Effect of bondline thickness on the static strength of structural adhesives under nearly-homogeneous shear stresses

An usual experimental observation retrieved in the technical literature is that the strength of an adhesive joint decreases by increasing the adhesive layer thickness. This well known behaviour is still not completely understood. All works found in literature consider a complex stress state in the adhesive with mode mixing, stress concentrations on the midplane and stress singularities at the interface occurring at the same time. This paper aims at estimating the effect of the adhesive thickness on its intrinsic static shear strength and evaluate whether this strength can explain the behaviour of a real bonded joint. A nearly-uniform shear stress distribution is obtained through an ad-hoc tubular butt joint subject to pure torsion. A standard single lap joint is considered as benchmark, due to its complex and singular stress field into the adhesive. The experimental campaign is focused on two brittle adhesives: a modified methacrylate and high strength epoxy. Four levels and three levels of the adhesive thickness were considered in the tubular butt joint and in the single lap joint respectively, all in the range between 0.05 and 0.4 mm. The effect of the adhesive thickness on the static strength of the adhesive is investigated by considering the type of failure and by comparing the structural stresses in the tubular butt joint with the ones on the midplane of the adhesive layer in the single lap joint. Moreover, the stress intensity factor in the single lap joint is calculated.