Tech Note: Shape Memory and Superelastic Technologies Conference and Exposition (Smst) 2022

The International Conference on Shape Memory and Superelastic Technologies (SMST) took place in Carlsbad, California on May 16th-20th, 2022. SMST brings together a diverse group of worldwide experts active in the field of shape memory and superelastic materials. There was be a full-day Nitinol workshop followed by a four-day technical program that covered a host of topics relevant to medical device manufacturers, including deep dives into fatigue behavior, Nitinol processing, corrosion, laser cutting, and many others. Below are the presentations given by Confluent’s own Dr. Harshad M. Paranjape and Dr. Behnam Amin-Ahmadi.

A Probabilistic Approach with Built-in Uncertainty Quantification for the Calibration of a Superelastic Constitutive Model from Full-field Strain Data
Presented by Dr. Harshad M. Paranjape
ABSTRACT: We implement an approach using Bayesian inference and machine learning to calibrate the material parameters of a finite element constitutive model for the superelastic deformation of NiTi shape memory alloy. The calibration scheme uses full-field surface strain measurements obtained using digital image correlation and global load data from tensile tests as the inputs for calibration. We use machine learning to create a surrogate model for the finite element constitutive model. We use the surrogate model to perform the Monte Carlo sampling as part of the calibration process. We demonstrate, verify, and partially validate the calibration results through various examples. We also demonstrate how the uncertainty in the calibrated superelastic material parameters can propagate to a subsequent simulation of fatigue loading. The machine learning surrogate model improves the computational efficiency of the calibration scheme and the use of full-field strain data improves the accuracy of subsequent simulations of local deformation.

Direct Experimental Evaluation of High-cycle Fatigue Indicator Parameters in Nickel-Titanium Shape Memory Alloys
Presented by Dr. Harshad M. Paranjape
ABSTRACT: A key step in the design of implantable Nitinol medical devices is to assess the fatigue safety limit of the underlying superelastic material. This is typically achieved using a combination of simulation and fatigue testing. Using simulation, mean strain and strain amplitude in surrogate specimens such as diamonds is estimated for a set of displacement boundary conditions. Fatigue testing is subsequently performed under these displacement boundary conditions to generate a strain-limit diagram, which is a plot of survival under fatigue loading vs. a fatigue indicator parameter such as the imposed strain amplitude. We have recently demonstrated that the phase transformation volume amplitude (PTVA) – an alternate fatigue indicator parameter for NiTi – provides a robust indication of fatigue life. In this work, we demonstrate a digital image correlation (DIC) based experimental method to directly quantify PTVA. We also show that there is substantial specimen-to-specimen variation in the measured PTVA values on diamond-type specimen. We build fatigue failure probability maps for NiTi using this method. Finally we demonstrate that PTVA allows connecting the fatigue fracture probability for a given expected cycle life to the statistics of impurity inclusion content in the NiTi material. This approach opens a new paradigm for directly measuring fatigue indicator parameters in NiTi device surrogate samples and estimating fatigue fracture probability based on such measurements.

Effect of ePTFE sintering post processing steps on surface quality and pitting corrosion behavior of NiTi stents
Presented by Dr. Behnam Amin-Ahmadi
ABSTRACT: Expanded polytetrafluoroethylene (ePTFE) as a bio-inert barrier cover on NiTi stents has been broadly used in recent years at medical industries. Using ePTFE prevents or delays the tissue ingrowth within the stent and therefore maintains luminal patency for longer periods after initial stent deployment. ePTFE cover normally sinters on the electropolished NiTi bare stent via applying pressure/heat. However, effect of sintering process on the surface quality of NiTi stent and consequently pitting corrosion behavior is not fully understood. In the present study, effect of different sintering temperature (250-550 C), times (10-60 min), strain and furnace atmosphere (air, Ar and vacuum) on the corrosion behavior of the electropolished NiTi stents will be investigated. The surface quality of the stents will be also characterized using scanning electron microscope. Preliminary results show that sintering the stents in the air furnace degrades the corrosion behavior (lower breakdown potential) compared with as-electropolished NiTi stent.