Hydroxyl-terminated polybutadiene liner curing process monitoring using non-collinear ultrasonic wave mixing technique

The Hydroxyl-Terminated Polybutadiene (HTPB) liner in solid rocket motors is a viscoelastic material formed through the polymerization and gelation of HTPB and toluene diisocyanate. The semi-cured state of the HTPB liner plays a critical role in determining the optimal timing for propellant loading. However, due to the thinness of the HTPB layer, traditional nonlinear ultrasonic testing methods suffer from insufficient resolution and time-domain aliasing of detection echoes, limiting their effectiveness in real-world applications. To address these challenges, this study proposes a novel Aluminum Plate - HTPB liner - Aluminum Plate detection structure for real-time monitoring of the HTPB curing process. First, the theoretical framework of nonlinear ultrasonic beam mixing is analyzed. Then, through both simulations and experimental studies on LY12 aluminum alloy, the most suitable mixing modes for beam mixing monitoring are determined. Based on these findings, a non-collinear mixing monitoring system incorporating the proposed detection structure is designed and implemented to track the curing progression of the HTPB liner in real time. Experimental results demonstrate that the mixed-frequency signals generated within the designed structure exhibit high sensitivity to changes in the curing state of the HTPB layer. In addition, this approach effectively prevents time-domain aliasing between the mixed initial wave and primary echo. Furthermore, the detected signal parameters align well with theoretical predictions, confirming the feasibility of this method for accurate monitoring of the HTPB curing process.