A Precision Statement for Fatigue of Solid Round Wire

ASTM E2948, Standard Test Method for Conducting Rotating Bending Fatigue Tests of Solid Round Fine Wire, is a procedure for rotating bending fatigue tests of fine wire to obtain the fatigue strength for metallic materials at a specified life in the fatigue regime where the strains are predominately and nominally linear elastic. This method is limited to the fatigue testing of small diameter solid round wire subjected to constant amplitude, completely reversed, periodic strain. The methodology can be useful in assessing the effects of inclusions, melt technique, and cold work processing. Data obtained in accordance with ASTM E2948 is subject to variability because of many factors. An inter-laboratory study (ILS) was performed to assess variability of the data and to develop a precision statement for ASTM E2948. Ten laboratories participated in the ILS. The samples consisted of two materials, Nitinol and 35N LT. Each material was supplied from a single lot from a single manufacturer. Samples were tested at three different strain amplitudes with ten replicates at each strain level. Variability of the resulting data was quantified as reproducibility limits, between laboratory variability, and repeatability limits, within laboratory variability. A precision statement for ASTM E2948 was formulated based on the reproducibility and repeatability limits determined from the inter-laboratory study. The viability of the limits is assessed with Monte Carlo simulations. Alternative methods of analysis using nonparametric statistics and bootstrap methods are discussed.