FATIGUE-CRACK PROPAGATION IN ARMOR STEELS
ARMY MATERIALS AND MECHANICS RESEARCH CENTER WATERTOWN MA
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Fatigue-crack propagation in high- and dual-hardness armor steels was studied at various fatiguing stresses and testing temperatures using a fracture mechanics analysis of the data. The armor steels tested were 1 a high- hardness XAR-30 steel 2 an ausformed cross-rolled H11 and X27 steel composite and 3 a dual-hardness composite of Ni-Cr-Mo steels with 0.34 and 0. 51 carbon. Crack-growth rates increased directly with increasing fatiguing stress level and decreasing testing temperatures. The fatigue-crack growth rate d2adN versus the change in stress intensity factor delta K followed an empirical relationship d2adN C delta K to the m power for the high-hardness and thin-section dual-hardness Ni-Cr-Mo composite steels. The crack-growth rate exponent m was 5.0 in the XAR-30 and 2.3 in the Ni-Cr-Mo steel. Crack-growth rates were generally higher in both the XAR-30 and Ni-Cr-Mo dual-hardness steels than in 4340 steel at the same strength level. The fracture mode for these steels was predominantly plane strain. The low fracture toughness component of the composite steel failed first and thereby caused rapid fatigue-crack growth acceleration and fast fracture in the remaining component.
- Metallurgy and Metallography