Standardization of shape memory alloy test methods toward certification of aerospace applications

The properties of shape memory alloys (SMAs) employed as actuators are especially attractive to engineers and designers in the aerospace industry [1, 2]; their status as the active material with the highest volume specific actuation energy density has enabled a number of adaptable aero-structural concepts [3, 4]. However, there are currently no industry or government-accepted standardized test methods for SMA materials when used as actuators. While there exists a broad range of potential actuation applications for SMAs on aerospace and automotive platforms, their transition to commercialization and production is hindered by a lack of accepted industry and regulatory testing and certification standards for SMA actuator materials and components. Even the most well-known SMA aerospace application to date, the hydraulic pipe couplers used on the F-14 [5], was limited to a non-actuating role on a military aircraft. Existing public standards are principally related to uses in the medical industry [6–10], which are based primarily on the superelastic response of these materials [11–13].

Due to the lack of universally accepted standards, the various research organizations pursuing the development of SMA actuator applications have been forced to identify and define key properties and institute their own (often conflicting) testing protocols and procedures. To rectify the current situation and provide a path forward, a new collaborative effort has begun⁸ to: (i) identify critical SMA material and actuator requirements, (ii) develop and propose baseline material and shape memory test methods, and (iii) to influence and inform the formation of public standards in collaboration with an established standards development organization (SDO). Note that, while aerospace applications exploiting the superelastic response of SMAs have been proposed, such development efforts have some recourse to the existing superelastic standards; this new effort focuses solely on the use of the material as a thermally induced actuator.

This pre-competitive standardization effort, though largely motivated by aerospace needs, should apply to a wide range of applications. The proposed standards are not intended to be directly associated to any specific alloy, family of alloys, or material form, nor are they intended to represent a total airworthiness certification strategy. The standardized experimental methods developed in this program are intended to describe tests that can be used to consistently evaluate new or existing alloys or alloys subjected to new processing steps or techniques, so long as the end use is as an SMA actuator. It is expected that the overall effort will be comprised of three phases requiring one year each to complete (as described in section 3).

The intent of this brief paper is to introduce the current effort to the active materials community at large, to provide a brief review of background motivation, and to encourage the input of other knowledgeable researchers in the area. The initial collaborative tasks are also described and some initial thoughts on material properties to be considered are presented.

The unique thermomechanical response of SMAs has made them an attractive option for the development of novel solid-state actuators. Efforts in the aerospace industry in particular have focused on the implementation of these thermally activated SMA actuators in the design of future aircraft and spacecraft actuation systems. SMAs can recover seemingly permanent strains developed under thermal or mechanical load through phase transformation from martensite (the low temperature phase) to austenite (the high temperature phase) and vice versa, where the strain recovery in particular occurs during heating. The attractiveness of SMA actuation components with respect to aerospace, automotive, and oil and gas exploration applications arises from their high actuation work output, low installation volume and silent and/or solid-state (i.e., robust) actuation. However, despite the ubiquitous use of the material in the highly regulated biomedical devices industry and the prior successes of SMA actuators in automotive and especially space applications [14–16], no SMA-based actuator to date has been certified for use in commercial aircraft. While the transformation-based properties that underlie the isothermal response of medical devices and the testing methods associated with them have been standardized for some time, the analogous properties associated with actuation have not been similarly standardized. Knowing and understanding the failure mechanisms and fatigue in SMAs is also necessary and useful for design and certification prior to commercial use⁹ . Finally, it is expected that such standardized actuation test methods will provide advantages to the SMA modeling community by regularizing the determination of material properties used in the calibration of computational analysis tools.

2.1. Airworthiness certification of materials

2.2. Existing ASTM standards for SMA materials applied to medical devices

As SMA materials gained acceptance for use in the medical industry, it became clear that some testing methods should be standardized. ASTM International has addressed this need by publishing standard test methods to guide SMA experimentation efforts [6–10]. Some standards are restricted to the standardization of terminology (ASTM F2005) or suitable compositions of NiTi alloys for medical use (ASTM F2063), but others address experimental methods directly. Standard test method ASTM F2004 addresses the details of differential scanning calorimeter testing, which is an important test for characterizing transformation temperatures in SMAs under zero stress. As an alternative method to estimate only the transformation temperatures in processed materials during heating, specification ASTM F2082 defines the 'bend and free recovery' test. This specification makes direct use of the shape memory effect, but is restricted to SMA wires with diameters from within a prescribed range. It is essential to note that both F2004 and F2082 define 'zero-stress' tests, whereby the transformation temperatures are assessed in the absence of externally applied loads. Further, F2063 and F2004 consider properties in the fully annealed state. Each of these restrictions contrasts with the growing use of SMA components as actuators, which are processed beyond an annealed state and by necessity undergo thermally induced transformation in the presence of loads (aero-structural and otherwise).

