Factors influencing the mechanical behaviour of small devices such as stents

Abstract

A coronary stent is a mechanical device designed to open arteries that have been occluded. The manufacturing of stents involves the laser cutting of specific designs in stainless steel tubes. In order to determine factors influencing the mechanical behaviour of stents, tensile tests were performed on struts laser-cut in tubes used for the manufacturing of stents. The hardening curves obtained show that the yield stress and Young’s Modulus increase when the width of the struts decreases, until a limit is reached where they start decreasing again; the breaking point varies inversely. To confirm the results a three-point bending test was conducted. Because the struts are cut in a tube, the resulting geometry makes it difficult to predict the mechanical behaviour. A model using Finite Element Analysis (F.E.A.) was therefore created and the mechanical behaviour identified by comparing the experimental results and the F.E.A. prediction. The hardening curves obtained confirm the conclusions reached in tension. In order to understand the phenomenon, the accuracy of laser cutting was investigated. An F.E.A. was conducted, showing that the inaccuracy of laser cutting can only explain the strain at breaking decreasing with the width. The geometry of the specimen being discarded, the microstructure was examined in order to understand the mechanical behaviour. The Heat Affected Zone (H.A.Z.) resulting from laser cutting was first observed. It was found that the size of the H.A.Z. is negligible when considering wide struts but becomes significant when dealing with struts smaller than a millimetre. Consequently, the strut behaves like a sandwich material partly explaining the experimental results. However, according to the sandwich theory, the yield stress, Young’s Modulus and strain at breaking should be linear functions of the width which is not the case. Therefore, another factor has to be considered. The number of grains in the width was then investigated and shown to explain the other part of the experimental results. In conclusion, the number of grains in the width and the H.A.Z. both influence the mechanical behaviour of the struts.