Analysis of dental shear bond strength tests, shear or tensile?

Abstract

The purpose of this research was to investigate dental ‘shear’ bond strength test methods using a test analogue. It was hypothesized that failure in many traditional dental ‘shear’ bond strength tests occurred as a consequence of tensile stress induced by the applied bending moment rather than by shear stress. Tensile bond strengths (‘maximum’ tensile stress to initiate failure) were fairly constant for a wide range of specimen shapes and moment arms, while average shear bond strengths were not. Based on the assumption that failure occurred due to tensile stress, theoretical predictions for failure loads followed experimental results closely. In contrast, the predictions based on the assumption that failure occurred due to shear stress did not. The results suggested that adhesive failure, in many dental ‘shear’ bond strength tests, does not occur as a consequence of shear stress but as a consequence of tensile stress induced by a bending moment. Therefore, calculation of shear bond strength is inappropriate for these test methods. The results of this investigation and others strongly suggest that dental adhesion researchers should be measuring the stress that initiates debonding rather than average stress.