Stability of arrays of multiple edge cracks

The creation and subsequent shedding of arrays of edge cracks is a natural phenomenon which occurs in heat-checked gun tubes, rapidly cooled pressure vessels and rock, dried-out mud flats, paint and concrete and in ceramic co atings and permafrost. The phenomenon covers five orders of magnitude in cr ack spacing and the driving mechanisms may include fast fracture, environme ntal cracking and fatigue crack growth. A simple model is developed which i ndicates that the shedding behaviour is governed by the behavior of individ ual cracks rather than global energy changes. The model predicts that all c racks will deepen until a crack-spacing/crack depth ratio (2h/a) of 3.0 is achieved, at which stage crack-shedding will commence. Two out of every thr ee cracks will be shed, leading to a new (higher) crack spacing/crack depth ratio at which stage growth of all currently active cracks will be dominan t. An approach based upon rapid, approximate methods for determining stress intensity provides good indications of behaviour provided near-surface str ess gradients are not excessive. In cases where stress gradients are high i t is shown that it is necessary to employ numerical techniques in calculati ng stress intensity. Two specific examples are presented, the first at very small scale (heat-check cracking in a gun tube, typical crack spacing 1 mm ) and the second at very large scale (permafrost cracking, typical crack sp acing 20 m). The predicted ratios for the proportion of cracks shed and for crack spacing/crack depth are in agreement with experimental evidence for gun tubes, concrete and permafrost. The ratios also appear to match experim ental observations of "island delamination" in ceramic coatings and paint f ilms. (C) 1999 Elsevier Science Ltd. All rights reserved.