Damage Tolerance Analysis of Grinding Burn Cracks in High Strength Steels

Landing gear are often made from high strength steels, such as 300M , which are known to have excellent damage resistance when properly manufactured and maintained but to have lesser damage tolerance when compromised. For example, the grinding process, when done improperly, can heat the steel excessively and cause undesirable microstructural transformations, called grinding burns, which often contain cracks. These burned microstructures are extremely detrimental to fatigue and stress corrosion resistance. The USAF seeks methods to incorporate grinding burns into damage tolerance methods. The Phase I program recently completed by APES illustrated that grinding burns can be readily incorporated into the ASIP process. Methods for systematic, repeatable generation of grinding burns are presented in this proposal, and an analytical framework for transforming these burns into defect criteria is defined. Analytical and physical damage correlation of grinding burns is supported by a research program featuring destructive metallography, non-destructive testing, fatigue testing, failure analysis, stress intensity development and validation, and crack growth modeling. Mitigation strategies for grinding burns were also scoped using laser peening as the solution path. Potential benefits of the technology to warfighter can be recognized through increased safety, reduction in unscheduled or unnecessary maintenance, and increased aircraft readiness rates.