In-Situ Strain Localization Measurements of Shape Memory Alloy Actuators during a Research Experience for Undergraduates Program
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The research experience for undergraduates (REU) program was completed by the author during the summer of 2012. This paper describes the research conducted and the preliminary results achieved during the development of a new measurement method to study the strain localization in shape memory alloy (SMA) actuators. With access to a diverse population of graduate students and professors from different specializations and institutions, the student was empowered with much knowledge and ideas to develop a virtual instrument for in-situ measurements. Development and implementation of this method shows a promising potential in understanding SMA actuator fatigue failure mechanisms. Valuable time was comprised of working on a challenging problem through integration of software, hardware, and algorithms to produce in-situ data. The program enriched the student's educational experience through development in research, problem solving, technical writing, and software knowledge required of a solid engineering education. Guidance from mentors prior to the REU program provided the student with the ideas to independently develop the method during the summer. The research objective was to develop a method to measure visual deformation and to acquire the real-time strain in regions of a specimen during high-cycle fatigue tests. The extent and location of the deformation helps to evaluate if localized strains contribute to early failure. A unique in-situ, non-contact extensometry method controlled by LabVIEW Vision Acquisition virtual instruments (VIs) was developed to measure strain in multiple regions during fatigue testing. A custom LabVIEW code and a webcam tracked and correlated markings on the specimen surface. The method was used in conjunction with a time controlled LabVIEW scheme. As a result, VIs performed image acquisition and distance measurements in real-time based on phase transformation timing. To test the image processing VI, images were acquired between cycles 0 to 1440 with specimen failure at 1683 cycles. The largest strain occurred during 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% in the central regions of the specimen, indicating the highest localized strains evolved near specimen failure. Strains during the austenitic, or heating, phase were lower as the specimen underwent shape recovery. The actuation strain during all cycles and for all regions measured remained nearly constant at 5% strain, indicating overall shape recovery. Average strains over the entire gauge length of a specimen were also compared between the data produced by the VI and a linear variable differential transducer (LVDT). Results were comparable, which concludes that LabVIEW VIs are effective in measuring deformation in multiple regions. American Society of Engeneering Education, 2013.