Nondestructive Measurements of Cold-Working Zones at Fastener Holes

In the aerospace industry, a fastener hole is considered as potential crack initiation site for structures that undergo cyclic fatigue loading because of the stress and strain concentration. However, the use of a cold expansion process to introduce a compressive residual stress field in the material surrounding the hole can enhance the service life of stressed aerospace components. Unfortunately, no method exists to quantitatively assess the effectiveness of the process. Use of the wrong expansion tool, out-of-round holes, and other factors can reduce the effectiveness of the process. Luna Innovations will develop a practical portable ultrasonic instrument capable of assessing changes in residual stress near aircraft fastener holes. The device will indicate both stress gradients and direction. This new instrument will be designed for use in the field by qualified inspectors without a high degree of training. The basis of the device is the small change that occurs in sound velocity in a metal under stress. Although this phenomenon has been understood for several decades, no company has achieved a practical device for fastener hole residual stress assessment. Luna has the capability to create such a device that can meet the Navy requirements including that of low cost and portability.