Dispersion behaviour of acousto-ultrasonic signals in HTPB based solid propellants

Acousto-Ultrasonic Testing (AUT) was performed on Hydroxyl-Terminated Polybutadiene (HTPB)-based solid rocket propellant samples to study the dispersion behaviour of mechanical waves within the material. Low-frequency mechanical waves, ranging from 10 kHz to 300 kHz, were used for testing. The results demonstrated that the stress wave velocity increased non-linearly with increasing frequency and sample thickness. Additionally, propellant samples subjected to natural ageing were analysed for mechanical properties, such as modulus, and these properties were correlated with stress wave velocity. This study aims to monitor variations in mechanical properties over time, revealing correlation between changes in mechanical properties and computed stress wave velocity. An average increase in wave velocity of approximately 25% is observed over a duration of 14 months. A corresponding enhancement in the elastic modulus, estimated at ~ 1.25 MPa (approximately 30%), is evident during the early stages of natural ageing. Experimental investigations on the variations in velocity with stress wave frequency and sample thickness are detailed.