ASTM F2514-08(2014)
(Guide)Standard Guide for Finite Element Analysis (FEA) of Metallic Vascular Stents Subjected to Uniform Radial Loading
Standard Guide for Finite Element Analysis (FEA) of Metallic Vascular Stents Subjected to Uniform Radial Loading
SIGNIFICANCE AND USE
4.1 Finite element analysis is a valuable method for evaluating the performance of metallic stents and in quantifying quantities such as internal stresses, internal strains, and deformation patterns due to applied external loads and boundary conditions. Many times an analysis is performed to correlate to and plan experimental tests. A finite element analysis is especially valuable in determining quantities that cannot be readily measured.
SCOPE
1.1 Purpose—This guide establishes general requirements and considerations for the development of finite element models used in the evaluation of the performance of a metallic vascular stent design under uniform radial loading. Suggested criteria are provided for evaluating the typical cases of metallic stents under uniform radially oriented and pulsatile loading. Recommended procedures for checking and validating the finite element model(s) are provided as a means to assess the model and analysis results. Finally, the recommended content of an engineering report covering the mechanical simulations is presented.
1.2 Limits:
1.2.1 This guide is limited in discussion to the finite element structural analysis of metallic stents of the following types:
1.2.1.1 Plastically deformable metal stents.
1.2.1.2 Self-expanding metal stents.
1.2.1.3 Plastically deformable metal portions of covered stents.
1.2.1.4 Metal portions of self-expanding covered metal stents.
1.2.2 The emphasis of the techniques described in this guide is intended for both elasto-plastic materials such as stainless steel, and superelastic materials such as nitinol. Unique concerns associated with stents designed for shape memory behavior are not addressed within this guide.
1.2.3 This guide does not consider changes to possible time varying conditions or different loadings related to vascular remodeling.
1.2.4 This guide is restricted to cases that involve the application of uniform radially oriented loading.
1.2.5 This guide does not provide guidance in the application or interpretation of FEA in determining fatigue life.
1.2.6 This guide is not intended to include complete descriptions of the finite element method, nor its theoretical basis and formulation.
1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
General Information
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: F2514 − 08 (Reapproved 2014)
Standard Guide for
Finite Element Analysis (FEA) of Metallic Vascular Stents
Subjected to Uniform Radial Loading
This standard is issued under the fixed designation F2514; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
This guide establishes general requirements and considerations for using finite element analysis
techniques for the numerical simulation of metallic stents subjected to uniform radial loading. These
stents are intended for use within the human vascular system.
1. Scope 1.2.4 This guide is restricted to cases that involve the
application of uniform radially oriented loading.
1.1 Purpose—This guide establishes general requirements
1.2.5 This guide does not provide guidance in the applica-
and considerations for the development of finite element
tion or interpretation of FEA in determining fatigue life.
models used in the evaluation of the performance of a metallic
1.2.6 This guide is not intended to include complete de-
vascular stent design under uniform radial loading. Suggested
scriptions of the finite element method, nor its theoretical basis
criteria are provided for evaluating the typical cases of metallic
and formulation.
stents under uniform radially oriented and pulsatile loading.
Recommended procedures for checking and validating the
1.3 The values stated in SI units are to be regarded as the
finite element model(s) are provided as a means to assess the
standard. The values given in parentheses are for information
model and analysis results. Finally, the recommended content
only.
ofanengineeringreportcoveringthemechanicalsimulationsis
presented.
2. Terminology
1.2 Limits:
2.1 Symbols:
1.2.1 Thisguideislimitedindiscussiontothefiniteelement
2.1.1 balloon expandable stent, n—a stent that is expanded
structural analysis of metallic stents of the following types:
at the treatment site by a balloon catheter. The purpose of the
1.2.1.1 Plastically deformable metal stents.
balloon is to plastically deform the stent material such that the
1.2.1.2 Self-expanding metal stents.
stent remains expanded after the deflation of the balloon.
1.2.1.3 Plastically deformable metal portions of covered
2.1.2 conceptual model, n—model produced by analyzing
stents.
and observing the physical system of interest composed of
1.2.1.4 Metal portions of self-expanding covered metal
mathematical models and equations representing that system.
stents.
1.2.2 Theemphasisofthetechniquesdescribedinthisguide 2.1.3 computational model, n—implementationofaconcep-
tual model in software.
is intended for both elasto-plastic materials such as stainless
steel, and superelastic materials such as nitinol. Unique con-
2.1.4 crimp, v—to secure the stent on a delivery system by
cerns associated with stents designed for shape memory
radially compressing the stent into a delivery device such as a
behavior are not addressed within this guide.
catheter or onto an expanding delivery device such as a
1.2.3 This guide does not consider changes to possible time
balloon.
varying conditions or different loadings related to vascular
2.1.5 delivery system, n—a mechanical system that is used
remodeling.
to deliver and deploy a stent at a target site.
