Wear Characterization of Nitinol Wire

Many cardiovascular and structural heart implants are made of braided nitinol wire. Wear marks have been observed from explant and bench test. Excessive wear reduces the cross-section of the braid wire and induces stress concentration at wear location. It is desirable to be able to predict wear rate of Nitinol wire for structural and fatigue analysis. Unfortunately, the publication on wear prediction model of Nitinol wire is limited. The aim of this study is to derive a practical wear prediction model.

Many Wear prediction models have been proposed for surface contact. Earliest widely used empirical wear prediction model was proposed by Archard:

W = K * s * P

where W is the worn volume, s is the sliding distance, P is the applied load. K may be described as the coefficient of wear.

A more complex empirical wear equation was proposed by Bayer with the following form:

W = K * s^m * Pⁿ

Where the exponents m and n are fitted by use of the experimental data which provide more options for curve fitting to experimental data. A wear model based on Bayer’s approach is used in this study.

Reciprocating wear tests on Nitinol wires with AF temperature of 30^oC were performed under varying normal load, cycles and stroke distance in saline with Nanovea tribometer. The wear depth/volume were measured for each test condition. An empirical wear depth prediction model is provided based on best fit of experimental data.