Full-field microscale strain measurements of a nitinol medical device using digital image correlation

Abstract

Computational modeling and simulation are commonly used during the development of cardiovascular implants to predict peak strains and strain amplitudes and to estimate the associated durability and fatigue life of these devices. However, simulation validation has historically relied on comparison with surrogate quantities like force and displacement due to barriers to direct strain measurement-most notably, the small spatial scale of these devices. We demonstrate the use of microscale two-dimensional digital image correlation (2D-DIC) to directly characterize full-field surface strains on a nitinol device coupon under emulated physiological loading. Experiments are performed using a digital optical microscope and a custom, temperature-controlled load frame. Following applicable recommendations from the International DIC Society, hardware and environmental heating studies, noise floor analyses, and inand out-of-plane rigid body translation studies are first performed to characterize the microscale DIC setup. Uniaxial tension experiments are also performed using a polymeric test specimen up to nominal stains of 5%. Sub-millimeter fields of view and sub-micron displacement accuracies (9 nm mean error) are achieved, and systematic (mean) and random (standard deviation) errors in strain are each estimated to be approximately 1,000 µ. The system is then demonstrated by acquiring measurements at the root of a 300 µm-wide nitinol device strut undergoing fixed-free cantilever bending motion. Lüderslike transformation bands are observed originating from the tensile side of the strut that spread toward the neutral axis at an angle of approximately 55°. Optical microscale 2D-DIC setups like that demonstrated herein will be useful in future studies for characterizing cardiovascular implant micromechanics, validating computational models, and guiding the development of next-generation material models for simulating superelastic nitinol.

Applying Full-Field Measurement Techniques for the Thermomechanical Characterization of Shape Memory Alloys: A Review and Classification

“…NiTi [111,179] Others 0.1 \ S \ 1 [ 41,107,126,152,199,205,209,218,222,229,261,312,314,321] 0.01 \ S \ 0.1 [216,227,258,270,318] S \ 0.01 [35,192,236,261,279,297,306,318,320]…”

Influence of Structural Porosity and Martensite Evolution on Mechanical Characteristics of Nitinol via In-Silico Finite Element Approach

“…NiTi processed with higher Ti content results in martensitic (B19’ monoclinic crystal structure) phase at room temperature, which translates into higher transformation temperatures and a prominent shape memory effect. A matrix with higher Ni content results in austenitic (BCC B2 crystal structure) phase at room temperature, possesses lower transformation temperatures, and exhibits superelastic properties [ 2 , 3 , 4 ]. Martensite is characterized by needle-like crystals arrayed in a herringbone shape.…”

An Automated Optical Strain Measurement System for Estimating Polymer Degradation under Fatigue Testing

“…Nowadays, it is one of the most common approaches for studying both strain and fracture processes in heterogeneous materials [1,2]. In addition, the DIC method found wide application in solving various scientific and technical problems in materials science [1,3,4], solid mechanics [5], biomedicine [6][7][8], civil engineering [9][10][11][12][13], non-destructive testing (NDT), structural health monitoring (SHM), etc.…”