Shear strength of adhesive bonds: comparison of lap-shear, asymmetric 4-point-bending and torsion tests

An important issue relevant to the design and use of adhesive bonding is the assessment of the shear strength in an objective manner. As it is well known, classical standard tests (e.g. ASTM D1002) give only a conventional strength useful for comparative purposes but unfit to quantify the intrinsic strength of the adhesive joint. The situation can be more complicated in case of non usual substrates, such as ceramics, which pose limitations in fabricating the desired specimen shape. In the present work, three test techniques aimed at assessing the shear strength are compared, namely: i) single-lap offset in compression (SLO), ii) asymmetrical 4-point bending (A4PB), and iii) torsion of hourglass shaped samples (THG). The adhesive adopted is a two-component structural epoxy of good ductility (6% elongation); three different substrates are considered, namely two ceramics and one steel. The goal of the work is to compare the shear strength values of the same adhesive, measured in the three different ways. In general, the lap-joint test is known to exhibit a markedly non-uniform shear and peel stress distribution. However, with the SLO used in this work the substrates are stiff and guided by special fixtures; this fact, together with the adhesive plasticity, generates a nearly uniform shear stress at failure, thus the ultimate load-to-area ratio can be representative of the strength. In the A4PB there is a shear-only, non-uniform, stress elastic distribution, but thanks to plasticity the stress at failure is again uniform. Similarly, THG has shear-only stress, non-uniform when elastic and uniform at failure. The obtained results show that consistent strength values are obtained in the three ways, if the correct testing conditions are met.