Nondestructive Evaluation for Assessing Cold Worked Countersunk and/or Filled Fastener Holes in Titanium and Steel Aerospace Alloys

The United States Air Force (USAF) has significant experience applying hole cold expansion (Cx) to extend fatigue life and commensurately increase inspection intervals for safety-of-flight structure. At the present time, however, the maximum allowable credit for Cx (per USAF guidelines) is limited to the improvement expected for a reduced initial flaw size in a crack growth assessment (e.g., a reduced 0.005-inch radius flaw versus a typical 0.050-inch radius flaw). This results in a benefit often referred to as “partial credit.” In the case of A-10 and T-38, for example, critical fastener holes are the primary driver for structural inspections, accounting for approximately 70% to 90% of the fatigue critical locations on the airframe. The "partial credit" is driven by the lack of Quality Assurance (QA) tools to ensure the correct processing occurred at the correct critical locations. The proposed program will focus on utilizing existing NDE technologies, e.g., electromagnetic testing (Eddy Current Testing (ET) or magnetic methods), Ultrasonic Testing (UT), or a combination of methods to evaluate the residual stress at countersunk and/or filled cold expanded fastener holes. The end product will be the delivery of a production-ready prototype system.