ADVANCED MATERIALS & PROCESSES | MARCH 2025 39 testing methods are required. Relevant material parameters include high internal latent heats, low mechanical hysteresis, and low transformation stress and strain, to name a few. While typical latent heat analysis has been conducted via DSC measurements, latent heats for elastocalorics need to be quantified while the material undergoes mechanical loading and unloading. Therefore, a new material testing regime has been proposed covering the complete thermo-mechano-caloric material characterization[24]. While there is intense research in the field of finding new SMA alloy systems for elastocalorics, standards in testing and characterization are needed for general comparisons and material classifications. SMA standards are in the infancy of their development and mostly limited to niche uses. Much can be learned from alloy systems with well-established industries. The steel, aluminum, and titanium alloy industries have thousands of standard material specifications and test methods. Where practical, these standards can be divided into subcategories based on their product form, intended application, service requirement, or other similar criteria[25]. Future work may include the development of standard specifications and test methods that address forming and post-processing into semi-finished and finished components, fracture toughness, application or component specific requirements, and other areas identified by the committees. Today, as a community, we should collectively look at the suite of standards available for phase change materials and develop best practices for their use. We should also identify areas for improvement and additional standards needed based on both current and future needs.~SMST For more information: Doug Nicholson, Ph.D., technical lead engineer, The Boeing Company, 6300 James S. McDonnell Blvd., Berkeley, MO 63134, douglas.e.nicholson@boeing.com. References 1. T.W. Duerig, Applications of Shape Memory, Materials Science Forum, 56, Trans Tech Publications Ltd., 1990. 2. A.R. Pelton, S.M. Russell, and J. DiCello, The Physical Metallurgy of Nitinol for Medical Applications, JOM, 55(5) p 33-37, 2003. 3. F. Sczerzenie, Consideration of the ASTM Standards for Ni-Ti Alloys, 2004. 4. ASTM F2004–24: Standard Test Method for Transformation Temperature of Nickel-Titanium Alloys by Thermal Analysis, ASTM International, West Conshohocken, PA, www. astm.org, 2024. 5. ASTM F2005–17: Standard Terminology for Nickel- Titanium Shape Memory Alloys, ASTM International, West Conshohocken, PA, www.astm.org, 2017. predominantly utilize NiTi material for its superelasticity, defined as the material’s elastic response to an applied stress. This is in stark contrast to the thermally induced shape memory effect utilized in actuation application. With so many potential branch paths and final forms it will be exciting to see how future specifications maximize the impact of NiTi-based material in a wide range of applications. WK 74640 Constant Force Repeated Thermal Cycling (CFRTC) of SMAs. This test method is being developed to address one of the most common types of SMA thermo- mechanical responses relevant to SMA actuation. That is, applying a constant load to the SMA and repeatedly thermally cycling it through the phase transformation. During CFRTC, the SMA materials actuation strain and transformation temperatures will typically evolve. This behavior is generally undesirable for actuator applications and is often attributed to “fatigue” in SMA. This is not to be confused with conventional material fatigue and failure from fracture under a fluctuating load, although in many cases it could be similar. The objective of this test method is to assess the ability of SMA material and components to meet life expectancy requirements in an actuator application. Life expectancy requirements may include structural and/or functional fatigue limits. It is well known that chemistry, processing, metallurgical structure, and cycling parameters, including stress, lower/upper cycle temperature, and heating/cooling rates will impact CFRTC structural and functional fatigue response in SMA. This was recently highlighted in an SMST NewsWire for NiTiHf actuator SMA[23]. This test method, in conjunction with the material specification (WK 82516) under development, seeks to quantify, assess, and standardize these parameters for SMA actuator material. FUTURE WORK The newest application field for SMA is the emerging technology field of elastocalorics. The elastocaloric effect is based on the release and absorption of latent heats during mechanical loading and unloading of superelastic SMA materials. This effect is utilized in next generation heating, ventilation, and air conditioning (HVAC) systems and heat pumps, using the SMA as a solid-state refrigerant in a thermodynamic cycle. Elastocalorics is drawing increasing attention as the solid-state refrigerant offers a climate- friendly cooling alternative and latest SMA material developments suggest material COPs of 30 and higher (COP is the coefficient of performance—latent heats released/ absorbed divided by mechanical work necessary). Transferring these high material COPs into commercial AC devices will cut down energy consumption for cooling (and heating) by factors. This new way of using SMA materials comes with completely new requirements for these materials and new 9 FEATURE
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