FEM study of the effects of interlayers and creep in reducing residual stresses and strains in ceramic-metal joints

Abstract

Finite element numerical models are used to compute residual stresses and strains that develop in a joined Al₂O₃Ni specimen during cooling from an assumed fabrication temperature. The concept of stress and strain control via interlayers is investigated by considering a homogeneous compliant interlayer, a composite interlayer, and a variety of functional gradient material (FGM) interlayers. Additionally, a power-law creep constitutive relation is incorporated into the model to investigate the effects and relative importance of time-dependent deformation during joining. Graded microstructures are treated as a series of perfectly bonded layers, with each layer having slightly different properties. Constitutive relations for the composite and gradient inter-layers are estimated using a modified rule-of-mixtures approximation. Stress and strain distributions are shown to be highly dependent upon interlayer properties, and stress reductions are only predicted when the interlayer is highly compliant, or when an “optimized” FGM microstructure is used. For reasonable rates of cooling, creep is shown to have a minor influence on the residual stress state achieved at room temperature; however, above approximately 700 K creep strains are significant, suggesting that creep should be included in models intended for predicting joint behavior at elevated temperatures.