Influence of joint component mechanical properties and adhesive layer thickness on stress distribution in micro-tensile bond strength specimens.

OBJECTIVES Differences in mechanical properties between adherents and adhesive result in stress singularity at the free edge for tensile test specimens. Beyond causing premature failures, these also trigger fracture initiation at the bond edge, where the use of the maximum stress at failure as a measure of strength is not a reliable parameter for obtaining bond strength. The aim of this study is to describe the pattern of stress concentration along the interface for micro-tensile bond strength (microTBS) specimens. Also, possible sources of variation in stress concentration (adhesive layer thickness and mechanical properties) were evaluated using the finite element method. METHODS One-fourth models of typical microTBS specimens were created using axisymmetric elements. A 15MPa load was applied at the outer edge and finite element calculations were performed within the elastic deformation range. In the first set of runs, the adhesive thickness was varied and in the second set, different nu/E ratios between the adhesive and the adherent were assigned. RESULTS When adhesive thickness increased, a significant increase in stress concentration at the free edge and in the length of the stress-affected area was observed. Increasing the nu/E ratio between adhesive and adherent lead to an increase in stress concentration at the free edge area without change in length of the stress-affected area. SIGNIFICANCE If mechanical properties and adhesive thickness expected for a specific composite-dentin bond combination in microTBS specimens are known, stress concentration at the interface can be predicted. The adhesive thickness should remain constant when comparing different joints, as it facilitates the interpretation of nominal strength values.