Relating Fatigue Endurance Limit and Fatigue Crack Growth Threshold
Abstract
Under fatigue loading, damage of a component is usually assessed by the S-N (or epsilon-N) or fatigue crack growth (FCG) da/dN-Delta-K approaches. Historically, these two approaches are treated as two separate domains. Such a separation is often justified by the notion that the local stress field ahead of a crack is vastly different from a constant stress field in a smooth sample. Despite these differences, a component in service spends around 80% of its total fatigue life in the regions near the endurance limit or FCG thresholds. During service a component experiences both mechanical and chemical effects on damage. At present, there are no reliable approaches dedicated to analyzing these regions in a synergistic way where both mechanical and chemical effects dominate damage due to the requirement of long-time exposure. In this proposal we focus to relate the near threshold behavior (da/dN)~10e-10 m/cycle for a cracked C(T)-sample to that from a S-N (N~10,000,000 cycle) fatigue tensile sample. The two corresponding thresholds Delta-K-threshold and Delta-sigma-endurance are interrelated through the average local stress at the process zone ahead of the crack with those in the smooth specimen. In addition, we also keep track of the variations between applied and local R-ratios for C(T)-sample corresponding to the two environments such as humid air and aqueous NaCl solution. The obtained result would characterize fatigue damage dueto both chemical and mechanical contributions. The proposed relationship between Delta-K-threshold and Delta-sigma-endurance will serve as a basis for the development of a new unified software relating fatigue endurance and threshold damage analysis. Such a software will help the users in the design of components for a given design stress or in the periodic inspection and in life prediction. The basic concepts developed under this proposal would apply to other families of Navy structural materials.The key findings of thisproposal would be:1. Relating Delta-K-threshold to Delta-sigma-endurance, at all R-ratios, for both lab air and NaCl solution,2. Developing a model relating applied and local R-ratios at the process zone for C(T) specimen,3. Validating the applied and local R-ratio relationship using elastic-plastic FEA simulations,4. We chose an alloy like 7050-type (or as suggested by Navy) to validate the method of analysis,5. Quarterly reports of progress,6. Presentations at conferences and Journal publications,7. Final report.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Apr 11, 2024
- Source ID
- N000142412177
Entities
People
- Daniel Kujawski
Organizations
- Office of Naval Research
- United States Navy
- Western Michigan University