In situ scanning electron microscopic study of structural fatigue of struts, the characteristic elementary building units of medical stents

The present study addresses the cyclic mechanical behaviour of strut assemblies, the elementary building units of medical stents. Strut assemblies were subjected to approximately 2500 cycles at room temperature in air at a frequency of 5 Hz. They were then integrated into an in situ test rig which allowed to perform pull-pull cycling in a scanning electron microscope. The stress-strain response associated with cyclic loading was monitored. And local failure events, which do not necessarily lead to a total loss of strut assembly integrity, were recorded. Early cracks always form at locations where the presence of notches results in high local stresses. These early failures can result in a redistribution of loads in the entire strut assembly. Miniature tensile testing is a useful method to show that pre-fatigue exposure weakens the mechanical resistance of a strut assembly. In most cases, crack nucleation seems to control the fatigue life of individual strut elements and strut assemblies in the austenitic state. It was often observed that micro-crack nucleation is directly followed by fast crack propagation and rupture. During tensile testing, surface flaws were identified as locations where cracks initiate. But we also found micro-cracks around surface inclusions which did not propagate during in situ pull-pull cycling or tensile testing.