Broadband Electromechanical Spectroscopy: characterizing the dynamic mechanical response of viscoelastic materials under temperature and electric field control in a vacuum environment

The viscoelasticity of a variety of active materials is controllable, e.g., by the application of electric or thermal fields. However, their viscoelastic behavior cannot be fully explored by current methods due to limitations in their control of mechanical, electrical, and thermal fields simultaneously. To close this gap, we introduce Broadband Electromechanical Spectroscopy (BES). For the specific apparatus developed, specimens are subjected to bending and torsional moments with frequencies up to 4 kHz and amplitudes up to 10^(−4) Nm (the method is sufficiently general to allow for higher and wider frequency ranges). Deflection/twist is measured and moments are applied in a contactless fashion to minimize the influence of the apparatus compliance and of spurious damping. Electric fields are applied to specimens via surface electrodes at frequencies up to 10 Hz and amplitudes up to 5 MV/m. Experiments are performed under vacuum to remove noise from the surrounding air. Using BES, the dynamic stiffness and damping in bending and torsion of a ferroelectric ceramic, lead zirconate titanate, were measured at room temperature, while applying large, cyclic electric fields to induce domain switching. Results reveal large increases of the specimen’s damping capacity and softening of the modulus during domain switching. The effect occurs over wide ranges of mechanical frequencies and permits lowering of the resonance frequencies. This promises potential for using ferroelectrics for active vibration control beyond linear piezoelectricity. More generally, BES helps improve current understanding of microstructure kinetics (such as during domain switching) and how it relates to the macroscopic viscoelastic response of materials

Highly non-linear creep life induced by a short close γ′-solvus overheating and a prior microstructure degradation on a nickel-based single crystal superalloy

Specific non-isothermal creep conditions were applied to a 1st generation single crystal nickel based superalloy at very high temperature. Creep tests were conducted under 120 and 160 MPa at 1050 °C with one overheating under stress at 1200 °C for 30 s. Results were compared to the ones already obtained under 140 MPa on the same alloy as well as on a fourth generation of alloy. It is shown that the residual creep life after a single overheating is optimal after a specific prior-creep time. The impact of the creep degradation prior to an overheating on the creep life is dependent on the magnitude of the effective γ/γ′ lattice mismatch. Indeed, an overheating performed when the effective γ/γ′ mismatch magnitude is maximum leads to longer creep lives, even better than without overheating

A microstructure-sensitive constitutive modeling of the inelastic behavior of single crystal nickel-based superalloys at very high temperature

The prediction of the viscoplastic behavior of nickel-based single crystal superalloys remains a challenging issue due to the complex loadings encountered in aeronautical engine components such as high pressure turbine blades. Under particular in-service conditions, these materials may experience temperature cycles which promote the dissolution of the strengthening γ′ phase of the material on (over)heating, and subsequent precipitation on cooling, leading to a transient viscoplastic behavior. Within this context, a model was recently developed by Cormier and Cailletaud (2010) to fulfill the effects of fast microstructure evolutions occurring upon high temperature non-isothermal loadings. New internal variables were introduced in the crystal plasticity framework to take into account microstructure evolutions such as γ′ dissolution/precipitation and dislocation recovery processes which are known to control the creep behavior and life. Nevertheless, this model did not consider the γ′ directional coarsening, one of the main microstructural evolutions occurring specifically at high temperature. In addition, no kinematic hardening was considered to describe the mechanical behavior, leading to a poor description of cyclic loadings. This paper details the development of a new model by introducing new internal variables for both modeling the γ′ directional coarsening and the evolutions of isotropic and kinematic hardening under complex loading paths. This model was calibrated using monotonous and cyclic experiments performed on [0 0 1] oriented single-crystal samples and both under isothermal and non-isothermal conditions. Thereby, it is able to predict microstructural evolutions for complex thermal and mechanical loadings as well as internal stress evolutions whatever the thermomechanical history. The model efficiency was highlighted by comparing FEM simulation and experimental results of a non-isothermal creep test on a notched sample (i.e. under complex mechanical stress state)