Parallel cracks

No common industrial metal is immune to corrosion fatigue since some reduction of the metal s resistance to cyclic stressing is observed if the metal is corroded, even mildly, by the environment in which the stressing occurs. Corrosion fatigue produces fine-to-broad cracks with little or no branching. They are typically filled with dense corrosion product. The cracks may occur singly but commonly appear as families of parallel cracks (Fig. 10.2). They are frequently associated with pits, grooves, or other forms of stress concentrators. Like other forms of... 

As part of a. basic study on M-8 proplnt, Revere (Ref 2lj using TEM, discovered that proplnt stored at —20° and —40° for 12 hour periods became wrinkled and exhibited definite parallel cracking and changes in crystalline structure, which was reflected in erratic burning... 

Thus, according to Rogers [117] when the surface of crystals which have been given a flash, but which have not exploded or broken down, is examined by an optical microscope, it can be seen that the crystal is much darkened on the irradiated face, and contains many irregular but parallel cracks. The cracks are not visible on the other side of the crystals suggesting that they penetrate only a short distance into it. 

Fig. 2. SEM micrographs of biotically aged polycaprolactone films a) parallel cracks and b) surface erosion on the PCL films...

Delannay and Warren have studied the interaction between parallel cracks by considering the energy release rate associated with the propagation of a new crack among an already existing crack network. 

Fig.5 Schematic diagram of expected R-curves with comparison of trends in nondimensional Kj for vertical and parallel crack.

Four different three-dimensional numerical infiltration experiments were carried out in a simulated porous medium with a central parallel crack as shown in Fig. 4-2. The lattice size is 10 by 100 by 150 sites in the x, y and z directions, respectively. Solid sites are represented in red. A probabilistic algorithm generated at random the solid distribution of the microporous matrix. The mean microporosity and macroporosity are 0.52, and 0.192, respectively, of the total volume of the medium. A gravity force was simulated as described in Di Pietro et al. (1994), oriented parallel to the crack in the z-downward direction. Void sites (white color in Fig. 4-2) are initially f ss are expressed in arbitrary lat-... 

Fig. 4-2. Simulated porous medium with a central parallel crack. Solid is represented in red.

Fig.4. Parallel cracks under tensile load on the side face of bending bars after bending at oxidation temperature (Tm = 1223 K) and cooling to RT (a rox = 20h e = 1C)-5s b fox = lOOh,...

Cracks propagate laterally when the stress intensity factor, Kt, exceeds the fracture toughness, Klc, of the scale. In a nonuniform stress field, also the stress intensity factor varies spatially according to formula (9). The position of the crack tip in a nonuniform stress field is determined by the equation Kx = /. .The last equation together with formula (9) permits, in principle, to derive the fracture toughness from the position of the crack tip provided the tensile stress is known as a function of the position (cf. Fig. 11). (In an accurate description, formula (9) has to be replaced by a corresponding relation /q(cr) which takes into account the interaction of parallel cracks.)... 

Parallel cracks pose a more serious threat to reinforcement than transverse cracks, as they sustain higher corrosion rates [11]. In any case, understanding of the corrosion mechanisms in relation to cracks is poor. In general, crack widths should be limited in particular in aggressive environments. 

CF results in fine-to-broad cracks with little or no branching, unlike SCC that exhibits branching cracks. The cracks appear as families or parallel cracks and are filled with dense corrosion product. The sample may have pits, grooves, or some other forms of stress concentrator. Transgranular fracture paths, often ramified or branched are more common than intergranular fracture with the exception of lead and zinc (Fig. 1.18). Some systems show a combination of transgranular and intergranular forms of fracture (8, 9). 

Crack branching Crack may subdivide into two or more roughly parallel cracks... 

This estimate is remarkably simple, given the complexity of the underlying boundary value problem. The left sides of the expressions in (3.114) are the curvatures normalized by the effective Stoney curvature based on the average film thickness hfb/p. Furthermore, the right sides of (3.114) do not involve the parameter p. In other words, p enters the estimate for curvature only through the effective Stoney curvature based on the amount of film material involved. When b = p, the response is identical to that for a periodic array of parallel cracks. 

The formation of a through-the-thickness crack in a film subjected to a residual or applied tensile stress relieves that stress in the film material at points adjacent to the crack path. At points in the film at some distance from the crack path, the stress remains unrelaxed due to the constraint of the substrate. Consequently, a long crack that is parallel to the first formed crack can also form. Indeed, an array of parallel cracks over the entire film surface is likely, and the point of the discussion in this section is to provide an estimate of the dependence of the spacing between cracks in such an array on the film thickness hf and the mismatch stress a. The discussion is limited to the case when the equi-biaxial mismatch stress is uniform throughout the film, the elastic properties of the film and substrate are nominally the same, and hg/hf is sufficiently large so that the behavior is insensitive to the substrate thickness hg. Furthermore, it is assumed that the cracks grow through the thickness of the film to the depth hj, but that they do not penetrate into the substrate. There is no fundamental barrier to relaxation of these limitations, but the relatively simple system is sufficiently rich in physical detail to reveal the principal features of behavior. 

For imiaxial stress systems, there will be an array of parallel cracks which are perpendicular to the direction of principal stress. Torsion loadings tend to produce a system of crisscross cracks at roughly 45° from the torsion axis. Corrosion fatigue cracks found in pipes subjected to thermal cycling will usually show a pattern made up of both circumferential and longitudinal cracks. 

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