Functionally graded adhesive joints under impact loads

Abstract

The industrial application of adhesively bonded joints has increased significantly in the last few years, driven by benefits such as the increased design flexibility, high vibration damping, the capability of joining dissimilar materials and the possibility of being used in combination with other joining techniques. However, the presence of stress concentrations at the overlap ends, especially in single lap joints, is one of the major issues associated with this technique, reducing joint strength. To solve this drawback, several techniques have been proposed, such as the use of adhesive spew, adhesive and adherend shaping, mixed adhesive joints and functionally graded adhesive joints. Functionally graded adhesive joints use an adhesive layer where the properties gradually change along the bondline, which results in the reduction of stress concentration peaks at the ends of the overlap, leading to a more uniform stress distribution. Multiple techniques for the creation of a functionally graded bondline have been presented in the literature, such as the inclusion of particles and nanoparticles and the use of functionally graded curing. However, the experimental works available in the literature only report results for quasi-static loading conditions, with the impact behaviour of these joints being an unstudied topic. The main objective of the present work is to fill this gap and study the mechanical behaviour of functionally graded adhesive joints loaded under impact conditions, using both experimental testing and numerical modelling. The results obtained show that, unlike what is found for quasi-static loads, graded joints do not offer significant strength improvement under impact loads. In contrast, energy absorption is significantly increased. This behaviour is explained by the completely different stress distribution on the adhesive layer for quasi-static and impact conditions, leading to the lower effectiveness of functionally graded adhesive joints under impact loads.