An experimental investigation on the effect of adhesive distribution on strength of bonded joints

Adhesive joints are widely used due to their higher strengths, lower weights, lesser expenses and ease of fabrication than other methods of joining. Hence, they are used extensively in aerospace and automobile industries. High quality bonds require the use of fixtures which are essential to ensure proper curing and to attain uniform thickness. Improper adhesive application and method of fixturing can cause irregularities in the distribution of the adhesive along the overlap which could affect the joint strength. This is especially critical for aerospace components as the replacement of parts can be costly and time consuming. This paper presents a nondestructive test (NDT) methodology to quantify the bonded joints where the adhesive does not completely cover the overlap area. Single lap adhesive joints with carbon fiber adherents were fabricated using a two-part epoxy based adhesive. The adhesive region was fabricated to have various shapes including elliptical and circular of different sizes and a joint with full coverage for comparison. Polytetrafluoroethylene (PTFE) cutouts were used to mask regions of adhesive to achieve the desired coverage pattern. After fabrication all samples were ultrasonically scanned with a 10?MHz spherically focused immersion transducer using pulse-echo ultrasonics to determine the actual as tested adhesive distribution shape. The scans were able to resolve the shape of adhesive distribution across the overlap region of 25.4?×?25.4?mm and were correlated with the actual adhesive distribution at the interface after lap shear tests. Lap shear tests were then performed on the samples fabricated and the samples were loaded to failure. It was found that the shape and the orientation of the adhesive shape relative to the loading direction had an effect on failure strength.