Iron-based shape memory alloys are currently of high interest for potential smart material applications in civil engineering constructions, but also other fields that do not require bio-compatibility but demand low cost of the SMA. The envisioned civil engineering application examples, for some of which prototypes have already been realized, include connectors in concrete, metal and timber structures, concrete reinforcement as well as isolators/dampers for earthquake resistant technologies.

In this contribution, we present first results regarding the experimental characterization of iron-based SMAs using a versatile miniaturized testing method, specifically the Small Punch Test (SPT). Its main advantages are that it is relatively simple, requires only small amounts of material, and allows testing under well-defined biaxial stress states (triaxiality of approx. 0.6). The SPT has been used to assess the effects of embrittlement, fatigue or aging in structural components that can potentially still be in service. It is further suitable for functionally-graded materials, welded joints, composite layers or coatings, for which the extraction of traditional bulk specimen is often not possible. In our lab, mainly metals (e.g., reactor pressure vessel and TRIP steels) and ceramics (e.g., alumina, carbon-bonded alumina) have been tested under controlled temperature conditions ranging between -180 and about 1.600 degrees centigrade. We have recently also gained experience with using the SPT for the characterization of NiTi shape memory alloys. The difficulty of identifying material parameters with this test and its variants (e.g. the ball-on-three-balls test) is that it always requires the numerical solution of an inverse problem, since the experiment records a structural response rather than a pure constitutive response. To address this, an in-house parameter identification tool has been developed, which combines finite element analysis, user-defined material models, and non-linear optimization algorithms. Building on this expertise, our current contribution investigates the employment of the Small Punch Test as an efficient methodology to characterize iron-based shape memory alloys (SMA) under cyclic bi-axial loading. Such data is of great interest to constitutive model developers for both calibration and validation purposes. Generally, sufficiently rich experimental data on SMA behavior, that goes beyond results obtained in one-dimensional tests (e.g., tension-compression-torsion or planar-biaxial loading) is scarce in the literature. The reason is that such tests are very expensive and require much larger amounts of material. Here we present, for the first time, SPT results of FE-based SMAs under cyclic mechanical loading at different loading amplitudes and test temperatures and the associated simulation-supported material parameter identification.

In prior work it was observed that aspects of local plasticity, damage and even fracture can occur in some loading regimes, which can lead to structural and functional fatigue or even complete failure of the SPT specimen. It is also known, that iron-based SMAs show response features that do not exist or play a minor role in NiTi, and vice versa, such as the occurrence of significant macroscopic plastic deformation in the austenite phase. Particularly the simultaneous evolution and interaction of plasticity and the martensitic phase transformation in some temperature regimes is challenging from both the modeling and the experimental characterization standpoint. The presented study therefore aims to identify test conditions and loading protocols that minimize the influence of friction and damage and at the same time allow a distinction between the other inelastic deformation mechanisms characteristic of iron-based SMA behavior.


Characterization of Iron-Based Shape Memory Alloys Under Multiaxial Loading Using a Miniaturized Test

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

SMASIS2020 - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.