Inclusions Size-based Fatigue Life Prediction Model of NiTi Alloy for Biomedical Applications

Abstract

Current standards consider the size and distri- bution of inclusions in semi-finished material, but do not place requirements on final biomedical devices made of NiTi shape memory alloys. In this paper, we analyze this by comparing the fatigue performances of NiTi supere- lastic wires obtained by different processes through a simple bilinear model of fatigue response in terms of strain life. The fracture surfaces of failed wires are analyzed through SEM microscopy and data regarding the presence of particles, and their morphology is recorded and analyzed using Type-I extreme value distribution. The results show a strong correlation between the fatigue limit of wires (in terms of strain) and the predicted extreme values of in- clusions at fracture origin. Then, following the concept of treating the inclusions as ‘small cracks,’ a simple rela- tionship between fatigue limit strain range and inclusion size is proposed based on DKth data from the literature. The model is compared with the fatigue data obtained from the tested wire

Elastocaloric effect vs fatigue life: Exploring the durability limits of Ni-Ti plates under pre-strain conditions for elastocaloric cooling

“…in rotary-bending tests) the Ni-Ti samples withstanding 10 5 cycles also reach 10 6 or even 10 7 cycles as shown in e.g. [32,37,40].…”

“…As the fatigue of SMAs is generally a major challenge for all its applications, it has been widely studied in the recent years. Following the pioneering work of Melton and Mercier [29], there are several state-of-the-art papers that address different crucial aspects of SMA's fatigue behaviour, such as surface finish [30], inclusions [31,32], pre-strain conditions [33][34][35][36], operating conditions (test temperature, frequency, strain-rate) [37][38][39][40], heat treatment during cycling (healing) [41], etc. Since the failure in these materials takes place shortly after the crack nucleation, the interest in the current state-of-the-art is mainly to control crack initiation that is usually caused by surface roughness and/or inclusions [42].…”

“…Launey et al [31] demonstrated that a Ni-Ti alloy with a low inclusion rate showed a 5-fold improvement in fatigue resistance (with an impressive 10 7 -cycle fatigue strain limit of 2.50%) compared with standard grade Ni-Ti. Urbano et al [32] performed a comprehensive study on the impact of inclusions in Ni-Ti wires on their fatigue life. They showed that, in particular, large inclusions (above 20 μm) of carbide and/or oxide near the surface of the sample, which effectively act as cracks, can drastically reduce the fatigue life.…”

Influence of Manufacturing Conditions on Inclusion Characteristics and Mechanical Properties of FeCrNiMnCo Alloy

“…It is generally known that oxide particles that have a high hardness could deteriorate the mechanical properties of alloys such as failure and fatigue behavior. This is especially true for ductile metals such as austenitic steels and Ni alloys with an FCC crystal structure [26][27][28][29][30][31].…”

Sodium Hypochlorite Treatment and Nitinol Performance for Medical Devices

“…Recognizing the importance of controlling inclusions such as Ti 4 Ni 2 O x and TiC, the industry standard for nitinol alloys used in medical devices and surgical implants, ASTM F2063, places an upper limit on inclusion size of 39.0 μm (Ref 8). Inclusions are an important research topic, and recent experimental and modeling efforts have been making progress toward better understanding how inclusions affect fatigue life (Ref 9–11). In one recent study, Robertson et al (Ref 10) examined the role of inclusions in fatigue by comparing the fatigue life of three standard nitinol production methods with that of two newer methods designed to reduce the size and/or number of inclusions and found that the newer process resulted in approximately a twofold increase in the strain limit at 10 million cycles.…”