Nondestructive Measurement of Cold-Working Effectiveness at Fastener Holes

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.