3.9(b) a craze is seen, created by a cellular structure of cavitated PBMA cylinders and plastic deformation of the PS matrix [21], Several types of deformation structures and crazes can be formed in block copolymers initiated by cavitation of one phase and plastic deformation of both phases. However, due to structures on much smaller length scales, there are a number of unique characteristics of the deformation mechanisms [6]. 

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