Rationale

ASTM 2082 does not cover testing of NiTi tubes above 3mm OD. As such, it implies that for tubes larger than 3mm, ASTM 2082 cannot be cited. Revisions are needed to clarify this. I can’t speak of the historical rationale behind limiting bending to 3mm but I can only assume that applying 2-2.5% strain on the outer fiber of a large tube or sheet would take a large force. Most semi-finished suppliers rely on the crush deformation method rather than bending for these products. Furthermore, considering 7.1 and 7.1.1: for specimen smaller than 0.3mm ( thickness or OD), shall non-contact method be used? With regard to small samples, it is extremely difficult to use an LVDT as the recovery forces would be very small hence why relying on non-contact method. Additionally ASTM F2082 does not cover testing of finished Nitinol components. In its latest revision, ASTM F2082/F2082M-23 states: “The test method uses a wire, tube, strip specimen, or a wire, tube or strip specimen extracted from a component; thus, it provides an assessment of a nickel titanium product in its semifinished or finished form”.    Furthermore, it also states that:   “For specimens not in the form of a wire, tube, or strip that are extracted from semifinished or finished components, a wire, tube, or strip shaped test specimen shall be made from the component such that the deformation mode in the test specimen is pure bending.”   The latter statement shall ensure that the strain recovery deformation occurs under pure bending mode so as to avoid any other deformations that may influence the outcome of the BFR test and measurement of the Af temperature. F2082/F2082M is the industry standard test method for the Af temperature measurement of finished Nitinol devices as a manufacturing in-line process control, and is used to ensure that the finished products meet the specified Af temperature specifications. ASTM F2082 was originally intended to assess the transformation temperatures of semifinished products (defined as wire, tube or strip material forms) by measuring the deformation recovered during heating of a specimen in bending with an outer fiber strain of 2 to 2.5% but was not intended for finished products such as stents. However, the standard was commonly used as a guide to obtain Af temperatures of finished products as part of process control and regulatory submissions. In fact, the USFDA Nitinol guidance document recommends ASTM F2082 for "devices that are expected to be load bearing”.   Most companies adapt ASTM F2082 to assess the transformation temperatures of large semi-finished and finished products. The finished device is often tested by flat plate compression of the stent in a controlled fashion to an oval shape. This leads to a local bending deformation of the surrounding struts of the stent; however, this method does not represent a pure bending load as recommended in ASTM F2082/F2082M-23. A pure (local) bending deformation mode would require extraction of a test coupon from the stent. Given the small sizes of the struts, an extraction of a single strut to apply pure bending would likely lead to erroneous and unusable results. Therefore, although the crush method does not strictly adhere to the pure bending recommendation in the latest version of the standard, the method provides recoverable bending strains during heating and might (or not) represents an acceptable assessment of the Af temperature. Furthermore, the USFDA Nitinol guidance document states that ASTM F2082 can be "modified to assess the strains the device is expected to experience in its final finished form”.   All is to say that there are loopholes in the standard that need to be addressed for both semifinished products in excess of 3mm AND finished devices. Deformation methods such as bending, radial crimping or crush might lead to different outcomes.