June 23, 2013
June 23, 2013
June 26, 2013
23.725.1 - 23.725.16
In-Situ Strain Localization Measurements of Shape Memory Alloy Actuators during a Research Experience for Undergraduates ProgramThe research experience for undergraduates (REU) program was completed by the author duringthe summer of 2012. This paper describes the research conducted and the preliminary resultsachieved after development of a novel measurement method to study the strain localization inshape memory alloy (SMA) actuators. With access to a diverse population of graduate studentsand professors from different specializations and institutions, the student was empowered withmuch knowledge and ideas to develop a virtual instrument for in-situ measurements.Development and implementation of this method shows a promising potential in understandingSMA actuators’ fatigue failure mechanism. Valuable time was comprised of working on achallenging problem through integration of software, hardware, and algorithms to produce in-situdata. The program enriched the student’s educational experience through development inresearch, problem solving, technical writing, and software knowledge required of a solidengineering education.A major challenge in using SMA actuators is understanding their fatigue failure mechanism so asto design actuators with longer fatigue life. The research objective was to develop a method tomeasure visual deformation and to acquire the real-time strain in regions of a specimen duringhigh-cycle fatigue tests. The extent and location of the deformation helps to evaluate if localizedstrains contribute to early failure. In addition, there is a possibility these regions correspond tocrack initiation sites or plastic strain accumulation. A unique in-situ, non-contact extensometrymethod controlled by LabVIEW Vision Acquisition virtual instruments (VIs) was developed tomeasure strain in multiple regions during fatigue testing. A custom LabVIEW code and awebcam tracked and correlated markings on the specimen surface. The code compared digitalimages of the test surface acquired before and after a full phase transformation. The method wasused in conjunction with a time controlled LabVIEW scheme. As a result, VIs performed imageacquisition and distance measurements in real-time based on phase transformation timing.Markings were recognized by machine vision VIs, tracking multiple displacements duringuniaxial constant stress actuation fatigue. To test the image processing VI, images were acquiredbetween cycles 0 to 1440 with specimen failure at 1683 cycles. The largest strain occurredduring the martensitic, or cooling, phase when the specimen elongated under constant loading.The last image processed at 1440 cycles showed a concentration of strains higher than 40% inthe central regions of the specimen, indicating the highest localized strains evolved nearspecimen failure. Strains during the austenitic, or heating, phase were lower as the specimencontracted to original length. As expected, the actuation strain during all cycles and for all sixregions measured remained nearly constant within 4.4 - 4.7% strain, indicating overall lengthrecovery. Average strains over the entire gauge length of a specimen were also comparedbetween the data produced by the VI and a linear variable differential transformer (LVDT).Results were comparable, which concludes that LabVIEW VIs are effective in measuringdeformation in multiple regions. This in-situ method can serve as a valuable tool to study fatigueof SMAs under various heat treatments, temperatures, and cycle times, leading to a greaterunderstanding of failure mechanisms.
Santa Cruz, J., & Lagoudas, D. C., & Hartl, D. J., & Shryock, K. J. (2013, June), In-Situ Strain Localization Measurements of Shape Memory Alloy Actuators during a Research Experience for Undergraduates Program Paper presented at 2013 ASEE Annual Conference & Exposition, Atlanta, Georgia. https://peer.asee.org/19739
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