Study and Analysis of Damages in Functionally Graded Adhesively Bonded Joints of Laminated FRP Composites: A Critical Review

Abstract

Adhesively bonded laminated FRP composite joints are widely used in many industries such as oil and gas, aerospace as well as automotive. In spite of several pertinent structural advantages, the adhesive bonding in composite structures suffers from severe peel stress concentrations in the bondline due to load path eccentricity which often act as a trigger for cohesive and adhesion (interfacial) failures. In addition, the FRP composite adherends are also prone to different conventional failure modes like interlaminar and intralaminar damages which tremendously affect the overall integrity of the bonded joint structure. There has been a considerable amount of research conducted in an attempt to reduce these stress concentrations through tapering the ends of the adherends, increasing thickness of the adhesive at the joint end, use of adhesive fillets, novel joint geometries, and joint insertions, to name a few. All of these methods involve local details of adherend geometry (except the adhesive fillets), which typically increase the complexity. Attempts have also been made for improving damage tolerance and load carrying capacity of the bonded joints through varying material properties of the bondline (visco-elastic, elastic, non-linearly elastic, etc). Functionally Graded Adhesives (FGAs) with nanoparticles distributed within the adhesive have been proposed recently in many published articles for providing suitable joint strength through minimization of stress concentration effects. Adhesives with functionally graded material properties are being considered for adhesively bonded joints to reduce the peel stress concentrations located near adherend discontinuities. Several practical concerns impede the actual use of such adhesives. These include increased manufacturing complications, alterations to the grading due to adhesive flow during manufacturing, and significant impact of varying the loading conditions on the effectiveness of the grading. The present article primarily focuses on a concise presentation of different research works carried out for studying the effect of functionally graded material properties on performance of adhesively bonded joint structures.