Functionally graded adherends in adhesive joints: An overview

A graded material can be defined as a material which has constituents and/or properties that vary in one or more directions throughout the material. In a functionally graded material (FGM), gradual changes in the mechanical properties are explored to obtain a better performance than a homogeneous material. Due to the unique capabilities they provide, FGMs are now used in several fields of application such as engineering, chemistry, dentistry, medicine and biology, areas where materials with highly specific characteristics that change through the space and volume are highly desirable. In the field of adhesive joining, adherends manufactured with FGMs offer the potential to increase the strength and decrease the weight of a bonded structure, improving structural efficiency. The joint strength increase mechanism associated with FGM adherends relies on creating a stiffness variation through the thickness and/or overlap length, reducing the shear and peel stress concentration at the joint edges and increasing the joint failure load in comparison to a homogenous material. The present work introduces an in-depth literature review of the existent scientific knowledge on bonded joints with FGM adherends, analysing a wide range of studies using analytical, numerical and experimental methods. This research shows that several different techniques for obtaining FGMs have been proposed in the literature, as well as many analytical and numerical models able to reproduce the graded distribution the mechanical properties of adherends in bonded joints. However, experimental works on the practical application of these techniques are still very limited, representing a major challenge and an open field for new research.