Comparison of kiss-bond and dis-bond defects in cfrp laminates by compression after impact testing

Kiss-bonds are structural defects, common to adhesive bonding applications, that feature a localised loss of structural continuity between two surfaces exhibiting intimate contact. Such defects are a problematic precursor to damage growth and structural failure but are notoriously difficult to detect. In order to better study the detection and influence of kiss-bonds in composite structures, a novel pre-curing approach has been developed for the manufacture of artificial kiss-bonds - which avoids the use of inclusions or contaminants that are common to previous studies. Small square defects have been manufactured in Carbon Fibre Reinforced Polymer (CFRP) samples, and evaluated using Non-Destructive Inspection (NDI) techniques and microscopy. The resulting defects were all found to be detectable using modified NDI practices but classified, based on industrial criteria, as either kiss-bonds (no detectable voids by conventional NDI) or dis-bonds (containing detectable voids). Samples with kiss-bond defects, dis-bond defects or no defects were then damaged with 10.5 J impact energy and re-assessed by ultrasonic C-scan. Impacted samples with pre-existing kiss-bonds resulted in damage areas around 30% greater than those of pristine samples, on average, and 15% greater than those of samples containing dis-bonds. Compression After Impact (CAI) testing has also been performed with specialised anti-buckling fixtures to assess any effect on mechanical performance. Kiss-bond and dis-bond samples were found to perform similarly, both showing only a slight reduction in compressive strength compared with the control samples. Ultimately, small kiss-bond and dis-bond defects of a detectable size (100 mm2) appear to have a negligible effect on the compressive performance of a thin laminate after a coincident low energy impact event.