Stress Analysis Of Bonded-interface Technique On Subsurface Damage Observations Of Brittle Porcelains

The bonded-interface technique (BIT) has been successfully used for examining subsurface damage of brittle materials in the Hertzian contact and sliding friction tests because it could solve the problem of distinguishing the cracks that develops accidentally during the preparing sample process from those actually produced in the tests successfully. In the BIT two polished half blocks was bonded together side-by-side with adhesive prior to tests. After test completion, the glue was dissolved in acetone to separate the blocks. The profile of wear scar was easily observed on the cemented section of the blocks without inducing any cracks.Objective: The aim of this study was to investigate the stress distribution of the BIT model in Hertzian contact and sliding friction modes and clarify the applicability of the BIT with the aid of finite element model and contact stress method.Methods: The stresses on the surface and internal of dental porcelains were calculated with the contact element method (CEM) by ABAQUS software. The model of rectangular dental porcelains and indented ball were developed based on sliding friction tests. The indented ball modeled in this study was of the diameter of 4 mm and 12 mm. The size of Vita VMK 95 and Cerec Vitablocs Mark II porcelain blocks was 5 mm wide, 2 mm thick and 15 mm long. Two blocks were bonded together side-by-side with cyanoacrylate based adhesive with the thickness of 10μm, 20μm and 30μm. The contact ratio was assumed to be 100%. The area of contact unit at the bonded-interface was the size of 8.6μm×9.4μm×10μm for 4 mm diameter balls and 17μm×15μm×22μm for 12 mm diameter balls. The parameters for this study are as follows: load (P), 3.5 N, 5 N, 7 N, and 10 N for 4 mm diameter ball and 11 N, 20 N, 30 N, and 40 N for 12 mm diameter ball; sliding displacement (D), 200μm, 500μm. The loading of the Si₃N₄ ball was modeled as a constant distributed vertical load applied across the bonding line of the porcelain. Stresses and displacements of all nodes of the model, especially at the bonding interface, were analyzed and compared.Results: The results of the study indicated that stress values correlated strongly to the applied loads. Stress distribution was symmetric about the bonded-interface plane. The maximum occurred in surface layer and the minimum on the bottom line in the porcelain blocks. Along the interface, stresses decreased with the distance from the surface of porcelain. On the other hand, high stresses focus on the surface part and go sharply down in the internal region along the depth direction. Among the parameters of sliding test on the stresses, the load effect is prominent. As for glue layer, the bond thickness plays a limited role on the overall stress field. The BIT alters the stress distribution in the neighborhood of the bonded interface. The high-stress zone shifts away from the interface, and extends to the surface.Conclusions: The BIT model experiences a different, non-axisymmetric stress field than that of real work-piece. So the BIT should not be considered as a reliable source of quantitative analysis but as a qualitative method for observing the form of the damage modes with more visible information than the conventional tools.