Nondestructive evaluation of interfacial adhesive strength in solid propellant grains using high-frequency phase-shifting ESPI and DA-UNet

Interfacial adhesion is critical to the structural integrity and service safety of solid propellant grains in solid rocket motors. Conventional nondestructive testing (NDT) methods often fail to detect weak interfacial bonding before macroscopic debonding, creating risks during long-term storage and service. This study presents an NDT method for interfacial adhesive strength based on high-frequency phase-shifting electronic speckle pattern interferometry (ESPI) and a dual-attention U-Net (DA-UNet) model. A real-time four-step phase-shifting technique shortens each measurement cycle to 9.1 ms and improves the environmental robustness of the ESPI system. The DA-UNet, integrating a convolutional block attention module and an attention gate, enables noise-robust phase unwrapping and accurate out-of-plane displacement extraction, achieving an MAE of 0.0151, RMSE of 0.0281, and SSIM of 0.9869. Calibration shows a displacement measurement error below 2.2%. Experiments on flat and cylindrical specimens reveal a significant negative correlation between adhesive strength and surface micro-deformation, with vacuum-thermal coupling providing the highest sensitivity for weak-adhesion identification. Mechanical tests and digital image correlation measurements validate the results. This approach offers a robust, high-precision optical NDT tool for early detection of weak interfacial bonding in solid propellant grains.