Fracture Toughness of Si3N4/S45C Joint with an Interface CrackLiedong Fu, Yukio Miyasita and Yoshiharu MutohCopyright AD-TECH.; licensee AZoM.com Pty Ltd.This is an AZo Open Access Rewards System (AZo-OARS) article distributed under the terms of the AZoOARShttp:/www.azom.com/oars.aspwhich permits unrestricted use provided the original work is properly cited but is limited to non-commercial distribution and reproduction.Posted: September 2005Topics CoveredAbstractKeywordsIntroductionExperimentalSpecimen PreparationExperimental ResultsOscillatory Singular Stress Field of The Interface Crack and The Maximum Normal Stress CriteriaElasto-Plastic Singular Stress Field at The Interface Crack TipFem Analysis and Evaluation of Fracture Path and Toughness Based on the Elasto-Plastic Stress Intensity FactorsConclusionsReferencesContact DetailsAbstractFracture toughness tests were carried out for Si3N4/S45C specimens with interface cracks of different lengths.It was found that the specimen with a crack of 4 mm has higher apparent fracture toughness than those with cracks of 1 mm and 2 mm due to the reduction of the residual stress.Fracture propagated into Si3N4from the crack tip in the direction of 40ofor cracks of 1 mm and 2 mm while it propagated along the interface for crack of 4 mm.Elasto-plastic analysis was carried out considering S45C as the linear hardening material and Si3N4as the elastic material.It was found that the stress around the crack tip is dominated by an elasto-plastic singular stress field, which is substantially the same as the elastic singular stress field of an interface crack.Evaluation of the fracture path and toughness was carried out based on the stress intensity factors of the elasto-plastic singular stress field.KeywordsInterface Crack, Fracture Toughness, Si3N4/S45C Joint, Thermal Residual Stress, Elasto-plastic AnalysisIntroductionThe ceramic/metal joints have been increasingly applied in a wide range of engineering fields because the ceramic has stable mechanical properties at high temperature and good resistance to wear, erosion and oxidation.However, the difference of material properties between metal and ceramic induces stress singularities at the interface edge.Moreover, high thermal residual stress will be induced during the cooling process due to the mismatch of the thermal expansion coefficients.The stress singularity together with the thermal residual stress degrades the strength of ceramic/metal joint and makes the evaluation of the strength difficult.Many works have been done about the residual stress and the strength evaluation of ceramic/metal joints.For example, Kobayashi et al. 1, 2 have investigated the bending strength and residual stress of Si3N4/S45C joint and the effect of the size of the specimen on the bending strength.Qiu et al. 3 have investigated the influence of residual stress and cyclic load on the strength of Si3N4/S45C joint.However, due to the complexity of the problem, a generalized evaluation method for the ceramic/metal joint has not yet been proposed.The elastic solution of the singular stress field of the interface crack has been studied since 1959 4-9.Rice 10 has summarized the work in this field and set up the elastic fracture mechanics concepts for interfacial cracks.Yuuki et al. 11, 12 have proposed the maximum normal stress criteria for predicting fracture path and strength of ceramic/metal joint based on the elastic theory.The plastic deformation of metal will inevitably appear near the crack tip due to the stress singularity.For most of the ceramic/metal joints, the plastic deformation of metal has a significant influence on the strength of the ceramic/metal joint.Due to the analytical complexity, the evaluation of the fracture path and strength of ceramic/metal joint based on the elasto-plastic theory has not yet been made.In this study, four point bending tests of Si3N4/S45C joint specimens with an interface crack were carried out.Evaluation of the fracture path and fracture toughness was attempted based on the elasto-plastic analysis.ExperimentalSpecimenPreparationFigure 1 shows the geometry and dimensions of Si3N4/S45C joint specimen.The silver based brazing alloy (wt% is: Ag, 71%, Cu, 27%, Ti, 2%) with 60 m thickness was used for the bonding between Si3N4ceramics and S45C steel.Brazing was carried in a vacuum furnace (2.5x10-5Torr).The temperature of the furnace was increased at a rate of 20oC/min up to the brazing temperature of 850oC and kept for 10 min, then decreased at a rate of 10oC/min.The joining surfaces were polished with diamond powder of 0.25 m diameter.During the brazing, a contact pressure of 0.002 MPa was applied.After brazing, an interface crack was introduced by the electric discharge method with the cutting wire of 0.1 mm diameter.Four specimens with different crack lengths were prepared.Two of the spe