Crack path

Stress corrosion is cracking that develops in sensitive aHoys under tensile stress which is either internally imposed or is a residual after forming, in environments such as the presence of amines and moist ammonia. The crack path can be either intercrystaHine or transcrystaHine, depending on aHoy and environment. Not aH aHoys are susceptible to stress corrosion (31). 

The transverse (circumferential) crack path reveals that the stresses responsible for SCC were axially oriented that is, the tube was pulled at its ends. Residual tube-forming stresses may also have contributed in this case. The specific cracking agent was caustic, which was apparently concentrated by evaporation when water flashed to steam in these locations. 

Figure 15.19 shows various crack orientations that can occur in connection and attachment welds. Applied stresses from external loading of these components can add to the residual weld stresses, producing still higher stress loads. This can increase the susceptibility to stress-corrosion cracking and can affect orientation and location of crack paths. 

Intergranular fracture along pre-existing paths Transgranular fracture along straingenerated paths Mixed crack paths by adsorption, decohesion or fracture of brittle phase ... 

Fig. 6.16. Crack paths al the bi-material interface (a) penetrating crack (b) singly deflected crack and (c) doubly deflected crack. After He and Hutchinson (1989).

FIGURE 1.4 Schematic diagram of the cracking path of a feed element in FCCU recycling operation. 

Fig. 29. Crack path in a Si3N4 ceramic a transgranular with low and b intergranular with high...

Indentation crack paths in alumina matrix composites containing 20 vol% of particles of (a) TiN and (b) Cr3C2. The alumina matrix is the darker phase in each case. 

Fig. 7. 13 Differences in crack wake contact and bridging mechanisms seen between static and cyclic fracture in AD 90 alumina at 1050°C. From Ref. 34. (a) Deflected crack path during static crack growth with no debris formation, (b) Deflected crack path with debris particles formed at a result of repeated rubbing between the crack faces under cyclic loading. Also seen are the debris and glass films which are squeezed out of the crack due to the pumping action of the crack walls.

Eressive stress parallel with the crack path which helps to keep the crack om running off to the side of the specimen. The critical separation is determined by measuring with a micrometer the total distance across the top of the specimen, dCi and by subtracting the thickness of the beams, 2h. The crack length, c, from where the stress is applied at the top of the specimen is measured with a cathetometer. 

The energy balance considerations in Griffith s original concept were later refined by Orowan and Irwin to include the effects of plasticity and elasticity for applicability to metals (Orowan, 1952 Irwin, 1957). Metals fail by ductile fracture, where the crack growth occurs in the direction of the primary slip system. When the slip plane is inclined to the crack, atoms across the slip plane slide past one another, relieving the stress, which results in a zigzag crack path. This is illustrated in Figure 10.14. 

Figure 10.14. The single-slip mechanism, in which crack growth occurs in the direction of the primary slip systems, results in a zig-zag crack path.

Fig. 22a—c. Microtome section of an intersperujitic crack path in bulk coarse spherulitic PP 1120 a. Figure b indicates as an SEM-micrograph taken from the surface of the specimen the interspherulitic craze formation prior to the cracking process, c shows a site of shear along a spherulite boundary oriented under an angle of about 60° to the horizontal crack direction... 

In addition to the described role of shear bands in crack initiation, the influence of spherulitic boundaries on crack propagation has to be considered, especially in coarse spherulitic, highly isotactic PP. Cracks which may have been formed at a particular point of a shear band according to the mechanism described above can leave their shear plane with further growth in order to move into an adjacent spherulite boundary (Fig. 36, Point I). This is especially probable when the boundaries are cut by the shear plane under a small angle. The selection of the particular crack path is determined by the partial fracture mechanical properties of the shear band and the spherulite boundary, respectively, as well as by the local geometrical conditions... 

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