A Study of Variance in Determining Transformation Temperature of Nickel-Titanium Shape Memory Alloys by Bend and Free Recovery

The bend and free recovery method (BFR), covered under ASTM F2082-06, is the current industry standard used by Nitinol wire manufacturers to determine active austenitic finish (A_f) transformation temperatures.  A study was conducted to determine statistical variance inherent to the testing in comparison to acceptable variance levels described in the standard.  For the internal data set, a total of 480 tests were performed on Nitinol material by utilizing two different wire diameters tested eight times at each of the ten stations on custom test equipment, and the tests were repeated by three operators.  Comparison of the internal data set was made to an external data set.  The external data are from Research Report F04-1009, ASTM Committee F04, Medical and Surgical Materials and Devices Subcommittee F04.15 on Materials Test Methods.  The statistical variance of external data was more than 10 times greater than the variance of internal data. This article will detail the statistical methods utilized in determining the results.