Quality Control for Advanced Residual Stress Inducing Surface Treatment Processes

Since the inception of laser peening, improved nondestructive test methods and sensors have been sought to ensure that laser peening produces the desired magnitude and depth profile of residual compressive stress in the part being treated. There are no reliable nondestructive evaluation (NDE) techniques generally applicable to any surface enhancement process to monitor residual stress, so it is important to have real-time process controls and monitors in place for these processes. Laser beam diagnostic sensors are used typically to verify that the correct laser pulse energy, temporal pulse width, and fluence spatial profile are delivered to the part for each laser spot applied. Test coupons, such as Almen strips, may be processed on a sampling basis, which provide a semi-quantitative measure of the effectiveness of laser peening. However, these techniques do not provide information measured from physical phenomena directly in the part during laser peening. Incorporating real-time process sensors, which monitor the laser beam interaction or the stress wave generated in the part on a spot-by-spot basis, will greatly reduce the probability of nonconforming process conditions. The real-time quality control system to be developed in this program will enable immediate corrective actions during processing, when needed.