Energy-based failure models

As seen, even relatively crude analytical energy balance models are helpful in understanding crack stability in adhesive layers. This energy-based perspective provides an intuitive understanding of the effect of in-plane stresses within an adhesive layer on crack path selection. Indeed, residual stresses have been shown to induce some very interesting failure modes in bonded joints, including spiral crack patterns [63]. 

Use of high energy radiation to create hot spots. Attempts have been made to initiate explosives by ionizing radiation such a-particles, high speed electrons, y-rays, Pions etc (Refs 8 9). No initiations were observed. Cerny Kaufman (Ref 9) take this absence of initiation to indicate failure of the hot spot model. However a crude preliminary calculation, based on the Friedman model (Ref 15), suggests that the dimensions of the Pion heated regions for Lead Azide (Fig 2 of Ref 9) and for RDX (Fig 3 of Ref 9) are smaller than the critical hot spot dimension at the corresponding temperatures... 

The fact that radius ratio arguments do not always predict the correct structure is sometimes regarded as a serious failure of the ionic model, and an indication that nonionic forces must be involved in bonding. Given the uncertainties in definition of ionic radii, however, and the fact that they are known to vary with CN, it is hardly surprising that predictions based on the assumption of hard spheres are unreliable. It also appears that for some compounds the difference in energy between different structure types is very small, and the observed structure may change with temperature or pressure. 

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