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Seven laboratories participated in an interlaboratory study to assess the applicability of ASTM E606, Test Method for Strain-Controlled Fatigue Testing, to strain-controlled fatigue of a tungsten fiber/copper matrix composite (Battelle Columbus, University of Iowa, IITRI, MCL, MarTest, Rockwell International Science Center and University of Twente). Because of the material and specimen fabrication methods, the number of fibers in the 9, 25, and 36 % fiber volume fraction specimens is approximately the same and the strain-life data are nearly identical for each of the volume fractions. Although the specimen aspect ratio is within the nominal range, the specimen diameter is less than the nominal value specified in ASTM E606. Laboratories reporting data noted that a small specimen diameter and a large gage section aspect ratio made specimen alignment difficult. Thus, specimen buckling was problematic at high strain ranges. The study data are reported and analyzed with nonparametric and semiparametric statistical methods to assess the effect of study covariates on fatigue life. The median fatigue life, median absolute deviation from the median, and interquartile range are reported as measures of central tendency and variability. ASTM E606 may be used as a guide to evaluate the fatigue response of this type of composite material if the length-to-diameter aspect ratio, specimen alignment in the fatigue machine, and specimen surface finish are closely monitored to ensure valid data.

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.

In addition to excellent biocompatibility and corrosion resistance, the unique flow behavior of NiTi pseudo-elastic alloys renders them ideally suited for several medical device applications utilizing minimally invasive technologies. For example, self-expanding stents are indicated chronic therapies for treatment of a variety of vascular diseases. Long-term device integrity is thus critical with the FDA recommending a minimum 10-year life. Historically, the majority of published fatigue data on NiTi alloys have been generated under strain-controlled conditions using wire or single-diamond cell samples designed to replicate the behavior of the repetitive unit within a stent. Resulting data are then presented on modified Goodman or Soderberg diagrams in order to define regions of acceptable device life. Consistent with their unusual monotonic flow characteristics these alloys are found to exhibit unique cyclic behavior in fatigue: published data clearly demonstrate that increasing mean strain often increases high-cycle fatigue life. This anomalous behavior is generally attributed to any of a number of possible microstructural or mechanical effects associated with the reversible stress-assisted austenite-to-martensite phase transformation. Although the majority of published work has focused on these macroscopic properties in order to characterize and adequately define device life, various efforts are currently underway to better understand the more fundamental metallurgical and mechanical aspects controlling fatigue life in this alloy. Such studies include crack initiation and propagation rates, determining the roles of absolute and relative inherent flow behavior of the transforming structures, mechanical instabilities resulting from the heterogeneous nature of the transformation, latent heat effects during cyclic transformation, as well as the effects of testing frequency, crystallographic texture, grain-shape anisotropy, and stress-state on fatigue life. A review of existing literature delineating these behaviors and current efforts is presented in light of known beneficial effects of transformation plasticity and toughening in enhancing fatigue and fracture properties of a variety of metallic and nonmetallic materials.