Use of ultrasonic Rayleigh-Lamb waves to detect defects in metal and rubber composites

Abstract

An investigation into the use of constant coupled ultrasonic non-modal Rayleigh-Lamb waveforms to detect defects within hatch seals was conducted to improve the current generation of ultrasonic leak detection devices.

An anechoic chamber was first constructed in order to isolate the emitted ultrasonic signal from the environment. The chamber walls reduced reflected 40 kHz signals emitted by ultrasonic transducers by 34.3 dB. Subsequently, an investigation was conducted into how effectively a varying hole size within metal plates, designed to mimic a non weather-tight seal, could be detected using 40 kHz ultrasonic signals. It was found that even at small distances from a hatch seal, using ultrasonic detection becomes unreliable. The decay rates of 40 kHz and 400 kHz ultrasonic signals were measured as they travelled through homogenous aluminium and steel plates in order to recommend a suitable number of emitters and receivers that would be required for consistent signal intensity. It was shown that 35 cm from the emitter, the signal did not decay significantly, suggesting that the maximum distance allowed between each subsequent receiver and emitter is greater than 35 cm. An investigation into how 40 kHz and 400 kHz ultrasonic waveforms traverse aluminium-rubber and steel-rubber non-adhered composites was then conducted and compared to composites with a varying number of air spacers present (which represent a non-defective and defective seal respectively). Although no statistically significant evidence showed that it is possible to distinguish between differing numbers of air spacers, it was established that a 400 kHz signal provides more consistent measurements than 40 kHz.

Finite element analysis (FEA) was conducted and compared to experimental results. In some cases, the analysis showed qualitative agreement with the findings. FEA was then used to guide preliminary experiments using artifically degraded rubber, showing strong indication that Rayleigh-Lamb waves could be used to measure the degradation of rubber within hatch seals.