A Torsion Based Approach For Fatigue Testing Of Nitinol Tube

Fatigue testing of NiTi wire is typically performed  by a zero mean strain rotating beam test.  Non-zero mean strain tensile-tensile fatigue testing of wire is also a commonly used method.  However, for fatigue testing of tube, the standard method consists of laser cutting and processing a complex diamond shape for tensile-tensile or tensile-compression testing.  This method can present difficulties in consistent specimen manufacture and strain analysis.  The new test method demonstrated here uses a simplified single strut test specimen that is laser cut from a tube.  The specimen can be thermally and chemically processed by any means desired to simulate final product.  Rotary motion is delivered by a brushless DC motor with a precise closed loop motion control system.  Any amount of mean rotation or alternating rotation can be programmed into the controller.    The maximum strain level for the rotation used is precisely calculated using finite element analysis (FEA) for the specific geometry of the test specimen.   A temperature controlled fluid bath can be used to eliminate any heating effect created in the specimen due to dissipative processes during cyclic loading (cyclic stress induced formation of martensite).  Break detection is accomplished by simply using continuity through the single strut test specimen.  Fatigue samples with different manufacturing and processing steps will be tested to show the variation in fatigue results with variations in processing parameters.