Particle Fracture Mechanisms

For a 2 rm long crack with tip radius equal to half the inter atomic distance (R = 10 i°m), K = 20l. 

Griffith (1921) proposed that for a crack in the surface of a body to propagate the following criteria must be satisfied  

Evans et al. (1961) showed that for a disc acted upon by opposing diametrical loads, there is a uniform tensile stress acting at 90° to the diameter. Under sufficiently high compressive loads, therefore, the resulting tensile stress could exceed the cohesive strength of the material and the disc would split across the diameter. Evans extended the analysis to three-dimensional particles to show that even when particles are stressed compressively, the stress pattern set up by virtue of the shape of the particle may cause it to fail in tension, whether cracks exist or not. 

Cracks are less important for tough materials (e.g. rubber, plastics and metals) since excess strain energy is used in deformation of the material rather than crack propagation. Thus in ductile metals, for example the stress concentration at the top of a crack will cause deformation of the material around the crack tip, resulting in a larger tip radius and lower stress concentration. 

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The manner in which a particle fractures depends on (i) the nature of the particle and (ii) the manner in which the fracture force is applied. A number of terms have been used to describe the different mechanisms of single particle fracture. The different terms considered here are abrasion, cleavage, shatter, and chipping. It may be pointed out that in practice these events do not occur in isolation. Real breakage involves a combination of these processes, with the proportions changing, depending on the equipment, and on the manner each particle is stressed within it. 

FRACTURE MECHANICS] (Volll) particle size reduction [SIZE REDUCTION] (Vol 22) in plastics [PLASTIC TESTING] (Vol 19)... 

During a fracture-mechanical test performed in mode I, the crack propagates in this mode from a macroscopic point of view. But the crack can be deflected locally by the rubbery particles and can also propagate in mode II. As for isotropic materials, GIIc is generally higher than GIc an artificial increase of the macroscopic GIc value will be then evidenced. 

Some reports have cited bridging across the relatively ductile metal particles as mechanisms which contribute to the increase in fracture strength.54 Other reports demonstrate cracks propagating at the metal-ceramic interfaces,... 

The behaviour of particle- and whisker-reinforced CMCs under conditions of thermal shock can be modelled successfully using the fracture mechanics methods outlined in the previous paragraph (e.g. Aldridge and Yeomans, 2001) while the thermal shock parameters (figures-of-merit) can also be useful for initial material comparison. 

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