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A Comparison of Zero Mean Strain Rotating Beam Fatigue Test Methods for Nitinol Wire

Dennis. W. Norwich, P.E.  Memry Corporation, Bethel, Connecticut, USA

Zero mean strain rotating beam fatigue testing has become the standard for comparing the fatigue properties of Nitinol wire. Most commercially available equipment consists of either a two chuck or a chuck and bushing system where the wire length and center-to-center axis distance determines the maximum strain on the wire. For the two chuck system, the samples are constrained at either end of the wire and both chucks are driven at the same speed. For the chuck and bushing system, the sample is constrained at one end in a chuck and rides freely in a bushing at the other end. These equivalent systems will both be herein referred to as chuck-to-chuck systems. An alternate system uses a machined test block with a specific radius to guide the wire at a known strain during testing. In either system the test parts can be immersed in a temperature controlled fluid bath to eliminate any heating effect created in the specimen due to dissipative processes during cyclic loading (cyclic stress induced formation of martensite) [1]. This study will compare the results of the same starting material tested with each system to determine if the test system differences affect the final results. The advantages and disadvantages of each system will be highlighted and compared. The factors compared will include ease of set up, operator skill level required, consistency of strain measurement, equipment test limits, and data recovery and analysis. Also, the effect of test speed on the test results for each system will be investigated.

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