Crack closure force

Up to this point, it has been assumed that increasing zone size will be beneficial in terms of toughness. It must, however, be realized that there is a limiting zone size, above which the toughness must decrease. For example, if the critical transformation stress is reduced to a value such that the stress-induced phase transformation occurs throughout the specimen, the transformation zone will no longer be constrained. This implies that the zone will no longer be left in residual compression and the crack closure forces will vanish. The overall trend is shown schematically in Fig. 8.63. The stress required to induce the phase trans-... 

Figure 8.7 Schematic of the DCB specimen healing process by manually applying the crack-closure force. Source [26] Reproduced with permission from Elsevier...

Figure 9.8 Dugdale s model for crack tip yielding under plane stress conditions. R, yielded zone length 5, crack tip opening. The shaded region is the yielded zone, while the arrows show the closure force on the hypothetical crack.

The stress intensity factor due to a pair of closure forces F, acting a distance x from the tip of a crack is... 

Figure 14.10 (a) Point load closure force, P, on crack flanks (b) Closure pressure, p, on crack flanks. 

Crack Bridging. A mechanism leading to higher toughness (q.v.) in ceramics and composites. Intact material behind the crack tip imparts closure forces on the crack. The crack is bridged behind... 

In structural ceramic composites, the principal effect considered was one of crack-face closure tractions, or cohesive forces, brought about, for instance, by bridging fibers. A rigorous evaluation of the crack tip fields where the crack faces are not traction free has not yet been attempted. However, an approximate approach for the small-scale creep case is to assume that the crack tip fields are not functionally altered by crack-face tractions, with the effect of the traction being only to introduce a zone of crack tip shielding. This allows for the development of preliminary models for creep crack growth which is inclusive of the role of crack bridging. These preliminary models predict that,... 

In the case of a screw closure, it should be evident that a closure which has one turn or more of thread engagement will suffer less thread-to-thread stress than one of, say, turn (i.e. the same force (torque) is spread over less area with the latter, hence stress cracking could be more likely). This torque force is also conveyed to the sealing surface of the container and the contact surface of the closure, hence areas receive and convey a greater force (onto the cap). Studying how a stress occurs with the closure—container system is therefore a useful part of avoiding stress cracking problems. 

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