Effect of Prestrain on the Fatigue Life of Superelastic Nitinol

Abstract

Three types of fatigue testing are performed to elucidate the effects of prestraining superelastic Nitinol on its subsequent fatigue lifetime: rotary bending and tension-tension testing of wire, and beam bending using diamond-shaped specimens fabricated from tubing. Results show that local plastic deformation during prestraining induces residual stresses that have a pronounced effect on fatigue performance, enhancing performance when the fatigue duty cycle is of the same sense as the prestraining (tensile prestraining followed by a tensile duty cycle, for example), and decreasing fatigue lifetime when the sense of the duty cycle is opposite to that of prestraining. This provides an avenue to increasing fatigue lifetime, but more importantly it highlights the need to fully understand the nature of the duty cycle: for example, prestraining a stent by crimping it into a delivery catheter induces favorable residual stresses with respect to subsequent pulsatile fatigue, but might accelerate fracture in other modes, such as axial or crush fatigue. Caution is also advised when trying to apply data from ''constant life diagrams'' derived from the literature (Ref 1, 2 for example) that may not properly reflect the strain history of the device being analyzed.

The Role of Parent Phase Compliance on the Fatigue Lifetime of Ni–Ti

“…Cycles to failure a , mean 1.10 9 10 6 1.79 9 10 6 6.04 9 10 6 1.12 9 10 6 Cycles to failure a , range 3.6 9 10 3 -8.6 9 10 6 5.3 9 10 3 -5.8 9 10 6 3.6 9 10 3 -4.0 9 10 6 7.4 9 10 3 -5. amplitudes ranging from 0.15 to 0.45%. Prior to cycling, all specimens were first loaded to 6% tensile strain (also described as ''pre-strain,'' which is recognized to influence durability performance [27]), then allowed to recover, before calibrating the load and displacement at each station. Testing included a total of 101 AMA specimens and 47 RMR specimens.…”

Advances in enhancing structural and functional fatigue resistance of superelastic NiTi shape memory alloy: A Review

“…NiTi components and devices in many applications are often subjected to dynamic stresses or deformations, resulting in fatigue failures. The phase transformation (Alarcon et al, 2015), thermomechanical treatments (Panton et al, 2019), pre-strain (Senthilnathan et al, 2019), asymmetric stress-strain response under tension, compression, and torsion results in remarkable complexity (Desroches et al, 2004) and (b) Typical stress-temperature phase diagram for NiTi. Loading path 1 represents SME, path 2 represents SE, and path 3 represents isobaric thermal cycling (Zeng et al, 2017).…”

Testing the State of Internal Load and Strength of the Facade Mesh Wire Subjected to the Corrugation Process

“…The replaceable jaws (2) are seated in the press warhead (4) and are fixed with screws (7). Vertical guides (6) have been placed in the warheads to ensure symmetry of the bends of the wire.…”

“…Load-bearing parts can be deliberately preloaded to reduce stress amplitude. The paper [6] presents the positive effects of introducing stresses in a plastically shaped stent segment. Residual stresses also arise in the machining process, which has been described, for example, in Ref.…”

“…[7]. In publications [6,7], the finite element method (FEM) was used to determine the residual stresses.…”