Session: 02-02: Shape Memory Alloy Actuator Material and Characterization Standards

Paper Number: 110739

110739 - A Unified Approach for Characterizing Mechanical and Actuation Fatigue in Smas 

Recent studies on Ni-Rich NiTiHf High-Temperature SMAs have showcased them as potential candidates for use in solid state actuators because of their high actuation work-output, fatigue resistance, transformation temperatures and cost effectiveness, making them ideal for use in harsh environments especially in the aerospace industry. Experimental studies have been carried out to characterize, model and predict actuation or thermomechanical fatigue response and lifetime of NiTiHf SMAs to assess their functional properties and longevity for optimal actuator design. For this, the Shape Memory Effect (SME) is exploited whereby the sample is cycled between an upper and lower cycle temperature at constant stress to failure. A drawback of actuation fatigue testing is the high cost and duration the tests require because of slow heating and cooling rates. This is especially applicable to high-cycle actuation fatigue which can take on the order of days to weeks to complete. On the other hand, mechanical or pseudoelastic fatigue is much quicker to carry out due to faster loading rates, with high cycle fatigue tests taking hours. For this, the Pseudoelasticity Effect (PE) is exploited whereby the sample is cycled between an upper and lower cycle stress at constant temperature to failure. This work aims to experimentally characterize the mechanical fatigue response and functional properties of three Ni-rich NiTiHf HTSMA’s (Ni_50.3Ti_29.7Hf₂₀, Ni_50.5Ti_33.5Hf₁₆, and Ni_50.6Ti_30.4Hf₁₉) under uniform testing parameters using automatic analysis algorithms, and further develop an empirical model to fit established actuation lifetime and work-output data with mechanical fatigue lifetime data for each material. For this, energy-based approaches to formulate a mechanical work ‘equivalent’ and power law formulations to model and predict mechanical fatigue life were used. Using this approach, similarity between the two fatigue types along different thermomechanical paths, geometry, and loading rates, providing for a unifying descriptor of tensile thermomechanical fatigue can be demonstrated. This approach showed that a similarity in the slopes of the mechanical and actuation fatigue lifetime trends is observed across the materials tested when using a mechanical work equivalent model similar to the actuation work output model, with actuation data having a consistently higher work output, leading to an overall linear offset of the fatigue curve. With similar slopes, to estimate actuation fatigue lifetime and loading conditions, one can conduct a series of fast mechanical tests to determine the slope, and a minimum of one actuation test to determine the offset and required shift, saving on run time and experiments.

Presenting Author: Hrishikesh Padalia Texas A&M University

Presenting Author Biography: Hrishikesh Padalia is a Master of Science student and Graduate Research Assistant under Dr. Dimitris Lagoudas at Texas A&M University with a background in experimental material science. He received his Bachelor of Science from Texas A&M University, where he majored in Aerospace Engineering. His research focuses on mechanical fatigue of high temperature shape memory alloys. He is committed to advancing the field of Aerospace Engineering, and is eager to continue his professional career in industry.