The effect of defects and imperfections on the structural integrity of adhesively bonded laminated composites.

The increased demand for long range, fuel efficient and high performance, high fuel economy automobiles has resulted in the growth of use of composites in aerospace and automotive sectors . Adhesive bonding is the preferred method to join composites as it does not require extra machining such as drilling of holes and avoids stress concentrations. As in other manufacturing processes, defects and imperfections may occur in bonded joints which can reduce the load carrying capacity of the joint. Nondestructive test methods such as ultrasonic scanning is used to determine the shape, size and position of defects and imperfections, and the reduction in strength due to defects has been predicted using both analytical and numerical methods. Previous studies on defects have primarily focused on defects within the adhesive bond such as voids and inclusions. Other bond imperfections such as improper adhesive distribution and thickness variations, occur in the bonded joints as well, which often result from improper fabrication techniques. The aim of the current study is to determine the effect of selected defects and imperfections on bonded joint integrity, which have not been previously addressed, and develop a suitable methodology for analyzing the strength of single lap bonded joints that have been compromised by the defects and imperfections. The primary focus here is to understand the effects of adhesive joint imperfections such as improper adhesive distribution and bond line thickness variations, and adherent defects in the form of inclusions, on the strength of adhesive bonded unidirectional carbon fiber reinforced polymer composite joints. Results show that a linear variation in bond line thickness decreases the failure strength of single lap bonded joints when compared with joints of uniform bond line thickness. The investigation with improper adhesive distribution across the overlap shows that the failure strength of bonded joints depends on the shape and size of the adhesive distribution. For the bonded joints with inter-ply inclusion defects, the location of the defect and the ply orientation of the adherend influences the failure strength of bonded joints. The study on the effect of increase in bond line thickness of single strap joints reveals a decrease in bond strength with an increase in bond line thickness. The failure strengths of bonded joints with uniform and varying bond line thickness along with that of single strap joints with uniform bond line thickness is predicted using the Critical Zone method. An insight into the change in stress states due to the presence of imperfections and defects studied, is established.