Physically-based modelling of the fatigue crack initiation life of stent components under cyclic loading employing the Finite-Element-Method (FEM)

Highlights

• The physically based TM formulation is a promising numerical method to predict the fatigue short crack initiation, specially for the stent components.

• By implementing the current numerical method we can predict the time before the nucleation of the first micro-cracks within the stent components.

Abstract

Most of the micro-components, such as coronary stents, consist of an oligo-crystalline micro-structure. This means that the detection of the few coarse grains which are columnar and parallel to the longitudinal ingot axis is possible. The deformation behavior of such micro-structures is obviously different from polycrystalline materials, since the anisotropic properties of one grain, in relation to its nearest neighbors should be considered. The goal of this study is the simulation of the crack initiation process under a cyclic loading situation in the oligo-crystalline micro-structure of the stent component made of X2CrNiMo-18-15-3. The crack initiation process is simulated employing the physically-based Tanaka-Mura model implemented in Finite Element Method (FEM). The available experimental result data have also been used as the input parameters to the current modelling approach. In this regard the grain size and the grain orientation are changed, and their effect together with the influence of the surface roughness on the fatigue life of the stent components has been studied.

Introduction

With a lethality of 35 %, the subarachnoid hemorrhage is nowadays one of the most dangerous intracranial bleedings [1]. At 60 % of the cases, the incidence is higher than average in people with the age between 40 and 60 years. Up to 25 % of the patients lose their lives and 30 % of those who survive, are left with severe neurological deficits and are dependent on a lifelong care. Less than one third of the survivors can master their daily life again and find back their independence [2]. Thus, adequate treatment of an intracranial aneurysm is of utmost importance both in case of incidental findings (unruptured) and in case of acute hemorrhage.

There are different ways of treating an aneurysm, depending on the severity of the bleeding that has already taken place or on the location and type of aneurysm that is still intact. The most common way to do this is to insert a medical stent which is made a metallic material (Fig. 1).

Stents have a wide range of applications and thus play an important role in medical care. Another example is the usage of stents in Cardiology. In industrialized countries, cardiovascular diseases are the most common causes of death within adulthood, especially coronary heart disease (CHD) [4].

Since these stents are long-term implants, they are exposed to cyclic stress over many years, which is due to the pulsating blood pressure in arterial vessels, combined with a static load due to existing vessel wall tension [5]. It is worth to mention that under such a cyclic loading situation, the material fails earlier compared to the pure tensile loading situation. Hence, it is of great importance that the medical stent components, are in the range below the fatigue strength, and have a high level of safety [6].

It has been observed by Mozaffarian et al. [7], that the cardiac contraction constantly moves the coronary arteries to the left, interior and anterior directions. Halwani et al., have observed that fracture in stent components happens at the curvature tip, especially at the band in the connecting links of the overlapping cypher stents (Fig. 2a,b). On the other hand upon carrying out fractography, the dominance of high cycle fatigue with multiple initiation sites, was quite observable from the fracture surface (Fig. 2b) [8].

A stent is a long term implement and that’s why its fatigue strength should precisely considered. The failure of such a component would cause serious problems for the patient. The most common complication is the in-stent restenosis, which is the renew occlusion of the vessel with already an implemented stent. This can be caused even by very small damages on the inner walls of the vessel [9].

The durability for such stents must be enormously high, as the load must be carried over a long time. Calculation of the cyclic loads over 10 years with a mean heart frequency of 76 beats per minute, results in a dynamic load of 400 million cycles [10]. This type of loading happens through the variation of the blood pressure during the ejection phase (Systole), when the heart contracts and also, during the relaxation phase (Diastole), when the heart chamber is filled with blood again. This pressure difference causes a change in the diameter of the arterial vessels (compliance). The mean blood pressure is counted as 13 kPa and has a maximum value of 16 KPa during the Systole [11].

A stent must fulfill many conditions that are often mutually excluding. Firstly, very little amount of material should be used. The smaller the surface of the foreign body, the less structure the material offers the body or the immune system to react or attack (Foreign Body reaction). Also, in the case of implantation, the stent should be kept very small, as no minimally invasive operating technique can be used. The thickness of the stent struts should also remain as thin as possible, so that there are no turbulences caused by the material inside the blood vessel. At the same time, there must be sufficient certainty of the radial strength and fatigue endurance, which requires, on the other hand, an additional amount of material [10].

Certain alloys can be used to produce the coronary stent, such as stainless steel (AISI 316L), Tantalum (Ta), Nitinol (NiTi). Among the aforementioned alloys, Austenitic 316L stainless steel is the most popular and widely used alloy because of its high corrosion resistance [12]. Apart from the good corrosion resistance, stainless steels fulfill the other aforementioned characteristics which are required for a stent material such as biocompatibility [13].

Apart from the chemical points, as it is mentioned also earlier, the failure in stent components can be also due to both the monotonic and cyclic loading during deployment and service, sequentially [14]. The main focus of this work is on the fatigue crack initiation under the cyclic loading condition. It is proven that a computational modelling method gives a very good assessment of the performance and fatigue life of stents [15].

In the present work, the fatigue crack initiation within the Oligocrystalline micro-structure of the stent components made of X2CrNiMo18-15-3 stainless steel under the cyclic loading condition is simulated. In this sense, the Physically-base Tanaka-Mura model is employed and the different parameters which are contributing to the fatigue crack initiation within the stent micro-structure, such as grain size, surface roughness etc., are studied.