14 P. A. Lara et al. Fig. 2.10 Wheeler and Willenborg retardation predictions of HF pulse of 50 Hz and 2900 N Wheeler model relies on the Irwin calculation to develop the radii of the overload zone and cyclic zones. The methodology of the Wheler model is that an overload plastic zone is created and retardation occurs as long as the reoccurring cyclic zone is within the bounds of the overload zone as the crack grows. The Willenborg model follows a very similar approach but relies on the stress intensity from the overload [15] and the calculation of a stress intensity reduction in order for retardation effects to seize. This behavior can be seen graphically showing how the residual stress intensity is accumulated after the HF pulse at 42 K cycles and how it diminishes over time to no retardation at 56 K cycles for the Willenborg model, Fig. 2.10. For the Wheeler model, the crack recovery can also be plotted to show when the radii of the cyclic plastic zone reaches the boundary of the overload zone and also hence ends the retardation cycle at 50 K cycles. One of the novel findings of this work was the observation of a crack kinking mechanism resulting from the application of certain HF pulses, outlined in the next section. 2.5 Crack Kinking Behavior An observed crack kinking behavior was mentioned in previous sections, during the experiments; this behavior was observed and tracked for the various specimen conditions outlined in Table 2.1. The microscope data shown in Fig. 2.11 show the crack growing from the left notch of each specimen, the orange arrow marks the direction of crack growth, the red mark shows the application of the applicable HF pulse, the green marks delimitate locations of data extraction in the fatigue cycle. While the microscope measurements provide a view of the crack propagation, a Barker’s etch method [21] was used for anodizing the aluminum alloy and then utilizing optical microscopy with polarized lights to observe the grain boundary and the propagated crack. This was evaluated at 500μm, 100μm, and20μm magnification levels, allowing the evaluation of the crack propagation through the grain, evaluating if there was a presence of large material inclusions, as well as investigating shielding effects such as contact and deflection on crack growth [22]. Fractography was also utilized to examine the surfaces through the crack depth on the specimen; several test specimens were carefully cut to allow views of the through thickness crack front. The results showed a distinct beach mark shape due to the HF pulse and a change in contrast near the edge surfaces due to the inclination associated with crack kinking. This evaluation also showed that the width of the beach mark is consistent with plastic zone and narrows beneath the surface as it is typical of plane stress to plane strain transition regions. This assessment did not indicate that there was a presence of a material inclusion that could cause the observed kinking behavior. The crack kinking behavior observed can be segmented into several areas associated with the kinking condition, as shown in Fig. 2.12. After the application of the HF pulse, there is a section when the crack continues to grow, and before it kinks, another section is when the crack is fully kinked and continues in that condition, and a last section where the crack kinking behavior has ceased and the crack has returned to its semi-original growth path. Length and angle parameters were gathered for the crack.L1 defines the crack length after the HF pulse and before the kink, L2 defines the length of the kink. The angle
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