Crack propagation, energy required

In real materials, much energy is used not only to create new crack surfaces, but is also dispersed in zones of plasticized or microcracked material. Therefore, outside the region where the threshold value of (j is reached (Figure 10.2c) rapid crack propagation without an energy input does not occur at all, or is preceded by slow crack propagation, which requires new energy to overcome obstacles in material structure and in material inelasticity. 

Griffith derived a similar equation using an energy balance approach, equating stored energy with the energy required for crack propagation ... 

Interleaving strips made from ductile short fibers, notably Kevlar fiber mat, and an adhesive (Browning and Schwartz, 1986) provide extra energy required during delamination crack propagation due to additional toughening mechanisms such as... 

Although it is relatively easy to understand why some of the macroscopic properties of liquids, especially their shape, can depend on surface properties, it is not so obvious for solids. However, the strength of a solid is determined by the ease with which micro-cracks propagate, when placed under stress, and this depends on its surface energy, that is the amount of (surface) work required to continue the crack and hence expose new surface. This has the direct effect that materials are stronger in a vacuum, where their surface energy is not reduced by the adsorption of either gases or liquids, typically available under atmospheric conditions. 

---