Crack formation and fatigue failure

6.11.4 Crack formation and fatigue failure

Extrusions, intrusions and fatigue cracks can be formed at temperatures as low as 4 K where thermally activated movement of vacancies does not take place. Such observations indicate that the formation of intrusions and cracks cannot depend on either chemical or thermal action and the mechanism must

be a purely geometrical process which depends on cyclic stressing. Two general mechanisms have been suggested. The first, the Cottrell ‘ratchet’ mechanism, involves the use of two different slip systems with different directions and planes of slip, as is shown schemat- ically in Figure 6.73. The most favored source (e.g. S 1 in Figure 6.73a) produces a slip step on the surface at P during a tensile half-cycle. At a slightly greater stress in the same half-cycle, the second source S 2 produces a second step at Q (Figure 6.73b). During the compression half-cycle, the source S 1 produces a surface step of opposite sign at P ′ (Figure 6.73c), but, owing to the displacing action of S 2 , this is not in the same plane as the first and thus an intrusion is formed. The subsequent operation of S 2 produces an extrusion at QQ ′ (Figure 6.73d) in a similar manner. Such a mechanism requires the operation of two slip systems and, in general, predicts the occurrence of intrusions and extrusions with comparable frequency, but not in the same slip band.

The second mechanism, proposed by Mott, involves cross-slip resulting in a column of metal extruded from the surface and a cavity is left behind in the interior of the crystal. One way in which this could happen is by the cyclic movement of a screw dislocation along a closed circuit of crystallographic planes, as shown in Figure 6.74. During the first half-cycle the screw dislocation

glides along two faces ABCD and BB ′ C ′

C of the band, and during the second half-cycle returns

D and A ′ D ′ DA. Unlike the Cottrell mechanism this process can be operated with a single slip direction, provided cross-slip can occur. Neither mechanism can fully explain all the experimental observations. The interacting slip mech- anism predicts the occurrence of intrusions and extrusions with comparable frequency but not, as

along the faces B ′ C ′ A ′

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Figure 6.73 Formation of intrusions and extrusions (after Cottrell, 1959; courtesy of John Wiley and Sons).

Internal cavity

Figure 6.74 Formation of an extrusion and associated cavity by the Mott mechanism.

is often found, in the same slip band. With the cross-slip mechanism, there is no experimental evi- dence to show that cavities exist beneath the material being extruded. It may well be that different mechanisms operate under different conditions.

In a polycrystalline aggregate the operation of several slip modes is necessary and intersecting slip unavoidable. Accordingly, the widely differing fatigue behavior of metals may be accounted for by the relative ease with which cross-slip occurs. Thus, those factors which affect the onset of stage III in the work-hardening curve will also be important in fatigue, and conditions suppressing cross-slip would, in general, increase the resistance to fatigue failure, i.e. low stacking-fault energy and low temperatures. Aluminum would be expected to have poor fatigue properties on this basis, but the unfavorable fatigue characteristics of the high-strength aluminum alloys is probably also due to the unstable nature of the alloy and to the influence of vacancies.

In pure metals and alloys, transgranular cracks initiate at intrusions in PSBs or at sites of surface roughness associated with emerging planar slip bands in low SFE alloys. Often the microcrack forms at the PSB/matrix interface where the stress concentration is high. In commercial alloys contain- ing inclusions or second-phase particles, the fatigue behavior depends on the particle size. Small particles ≈0.1 µm can have beneficial effects by homogenizing the slip pattern and delaying fatigue- crack nucleation. Larger particles reduce the fatigue life by both facilitating crack nucleation by slip band/particle interaction and increasing crack growth rates by interface decohesion and voiding within the plastic zone at the crack tip. The formation of voids at particles on grain boundaries can lead to intergranular separation and crack growth. The preferential deformation of ‘soft’ precipitate-free zones (PFZs) associated with grain boundaries in age-hardened alloys also provides a mechanism of intergranular fatigue-crack initiation and growth. To improve the fatigue behavior it is therefore

380 Physical Metallurgy and Advanced Materials

Figure 6.75

A schematic fatigue fracture.

necessary to avoid PFZs and obtain a homogeneous deformation structure and uniform precipitate distribution by heat treatment; localized deformation in PFZs can be restricted by a reduction in grain size.

From the general appearance of a typical fatigue fracture, shown in Figure 6.75, one can distinguish two distinct regions. The first is a relatively smooth area, through which the fatigue crack has spread slowly. This area usually has concentric marks about the point of origin of the crack which correspond to the positions at which the crack was stationary for some period. The remainder of the fracture surface shows a typically rough transcrystalline fracture where the failure has been catastrophic. Electron micrographs of the relatively smooth area show that this surface is covered with more or less regular contours perpendicular to the direction of the propagation front. These fatigue striations represent the successive positions of the propagation front and are spaced further apart the higher the velocity of propagation. They are rather uninfluenced by grain boundaries and in metals where cross-slip is easy (e.g. mild steel or aluminum) may be wavy in appearance. Generally, the lower the ductility of the material, the less well defined are the striations.

Stage II growth is rate controlling in the fatigue failure of most engineering components, and is governed by the stress intensity at the tip of the advancing crack. The striations seen on the fracture surface may form by a process of plastic blunting at the tip of the crack, as shown in Figure 6.76. In (a) the crack under the tensile loading part of the cycle generates shear stresses at the tip. With increasing tensile load the crack opens up and a new surface is created (b), separation occurs in the slip band and ‘ears’ are formed at the end of the crack. The plastic deformation causes the crack to be both extended and blunted (c). On the compressive part of the cycle the crack begins to close (d). The shear stresses are reversed and with increasing load the crack almost closes (e). In this part of the cycle the new surface folds and the ears correspond to the new striations on the final fracture surface. A one-to-one correlation therefore exists between the striations and the opening and closing with ear formation. Crack growth continues in this manner until it is long enough to cause the final instability when either brittle or ductile (due to the reduced cross-section not being able to carry the load) failure occurs. In engineering alloys, rather than pure metals, which contain inclusions or second-phase particles, cracking or voiding occurs ahead of the original crack tip rather than in the ears when the tensile stress or strain reaches a critical value. This macroscopic stage of fracture is clearly of importance

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New surface

New surface

Figure 6.76 Schematic illustration of the formation of fatigue striations.

to engineers in predicting the working life of a component and has been successfully treated by the application of fracture mechanics, as discussed in Chapter 7.