method i.e. the difference between the opening stress and the stress acting parallel to the crack plane. xxσ was calculated in the direction θ = 180 ° (i.e., in the crack rear back direction), and the T-stress was defined as the value of xxσ in region where this value is constant see fig 4. Stress distribution is computed with stress strain behaviour at transition temperature where material is quasi elastic. Therefore T stress is not sensitive to temperature. For each test, critical load has been chosen at transition temperature at which brittle fracture is assumed under linear elastic behaviour. The considered T-stress value is the effective value given by Equation (11) for CT and Charpy specimens. For tensile specimen, T-stress is simply the stress difference (σxx- σyy) using a Poisson ratio equal to ν = 0.3. Table 7 summarizes values of T-stress and associated transition temperature. Transition temperatures obtained with the 3 different specimens are plotted against temperature and reported in Figure 5. One notes a linear increase of transition temperature with T-stress according to the following relationship: 182 0.0607 + = ef t T T (12) where Tt is the transition temperature in Kelvin and Tef the effective T-stress in MPa. Transition temperature is not intrinsic to material. It decreases with loss of constraint and one notes that the choice of TK27 as reference temperature in Equation (2) is the most conservative. Table 7. Specimen geometry T-stress and associated transition temperature. Specimen Charpy CT Tensile Tef (MPa) -220 -330 -998 Transition Temperature (K) 174 156 123 Figure 5. Evolution of transition temperature with effective T-stress (Tef).
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