New approaches in nondestructive characterisation of the interface in metal - CFRP hybrid structures

With regard to cost-efficient lightweight design there is an obvious trend to join different materials resulting in hybrid structures, such as joining metal with CFRP (Carbon Fibre Reinforced Polymer) structures. In order to reduce the gap in stiffness between the metal and CFRP structure a thermoplastic polymer component can be inserted, which is capable to fulfil additional functional properties. To characterise the used materials and defects in the hybrid, nondestructive testing (NDT) approaches in combination with mechanical testing are shown. Especially the interface between the metal structure and the thermoplastic polymer component as well as the interface between the thermoplastic polymer component and the CFRP structure are investigated because previous works have shown for adhesively joints that cracks initialising within the adhesive zone lead to ultimate failure by fracture of the CFRP ply adjacent to the adhesive. Thus ultimate failure of the hybrid joint is significantly determinated by the crack resistance of the CFRP. Therefore investigations will be presented with active thermography and ultrasonic (US) testing on CFRP samples with defined defects, which are artificially implanted within the preforming process. Furthermore passive thermography during mechanical testing will be applied to characterise the effect of the artificial defects in the CFRP samples whilst loading. By combining different NDT methods, the characterisation of defects shall be improved. Additionally electromagnetic acoustic transducers (EMAT) are used to evaluate the quality of the adhesive bonding of the metal and the thermoplastic polymer component.