Mist and Velocity Hackle

Rgure9.8 Schematic showing dependence of crack velocity, V, on stress intensity, K. 

Experimentally, it has been observed that, for a crack that grows in a uniform stress field, the boundaries of the mist and hackle regions are semicircular. Furthermore, if the origin is a well-defined semicircular flaw, the radii of those boundaries bear fixed relationships to the radius of the flaw. Equivalently, the apphed stress at failure and the boundary radius are related by equations with the form 

There is no consistent explanation for the generation of these features at particular values of. Nevertheless, the fact that a value of ATj can be inferred from a measurement of Rj is useful in analyzing failure. Mecholsky and Freiman have shown how to use these features to quantify the stress state in the part, including localized residual stresses, to determine whether subcritical crack growth occurred, and to quantify the severity of the flaw. 

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The features described above can be generated by either a slowly or a rapidly moving crack. There is, however, a class of features called mist and velocity hackle that are produced only when the crack velocity approaches that of acoustic waves in the material. Figure 9.7 is a micrograph of a fracture surface in glass with examples of mist and velocity hackle. This fracture initiated at the point marked O and grew... 

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