Shearography non-destructive testing of a composite ship hull section subjected to multiple impacts

Abstract

The use of thick composites and sandwich structures is increasing rapidly in marine, aerospace, and wind energy industries [1–3]. For example in the marine sector, sandwich structures consisting of glassfiber laminate skins bonded to a foam core are attractive because of the advantages of being light-weight, resistant to corrosion and underwater shocks, and cost-effective [4]. The thickness of these structures can be more than 50 mm. Nevertheless, various defects including delaminations and fiber breakage tend to occur in thick composites because of material complexity. These defects can arise from extreme loads such as impact and blast and can degrade material properties and structural integrity significantly. Hence, it is important to advance non-destructive testing (NDT) towards composite structures of significant thickness. The objective of this study is to perform shearography NDT of a large-scale thick composite structure, specifically a composite ship hull section in a shipyard environment. Shearography is a full-field and non-contact optical NDT method. It reveals defects by comparing two states of deformation of a test object. By applying a suitable loading, the defects can be revealed by looking for defect-induced anomalies in fringe maps or phase maps, which can be related to surface strain components. The composite ship hull section is a RAMSSES (www.ramsses-project.eu) demonstrator at Damen Shipyards. Before shearography inspection, multiple impact tests surpassing helicopter emergency landing loads (https://vimeo.com/522716506) have been performed on the hull shell and its composite helicopter deck for proving the resilience of composites to harsh marine environments. We will present our experimental results on shearography inspection of the impact damage in this large-scale composite structure. A total area of about 1×1.5 m2 was inspected by stitching six fields of view of 0.6×0.6 m2. Different heating scenarios including step heating as well as a mechanical loading were performed for shearography NDT. A brief comparison between thermal loading and mechanical loading on thick composite inspection with shearography will also be reported. Our previous work with a 51 mm thick marine laminate [5] showed that defects at 5 to 20 mm depth can be detected successfully using shearography with thermal loading. Here we aim at bringing the technique out of the laboratory and extending shearography to applications to composites with a thickness of more than 50 mm.