EFFECT OF SURFACE MODIFICATIONS ON THE LAP SHEAR STRENGTH OF ADHESIVELY BONDED JOINTS

Automotive and aerospace industries are rapidly replacing aluminum components with light weight carbon composites. However, it is difficult to replace complex aluminum parts such as threaded or grooved structures with carbon composites because carbon composites are difficult to machine. As such, an alternate approach is the use of hybrid joints. In a hybrid joint, a strong adhesive is used to join a carbon fiber composite with a complex aluminum structural component. In this work, hybrid joints involving aluminum plate and laminated polymer carbon composite were bonded adhesively using a two-part epoxy, and were investigated. To improve the strength of adhesive bond, several investigators have attempted various methods such as modifications of the adhesive surfaces by mechanical abrading, chemical etching and plasma etching. This work presents two methods for the surface modifications. The first method involves dimpling technique, a form of mechanical abrading, and second method involves an introduction of a sacrificial layer of carbon prepreg on the surface of the aluminum to improve the lap shear strength in the hybrid aluminum/composite joint. Using these two methods, lap shear test coupons were fabricated and tested using ASTM 3165 standard. With the given surface preparation, results clearly indicated that the strengths were less but more consistent with 30 dimples. Dimpling with 15 dimples had more average strength than undimpled test coupons. Investigation also showed that when a sacrificial carbon fiber layer with 0 and 45 degree fibers is adhered onto both the aluminum and composite adherend, the lap shear coupon always failed on the 45 degree fibers. The stronger failed coupons always exhibited fiber bridging and cohesive failure. This study is a step forward for significant improvement of joint surfaces through surface modification. This study shows the behavior of joints with carbon fiber sacrificial layers containing 0 and 45 degree fibers.