Analyzing Nitinol Stents Using Abaqus: A Comprehensive FEA Guide

Self-expanding microdevices, particularly Nitinol stents, have revolutionized the cardiovascular medical industry. However, designing these life-saving devices requires overcoming significant engineering hurdles. Analyzing Nitinol stents using Abaqus allows engineers to predict the complex superelastic behavior, fatigue life, and stress distribution of these devices before they ever enter a human body.

In this guide, we will explore the critical challenges of Nitinol simulation and how finite element analysis (FEA) ensures the safety and efficacy of medical stents.

Why Use Nitinol for Medical Stents?

Nitinol (Nickel-Titanium Naval Ordnance Laboratory) is a shape memory alloy (SMA) renowned for two unique properties:

• Shape Memory Effect: The ability to return to a pre-defined shape upon heating.
• Superelasticity: The ability to undergo massive, reversible deformations (up to 8% strain) without permanent plastic damage.

For cardiovascular stents, superelasticity is the key. The stent is crimped (compressed) into a tiny catheter, navigated through the human vascular system, and then deployed. Upon release, it automatically expands to its original shape, gently keeping the artery open.

The Core Challenges in Analyzing Nitinol Stents Using Abaqus

While Abaqus is an incredibly powerful solver, simulating Nitinol stents introduces highly non-linear complexities:

• Complex Material Calibration: Nitinol does not behave like standard steel. Its stress-strain curve features distinct loading and unloading plateaus caused by the phase transformation between Austenite and Martensite. Manually defining these parameters in Abaqus requires complex data lines. (💡 Solution: You can completely automate this process using our Calibrate Nitinol Material Behavior Abaqus Plugin. It generates the exact keywords needed based on your uniaxial tension test data).
• Severe Contact Interactions: When a stent is crimped, its delicate struts press against the crimping tool and often against each other. Defining robust self-contact and surface-to-surface contact in Abaqus is vital to prevent penetration errors.
• Large Deformations and Meshing: The ultra-thin struts of a stent undergo massive rotational and bending strains. Using incompatible or overly coarse elements (like standard C3D8) can lead to sheer locking or divergence.

Nitinol Material Model

Most stents are modeled with either an elastic-plastic or Nitinol superelastic (-plastic) material model. For an elastic-plastic material, a proper amount of hardening should be defined to aid convergence. Note that perfect plasticity is assumed if the plastic strain exceeds the last yield point provided, which will sometimes lead to convergence difficulty. Similarly, for the Nitinol superelastic material model, a small positive slope should be provided for the transformation between the austenite and martensite phases, even though in reality the transformation zone slope is close to zero or sometimes negative.

Due to the difference between the loading and unloading curves, convergence difficulties can arise at the junction of the start or the end of the transformation zone if a large increment is used. Normally, this is not a problem for stent analysis as the increment sizes are usually very small. However, for a simple coupon test, the maximum increment size might need to be reduced.

The superelasticity model has recently been added to Abaqus. Previously, this material model was available as UMAT and VUMAT subroutines as built-in materials for the elastic-plastic simulation of Nitinol. The superelasticity model provided in Abaqus is intended for modeling nitinol-type materials that undergo solid-solid, martensitic phase transformation and exhibit superelastic response. It can be used to model the stress-induced transformation of austenite to single-variant martensite, the stress-induced transformation of martensite to austenite, reorientation of martensite, and the shape setting process. This model is based on the work of Auricchio and Taylor (1996, 1997) and is based on the concept of generalized plasticity and physical principles.

If you use physical test data for modeling in Abaqus, it is necessary to determine the coefficients of the Superel asticity material model using the calibration of these data. For this, you can read this Abaqus Tutorial: Abaqus/CAE Plug-in Utility to Calibrate Nitinol Material Behavior.

Blood Vessel Material Model

It is commonly represented as a hyperelastic matrix containing embedded fiber families. The Holzapfel-Gasser-Ogden model, which is implemented in Abaqus, is designed for this specific application and is utilized in this instance.

The Three Pillars of Cardiovascular Device FEA

Successful shape memory alloy simulation requires mastering three distinct phases to ensure patient safety and regulatory approval:

1. Stent Crimping Simulation: Compressing the stent into a micro-catheter demands a robust superelastic material model in Abaqus to handle extreme self-contact and massive structural strains without artificial yielding.

2. Stent Deployment in Abaqus: Accurately simulating the release of the device inside a hyperelastic artery model to capture the exact radial spring-back forces.

3. Fatigue Analysis of Nitinol Stents: Applying cyclic pulsatile blood pressure loads (systolic/diastolic) to predict long-term fracture risks and validate the stent’s infinite life cycle.

