Shear versus micro-shear bond strength test: a finite element stress analysis.

OBJECTIVES: This study aimed at comparing the stress distribution in shear and micro-shear test set-ups using finite element analysis, and suggesting some parameter standardization that might have important influence on the results.
METHODS: Two-dimensional plane strain finite element analysis was performed using MSCPatran and MSCMarc softwares. Model configurations were based on published experimental shear and micro-shear test set-ups and material properties were assumed to be isotropic, homogeneous and linear-elastic. Typical values of elastic modulus and Poisson's ratios were assigned to composite, dentin and adhesive. Loading conditions considered a single-node concentrated load at different distances from the dentin-adhesive interface, and proportional geometry (1:5 scale, but fixed adhesive layer thickness in 50microm) with similar calculated nominal strength. The maximum tensile and shear stresses, and stress distribution along dentin-adhesive interfacial nodes were analyzed.
RESULTS: Stress distribution was always non-uniform and greatly differed between shear and micro-shear models. A pronounced stress concentration was observed at the interfacial edges due to the geometric change: stress values farther exceeded the nominal strength and tensile stresses were much higher than shear stresses. For micro-shear test, the relatively thicker adhesive layer and use of low modulus composites may lead to relevant stress intensification. An appropriate loading distance was established for each test (1mm for shear and 0.1mm for micro-shear) in which stress concentration would be minimal, and should be standardized for experimental assays. SIGNIFICANCE: The elastic modulus of bonded composites, relative adhesive layer thickness and load application distance are important parameters to be standardized, once they influence stress concentration.