Tensile stress crazing

Physical ageing in thermoplastics can lead to crazing, a failure process normally produced simply by high tensile stress. Crazing involves the prior development of regions called crazes, resembling cracks bridged by several strands of oriented polymer. 

Crazing requires a stress field that must have at least one tensile component where in unoriented homo-polymers crazes form and grow normal to the maximum principal tensile stress. Craze microstructures in homo-polymers that have been widely studied consist of drawn polymer fibrils a few nanometers in diameter and have extension ratios of 2-4 that bridge the two faces of a craze and result in a density reduction of up to 0.75 locally. 

Optical and mechanical study of the effect of various variables, such as magnitude and duration of stress on initiation and development of crazing. Crazing is a mechanical separation of polymer chains or groups of chains under tensile stress. Crazing does not occur in materials oriented along the draw direction. Hsiao and Sauer (781 PS... 

Brittle fracture is normally preceded by crazing, i.e. a running crack is preceded by a zone of crazed material (see Chap. 13, Fig. 13.81). Like cracks, crazes in isotropic materials grow at right angles to the principal tensile stress and only propagate if the stress at their tip exceeds a certain value. The craze can be described as an "open cell foam" with voids of the order of 10-20 nm in diameter and centre-to-centre distances of 50-100 nm. 

Crazes usually form under tensile stress when a critical strain is surpassed they do not occur under compressive stress applying hydrostatic pressure during tensile deformation can even inhibit their development. Crazes always nucleate preferentially at points of triaxial stress concentration. It is the dilatational strain which initiates crazes and cracks. 

Mechanically stressed polystyrene without the application of any chemical compound can also produce crazing. Figure 23 shows crazing and its diffraction pattern attributed to bending the polystyrene sheet surface crazing appears on only one side. This contrasts the development of crazing in depth by simple tensile stress (see Figure 24) where crazes developed on both sides of, and sometimes in between, the surfaces. 

Most chemicals including water, but excluding glycerin, will weaken some locations of a polystyrene molecular system and will initiate parallel crazes perpendicular to the applied tensile stress field. The density... 

In discussing shear deformation, it is convenient to distinguish between the initial elastic and viscoelastic response of the polymer to the applied load and the subsequent time-dependent response. However, the distinction is somewhat arbitrary and is not as fundamental as that between elastic volume response and crazing. Viscoelastic shear deformation continues throughout the period under load. The observed time-dependence of lateral strain reflects both generalized viscoelastic relaxation and shear band formation. Since crazing consists simply of displacement in the tensile stress direction, it makes no contribution to lateral strain therefore —e specifically measures deformation by shear processes. 

We observed in crazed tensile samples that particles are elongated in the direction of the tensile stress field and show what appears to be a microvoid associated with each of the large particles (Figure 7). We did not observe the fibrous crazes reported elsewhere (14,15,16) for the thermoplastics. 

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