Finally, specification ASTM F2516 addresses the tensile testing of SMA specimens with a focus on those exhibiting superelasticity at room temperature. While some aspects of the standard might apply to the characterization of material intended to provide thermally induced actuation, the utility of its content in the context of current aerospace and/or automotive industry goals is limited. The current ASTM standards aim for a basic understanding of certain aspects of SMA behavior, mostly relevant to superelastic medical applications; they do not address the full thermo-mechanical characterization of these materials. This is best illustrated in figure 1, which shows the three most important SMA responses (superelasticity, traditional shape memory effect, and load-biased shape memory effect). Only assessment of the first is currently considered in the industry standards described above; our new effort aims to standardized characterization of the second and third responses.

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2.3. Prior work on standardization of SMA actuation testing

It has been determined that the overall SMA specification and standardization effort will require three phases spread over approximately three years in order to deliver the first ever FAA-accepted material specification and test standards for SMAs as employed as actuators for commercial and military aviation applications. The three phases to be pursued are as follows.

• (Current effort.) Standardized test methods are being developed and documented for determining chosen material and shape memory properties of existing and new alloys. Developed test methods are being assessed via preliminary trial experimentation. Potential SDOs for collaboration will be identified and their level of interest and comments on eventual publication of SMA standards are solicited.
• A draft of the test methods will be submitted to industry, regulatory, and expert organizations for comment and to begin the process toward standards publication. An established SDO will be selected for detailed collaboration and final public release of the test methods. During this process, the test methods will be independently evaluated via a greatly expanded 'round robin' experimental effort (see [20] for a similar example associated with SMA model development). The experimental effort will consider a statistically significant number of samples and a number of laboratories.
• In partnership with a selected standards body, the standardized test methods will be finalized, resulting in a formalized recommendation to regulatory agencies.

In the following, we briefly describe the efforts currently underway toward the completion of phase 1, which initiates the overall effort. Essential to the success of this project is the participation and input from a number of organizations and individuals, which by design includes engineers and designers working in materials and processing development, application design, SMA component fabrication, and testing at the component and system level. Further, strong consensus among this diverse body of participants is needed to advance standardization concepts developed herein for universal adoption by the greater aerospace community and especially regulatory bodies. Additionally, in order to assure broad acceptance of the SMA test methods, the development and release of public standards will be done in collaboration with an established SDO such as ASTM, SAE, etc.

3.1. Determination of SMA properties included in final specification

3.2. Determination of material characterization methods for assessment of chosen SMA properties

3.3. Preliminary experimental assessment of proposed tests

3.4. Evaluation of existing SDO for collaboration

As this SMA testing standardization effort begins, researchers have focused on properties and associated tests. Property discussions have addressed the need of material suppliers and application developers to consider fundamental properties such as element composition (including trace elements) and microstructural properties such as grain morphology and inclusion characteristics. The effect of these on actuation performance and even fatigue is especially strong in phase transforming materials such as SMAs. Further, standard thermoelastic properties such as tensile and shear moduli and thermal expansion coefficient have also been discussed, though it is evident that recourse will be made the existing testing standards for metals (e.g., ASTM E8 for Tension Testing for Metallic Materials).

However, of greatest interest to the SMA research community is the determination and description of important thermally induced shape memory response features. Two experiments (and the response features they elucidate) have been down-selected as being both the most critical and most common characterization methods used throughout the aerospace community: (i) the tension/compression free recovery test, and (ii) the constant force thermal cycling test. The former is associated with the conventional shape memory effect (cf figure 1(b)) and is analogous to ASTM F2082 on 'bend and free recovery,' while the latter directly addresses the capability of an SMA material or processed component to perform actuation work (i.e., to recover strain under load). The two loading paths associated with these first tests to be standardized are shown in figure 2, where the critical stress–strain–temperature points to be obtained from these tests are schematically illustrated. Note that such aspects as recoverable and irrecoverable strains and critical transformation temperatures are measured in various ways.

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Draft testing standards associated with these two tests are currently being authored, and these have been and will continue to be shared with the wider SMA actuation community via conference presentations and articles such as this. Preliminary experimentation is also being initiated to assess the robustness of these methods across a range of specimen types, materials systems, experimentalist experience levels, etc. In the coming years, and especially as the SDO process begins, it will be essential that more organizations with a vested interest in the ability of these important materials to be flight-certified choose to participate in these developments, in addition to those in the areas of automotive applications and even consumer goods. This article has provided a description of the effort in its current form and serves as an invitation to other interested entities to join and contribute. It is only through such community-wide effort that SMA actuator applications will reach their full market potential and functional impact.

This work has been organized through the Aerospace Vehicle Systems Institute (AVSI). It is being supported by equal contributions from the following current project members: ATI Specialty Metals, Boeing, Embraer, NASA (Glenn and Langley Research Centers), Rafael Advanced Defense Systems Ltd, Rolls-Royce plc, and SAES Getters. Though not listed as co-authors, Ron Noebe (NASA) and Frank Sczerzenie (SAES Smart Materials) contributed to the generation of this manuscript and their input is gratefully acknowledged. Finally, the project planning input of Gregory Saunders, former Chair of the AVSI Executive Board and Director of the Defense Standardization Program Office in the Office of the Secretary of Defense, is also gratefully acknowledged.