Abstract
Acoustic emission (AE) monitoring shows promise as one of the most effective methods for condition monitoring of adhesively-bonded joints. Previous research has demonstrated its ability to detect, locate and classify adhesive joint failure, though in these studies little attention appears to have been paid to the differences in AE wave propagation through the bonded and un-bonded sections of the specimens tested, or to the effects of the wave modes excited or the propagation distances. This paper details an experimental study conducted on large aluminium sheet specimens to identify the effects of the presence of an adhesive layer on AE wave propagation. Three specimens are considered; a single aluminium sheet, two aluminium sheets placed together without adhesive, and an adhesively-bonded specimen. A pencil lead break (PLB) is used as a simulated AE source, and is applied to the three specimens at varying propagation distances and orientations. The acquired signals are processed using wavelet-transforms to explore time-frequency features, and compared with modified group-velocity curves based on the Rayleigh–Lamb equations to allow identification of wave-modes and edge-reflections. The effects of propagation distance and source orientation are investigated while comparison is made between the three specimens. It is concluded that while the wave propagation modes can be approximated as being constant throughout all three specimens, there is a significant change in the received waveforms due to the attenuation of high-frequency components exhibited by the bonded specimen. These findings may be utilised to provide a deeper understanding of acquired AE data, improving the current abilities to identify, locate and characterise damage mechanisms occurring within adhesive joints, ultimately improving safety in the use of adhesive bonding for critical applications.
Highlights
Many techniques to assess adhesive bond quality have been developed, with varying levels of success and differing merits in terms of accuracy and practicality
The aim of this study is to investigate the Acoustic emission (AE) wave propagation in a single layer, an un-bonded double-layer and an adhesively-bonded aluminium joint, with both in-plane and out-of-plane sources, to provide more insight into the propagation of AE waves excited by defect propagation in bonded joints
The essential experimental approach was to carry out a systematic investigation of AE wave propagation in large aluminium sheets, that investigation spanning over a single sheet, two identical sheets placed on top of each other without adhesive, and an adhesivelybonded specimen. 1050 A H14 aluminium substrate sheets of average thickness 1 mm (Grampian Steel Services, UK) were cut into sections of 500 mm × 500 mm
Summary
Many techniques to assess adhesive bond quality have been developed, with varying levels of success and differing merits in terms of accuracy and practicality. While they are extremely effective in certain situations, they can be limited by aspects such as; requiring access to both sides of the bond (for through-transmission), the limited depth that can be inspected by single-sided approaches, the necessity for sensor coupling (usually achieved by waterjet or immersion bath), and the inability to detect certain defects such as zero-volume disbonds These techniques are reliant on scanning of the entire area being inspected, an extremely time-consuming process for large areas, with areas of several square metres potentially taking over an hour to scan, depending on the desired resolution [1].
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