2.1.6 elasto-plastic material, n—a material behavioral
model that exhibits elastic behavior (recoverable) up to its
This guide is under the jurisdiction of ASTM Committee F04 on Medical and
yield point and plastic behavior (irrecoverable) above its yield
Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.30 on Cardiovascular Standards. point.
Current edition approved March 1, 2014. Published April 2014. Originally
2.1.7 endurance limit, n—stress or strain level at which the
approved in 2008. Last previous edition approved in 2008 as F2514 – 08. DOI:
10.1520/F2514-08R14. material is considered to have “infinite” life.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2514 − 08 (2014)
2.1.8 finite element analysis (FEA), n— a general purpose 2.1.20 stent, n—a tubular structure that is permanently
numerical technique. implanted in the native or grafted vasculature and that is
intended to provide mechanical radial support to enhance
2.1.8.1 Discussion—In this guide, the structural continuum
vessel patency. For the purposes of this guide, a stent is
is discretized into regions known as elements, in which the
metallic and may be covered by a coating, synthetic textile, or
mechanical behavior is defined. Continuity is enforced at the
tissue graft material.
vertices of the elements where node points are defined. The
mechanical behavior of the continuum is then defined accord-
3. Summary of Practice
ing to mathematical expressions of physical laws at the node
points. This results in the definition of a set of simultaneous 3.1 This guide addresses the use of the finite element
equations that are solved for state variables from which such
method for structural analysis of metallic vascular stents under
important quantities as displacements, stresses, and strains can various types of simulated uniform radial loading.The purpose
be derived.
of a structural analysis of the stent is to determine such
quantities as the displacements, stresses, and strains within a
2.1.9 geometrical nonlinearity, n—a type of nonlinearity
device resulting from external loading. This includes stresses
related to structural deformation where the relation between
and strains potentially due, but not limited, to manufacturing
strain and displacement are not linearly proportional.
processes, to delivery in the body, and to pulsatile loading in
2.1.10 linear elastic material, n—a material in which the
vivo.
stress resulting from an applied force is directly proportional to
3.2 Current United States government guidelines (1) rec-
the corresponding strain it produces. Thus, linear elastic
ommend structural analysis of a proposed device under physi-
materials do not retain any stress or strain when all external
ologically appropriate loading. The analysis technique dis-
loads and boundary conditions are removed and all deforma-
cussed in this guide is restricted to the finite element analysis
tions are recoverable.
technique (2-5), although other techniques may be equally
2.1.11 model calibration, n—the process through which the
suitable for the required analysis.
parameters of a computational model are checked or adjusted
3.3 Prior to the finalization of a device design, rigorous
to create a model with the proper measure of accuracy.
experimentaltestingisrecommendedtocomplementtheanaly-
2.1.12 model validation, n—the process of determining the ses performed. During these tests, care should be taken to
degree to which a computational model accurately represents represent the loading and boundary support conditions consis-
the real world behavior it was intended to represent. It is an tent with those used not only in the finite element analysis and
evaluation of the fidelity of the computational model and the experimental tests but also those expected in clinical use.
real world. Experimental tests should be carefully monitored. Any behav-
iorthatwasnotcapturedbythenumericalsimulationshouldbe
2.1.13 model verification, n—the process of assessing that
identified and evaluated for its effect on safety and reliability.
the implementation of the computational model accurately
represents the engineer’s conceptual model and of the solution
4. Significance and Use
tothemodel.Itisanevaluationofthefidelityoftheconceptual
4.1 Finite element analysis is a valuable method for evalu-
model and the computational model.
ating the performance of metallic stents and in quantifying
2.1.14 nonlinear material, n—a material behavior in which
quantities such as internal stresses, internal strains, and defor-
the stress resulting from an applied external load is not directly
mation patterns due to applied external loads and boundary
proportional to the induced strain.
conditions. Many times an analysis is performed to correlate to
and plan experimental tests. A finite element analysis is
2.1.15 permanent deformation, n—residual or irrecoverable
strain and deformation in a structure after all loads and especially valuable in determining quantities that cannot be
readily measured.
boundary conditions are removed.
