Crack slow subcritical

Very slow subcritical crack growth can be approximately characterized by an empirical law of the form... 

Usually, there are no visual cracks in cathode blocks at startup. But this doesn t mean that these cracks (pre cracks) are not formed inside the blocks. The strains are quite sufficient for the formation of pre cracks of subcritical dimensions. According to the slow crack growth model [67], the strains for the slow propagation of cracks are lower than strains for initiating the cracks. Usually, thermal strain cracks reach the surface of cathode blocks, and the distance between them is 80-100 sm (Fig. 2.45). Normally, they are parallel to each other and normal to the surface of the cathode. However, it is necessary to remember that these cracks reach the surface after 6-36 months although it is most likely that they appear during preheating and startup. 

The slow subcritical crack growth in a polymer is due to thermomechanically activated processes of the same nature as described in the last chapter for the homogeneous fracture, i.e. chain stretching, plastic deformation, void opening, chain disentanglement, and chain scission. The rate with which these processes occur is, however, in the strain field surrounding a crack tip larger than elsewhere in the material. 

The step-modified slow strain-rate or Rising Step-Load (RSL ) test produces an increasing stress intensity that is different at each step-load but remains constant with crack extension for a Chaipy-sized SE(B) four-point bend specimen while each load level is maintained constant for a period of time. The rising step-load test was developed as an accelerated low-cost test for measuring the resistance of steels and weldments to IHE or EHE [45,46 ]. The load is constantly increased under displacement control, but incrementally in steps with each step being sustained for a period of time so that the onset of subcritical crack extension can be detected, which is the threshold for the onset of IHE or EHE. The RSL test differs from the SSRTT, which measures the differences in ductility parameters over the... 

Figure 4.33 shows the rate of slow, time-dependent subcritical crack growth, measured using the S5rmmetric four-point flexure specimen geometry, along the TiN-Si02 interface in the multilayer structure as a function... 

Willis, J.R. (1967b) Crack propagation in viscoelastic media. J. Mech. Phys. Solids 15, 229- 240 Wnuk, M.P. (1971) Subcritical growth of fracture (inelastic fatigue). Int. J. Fract. Mech. 7, 383-407 Wnuk, M.P. (1973a) Slow growth of cracks in a rate sensitive Tresca solid. Eng. Fract. Mech. 5, 605-626... 

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