Unusual nonlinear response of some metallic materials

An anomalous material response has been observed under the action of compressive loads in fibrous silicon carbide/titanium (both Ti-6242 and Ti-15-3 alloys) and the nickel-base alloy Inconel 718 in the aged (precipitation hardened) condition. The observed behavior is an increase, rather than a decrease, in the instantaneous Young's modulus with increasing load and is referred to as stiffening. This increase is small, but can be significant in yield surface tests where an equivalent offset strain on the order of 10 με (10×10^-6 m/m) is being used. Stiffening has been quantified by calculating offset strains from the linear elastic loading line. The offset strains associated with stiffening during compressive loading are positive and of the same order as the target offset strains in yield surface tests. Stiffening appears to be nonlinear elastic and for SiC/Ti-15-3 decreases as the test temperature increases, suggesting a temperature-dependent mechanism. No stiffening occurred in SiC/Ti-15-3 above approximately 500 °C. In addition, the tensile response of aged Inconel 718 exhibits a nonlinear elastic response (not stiffening) prior to the initiation of plastic deformation. The nonlinear elastic response in both tension and compression agrees with a second order stress-strain relation. The higher order terms may be important due to constraint interactions associated with a strengthening second phase such as precipitates or fibers.