Accuracy of laser-cutting and its influence on the mechanical behaviour of stents

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 the mechanical behaviour of stents, tensile tests are performed on straight beams. The tensile test specimens are laser-cut into the same tubes as used in the manufacturing of stents. The hardening curves obtained show that the yield stress increases when the width of the struts decreases, whereas the breaking point increases when the width increases. In order to understand this phenomenon, the role of laser-cutting is investigated. Firstly, it is shown that the heat-affected-zone, associated with laser-cut items, is very small compared to the size of the specimen. Therefore, the influence of the heat-affected-zone is considered as negligible. The inaccuracy of laser-cutting is then studied by measuring a succession of straight beams. A difference in width between the two extremities of the strut is observed. The error increases when the width of the beam decreases. An F.E.A analysis of the tensile test shows a stress concentration at the smaller extremity, explaining why the true strain at breaking point decreases with the width of the struts.