Fatigue properties of aerospace structures are improved significantly by the application of cold-working processes, which impart favorable compressive residual stresses at and near the surface of the metal. While cold-working production processes are tightly controlled to ensure the effectiveness of the process, human errors are known to occur and the degree of fatigue or damage resistance may be less than the design requirements. Today, no means exists to non-destructively measure the residual stress state of cold-worked fastener holes. Proto has years of experience in measuring the effects of surface enhancement techniques and believes that x-ray diffraction (XRD) technology can provide the NDI capability needed by the USN. Proto intends to prove that XRD is a feasible and cost-effective solution for the challenge posed by cold-worked fastener holes. Proto will perform numerous laboratory measurements on cold-worked coupons in the hoop and radial directions at the surface and subsurface, including the internal diameter of the holes, using its patented XRD tools. Proto will map these RS distributions and correlate that data with FEA and strain gauge data to establish the technical relationships for determining the effectiveness of the cold-working processes. In Phase II, Proto will deliver and demonstrate a rugged, portable XRD NDI system for measuring the effects of cold-working as well as the effectiveness of the cold-working processes. It will support an operational evaluation by trained NAVAIR personnel in a depot quality control environment.
Benefit: The proposed work will validate the feasibility of XRD technologies to measure and map residual stress distributions in cold-worked fastener holes in aluminum alloys. It will correlate measured surface and subsurface residual stress (RS) distributions with the results of finite element analyses and strain gauge data and validate the suitability of XRD for this application. This will lead to a Phase II XRD RS system demonstration and operator/user evaluation of an effective, production-quality XRD RS system in a NAVAIR depot environment. The XRD RS system will provide the USN, other Services and aerospace industry an important NDI tool optimized for quality control and for quantitative verification of cold-working during depot repair and initial production.
Keywords: depot factory implementation, depot factory implementation, cold-work, fastener holes, Residual Stress, cold expansion, Non-destructive inspection, X-ray Diffraction
