Characterization of bonding quality in a multilayer structure using segment adaptive filtering

Ultrasonic testing is widely used in detection of disbonds in multilayer structures such as solid fuel rocket motors, which consist of steel, rubber laminate, and solid fuel. However, only a small fraction of ultrasonic waves can transmit through the steel-rubber interface because of their large difference in acoustical impedance. Very little ultrasound is reflected back by the weak bond interface within the rubber laminate or from the interface between the rubber and solid fuel. Consequently, the interface bond degradation can only produce a very slight variation in the received ultrasonic echo sequences, which are too weak to be detected effectively. In this paper, ultrasonic pulse reflection from the interfaces is considered to evaluate the interface bond condition. A multilayer model with spring boundary condition is used to describe the weak bond, and ultrasonic reflection response is obtained for the structure in the immersion mode. After analysis of the ultrasonic echo sequences of the steel-rubber structure, the segment adaptive filtering method is used to separate the simulated echo sequences and to obtain the interface signals. With the separated interface signals, the weak bond of all the interfaces, including the one within the rubber laminate, can be detected. The procedure has been applied to signals measured from the steel-rubber samples, and the amplitude and envelope of various interface signals agree well with those from the corresponding simulated signal. The disbond and weak bond can be detected and evaluated by the amplitude of corresponding signal of interfaces.