Abstract
It is well established that fatigue crack nucleation and small crack growth in high strength aluminum alloys are highly influenced by the surrounding microstructure including grain boundaries, texture, inclusion barriers, among other factors. As such, specific and targeted experimental and computational methods are necessary to accurately capture and predict the discrete behavior of microstructurally small fatigue cracks. In this study, surface fatigue crack nucleation and microstructurally small crack growth in high strength aluminum alloys, commonly used in aerospace applications, are evaluated through a holistic approach encompassing fatigue testing, crack measurement, and computational prediction of crack growth rates. During fatigue testing, crack shapes and growth are quantified using a novel surface replication technique that is applied to investigate crack nucleation, as well as to collect validation data that includes an accurate description of crack shape during crack propagation, a challenging and essential component in predicting crack growth. Computational simulation of fatigue crack growth in non-straight, complex surface crack arrays typically requires high fidelity analysis using computationally expensive methods to account for the mathematical and geometrical complexities inherent in the solution. A dislocation distribution based technique has been previously demonstrated to rapidly and accurately predict the stress intensity factors for through cracks of complex shape. This method was expanded and investigated as an approach for rapidly predicting the crack growth rate of kinked and tortuous surface crack arrays, using the crack configuration and bulk material properties as inputs. To investigate the accuracy and effectiveness of this characterization approach, surface crack growth in AA7075-T7351 was experimentally analyzed and modeled under high cycle and low cycle fatigue conditions. This comprehensive approach was determined to be an expedient and applicable method for characterizing and evaluating the nucleation and crack growth rate of non-planar microstructurally small and short crack configurations.
Highlights
Predicting the crack growth rate and direction in the early stages of crack growth is challenging due to several phenomena
The present study focuses on using the novel fast drying two-part silicon rubber replica method to quantify the fatigue crack nucleation and microstructurally small crack growth mechanisms of AA7075-T7351
The two low cycle fatigue (LCF) specimens closely compare, where the dominant crack spent the majority of the fatigue life under a crack length of 250 μm
Summary
Predicting the crack growth rate and direction in the early stages of crack growth is challenging due to several phenomena. The rate and direction of crack growth immediately following initiation is heavily dependent on microstructure (Hussain 1997; Stephens et al, 2001), and both can change rapidly as a crack grows across grain boundaries, through imperfections, and around inclusions. This behavior can cause microstructurally small cracks to kink or for several smaller cracks to form, later coalescing into a single dominant crack, in the early stages of crack growth.
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