Failure behavior of bonded composite joints

 
With sufficient cohesive strength of the adhesive layer, a main influence upon the failure behavior of FRP-joints is the delamination resistance of the top layer. This resistance is mostly affected by the adhesive properties, the stacking sequence, the top resin layer thickness and the surface pre-treatment method. In the case of adhesive variation it could be verified that with a more brittle and high-modulus adhesive the fiber delamination becomes the dominating failure while an elastic adhesive more often shows adhesion failure. In the topic of the influence of the stacking sequence it has been observed, that not only the orientation of the top layer, but up to the first four layers may have an influence on the failure behavior. In terms of the thickness of the surface resin layer it could be observed that with increasing thickness the strength decreased and the failure went from former cohesive to structural in the surface resin layer. While the failure of untreated CFRP-joints was mainly adhesively dominated it could be shown that with different methods (laser, low-pressure blasting, etc.) the failure becomes more cohesive which results in higher lap shear strengths.