Use of a Ni60Ti shape memory alloy for active jet engine chevron application: I.Thermomechanical characterization

Abstract

A shape memory alloy (SMA) with a composition ofNi60Ti40 (wt%) was chosen for the fabrication of active beam elements intended for use ascyclic actuators and incorporated into a morphing aerospace structure. The activestructure is a variable-geometry chevron (VGC) designed to reduce jet engine noisein the take-off flight regime while maintaining efficiency in the cruise regime.This two-part work addresses the training, characterization and derived materialproperties of the new nickel-rich composition, the assessment of the actuationproperties of the active beam actuator and the accurate analysis of the VGC andits subcomponents using a model calibrated from the material characterization.The characterization performed in part I of this work was intended to providequantitative information used to predict the response of SMA beam actuatorsof the same composition and with the same heat treatment history. Materialin the form of plates was received and ASTM standard tensile testing couponswere fabricated and tested. To fully characterize the material response as anactuator, various thermomechanical experiments were performed. Properties suchas actuation strain and transformation temperatures as a function of appliedstress were of primary interest. Results from differential scanning calorimetry,monotonic tensile loading and constant stress thermal loading for the as-received,untrained material are first presented. These show lower transformation temperatures,higher elastic stiffnesses (60–90 GPa) and lower recoverable transformation strains (≈1.5%) when compared to equiatomic NiTi (Nitinol). Stabilization (training) cycles were appliedto the tensile specimens and characterization tests were repeated for the stable (trained)material. The effects of specimen training included the saturation of cyclically generatedirrecoverable plastic strains and a broadening of the thermal transformation hysteresis. Aset of final derived material properties for this stable material is provided. Finally, theactuation response of a structural beam component composed of the same material giventhe same thermomechanical processing conditions was assessed by applying a constantbias load and a variable bias load as thermal actuation cycles were imposed.