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Session: Session 5: Fatigue crack growth and life prediction methods  II
Session starts: Tuesday 27 June, 10:50
Presentation starts: 10:50
Room: Theatre room: plenary

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Marcus Stanfield (Southwest Research Institute)
Laura Hunt (Southwest Research Institute)
Isaac Grothe (Southwest Research Institute)

Abstract:
With the T-38 Talon entering its fourth 20-year lifetime, engineering has increasingly relied upon a robust risk analysis to ensure safety of flight. This presentation will outline how the T-38 engineering team has implemented a successful risk management plan that utilizes teardown results from fleet condemnations, retirements and major modification programs to achieve structural safety. Along with teardown failure analysis of large cracks from condemnations, T-38 engineering has identified and executed numerous retired wing teardowns to measure and record the smaller crack findings as well. In addition, simple teardown and failure analyses were conducted on structural items removed and replaced during a major modification fuselage life extension program. These efforts were able to capture valuable data on fatigue critical, life limited parts that would have otherwise been discarded. A thorough failure analysis of detected cracks allows an analyst to build a robust Equivalent Initial Flaw Size (EIFS) distribution dataset for probabilistic risk analysis. This marked a move to a more proactive, rather than reactive, risk management plan. Combined with maintenance records, T-38 now has an extensive library of thousands of findings covering several wing and fuselage Fatigue Critical Locations (FCLs). This allows the creation of FCL-specific EIFS distributions rather than a general detail (e.g. fastener hole) or purely material-based distributions. The risk analysis process, including gathering teardown failure analysis data, building an EIFS distribution, and PROF (Probability of Fracture software) analysis, will be shown in detail for three locations. For the first case study, a set of condemnation data was used to develop an EIFS distribution that resulted in a Uniform distribution versus the typical Weibull distribution. The second case study at a wing FCL had teardown findings contrary to previous damage tolerance assumptions. Incorporating the teardown findings produced a probability of failure much lower than previously calculated. The third case study will show how historical teardown data was used to enhance the risk analysis for a previously unknown fuselage FCL.