Craze propagation

The extremely high energy dissipation in the craze layer, 5 X 108 ergs/gram, would lead to an adiabatic temperature rise of about 24°C during crack/craze propagation, which is insufficient to cause most matrix polymers to pass through their T0 at usual environmental temperatures. Introduction of rubber particles can only lower this temperature rise since rubber secant modulus is very low and rubber deformation does not exceed the 300% or so of the deformed matrix. 

In an isotropic medium, cracks do not move faster than half the shear wave velocity Vu so the implications of the 0.8V curve in Figure 4 were not explored. In the two-phase ABS system, however, one can imagine cracks or crazes propagating rapidly in the matrix (V /2 <—B20 meters/sec), and thence into the rubber particle [at 23°C, polybutadiene (V /2 /—29 meters/sec)] where violent branching would occur. 

If Vt 1240 meters/sec in the matrix and branching will occur in the rubber at 29 meters/sec, we calculate A/Co = 0.047. Thus, branching can occur after a matrix crack acceleration distance of only 2 to 5/x (assuming a Griffith crack length of 50-100fi) hence, ample room for the development of fast cracks or fast crazes exists in the ABS structure. Note that the expressions for craze instability, acceleration, and speed (Equations 1, 6, 7) show that the macro strain rate of the specimen is irrelevant— fast cracks and crazes propagate in specimens strained even at slow creep rates. 

When the craze propagates over a certain length, the fibril located in the central part (midrib) of the craze breaks, yielding a crack in the middle of the craze. Such a craze fibril breakdown also occurs in the craze ahead of a crack tip and results in a crack propagation. The broken down fibril parts retract on each crack surface and can be observed on fracture surfaces. The fibril breakdown mechanisms will be described later on in this section. 

A convenient technique for studying the crack tip craze propagation in amorphous polymers deals with optical interferometry. It has been applied to the examination of PMMA behaviour at various temperatures and crack speeds under conditions of stable propagation [44,45]. 

Finally, it should be noted that the transition temperatures cited above for the craze phenomenology correspond roughly to the secondary relaxation peak P for the 4 polymers. The p temperature depends on the testing frequency which is not precisely defined in the craze propagation experiment, and therefore the correspond-ance with the craze transition can only be approximate. 

Figure 42 shows the craze stress versus craze propagation velocity. The fibrils diameter being unknown, the lower mean stress on the craze surface does not necessarily mean a lower stress in the fibrils, but it may be imderstood as an... 

While in the first case the results are mainly qualitative, the observed crack-tip craze propagations have led to the following interpretations in PMMA ... 

Figure 3.17. Microhardness Hexp derived from different indentations in PMMA along the direction of craze propagation x. (From Michler et al, 1999.)...

The initiation point of crazes is directly at the surface of the rubber particles. Along with the formation of crazes, there is the creation of microvoids, which increase the stress concentration at the craze tip. By this stress concentration, the polymeric material at the craze tip is transformed into the craze. Thus, with craze propagation the stress state necessary for craze initiation is reproduced continuously at the craze tip in the matrix (Figure 10). 

Socrate et al. (2000) considered an axially symmetric problem, with a rubber sphere in the centre of a short cylinder of matrix the spheres are in a row, aligned with the tensile stress axis. The potential positions of crazes were predetermined, initially running radially from the material interface, then becoming normal to the tensile stress along the cylinder. The initial stress concentration is greatest in the polymer near the equator of the sphere (Fig. 4.11a). The model, for a 20% volume fraction of rubber, predicts a yield point in the tensile stress-strain curve at an average strain of 1%, and 24 MPa stress, when the first craze propagates across the section. However, this relieves the stress in the polystyrene, and a tensile stress concentration... 

Impact modifiers function by dispersing a damping phase capable of absorbing energy (to stop craze propagation) into the brittle matrix, and in general, elastomerics are preferred and used. 

Fig. 13.22 A sketch of a craze propagating through pools of a low-molecular-weight PB diluent in a PS/PB blend (from Argon et al. (1990) courtesy of the ACS).

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