Residual Stress Effect on the Strength Estimate of Base-Coating Systems

Abstract

The effect of manufacturing-induced residual stresses in layered isotropic composites of basecoating type on the latter strength is studied. The conventional method, based on the principle of additivity of the initial properties of layers of a symmetric composite, is applicable to uniaxial tension or compression, but not to bending, where the latter principle is not valid. Therefore, in the latter case, stress calculation has to be based on the method of equivalent bending stiffness linking the derived strength characteristic with operating stresses in the composite layers. Residual stresses in strength assessment are taken into account by substituting their analytical expressions into respective calculation models. The reliability of mostly used relationships for determining residual stresses is analyzed. To this purpose, an independent derivation of engineering formulas for axial residual stresses was carried out, varying elastic parameters, thickness, and number of layers of the composite. The substitution of the above formulas into the additivity relation for an isotropic symmetric system under uniaxial loading does not change the latter form, due to self-balancing of residual stresses and, therefore, residual stresses do not affect the strength estimate. As an example, the axial tensile strength of an aluminum alloy clad with corrosion-resistant steel was calculated for various ratios of cladding layer-to-steel layer thickness. The results on the composite hardening are presented. It is shown that, in contrast to the uniaxial tension of a composite, residual stresses in bending loading, significantly affect the strength assessment results. Numerical results on such an assessment are presented for a case study. A scheme for determining the endurance limit under cyclic bending was elaborated based on the Goodman hypothesis and experimentally verified. All analytical derivations implied the assumptions of homogeneity and isotropy of the physicomechanical properties of the composite constituents, their ideal adhesion, and elastic stress-strain state described by Hooke’s law.