Crack acceleration

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. 

In the high crack velocity regime three different values of Kid can be assigned to one rate of crack propagation depending on the state of crack acceleration. This behaviour was ascribed to inertia effects associated with crack acceleration and deceleration. Such a hypothesis is corroborated by the computed K data (also shown in Fig. 9), which were obtained from a finite element model, taking into consideration the mentioned transient dynamic linear elastic effects [35]. 

Figure 8.75 Fracture surfaces in brittle materials generally show a smooth region that surrounds the failure origin (mirror region) but the surface increases in roughness as the crack accelerates (mist region) until crack branching occurs. The branched region contains ridges known as hackle. (Optical micrograph courtesy of Matt Chou.)...

Figure 16.7. (a) Peeling force versus crack speed for uniform rubber film of different thickness and elastic modulus, (b) Crack slowing at an interface with sliffer rubber film, (c) Crack acceleration at interface with compliant rubber. 

A side effect of the anodic dissolution is often, particularly at crack tips (see Sec. 5.2.5), the production of hydrogen by the reduction of water. Some studies have shown that hydrogen absorption can favor local plasticity, due to enhanced dislocation velocities with hydrogen. In the presence of a crack, accelerated hydrogen penetration can occur very near to the crack tip region by stress-assisted diffusion and dislocation transport. These effects will be discussed later. 

The surface of fibers was investigated by the methods of light and electronic microscopy in order to clarify the reasons for the loss of mechanical properties of fibers and increase in their brittleness. At an earlier time of implantation fibers morphology remains basically unchanged. Cracks appear after 5 months. The development of cracks accelerates on the surface of libers during the first year. 

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