The effects of testpiece thickness (B) on fibrous fracture and fatigue-crack propagation have been studied using single edge-notched test pieces of two low-strength steels. Most tests were carried out in four-point bending, but some fatigue tests were carried out in tension or three-point bending. The results for fibrous fracture show that a higher crack-tip displacement (COD) is necessary for crack initiation in a thin testpiece than in a thick. It is shown by using micro hardness measurements that the change in critical displacement is consistent with the difference in the strain distributions about crack tips found in thick and thin testpieces. An analogous effect was observed in fatigue-crack propagation in bending, where lower growth rates were found in thin specimens. The magnitude of the alternating COD should be greater in a thin specimen for a given level of alternating stress intensity, ∆K, and the lower growth rate is again attributed to the change in crack-tip strain distribution with thickness. Fatigue-crack propagation rates in tension were found to be higher than in bending for thick specimens, and this is explained in terms of the errors arising in the stress-intensity factor for different geometries, when there is a small amount of crack-tip plasticity. In tension it was found also that fatigue cracks grow faster in thin testpieces, and this is considered to occur because this loading geometry allows out-of-plane sliding and an antiplane (K III) mode of fracture to operate. This was confirmed by fractography.
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