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Transformation Induced Cyclic Behavior and Fatigue Properties of Nickel Rich NiTi Shape Memory Alloy Actuators

In the midst of a new emergence of industrial and commercial applications employing multifunctional materials, the design of active structures using Shape Memory Alloys (SMAs) has become predominant. Boeing has recently been working on a new composition for the binary NiTi, Ni60Ti40 (wt.%) for actuation applications. The high nickel content leads to the formation of different intermetallics such as Ti3Ni4, Ti2Ni3, and TiNi3, resulting in shape memory effect without any cold work. The nickel-rich SMA is used as a bending actuator in opposition to a chevron structure mounted on the nozzle of a jet-engine. This active structure, also called the Variable Geometry Chevron (VGC), is able to cause deformations with temperature variations. The result is different mixing conditions between the hot stream from the nozzle and the cold air flowing around it. The purpose is to reduce noise and vibrations caused by this turbulent flow during take-off and landing. In order to see such design implemented in commercial airplanes, the SMA actuators need to comply with safety standards, the most important being the fatigue life of the component. In the present work, the transformation induced fatigue behavior of 60Ni40Ti is being investigated. From the loading conditions in service of the VGC, thermally induced constant stress fatigue testing was conducted. A stress range of 100 MPa – 250 MPa was applied to cover the maximum operational applied stresses. The fatigue life results ranged from about 5,000 cycles at 250 MPa load to about 60,000 cycles at 100 MPa. The thermomechanical transformation cycles generated a large amount of irreversible strain. For this study, two key parameters were selected to be investigated, i.e., heat treatment and specimen thickness, in addition to applied stress. The fatigue test results are presented for different values of these parameters and a post failure analysis to identify the micro-mechanisms leading to fatigue failure are discussed.