Crack growth tests under cyclic loading conditions were conducted on samples from different lots of dual hardness steel (DHS) armor. The results showed that the growth rate increased with stress level, and the cracks grew at a different rate in the frontal and backup layers of the armor. This difference in growth rate became progressively greater with increases in the stress level. The crack growth rate also increased with reductions in the frequency of loading and temperature increases over the range of -60 to +200°F.
Attempts are made to explain the crack growth behavior of the composite in terms of the behavior of each component. The growth rates in the composite are related to the stress intensity factor, ΔK, calculated from the surface crack length assuming that there was no interaction between the layers and the displacement in each layer was the same on the crack line. The latter procedure yielded better results and suggested that the interface plays an important role in retarding crack growth in the frontal layer of DHS.