Statistical Review of Equivalent Initial Flaw Size Trends in 7075-T6 and Ti-6Al/4V

Research in fatigue and crack growth behavior has shown that microcracks can nucleate from natural material inclusions, voids, and dislocations early in the "fatigue life" of the metallic materials used in aircraft structures [1]. In addition, cracks also may readily be nucleated sooner from discrete service-induced flaws such as corrosion pits, scratches, and gouges. Inclusion particles and service induced anomalies are a normal part of aluminum metallurgy and thus must be accounted for and modeled during design and analysis. Research by Alcoa [2] has shown that the "fatigue life" for aluminum can be correlated to inclusion particle size. Since the evidence is that microcracks occur early in the fatigue process, it is common to assume crack growth models may be more appropriate for "fatigue life" evaluation than empirical S-N curve / Miner's cumulative damage models. Converting voids, corrosion pits, etc into "cracks" can be done through the use of and Equivalent Initial Flaw Size (EIFS). The EIFS argument is that fracture mechanics is more physics based thus not reliant on the numerous empirical factors needed in S-N analysis. What is often not emphasized enough is that many of the same SN-curve factors still remain in fracture calculations but become hidden in the EIFS calculation. The EIFS calculation thus becomes as important as loading, yet many times does not receive the same attention. Chaos, Continuum Mechanics, and Pertinent defining variables all need to be addressed when using EIFS methods. As part of a National Rotorcraft Technology Center (NRTC) project, Sikorsky Aircraft performed testing and crack growth analysis for several common rotorcraft materials to address EIFS influence on stability of crack growth models for Al7075-T73 and Ti-6Al-4V beta-STOA. Surface conditions included polished, machined (63 rms), and 0.125 and 1.0 mm corrosion pits.