PSI - Issue 45

James Martin Hughes et al. / Procedia Structural Integrity 45 (2023) 44–51 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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White et al. (2018) previously investigated the crack growth rate of a CT specimen manufactured from a similar aluminium alloy and subjected to the same variable amplitude loading sequence. The response of several strain gauges placed on the front face of the specimen in close proximity to the crack tip (plane stress) and a back face strain gauge (plane strain) were recorded. The measurements from these gauges indicate similar trends in closure load to those found in this study (see Fig. 2), whereby during the initial portion of crack growth there is little crack closure, followed by a relatively stable increase in closure level before the opening load ratio rises for longer cracks. The White et al. (2018) data, however, does not show a great deal of distinction between the closure loads across different stress ratios. This is likely due to the inferior resolution of the strain gauges used. Using a piezoelectric strain sensor to capture data represents a significant improvement over traditional closure measurement techniques. This enhanced ability can facilitate a more accurate collapse of the fatigue crack growth rate curve into a master curve. The White et al. (2018) study also uses fractographic imaging to draw conclusions regarding the influence of crack closure on the growth rate of the fatigue crack. Their analysis shows that the stress ratio effect is similar at the very start of loading and after the crack has grown 5 mm, leading them to conclude that closure is not the dominant cause of the stress ratio effect. The results gained in this study do not support this conclusion, and instead indicate that the stress ratio effect can be almost entirely attributed to crack closure. Using the closure results gathered in this study, the crack growth rate curves presented by White et al. (2018) can be collapsed in a similar fashion with high accuracy. This provides a substantial indication that crack closure is the dominant cause of the stress ratio effect. 4. Conclusion The crack closure concept provides a plausible explanation of fatigue crack growth phenomena such as the stress ratio and thickness effects (Codrington and Kotousov, 2007). Previous studies have been largely focused on crack closure evaluation under constant amplitude loading. These closure loads were utilised to evaluate the effective stress intensity factor range, which is often considered as a fatigue crack driving force. However, few experimental investigations have examined the role of crack closure under a variable amplitude loading. Those that have generally only measure the crack opening load for a few cycles. This study presents outcomes of the evaluation of crack closure in CT specimen using an advanced piezoelectric strain gauge for an entire transport aircraft load spectrum. The piezoelectric strain gauge has previously been shown to be highly sensitive for measurement of the specimen compliance non-linearities, and, in turn, crack closure/opening load values during cyclic loading. A compact tension specimen outfitted with the piezoelectric strain gauge was subjected to a repeated variable amplitude load sequence which contained blocks of constant amplitude loading at stress ratios between R = 0 and R = 0.5. The purpose of the study was to demonstrate the effectiveness of the piezoelectric strain gauge at measuring crack closure across the entire fatigue life, and the utilisation of these closure loads to collapse the fatigue crack growth rate curves. The fatigue crack growth rate was measured using an optical microscope at various locations along the crack front, and the raw data displayed a similar trend seen in the literature. The results of this study indicate that during the early stages of fatigue crack growth the closure load is close to the minimum load in the applied load cycle, and as the fatigue crack progresses, the crack remains closed for a greater portion of the cycle: an effect which is more pronounced for lower stress ratios. The minimum crack opening load ratio across all stress ratios is reached at a crack length ratio of a/W = 0.28, after which the effects of closure steadily reduce. The main outcome of this work is the utilisation of the experimentally measured closure values to collapse the crack growth rate curve, which is highly successful. Finally, the results are compared to historical studies which mainly utilise constant amplitude loading and display good agreement. The results gathered in this study have significant implications for spectrum truncation and compression algorithms, which require a detailed knowledge of crack closure loads. Acknowledgements The authors would like to acknowledge the funding and support provided by the Australian Defence Force, the Australian Defence Aviation Safety Authority, and the Defence, Science and Technology Group.