PSI - Issue 39

N.A. Makhutov et al. / Procedia Structural Integrity 39 (2022) 247–255 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 6. The shape of the crack fronts under various test conditions (modes 1 - 4) for the half of the dangerous cross-section 1 – mode 1: static, +20 0 C; 2 – mode 2: static, -80 0 C; 2 – mode 3: dynamic, +20 0 C; 4 – mode 4: static, -80 0 C. The data related static and dynamic tests for modes No. 1-4 at t 0 =20 0 C (modes 1, 3) t= -80 0 С (modes 2, 4) are given in Table 1 (Makhutov, Reznikov, 2019a).

, 252

̅ 619 487 244

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Table 1. Strength and ductility characteristics. No Mode of loading t , 0 С

m

1 2 3 4

Static Static

+20 -80 +20 -80

0,225 0,195 0,127 0,105

0,78 0,77 0,65 0,32

1,00 1,26 2,11

317 533 665

Dynamic Dynamic

96 2,64 The front of the growing crack that starts from its initial position l (Fig. 6) was determined according to the assumption on the constancy of K I along the z -axis (Fig. 4) in the direction of the specimen thickness b (Fig. 1), taking into account the stress triaxiality according to (15) and the ultimate plasticity according to (16). At low temperatures (-80 0 С) and dynamic loading (mod 4), the maximum crack growth ∆ l occurs in the central part of the specimen, where the highest stress triaxiality state takes place (front line 4 in Fig. 6). The crack propagation on the free lateral surface of the specimen is only 0.05 of the maximum value that is reached at the center. At impact bending at room temperature ( t =+20 0 C, mode 3), the crack increment in the center of the specimen is 0.2 of that in mode 4 at t =-80 0 C (front line 3 in Fig. 6). Static bending at low temperatures ( t =-80 0 С ) leads to insignificant crack growth at the level of 0.05 of the maximum crack increment at impact bending. Under static bending at room temperature t 0 (front line 4, Fig. 6), the crack growth is about half of that under impact loading at t 0 . 7. Conclusions Strain-based criteria of nonlinear fracture mechanics allow analyzing the initial stage of fracture with the determination of the value of crack increment ∆ l , depending on the coordinate z, temperature t , and the type of loading (static or dynamic) (Fig. 6). The analysis of further crack propagation under static and impact bending is a more complicated task due to the development of local plastic deformations under impact bending at low temperatures and general plastic deformation under static bending at both room and low temperatures. In addition, the formation of plastic deformations under impact loading can be accompanied by an increase in local temperatures at the tip of a growing crack due to internal