Development of non-destructive methods to evaluate defects in pressure composite tanks: Ultrasonic and acoustic emission techniques

This paper explores the application of ultrasonic and acoustic emission techniques in detecting defects within pressure composite tanks. The objective was to enhance non-destructive testing (NDT) methodologies for identifying defects and delamination at the interface between thermoplastic liner and glass fibre-reinforced polymer (GFRPs) layers. For this purpose, two sample types were prepared: flat specimens and cutouts from two types of tank. The first tank was manufactured using polyester fabric on the layer’s boundary and the second one was manufactured without fabric. The flat samples underwent ultrasonic testing using Phased-Array probes at 3,5 and 5 MHz. The use of a 5 MHz probe provided more accurate imaging of delamination. Moreover, the analysis focused on two approaches: direct signal examination from the liner-GFRPs joint (defect echo evaluation) and the liner backwall echo evaluation. Both methods aimed to precisely pinpoint delamination at the liner-GFRP interface. The use of polyester fabric improves the adhesion between the GFRPs and PE layers of the tank wall. The results confirmed the choice of testing method should be influenced by factors such as GFRP layer thickness and surface roughness. The PAUT results enabled the selection of appropriate test parameters, including probe work frequencies, and the selection of a methodology based on the liner backwall echo evaluation, leading to the detection of defects occurring in double-layer pressure tanks. The study concludes that while the developed methodology is effective, customization to each tank type is required for optimal results. In the next stage, acoustic emission (AE) analysis was conducted during pressure testing on the tank, revealing instances of damage to the shell and bottom. Acoustic emissions with signal amplitudes exceeding 75 dB were observed when tank pressures surpassed 2 bar. The Felicity effect was evident, while signals ranging from 80 to 99 dB indicated separative cracking, suggesting delamination within the tank structure. Additionally, a few signals with amplitudes above 99 dB were related to fibre cracking. From these observations, AE parameters were divided, correlating to specific damage observed within the tanks.