Fatigue Analysis of Nitinol Stents

The main concern with medical implants is their fatigue life. Traditional fatigue testing methods are often impractical due to their time-consuming nature, making them incompatible with conventional design cycles. Finite element analysis offers a way to obtain quantitative measures of stress and strain without physically constructing the devices, enabling design optimization and predicting device lifespan.

Materials or components being tested should withstand a specified level of cyclic loading without failure for at least 1 million cycles. For example, heart stents are expected to endure approximately 600 million cycles, equivalent to 15 years of alternating loading in vivo. As fatigue curves become relatively flat over such extended lifespans, minor variations in strain/stress amplitudes can significantly impact the calculated life.

Once in use, the stent undergoes cyclic loading due to pulsatile blood flow. With a normal heart rate of 60-80 beats per minute and a stent lifespan of 10 years, the stent must endure hundreds of millions of cycles. The assessment of fatigue behavior during this period is crucial, and simulations offer an alternative for studying this behavior.

For this purpose, you can use the Nitinol fatigue fe-safe plugin in Abaqus.

Image credit: Giovanni De Morais Teixeira

Workflow: How to Simulate a Stent in Abaqus

A standard FEA workflow for a Nitinol stent involves three main simulation steps:

1. Flexibility Test: This test assesses a material’s or component’s ability to bend or flex without breaking. It is important for materials used in applications where flexibility is critical, such as stents.
2. Crimp Simulation: This refers to simulating the process of crimping, which involves deforming a material or component to hold it in place or secure it to another object. Crimp simulation is important for evaluating the performance and durability of crimped connections, such as those found in medical devices, like stents.
3. Expansion Simulation: This involves simulating the expansion of a material or component under specific conditions, such as pressure, temperature, or mechanical loading. Expansion simulation is important for understanding how materials will behave in applications where expansion is a critical factor, such as in inflatable structures, seals, or medical implants.
4. Virtual Stenting: This refers to using computer-aided design and simulation tools to model the behavior of stents in a virtual environment. Virtual stenting allows for the evaluation of stent performance under various conditions, such as different vessel geometries, blood flow rates, and mechanical loads. It is an important tool for optimizing stent design and assessing its effectiveness in treating vascular diseases.

Abaqus Plugins/Utilities

It’s time to introduce Abaqus plugins that are used to analyze stents in Abaqus and make the process easier for Abaqus users. I will try to update these types of tools over time.

Well, the one that I introduced earlier was for fatigue analysis and called the fe-safe plug-in for Nitinol fatigue.

Wrap Mesh Plug-in

This plug-in is used to model tubes with lattice structures, and to prepare it, you should read the DS Knowledge Base article “Abaqus/CAE plug-in to wrap a flat 3D mesh into a cylindrical shape“.

Calibrate Nitinol Material behavior

For this plugin, I previously wrote a step-by-step guide on how to use Nitinol test data calibration in Abaqus.  Also, you can download it for free here.

Subroutines

SMA UMAT

A user-defined material subroutine for polycrystalline shape memory alloys under large deformations. [Github]

USUPERELASHARDMOD / VUSUPERELASHARDMOD

To specify the variation of the transformation stress levels of a superelastic material as a function of the plastic strain dependence of the transformation stress levels on plastic strain in user subroutine USUPERELASHARDMOD (for Abaqus/Standard) or VUSUPERELASHARDMOD (for Abaqus/Explicit), for defining superelastic hardening modifications in Abaqus.

VUMAT/UMAT for Superelasticity and Plasticity of Shape Memory Alloys

For Releases 6.5-1 to 2019 of Abaqus, a constitutive model to simulate the superelastic behavior of alloys such as Nitinol (a nickel-titanium alloy commonly used in medical devices, such as cardiovascular stents and orthodontic wires) at finite strains is provided in the form of a built-in user material model for both Abaqus/Standard and Abaqus/Explicit. This model has been well-tested and performs robustly for all applicable elements.

This routine was compared with the superelastic material model that was added in Abaqus 2019, and nothing is missing!

Need Expert Help with Cardiovascular Device Simulation?

Simulating hyperelastic human arteries interacting with superelastic Nitinol stents is a highly specialized field. If your R&D team is facing convergence issues, meshing errors, or needs to validate a new stent design rapidly, BanuMusa R&D is here to help.

We provide specialized engineering solutions for the medical industry, from structural optimization to stent corrosion modeling. 👉 Explore our FEA & CFD Consultancy Services today or contact our experts to discuss your project requirements.