8 Coalescence and Growth of Two Semi-Elliptical Coplanar Cracks in API-5L Grade B Steel 61 Table 8.2 Testing conditions Loading cycles Marker band cycles Stress (MPa) Stress (MPa) Max Min Loading ratio # of cycles Max Min Loading ratio # of cycles Frequency (Hz) 270 27 0.1 13,000 270 215 0.8 2,000 2 280 28 280 220 0.2 280 28 280 220 2 290 29 290 230 0.2 290 29 290 230 2 smin smax C cycles Loading cycles 0.1 Marker Band cycles R=0.8 Load (s) M cycles Time (t) Fig. 8.6 Loading pattern with the marker band loads [12] of the maximum stress. According to [12], it was observed that marking block with high stress ratio yields a better marking impression which is of better visibility. A typical loading ratio used in marker blocks varies from 0.5 to 0.8 (Fig. 8.6). According to [13], one of the methods to determine the crack length at different numbers of cycles along with monitoring the fatigue crack front history after testing is to use the marker bands technique. A selective marking of the fatigue surface could do this with a coded marker bands generated by applying a series of fatigue under-loads predetermined intervals. Also, as stated by [13], the marker bands are groups of microscopic striations that when generated in the proper fashions are readily identifiable by optical and scanning electron microscopy. 8.4 Results and Discussion There are many factors affecting crack growth including the type of stresses affecting the structure, microstructure and the environment. Figure 8.7 illustrates different factors that affect fatigue crack growth in different fatigue stages. When neighboring surface cracks are in close proximity, they can interact and grow, achieving what is called cracks coalescence. This phenomenon adds significant complications to the crack growth analysis. Neighboring cracks interaction and coalescence lower the fatigue life of structures and components. Life decrease with increasing cracks density which results in interactions among different cracks (Fig. 8.8). 8.4.1 Surface Crack Monitoring Measurements of the surface crack growth around the two cracks during the tests were recorded. A real-time microscopy method was employed during the fatigue tests to keep a record of the surface crack growth both between the two cracks and near the sample edge. Figure 8.9 shows the gradual interaction between the two cracks until they achieve coalescence. However, Fig. 8.10 shows the crack on the right growing towards the sample edge.
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