The Effect of Crimp Strain on the Fatigue Performance of Nitinol

Nitinol continues to be used extensively for cardiovascular devices given its super-elastic behavior and favorable corrosion and fatigue performance.  Typically, the highest strain these devices will see in service occurs during crimping onto the catheter.    These crimping strains may induce latent defects or residual stresses that can comprise fatigue performance.  Therefore, the purpose of this study was to evaluate the fatigue performance of Nitinol wires after exposure to a range of peak strains comparable to those induced during crimping in transcatheter devices.

Nitinol wire apex specimens with a wire diameter of 0.51 mm and an apex bend radius of 0.75 mm were compressed to peak tensile strains ranging between zero and 14%.  The corresponding peak compressive strains on the intrados of the apex ranged between zero and 16%.  After crimping, specimens were tested to characterize the effect of crimp-strain magnitude on strain-life fatigue performance (to 10-million cycles).  The specimens were tested so that either the intrados was in constant tension or the extrados was in constant tension during the fatigue testing.  The results of the testing demonstrated that the crimping affected the fatigue performance when the intrados was in tension, but not when the extrados was in tension.