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Thin-ply laminates exhibit a higher degree of freedom in design and altered failure behaviour, and therefore, an increased strength for unnotched laminates in comparison to thick-ply laminates. For notched laminates, the static strength is strongly decreased; this is caused by a lack of stress relaxation through damage, which leads to a higher stress concentration and premature, brittle failure. To overcome this behaviour and to use the advantage of thin-ply laminates in areas with high stress concentrations, we have investigated thin-ply hybrid laminates with different metal volume fractions. Open hole tensile (OHT) and open hole compression (OHC) tests were performed with quasi-isotropic carbon fibre reinforced plastic (CFRP) specimens. In the area of stress concentration, 90° layers were locally substituted by stainless steel layers of differing volume fractions, from 12.5% to 25%. The strain field on the specimen surface was evaluated in-situ using a digital image correlation (DIC) system. The embedding of stainless steel foils in thin-ply samples increases the OHT strength up to 60.44% compared to unmodified thin-ply laminates. The density specific OHT strength is increased by 33%. Thick-ply specimens achieve an OHC strength increase up to 45.7%, which corresponds to an increase in density specific strength of 32.4%.

The study deals with an online monitoring approach for adhesively bonded composite joints. A modification of epoxy-based adhesive films with carbon nanotubes allows for electrical resistance measurements through the bonding via inkjet-printed silver conductive circuits on the composites structure. Impact damages are introduced into adhesively bonded glass fiber–reinforced polymer specimens. In-plane and through-thickness electrical resistance measurements show the possibility of accurate damage detection and damage localization of the introduced damages in one or two dimensions, depending on the conductive path designs. The measured electrical resistance changes are compared with results from ultrasonic inspections and light microscopy observations. Furthermore, a linear correlation of electrical resistance and bond line thickness was found. The results demonstrate the applicability of the presented method in a structural health monitoring system.

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