A constitutive model for a rate and temperature dependent, plastically anisotropic titanium alloy
Résumé
This study focuses on the design of fan blades regarding impact loading resulting mostly from bird strike or engine fan blade loss and involving large deformation, high rate of deformation, non-proportional loading paths, plastic dissipation induced heating and potential damage and fracture. Due to their high strength-to-weight ratio and good toughness, Ti-6Al-4V titanium alloys are promising candidates for the leading edge of multi-component fan blades. To get a reliable prediction of the resistance of the whole engine structure, an extensive experimental campaign has been carried out and a constitutive model has been developed for a grade of Ti-6Al-4V titanium alloy provided in the form of cold rolled plates. The thermo-mechanical characterization, consisted of tension, compression and shear tests at various temperatures, (quasi static and dynamic) strain rates and (monotonic and alternate) loading paths, has evidenced a strong temperature and rate dependence as well as an orthotropic behavior with a significant tension/compression dissymmetry and a combination of isotropic and kinematic strain hardening. A constitutive model has been accordingly developed accounting for the combined effect of nonlinear, isotropic and kinematic strain hardening, strain rate hardening, tension/compression dissymmetry and machining direction. For that purpose, orthotropic plasticity models have been extended within a rate-dependent and isotropic vs. kinematic hardening formulation. The identification of the constitutive model constants has been conducted by means of the commercial software Zset. The constitutive model has then been implemented as a user material subroutine into the commercial finite element computation code LS-DYNA. Numerical simulations have been conducted considering some basic cases involving representative volume elements as well as structures such as the specimens used for the experimental campaign.