On the ratcheting of the VT6 alloy in a range of loading scenarios

Abstract

A set of experimental results concerning the accumulation of inelastic strain in the VT6 titanium alloy is presented. The considered tests include cyclic stress-controlled loading with monotonically increasing stress amplitude and constant mean stress. The focus of the study is on the accurate phenomenological modelling of the complex material behaviour. Four different material models are employed to capture the phenomena of ratcheting and nonlinear kinematic hardening. Two models are of the Armstrong-Frederick type and the remaining two models are of the Ohno-Wang type. Since all the considered models contain a big number of material parameters, a sophisticated nested procedure is used for the parameter identification; the obtained material parameters are validated by additional experiments. It is shown that all the models yield nearly the same accuracy; the limitations of the phenomenological approach are highlighted and discussed.

Inspection of ratcheting models for pathological error sensitivity and overparametrization

“…Here, η > 0 and m Perzyna > 0 are parameters of Perzyna's viscosity law; f 0 = 1 MPa. In the previous paper [19] it was shown that the classical Voce's law of isotropic hardening is inappropriate for the ratcheting of the VT6 alloy. Therefore, the evolution of the isotropic hardening R is modelled by a new law, which accounts both for hardening and softening:…”

“…Remark. Differentiating (12) with respect to time and using (19), we obtain a well-known form of the Armstrong-Frederick equation:…”

“…Although (19) and ( 20) are equivalent, we prefer using strain-based relation ( 19) rather than the stress-based formula (20). First Ohno-Wang hardening.…”

“…We use experimental data from [19] on the ratcheting of samples from the titanium alloy VT6. In each test, the loading program consists of four stages (Fig.…”

“…AF-type models contain 7, 9, and 11 unknown parameters for variants with two, three and four Maxwell branches, respectively. The number of free parameters in models of OW-I-type is [19] on the ratcheting of the VT6 alloy. always less by one than in corresponding AF-models.…”