Fatigue cracks examples

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). 

Separate the metal from the environment with a physical barrier. Many corrosion inhibitors make use of this principal to protect metals. Proper use of an appropriate inhibitor may reduce or eliminate pitting. Pits are frequently initiation sites for corrosion-fatigue cracks. The effectiveness of inhibitors depends upon their application to clean metal surfaces. An example of this method is the use of zinc coatings on steel to stifle pit formation. 

Example 2.22 A certain grade of acrylic has a Kc value of 1.6 MN and the fatigue crack growth data as shown in Fig. 2.77. If a moulding in this material is subjected to a stress cycle which varies from 0 to 15 MN/m, estimate the maximum internal flaw size which can be tolerated if the fatigue endurance is to be at least 1(P cycles. 

The complexity of these chemical and mechanical interactions is such that each metal-environment system must be examined on an individual basis to determine the important processes influencing corrosion fatigue crack nucleation and growth rates. Thus, in the ensuing sections, examples are quoted to illustrate commonly occurring phenomena or establish more general principles with reasonably wide applicability for particular classes of metal/environment combinations. It should be noted, however, that when... 

FIGURE 25.6 Examples of variable ampUtude fatigue crack growth test signals applied to pure shear specimens to investigate the effects of (a) load severity, (b) load sequence, (c) R-ratio, and (d) dwell periods on crack growth rates. A, B, and C denote peak strain levels. 

Fig. 7.11 (a) An example of a SiC whisker bridging the faces of a fatigue crack in an alumina/SiC composite (R = 0.15, T = 1400°C, and vc = 0.1 Hz), (b) Scanning electron fractography showing pulled-out SiC whiskers which have been oxidized to form glass. From Refs. 4 and 61. 

Fig. 7.12 (a) A schematic representation of the pinning of grain boundaries by whiskers, (b) An example of the process shown in (a) in the fatigue crack tip region of the alumina/SiC composite subjected to fatigue fracture at 1400°C. From Ref. 61. A refers to alumina grains. 

Examination of the crack profiles after cyclic fatigue revealed extensive branching at the crack tip and a substantial damage zone in the vicinity of the (macro)crack tip. Increases in test temperature caused an increase in the size of the zone. Close examination revealed bifurcation of cracks and ligaments extended across the crack faces. An example of a typical fatigue crack profile appears in Chapter 7 by Suresh. 

By definition, ESC is influenced by the level of the applied multiaxial stress, ft is expected that below an assigned value of stress in a specific medium ESC will not occur. Fatigue crack growth experiments at various constant levels of A K can be applied to study the influence of the crack-tip loading on the ESC behavior in a systematic way. As an example, cast CT specimens of PMMA (Mn = 4.6 x 105) were tested in air and IPA at different levels of AK (0.6, 0.7, 0.8, 0.9 MPa -v/m). The tests were performed as described earlier, with the application of the medium after a certain time of cyclic loading in air. From the plots of the crack length ratio vs. number of cycles, the following observations can be made (Fig. 23) ... 

Fig. 31. Example of interference fringe patterns measured during fatigue crack propagation (

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