Fatigue striations

Figure 7.12 Fatigue striations observed on the fracture surface...

Fig. 40. Fatigue striation on the fracture of PC (lower part) due to different cyclic loading (upper part) [Jacoby, G., Cramer, Ch., Rheologica Acta 7, 23 (1968), Dr. Dietrich Steinkopff Ver-lag]...

Fig. 41. Formerly proposed mechanism for the forming of fatigue striations in polymers...

Fig. 43. Optical interference photograph showing fatigue striations on the fracture surface and craze zone in a partially broken PMMA specimen...

It can thus be concluded that during the so-called continuous mode of fatigue crack propagation there is no simple correlation between fatigue striation spacing and craze dimensions. 

Figure 7. Fine and shallow fatigue striations observed in the region of early crack growth in...

Figure 8. Fatigue striations in nylon 66 equilibrated to contain 5.7 wt % water. Arrow shows direction of fatigue crack growth.

Markings believed to be classical fatigue striations (Figure 8) were, however, observed in specimens having water contents in the range between 2.2 and 5.7 wt %, inclusive. 

Classical fatigue striations are observed at intermediate water contents, confirming earlier findings. However, no evidence was found (at any water content or AK level) for discontinuous growth bands, which are found in many other polymers. 

Fig. 7.14 Fatigue fracture surface of 3Y-TZP for a = 0.8 and b, c R = 0.01. In both cases, fatigue striations appear during the last scores of cycles before catastrophic failure, d Schematic of fatigue striations in terms of two growth steps [18]. With kind permission of John Wiley and...

ESEM was needed to search for possible defects not detected in tlie optical microscope. An oblique shot confirmed the fracture to be relatively featureless (Fig. 9.20). There was no evidence of fatigue striations on the surface, therefore slow intermittent failure across the catheter could be excluded. The zone near the join with the bag showed many defects, and indeed some can be just seen at a deep cleft on the right (G) where the bag joins the tube. There appeared to be microcracks present here, which might explain how failure occurred. A more extensive survey revealed more cracks wherever the catheter met the bag, suggesting that either severe stress had created the cracks, or that these zones were inherently weak (Fig. 9.21). 

In many polsrmers, fatigue striations can be observed at relatively high AFC levels. These striations correspond to the successive location of the advancing crack front after each loading cycle or local crack jump. There is a clear distinction between macroscopically observed clamshell markings, which represent periods of... 

On the other hand, it is important to note that discontinuous crack growth under a static load may yield fracture surfaces, which may look quite similar to fatigue striations. To observe this phenomenon, the material has to be able to slowdown or even stop the crack propagation after the static load initiated the... 

Fig. 9. Tensile fatigue striations in SAN with residual force fracture at elevated load.

Genuine Ductile Fatigue Fracture. D5mamic fatigue fracture may propagate on different fracture paths and expand at different times. Materials relatively easy to deform yield fold-like, rounded, cloth-like residual fractures between the fracture paths (Fig. 10). The fatigue striations too are more fold-like and rounded and the fracture plane contains exposed inclusions. 

Fig. 10. Propagation of dynamic fatigue fracture in offset fracture paths with fatigue striations and cloth-like residual fractures, (a) Schematic and (b) example for nylon-6,6 (PA 66).

Pig. 11. Dynamic fatigue fracture in brittle poljrmer with sharp-edged d)mamic fatigue fracture paths, fatigue striations, and secondary cracks, (a) Schematic (b) styrene-butadiene copolymer with severed butadiene spheres. 

Frequently, the crack propagation can be seen on the fatigue crack surface as so-called fatigue striations (figure 10.12), with a distance that depends on the load and is usually between 0.1pm and 1pm [90]. This distance is an approximate measure of the crack propagation per cycle. Macroscopically, the striations are invisible and the fatigue crack surface appears rather smooth. The formation of striations is due to the plastic deformation near the crack tip shown in figure 10.11. 

Figure 4.61 Fatigue striation in a 7178 aluminum alloy. (From Pelloux, M.N. (1965). In Metals Engineering Quarterly, ASM, November)...

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