This paper presents a study on the fatigue life of shape memory alloy actuators undergoing thermally induced martensitic phase transformation under various stress levels. A microstructural study characterizing specific damage patterns is conducted in the current work. A highly stressed state with formation of different types of microcracks has been observed, showing a superficial micro cracking, responsible for the growth of circular cracks localizing the failure of the specimens. The influence of the interactions between the micro cracking pattern and the corrosion occurrence is also studied. Finally, a spallation oxidation occurring at the surface of the actuator, which damages its properties, is also investigated. The failure pattern observed provides information necessary to introduce a correction to the classical Wohler curve for fatigue life of a material undergoing cyclic loading.
Within the last decade, the development of compact SMA actuators has led to the design of smart structures such as the Variable Geometry Chevron (VGC), designed by Boeing engineers. The chevrons are aerodynamic devices actuated by SMA beam actuators and placed along the trailing edge of a jet engine to provide noise reduction. The SMA actuators are clamped on an elastic substrate that provides a biasing force allowing repeated one-way shape memory effect under cyclic thermal actuation. In this work, a comprehensive characterization of thermally induced fatigue behavior of nickel-rich NiTi SMA actuators subject to different constant applied stresses is presented. The influence of various parameters is studied in order to assess the fatigue behavior of nickel-rich NiTi, namely: two heat treatments, two heat treatment environments, three fatigue test specimen thicknesses and four stress levels. The purpose of this thermomechanical fatigue study is to evaluate the shape recovery stability, the influence of large applied stresses, the amount of permanent deformation and the resulting failure mechanisms. Fatigue limits of ~ 5,000 to ~ 60,000 cycles were found for applied stress levels ranging from 250 MPa to 100 MPa.
The recent development of various aerospace applications utilizing Ni-rich NiTi Shape memory Alloys (SMAs) as actuators motivated the need to characterize the cyclic response and the transformation fatigue behavior of such alloys. The fatigue life validation and certification of new designs is required in order to be implemented and used in future applications. For that purpose, a custom built fatigue test frame was designed to perform isobaric thermally induced transformation cycles on small dogbones SMA actuators (test gauge cross-section up to: 1.270 x 0.508 mm2). A parametric study on the cyclic response and transformation fatigue behavior of Ni-rich NiTi SMAs led to the optimization of several material/process and test parameters, namely: the applied stress range, the heat treatment, the heat treatment environment and the specimen thickness. However, fatigue testing was performed in a chilled waterless glycol environment maintained at a temperature of 5°C that showed evidence of corrosion-assisted transformation fatigue failure. Therefore, it was necessary to build a fatigue test frame that would employ a dry and inert cooling methodology to get away from any detrimental interactions between the specimens and the cooling medium (corrosion). The selected cooling method was gaseous nitrogen, sprayed into a thermally insulated chamber, maintaining a temperature of -20°C. The design of the gaseous nitrogen cooling was done in such a way that the actuation frequency is similar to the one obtained using the original design (~ 0.1 Hz). For both cooling methods, Joule resistive heating was used to heat the specimens. In addition and motivated by the difference in surface quality resulting from different material processing such as EDM wire cutting and heat treatments, EDM recast layer and oxide layer were removed. The removal was followed by an ultra-fine polish (0.05 μm) that was performed on a subset of the fatigue specimens. Experimental results are presented for full actuation of the SMA actuators and are given in terms of applied stress, accumulated plastic strain and number of cycles to failure. In addition, the assessment of the influence of the surface quality is supported by fatigue tests results and post-failure microstructure analysis.
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