8 Detecting Fatigue Crack Closure and Crack Growth Delays After an Overload Using DIC Measurements 63 Fig. 8.11 (a) COD measurements along the crack flanks and (b) strain field map for the point OLC793 kcycles Fig. 8.12 (a) COD measurements along crack flanks and (b) strain field map for the point OLC953kcycles Fig. 8.13 (a) COD measurements along crack flanks and (b) strain field map for the point OLC1055 kcycles Figure 8.14 shows the strain behavior of the material near the OL location, for two different crack lengths. It can be seen that the strains have the same behavior and tend to stabilize at a value of 1300 ". In this way, the experimental results showed that the residual hump of stretched material at the OL location is the cause of the discontinuous closure observed in this fatigue test. This phenomenon leads to high levels of crack opening loads in post-overload behavior as shown in the analysis, increasing the retardation at about 3 times the theoretical OL-induced plastic zone for this case.
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