Shear bond strength (SBS) testing is a commonly used method for evaluating different dental adhesive systems. Failure mode analysis provides valuable information for better interpretation of bond strength results. The aim of this study was to evaluate the influence of specimen dimension and loading technique on shear bond strength and failure mode results. Eighty macro and micro flowable composite cylindrical specimens of 1.8 and 0.8 mm diameter, respectively, and 1.5 mm length were bonded to dentinal substrate. Four study groups were created (n = 20): Macroshear wireloop, Gp1; Microshear wireloop, Gp2; Macroshear chisel, Gp3; and Microshear chisel, Gp4. They were tested for SBS using chisel and wireloop loading devices followed by failure mode analysis using digital microscopy and SEM. Two- and one-way ANOVA were used to compare stress at failure values of different groups while the Kruskal–Wallis test was used to compare between failure modes of the tested groups. Gp4 recorded the highest mean stress at failure 54.1 ± 14.1 MPa, and the highest percentage of adhesive failure in relation to the other groups. Specimen dimension and loading technique are important parameters influencing the results of shear bond strength. Micro-sized specimens and chisel loading are recommended for shear testing.

The aim of this study was to develop a numerical model for simulating shear bond strength tests with different specimen sizes and loading techniques. A finite element model was generated consisting of a composite specimen bonded to dentin substrate surrounded by enamel, acrylic resin and polypropylene tube. Four models were created simulating macro (diameter 1.8 mm) and micro (0.8 mm) sized specimens loaded by either a chisel or a wire loop. Experimental data from a previously published study using the identical specimen diameter and shearing tools were used as reference. Four groups were established: macro shear wire loop (group 1), micro shear wire loop (group 2), macro shear chisel (group 3), and micro shear chisel (group 4). In the simulations, contact-based glue failure based on shear contact stresses (series 1) or a combination of shear and normal contact stresses (series 2) were used to simulate the progressive failure of the specimens. Shear and normal failure stress limits were fitted to the experimental results in sensitivity analyses by varying both stresses. Experimental failure forces could be reproduced using group-specific shear stress limits of 71 (group 1), 48 (group 2), 106 (group 3), and 131 MPa (group 4) in series 1. However, when also considering normal stresses, no single, unique pair of shear and normal failure stresses can lead to the experimental failure force values for all groups. In conclusion, no unique pair of shear and normal stresses can provide the same failure force values for different shear setup geometries.