The deformation behavior, mechanical properties, and microstructure of Fe-Cr-Mn-0.53%N austenitic stainless steel were studied at a temperature range of 77 up to 293 K. The dynamics of the steel elongation were non-monotonic with a maximum at 240–273 K, when peaks of both static atom displacements from their equilibrium positions in austenite and residual stresses in the tensile load direction were observed. The results of X-ray diffraction analysis confirmed that the only stress-induced γ→ε-martensite transformation occurred upon deformation (no traces of the γ→α′ one was found). In this case, the volume fraction of ε-martensite was about 2–3%. These transformation-induced plasticity (TRIP) patterns were discussed in terms of changes in the phase composition of steel as the root cause.

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A new method is proposed to determine fracture toughness of structural materials according to the test data of non-standard small-size chevron-notched specimens. During the tests, loading diagrams and photographic images of the specimens taken in time intervals are obtained. The crack length is measured in the process of its initiation and propagation. The analytical expressions are obtained being based and derived from the constitutive equations of engineering fracture mechanics to determine the crack-driving force (specific fracture energy) and the stress intensity factor. The method allows us to exclude the periodic unloading of the specimen applied under standard test conditions to determine the change in specimen compliance, which is taken into account in constitutive equations at crack length increase. All necessary calculation parameters are determined according to the experimental data. The method allows us to certify fracture toughness of the material without restrictions regarding the amount of plastic deformation and in front of the crack tip and in the specimen as a whole. The examples are given to calculate the fracture toughness criteria for a number of structural materials characterized by the ability to plastic deformation and by the Young's modulus value.

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