Interfacial Mechanics and Macroscopic Failure in Titanium-Based Composites

Abstract

Results are presented for Ti-6Al-4V containing silicon carbide (SiC) monofil-aments having duplex (carbon plus TiB2) coatings. Single fiber pushout testing has been used to study the debonding and frictional sliding characteristics under shear loading, with and without the residual radial compressive stress being reduced by applied in-plane tension. It is shown by finite element method (FEM) modeling that it is important to take account of thermal residual stresses when interpreting data from these tests. Prior heat treatments causing interfacial reaction tend to raise the resistance of the interface to debonding and sliding. This is correlated with data from tension testing of composites under axial and transverse loading, with continuous Poisson's ratio monitoring. The behavior under transverse loading is particularly sensitive to the mechanical response of the interface. The presence of interfacial reaction layers tends to inhibit the development of interfacial debonding and void formation under transverse loading, but causes embrittlement and leads to a reduced strain to failure. The behavior under transverse load has also been studied with superimposed thermal cycling between 400 and 700⪤gC. Strain histories have been monitored using scanning laser extensometry. Under these conditions, thermal stresses have a pronounced influence on the behavior. Many of the steady-state creep characteristics can be successfully modeled on the basis of matrix creep being controlled by volume-averaged stresses (predicted using the Eshelby method), with stress relaxation processes simulated via a variable stress-free temperature. However, the behavior tends to be influenced from a relatively early stage by interfacial damage development. Interfacial debonding and damage, promoted by a combination of opening mode stress from the applied load and shear stress from differential thermal contraction (particularly towards the specimen edges), soon starts to influence the strain history and has a strong influence on the rupture strain. In sharp contrast to the room temperature behavior, the heavily reacted specimens exhibited delayed onset of interfacial damage and a much longer lifetime, although the final strains to failure were similar. These results are considered in terms of stress fields and interfacial properties.