Developing experimental procedures to minimise parasitic effects (such as peel stresses and bending moments) remains a major challenge in characterising shear behaviour of adhesive joints. This paper investigates the Asymmetric Four-Point Bending (AFPB) test as an alternative configuration. Analytical modelling confirms zero bending moment along the adhesive layer centreline. However, Finite Element (FE) simulations show low-magnitude peel stresses and local stress singularities at the adhesive extremities, particularly at substrate interfaces. Utilising stiffer substrates enhances shear stress uniformity, while chamfered edges effectively reduce stress concentrations. Experiments on a structural adhesive using Digital Image Correlation (DIC) demonstrate high repeatability for shear stress-strain analysis. Although minor parasitic peel stresses persist and stress singularities are present at the edges, especially at the interface with the substrates, these results establish AFPB as a promising and robust protocol for shear characterisation of adhesive joints.
Adhesive bonding is an effective technique for joining dissimilar materials and reducing structural weight in high-performance applications, including aerospace structures. However, reliable and practical methods for evaluating the strength of adhesively bonded structures have not yet been fully established. Strength evaluations based on peak stresses near adhesive layer edges are affected by stress singularities and mesh dependence, whereas damage-based approaches require detailed material parameters, fine meshes, and substantial computational effort. This study proposes a simple and practical stress-based method for evaluating adhesive joints subjected to combined normal and shear stresses. The method relates the characteristic distance to the extent of the process zone near the adhesive layer edge and defines the end of this zone as the stress evaluation point. A mixed-mode fracture criterion is then applied to the normal and shear stresses at this point. Material strengths under pure tension, pure shear, and combined tension–shear loading were obtained from Arcan tests, and an elliptical mixed-mode fracture criterion was established. The evaluation point was first determined for a single-lap shear joint and then applied to double-beveled lap, double-lap shear, and three mixed-mode flexure-type joints. Experimental tests and three-dimensional finite element analyses were conducted for all six joint configurations. Across these configurations, the normal-to-shear stress ratio at the adhesive layer edge at fracture ranged from 0.5 to 6.6, while the stresses at the evaluation points were close to the mixed-mode fracture criterion. These results support the consistent applicability of the proposed method to joints with different geometries over the investigated range of stress states.
