13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- x R T T (2) where x = KI/√(2 r) is the crack tip local stress component parallel to the crack plane and r is the distance to the crack tip. Pook [23] recommends that TR is evaluated at a characteristic distance r = rch, which is the same order of a microstructural feature size. Taking rch = 0.0159 mm [23], using Eq. (2) and the MN-m units, find: I R K T T 0.01 (3) According to [15, 23], when TR exceeds its critical value TRC, crack will deviate from its original path. In [15] Pook stated a typical TRC value suitable for many metals as at least 0.021 (unit: m-1/2). Calculated TR values for various k ratios are presented in Fig. 6. For the test case of k =1, at half crack length a = 50 mm, calculated TR according to Eq. (3) is 0.038 m-1/2, which is greater than the suggested TRC value. Therefore, under tension biaxial loading at ratio k=1, calculated crack deviation was expected, however the magnitude of deviation is small, just 3 mm away from the original crack path (Fig. 4b), which is not a major loss of directional stability. Measured crack deviation was nearly doubled at 5 mm, and crack deviation started at much earlier in shorter crack length (Fig. 4b). This discrepancy could be due to the changes in weld metal microstructure and the anisotropy property of the 2198-T8 alloy as reported in [24-25]. Material microstructure change and anisotropy properties were not modelled in this study. Fig. 5 T-stress (normalised by applied stress y). Fig. 6 Calculated TR distribution. 4.4.3. Influence of weld-induced residual stress Test measured crack turning is more pronounced than FEA predicted, Fig. 4b. One possible reason could be that the longitudinal residual stresses were present in the test specimen making the actual biaxial load ratio much higher than 1, but this residual stress was not considered in the prediction model. In this work, KI, KII and T are calculated for biaxial load ratio k = 1 without considering the welding-induced residual stresses. If the longitudinal residual stress (e.g. 150 MPa maximum magnitude according literature) is added to the applied lateral stress x, then the biaxial stress ratio k
RkJQdWJsaXNoZXIy MjM0NDE=