PSI - Issue 59

V. Sidyachenko et al. / Procedia Structural Integrity 59 (2024) 265–270 V. Sidyachenko and V. Pokrovskii / Structural Integrity Procedia 00 (2019) 000 – 000

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The investigations on characteristics of fracture toughness under conditions of in-plane shear, as well as on the influence of WPS on the fracture toughness characteristics have been performed on a Schenck servohydraulic testing machine. Practical implementation of the loading conditions and the dependences used to determine stress intensity factor (SIF) was described by Pokrovskii et al. (2013). The investigations on characteristics of fracture toughness under conditions of in-plane shear were conducted on prismatic specimens of dimensions 270 mm (length) х 40 mm (width) х 20 mm (thickness) with a thickness of the working part of the specimen thinned to 10 mm and with a preliminary grown (under mode I loading) fatigue crack of length 0.5W. The tests were conducted under severe loading conditions at a constant rate of displacement of the active grip of about 0.5 mm/min. In the process of testing, the diagram “load vs. displacem ent of active grip” was recorded. The load determined by the 5% secant on the abovementioned diagram and crack length corresponding to the crack initiation, which was measured after specimen fracture, were used in the SIF calculation. Thus, in this paper fracture toughness is meant the conventional value of SIF corresponding to the crack initiation. WPS was performed by the scheme with complete unloading, namely: heating of the specimen to a temperature higher than the brittle-to-ductile transition temperature, loading to a level of 85%-90% K c under an appropriate mode of loading, cooling and fracture. While doing this, the following scenarios of the influence of WPS on the fracture strength were studied: WPS under mode I – mode-II fracture; WPS under mode II – mode-I fracture; WPS under mode II – mode-II fracture. To investigate all possible combinations of mixed loading modes (I+II+III), a rectangular cross-section single edge specimen was used (Fig. 1b).

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Fig. 1. (a) loading scheme, (b) specimen, ( с ) device for mixed mode loading.

When changing the angles α, β between the axial load and the normal to the crack plane (Fig. 1a), various combinations of mixed loading modes can be realized, and the SIFs are calculated based on Richard et al. (2017). 2.2. Numerical procedure We have been conducted numerical investigations of the influence of WPS under mode II on the mode-I fracture toughness. The FE model of the specimen with a crack subjected to four-point bending was developed accounting for the plane strain state, elastoplastic behavior of the material with kinematic hardening law and geometric nonlinearity. The data regarding the material properties presented in Table 1 in the coordinates “ true stress logarithmic strain ” were used in the calculations. The crack was modeled with a mathematical cut. The crack-tip region was represented by six- node triangular finite elements of 125 μm in size, which approximately corresponds to the grain size for the given material. 3. Results and discussions The diagrams of limit states obtained at temperatures +20 ° C, +325 ° C, -100 ° C as a result of studying crack resistance in mixed modes I+II, including taking into account WPS are shown in Fig. 2 . The values of angles α and