2.1.16 plasticity, n—material behavior characteristic where
5. Overall Technical Approach
permanent or irrecoverable deformation remains when the
5.1 The application of finite element analysis is intended for
external loading is removed.
the development of a quantifiable level of confidence in the
2.1.17 pulsatile, adj—recurring alternate increase and de-
stent design. The overall approach described in this guide
crease of a quantity such as the pressure that would occur in an
focuses on the development of a systematic technical approach
artery.
to using the finite element analysis technique to evaluate stent
performance. The basic process includes:
2.1.18 self-expanding stent, n—a stent that expands at the
5.1.1 Detailed definition of the geometry of the stent being
treatment site without mechanical assistance. The material
evaluated.
typically used for the stent has the ability to return either
5.1.2 Thedetermination,quantificationandvalidationofthe
partially or fully to a previous size and shape and remain
important mechanical material properties.
expanded after the delivery system is removed.
2.1.19 solution sensitivity, n—a measure of the relative
change in solution results caused by changing one or more
The boldface numbers in parentheses refer to a list of references at the end of
parameters in a computational model. this standard.
F2514 − 08 (2014)
5.1.3 Selection of the appropriate finite element tools and 6.1.2 Preliminary Models—During the preliminary design
programs to ensure effective and reliable representations of the phase, detailed geometric and/or material data may not be
stent being evaluated. warranted and/or readily available. In these cases, it is appro-
priate to use initial design geometries and material data from
5.1.4 Selection and validation of the appropriate finite
standard engineering references. The results of such simula-
element model and type of element(s) used.
tions will be considered preliminary results.
5.1.5 Calibration, validation, and verification of model
6.1.3 Material Property Tests:
input, parameters for the numerical simulation, solution results
and comparison to experimental tests. 6.1.3.1 Mechanical properties of the material should be
determined from rigorous experimental testing of the material
5.1.6 Definition of all important loading steps.
that has undergone all pertinent manufacturing processes
5.1.7 Selection and application of appropriate boundary
including finishing, cleaning, and sterilization, if appropriate.
conditions, such as symmetry.
Themechanicalmaterialpropertiesforafiniteelementanalysis
5.1.8 Effective and proper application of the finite element
are most often determined through tensile testing of the
analysis program for the intended evaluation.
material. During the test, load and displacement data is to be
5.1.9 The generation and interpretation of results to perform
collected to define the entire material curve. All relevant
an effective evaluation.
hysteresis and/or temperature effects on the material response
5.1.10 Documentation of the analysis, including all support-
must be included.
ing citations and references, analysis methodology, and
6.1.3.2 When testing for material properties, extreme care
assumptions, results interpretation, and overall stent design
should be taken to ensure accurate measurements using suit-
evaluation.
able fixturing and appropriately calibrated devices for measur-
ing both load and displacement.
6. Input Data
6.1.3.3 If warranted by the material, the material curve(s)
6.1 Finite element analysis is a numerical technique use for should be measured at the appropriate temperature(s) of the
simulating the mechanical response of structures. A finite
intended use. The effects of temperature on the material
element structural analysis requires input to numerically rep- response are extremely critical for superelastic alloys. Differ-
resent geometric and material information, as well as mechani- ences in the material behavior in tension and compression
cal support and loading conditions. Two important parts of any should also be considered along with any load history depen-
finite element analysis is the proper representation of material dent tension/compression asymmetry phenomena or work
properties and the definition of load cases and boundary hardening of the material.
conditions. These must reflect the entire process and perfor-
6.1.4 Material Property Validation:
mance history and environment of the device. The load history
6.1.4.1 The material mechanical property values must be
should include all relevant manufacturing loads and all steps of
converted into a format and form consistent with the finite
the intended clinical end use of the device. If all steps are not
element representation.
included, the reason for the omission should be described.
6.1.4.2 Validation tests should be performed to validate the
6.1.1 Geometric Data:
material model used in the analysis. The effects of the test
6.1.1.1 Finite element models are based on a geometric specimen size or shape (tube, wire, sheet) must be considered
representation of the device being studied. The source of the
in applying the material model to the validation model.
details of the geometry can be drawings, computer aided
6.1.4.3 Amaterial validation test could include the determi-
design (CAD) and solid models, preliminary sketches, or any
nation of the load-displacement behavior of a finite element
other source consistent with defining the device model geom-
model of a simple tensile test. For example, a model is first
etry.
created of a simple geometric specimen of material using the
6.1.1.2 Finite element modeling is used extensively in the element type for which the validation is being performed. The
design phase of product development, many times before any geometry and number of elements in the validation model
prototyping has occurred. As such, models are often based on should be sufficient to enable the definition of proper loading
preliminary designs from CAD drawings. Changes associated
and constraints, yet simple enough to isolate the key load-
with the progre
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