Under WPS effect is understood ratio of stress intensity factor at fracture after WPS (Kfr) and the “usual” fracture toughness KIc. Theoretical models of WPS effects were derived for Small Scale Yielding (SSY) conditions. For comparison of experimental results with theory, classical Chell model was chosen: real Kfrexper is compared with predicted KfrChell. For characterization of pre-stress level in case of small specimens, it is convenient to use elastoplastic quantity KJWPS corrected to the equivalent KJWPS in SSY conditions; this quantity is denoted by symbol KJWPS(SSY). For material with certain stress-strain curve σ = f(ε), universal dependence of Kfrexper/KfrChell on KJWPS(SSY) was obtained, which describes behaviour of specimens of different sizes. If another stress-strain curve has the same shape (i.e. if σ = const.f(ε)), KIcfrexper/KfrChell lies (up to the accuracy of experiments) on the universal dependence curve Kfrexper/KfrChell vs. KJWPS(SSY)y, where KJWPS(SSY)y is a quantity proportional to the square root of the plastified zone size in SSY conditions. Different stress-strain curve shapes lead to smaller or greater Kfrexper/KfrChell. Stress-strain curves cannot be generally expressed by power law hardening, but the Curry-Smith model derived with the use of Ramberg-Osgood law qualitatively predicts the increase or decrease of Kfr in dependence on the strain hardening exponent.

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