Functionally Graded Adhesively Bonded Joints : critical review

Adhesively bonded joints exhibit a common problem, called edge effects, arising from peel and shear stress concentrations occurring around the free edges of the adherend-adhesive interfaces and affecting the overall joint strength considerably. Some joint geometry-specific measures were considered for adjusting the stiffness of joint members around these free edges in order to improve the joint strength. Functionally graded materials appear in nature with a role of reducing stress concentrations along bi-material interfaces. Biological interfaces, such as dentin-enamel junction or tendon to bone, utilize the concept of functionally graded materials. Today, this concept is considered to reduce stress concentrations appearing along the adherend-adhesive interfaces of the adhesive joints serving under static, dynamic and thermal loads. This review first discusses the mathematical models, solution methods related to the adhesive joint problem, the free edge effects, and measures to relieve these effects based on the current literature, and later evaluates how the concept of functionally graded materials can be implemented in adhesive joints, i.e. use of functionally graded adherends and adhesives.