PSI - Issue 32

A.Yu. Iziumova et al. / Procedia Structural Integrity 32 (2021) 93–100 Author name / Structural Integrity Procedia 00 (2019) 000 – 0 0

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advanced high strength steel sheet. The theory of critical distances (TCD) is used by Terekhina et al. (2017) and Benedetti et al. (2019) to estimate the magnitude of the effective stress. The central idea in the TCD is the definition of an effective stress based on the stress at a point located at a critical distance from the stress concentrator (Point Method). Failure is expected to occur if effective stress exceeds a reference material strength. Conservativity of elastic approach in elastic-plastic fracture mechanics from a viewpoint of the two-parameter J-A fracture criterion has been analyzed by Matvienko and Nikishkov (2017). Fracture criterion based on generalized stress intensity factors is proposed by Zappalorto (2019) for notched orthotropic plates under mode I. The choice of the fracture criterion has a number of requirements, which include determinability by direct or indirect measurement methods, versatility under different loading conditions, and applicability in engineering calculations. The thermodynamic approach is one of the most versatile to study of complex systems (including fatigue loading). Schipachev (2018) notices, that thermodynamic approach can be used to integrally assess the solid state, its damage and fatigue strength characteristics. In the late seventies of the twentieth century Fedorov (1979) presented his thermodynamic theory of strength and fracture of solids. The main idea of this theory is an energetic analogy between the processes of melting and mechanical destruction of metals. According to the thermodynamic theory of strength, the measure of damage is internal energy stored in material as a result of deformation. Now the energy based approach is used by many researchers to predict the fatigue life of materials, to monitor the state of the material, to assess the kinetics of fatigue crack growth and the features of its propagation (Ranganathan et al. (2008), Palumbo et al. (2017), Wang et al. (2017), Mareau et al. (2013), Jang and Khonsari (2018), Iziumova et al. (2021)). In this work the energy balance and stored energy were estimated experimentally in Charpy V-notch specimens of titanium alloy Ti-0.8Al-0.8Mn under cyclic deformation with constant maximum loading of 10.5 and 11 kN. The plastic work was calculated by the area of the hysteresis loop obtained on the basis of contact extensometer data. The heat dissipation rate near the crack tip was registered by an original contact heat flux sensor. The stored energy was calculated as the difference between the plastic work and heat dissipation. 2. Materials and conditions The titanium alloy Ti-0.8Al- 0.8Mn was employed in the testing program. Such pseudo α alloys are widely used as structural materials for the manufacture of almost all types of semi-finished products, parts and structures, including welded ones. Their most effective application is in aerospace engineering, in chemical engineering, in cryogenic engineering, as well as in units and structures operating at temperatures up to 300- 350 ° C. Experiments were carried out on a 100kN servo-hydraulic testing machine Instron 8802 under constant maximum loading of 10.5 and 11 kN at a stress ratio R=0.1 and loading frequency 10 Hz, using Charpy V-notch (CVN) specimens (Fig. 1a). a b

Fig. 1. (a) geometry of studied titanium alloy specimens, thickness of specimens is 4 mm (all dimensions are in millimeters); (b) true stress-true strain curve of titanium alloy Ti-0.8Al-0.8Mn. The crack length was measured by an electrical potential drop method (PDM) which was described in detail by Hartman and Johnson (1987). The quantitative measurements of the heat dissipation rate at the crack tip area were