Exploring Crack Closure and Load Effects in Fatigue

Addressing challenges of crack closure and load effects in fatigue for enhanced structural integrity in the aircraft industry.

Crack tip closure has gained significant popularity in the aircraft industry due to its relevance to the behavior of cracks under the influence of random loads. However, there are several weaknesses and unanswered questions surrounding the reliance on experimental techniques and the measurement location of crack tip displacement, as well as the understanding of crack closure at high stress ratios and under spectrum loading conditions. In order to address these challenges and advance the understanding of fatigue crack growth, further research is needed.

Question 1: In the aircraft industry, crack tip closure has gained significant popularity due to the influence of random loads. A critical question arises: does the reliance on experimental techniques and the measurement location of crack tip displacement pose weaknesses to this concept?

Recent research on crack-opening load measurements has shed light on certain challenges associated with the ASTM E647 approach. Notably, studies have shown that while remote measurements such as remote Crack Mouth Opening Displacement (CMOD) and Backface Strain (BFS) gauges yield satisfactory results at low stress ratios (R), they fail to accurately capture opening loads at high R. Consequently, researchers have reverted to a "local" method similar to the original approach. Furthermore, it has been found that the ASTM 2% offset method introduces dependency on the measurement location, affecting the determination of opening loads. It is important to note that the weakness lies in the shift from local to remote methods, rather than the fundamental concept of crack closure. Additionally, the load-shedding test method specified in ASTM E647 induces remote closure, similar to what occurs under variable-amplitude loading conditions. However, the measurement of crack-opening loads under spectrum (or random) loading has not received extensive attention in research. Some work on single spike overloads has provided valuable insights. Nonetheless, measuring the correct crack-opening loads under spectrum loading poses challenges. This highlights the pressing need for further research in this domain. Question 2: Can the mean stress effect on fatigue crack growth (FCG) at high stress ratios be explained despite the absence of crack closure at high loads?

As previously mentioned, when employing a local measurement method, crack closure is observed at high stress ratios. Extensive investigations have demonstrated the presence of high-R closure across various materials. It appears that this high-R closure may be attributed to surface roughness and/or debris-induced closure, although plasticity remains the prevailing mechanism even at high stress ratios. Question 3: Specifically for aluminum alloys, does the ASTM E647 load-shedding method adequately capture the effective stress intensity range associated with crack closure to establish a correlation with fatigue crack growth near the threshold regime?

Once again, the utilization of the ASTM E647 load-shedding method presents challenges in comprehending the threshold behavior due to remote closure. Over the past decade, researchers have made notable progress in developing a novel method, in collaboration with their colleagues, to generate test data at low rates without inducing remote closure through compression pre-cracking. It is widely recognized that the threshold regime involves crack closure induced by surface roughness and/or debris, with plasticity still playing a crucial role.

Question 4: How can we explain the paradoxical observation that in vacuum tests, where crack tip closure is insignificant, fatigue crack growth appears to be slower compared to air tests, contrary to expectations?

This intriguing question raises the need for further investigation. Despite the absence of specific references, it remains uncertain whether enough testing and measurements have been conducted under vacuum conditions to provide a comprehensive understanding. Previous studies have suggested the existence of a shift in the R-value in recent experimental results from vacuum tests. A deeper analysis is required to unravel the underlying factors contributing to this phenomenon. Question 5: Is it erroneous to model the crack opening behind the crack tip by symmetrically incorporating a residual stress distribution analogous to the one existing ahead of the crack tip?

In closure modeling, the presence of residual stress in front of the crack tip plays a vital role in determining the distribution of contact stress along the crack surface, which is subsequently utilized in calculating the crack-opening load. To assess the factors contributing to material damage and crack growth, it becomes necessary to examine the cyclic plastic strains and hysteresis energies in the crack-front region. Consequently, we believe that the concept of crack closure provides a suitable framework for characterizing the damage occurring at the crack tip. Question 6: How can we elucidate the impact of the compressive component in the load history on fatigue crack growth by leveraging the concept of crack closure?

The influence of compressive loading is contingent upon the specific configuration of the crack. In the case of a single crack within a sizable body, where it encounters a positive load, a substantial portion of the compressive loading may prove ineffectual. However, when dealing with a small crack propagating at a hole or notch, the crack has the potential to open under compressive loading. The treatment of compressive loads does not differ from positive loads, necessitating the calculation of contact stresses under the intended loading conditions.

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