Fatigue propagation

Figure 11.27 Fatigue propagation in plastics. Region I, substantially nil crack growth. Region II, figure crack propagation proceeds with thousands of cycles required to produce failure. Region III, failure after only a tew cycles.

Liu et al. (2002) studied the influence of different inhibitors on the fatigue propagation crack rate in high strength aluminum alloy AA7075-T76, in a 3.5% NaCl solution These authors observed, in the presence of phosphate, molybdate, citrate or thiazole/imidazole, a reduction in the rate of crack growth, mainly in the first stages of its propa tion, as a consequence of the diverse mechaiusms of inhibition of each of the used compounds. 

This optimum condition is designed to ensure that the botes of all components yield at the same time. If the cylinder is subjected to fatigue conditions, it has been suggested (39) that a better design criterion would arrange for the maximum normal stress, which controls fatigue crack propagation, to be the same in each component. 

The fracture surfaces, revealed when the tube is broken open, are found to be smooth with a rippled appearance characteristic of fatigue. This type of behavior is sometimes known as leak before break. On the other hand, if the material lacks toughness, the propagation of the fatigue crack may be intermpted part way through the wall by the intervention of fast fracture, resulting in what is sometimes known as the break before leak mode of failure. 

The life of a component, as measured in a fatigue test, is the number of cycles needed to initiate a crack and cause it to propagate across the wall until it intersects the outside surface or until fast fracture intervenes. 

Fracture Mechanics. Linear elastic fracture mechanics (qv) (LEFM) can be appHed only to the propagation and fracture stages of fatigue failure. LEFM is based on a definition of the stress close to a crack tip in terms of a stress intensification factor K, for which the simplest general relationship is... 

One aspect of pressure vessel design which has received considerable attention in recent years is the design of threaded closures where, due to the high stress concentration at the root of the first active thread, a fatigue crack may quickly initiate and propagate in the radial—circumferential plane. Stress intensity factors for this type of crack are difficult to compute (112,113), and more geometries need to be examined before the factors can be used with confidence. 

In the case of the fibrous laminate not much work has been done, but it has been observed that a significant loss of stiffness in boron—aluminum laminate occurs when cycled in tension—tension (43,44). Also, in a manner similar to that in the laminated PMCs, the ply stacking sequence affects the fatigue behavior. For example, 90° surface pHes in a 90°/0° sequence develop damage more rapidly than 0° pHes. In the case of laminates made out of metallic sheets, eg, stainless steel and aluminum, further enhanced resistance against fatigue crack propagation than either one of the components in isolation has been observed (45). 

Fatigue Crack Propagation, ASTM STP 415, American Society for Testing and Materials, Philadelphia, Pa., 1967. 

Highly ductile materials tend to be more resistant to thermal fatigue and also seem more resistant to crack initiation and propagation. 

Several theories have been proposed to explain the corrosion-fatigue phenomena. One is that cyclic stressing causes repeated rupture of protective coatings. Corrosion-fatigue cracks propagate as the coating is successively reformed and ruptured along a plane. 

Data on fatigue crack propagation are gathered by cyclically loading specimens containing a sharp crack like that shown in Fig. 15.7. We define... 

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