Discontinuous crack propagation

Setting aside the specifics of these localized fiacture mechanisms in the process zone for the present, it is apparent that hydrogen embrittlement models predict discontinuous crack propagation at an average rate of... 

Linkage of these microcracks to give a discontinuous crack propagation over and above that due to mechanical fatigue failure. 

Discontinuous crack propagation Liberation of hydrogen upon cathodic polarization... 

Type 5 takes place when transverse discontinuities are present between structural units of the fiber and are sufficient to stop direct crack propagation. Breaks then occur with a granular surface roughly perpendicular to the fiber axis, as shown in our example of wool-fiber tensile failure (Figure 2). Once again, split-level transverse breaks are possible (5). 

Basically both types of fatigue crack propagation are discontinuous hence the so-called discontinuous type would be better characterized as retarded crack growth >. 

Fig. 55. Discontinuous growth bands in PVC Crack propagation direction from left to right normal to the bands. AK, = 17.5 N/mm ...

Retarded fatigue crack growth (RCG) was first observed by Elinck et al. in PVC, and was called discontinuous initially. They noted that the crack propagated step-wise and that a great number of fatigue cycles was needed for the formation of successive markings on the fracture surface. 

BPA Bisphenol-A CTPZ Crack tip plastic zone DCG Discontinuous crack growth FCP Fatigue crack propagation... 

Under normal fatigue crack propagation (FCP) conditions, the crack advances on sach load cycle. Below a critical threshold value, no crack propagation occurs. This is denoted by the threshold value in Fig. 3. Unique to polymers, however, is a cumulative dama process at the fatigue crack tip which enables discontinuous crack growth (DCG), i.e., crack advance after a multiple number of load cycles This DCG process is sub-critical and appears below the threshold... 

At the ductile-to-brittle transition, crack propagation changes from unstable to stable crack growth. Both at low and high speeds, a ductile-to-brittle transition can be observed. The high-speed transition cannot be obtained by extrapolating from the low-speed data. Also, the low-speed transition is not a discontinuous transition and depends on different structural parameters, like... 

Figure 7 shows clearly the effect of number of joints on strength and deformation behaviour of POP specimen under unconfined condition. The strength decreases drastically and further the strain at failure increases. It can be seen that, as the joint frequency increases, the ductile nature of the specimen is more pronounced and atransition between brittle to ductile behaviour is observed from intact to specimen with three joints (Figure 7). The reduction in the strength for one joint at P=90° i.e., 13 joints per meter, is around 10% (Figure 7). The reduction is around 30% and 40% for 26 joints and 39 joints per meter respectively. Although the strength decreases with an increase in the number of joints, failure modes are almost similar for a specific inclination and the crack propagation usually interrupted due to the presence of discontinuity. Similar observation was reported by Arora (1987) and the reduction of strength was aroimd 7% for 10 joints per meter...

This model can also be used to explain the crack arrest markings and cleavagelike facets on the fracture surface, as well as the discontinuous nature of crack propagation [32, 33], although more research into surface films and brittle fracture is required before this model can be thoroughly evaluated [34],... 

Delay in fracture apparently results because of the time required for hydrogen to diffuse to specific areas near a crack nucleus until the concentration reaches a damaging level. These specific areas are presumably arrays of imperfection sites produced by plastic deformation of metal just ahead of the crack. Hydrogen atoms preferably occupy such sites because they are then in a lower-energy state compared to their normal interstitial positions. The crack propagates discontinuously because plastic deformation occurs first, and then hydrogen dif-... 

---