X-ray method of evaluating local state of stress of material at crack tip in one-time loading

The article deals with the fractographic traits of failure and the regularities of the formation of zones of plastic deformaton in materials with bcc lattice (steels 20, 40, 45, St. 3, 15Kh2MFA) and with fee lattice (alloy D16 and the austenitic steels 40G18F, 40Kh4G18F, 03Khl3AG19) under different kinds of one-time loading: static, impact, high-speed impulsive. It is shown that the local state of stress of the material at the crack tip with all the mentioned kinds of loading can be evaluated by the single criterion h_max/t (the ratio of the maximal depth of the zone of plastic deformation under the fracture surface to the thickness of the specimen). Three regions of local state of stress are identified: h_max/t < 10^–2 close to plane strain (PD), h_max/t > 10^–1 close to plane stress (PD), and 10^–2 < h_max/t < 10^–1 the transient region from PD to PS. In static crack resistance tests of materials the known criterion t(k/_0.2)² 2.5 separates PS from the transient region in materials with fee lattice, and in materials with bcc the transient region from PD. In materials not undergoing phase transformations at failure, the correlation of the criterion h^max/t and the ratio /₀ is described by a single curve, independently of the form of loading. In regions close to PD and PS the ratio /₀ can be used for evaluating the local state of stress of the material at the crack tip.Translated from Problemy Prochnosti, No. 11, pp. 25–32, November, 1991.