Simulation vs. Testing: Getting the Best of Both Worlds

Simulation vs. Testing: Getting the Best of Both Worlds

T. Kim Parnell, Ph.D., P.E.Janet Neo

PEC – Parnell Engineering & Consultingwww.parnell-eng.com

IntroductionSimulation vs. TestingWhat are the issues?

Benefits of Synergizing Simulation and TestingIllustrations & Case StudiesConclusionsQuestions??

Outline of Presentation

Introduction

• Finite element analysis (FEA) and physical testing are complementary

• A comprehensive program needs to include both components

• With judicious experimental validation, FEA can be used to reduce the amount of physical testing that is needed and shorten the design cycle

The Challenge for Medical Device Development

• Reduce development time• Increase confidence of success• Avoid surprises and delays

Prototype Development

1. Physical prototypeCost and lead time is often a limitationEssential for animal testing and determining needed characteristicsWant to reduce the number of design iterations that are prototyped

2. Virtual prototypeAssess more design options Compare alternatives

Testing Is Essential for:

• Detailed characterization of the material; getting data needed for the analysis

• Fatigue testing taking into account surface finish, processing steps

• Validation

Sensitivity by Analysis

• Material• Tolerance• Variability of the body/target environment• Atypical applications

Validation of Model by Test

• Analysis of tensile test to confirm ability to predict material behavior

• Validation tests for stents might include:- Flat plate loading- Radial expansion- Radial compression

Example:Flat Plate Loading Using Contact

Note: This “pinching” loading mode is distinct from “radial” loading

Are the Assumptions Satisfied?

Make adjustments/corrections as needed so that the model is predictive of the test

Additional Information and Insight From Analysis

Get information not available from device testing aloneInternal conditions: stress levels, degree of plasticity, residual stress, transformation fraction

Balloon Expandable Stent

1. Basic steps:Roll-down for catheter insertionInflation and DeploymentCyclic pulsation loading

2. Fatigue testing of full device to FDA required 400M cycles is a long process

Fatigue and Life Testing

• Long test times for full device• Reduce testing of multiple design iterations• Get insight more quickly• Need both analysis and testing

Cyclic Testing of Sub-specimen1. Before fatigue testing full device, get more

information in less time with sub-specimenHigher loading frequency, reduced test timeCycle to failure for a range of loadsDevelop part-specific S/N data

2. Extend with analysis, develop and interpret test conditions in terms of stress & strain

3. Make predictions for full device

Stent Segment

Sub-specimen

Stent Segment and Sub-specimen

Parnell, (2000)

Stent Segment Sub-specimen

Material Testing: Elastic/Plastic

• Need more detail than basic data from manufacturer (for example, Min. Yield, Ultimate, Elongation)

• Elongation is sensitive to the gage length tested• Reduction of area very useful, particularly for highly

ductile materials• Need full stress/strain curve with additional data like

reduction of area

Tensile Response of Elastic/Plastic MaterialE’

E

D

CB

A

0 ε

σ

Proportional limit

Yield stress

Ultimate stress

Linear Plastic Strain hardening

Significant necking

True Stress

Eng. Stress

Anderson (2002), Biomaterials

Typical stress/strain curve for steels. Strains become localized when necking occurs. Standard elongation highly dependent on gage length. Measured area reduction gives correct local strain.

Shape Memory Material (SMA) Applications

• Unique characteristics• Large recoverable strain range• Super elastic vs. Shape Memory (thermally

activated)• Self-expanding devices• Conditions after partial unloading• Load predictions

Applications for Shape Memory Alloys Materials that return to some shape upon appropriate temperature change

Applications:

Medical

IndustrialApplications

Home Appliance

Accessories clothing

Sports

Communi-cation

ShapeMemory

Shape Memory Material Properties

• DSC to determine transformation temperatures• Tensile test• Behavior as function of temperature • Super elastic material behavior

- General features (T > Af )- Stress-induced martensite and reverse

• Shape memory (reverting to learned shape)

NiTi Response to Temperature

T< Ms Shape Memory(residual strain recovered by heating)

Ms <T< Af Shape Memory(residual strain recovered by heating)

Af <T<Tc Superelastic (SIM)(full strain recovery)

T>Tc Plasticity before SIM(permanent residual strain)

As [K] Af [K] Ms [K] Mf [K]

188 221 190 128

Transformation Temperatures

Miyazaki, et.al., (1981)

Variation of SMA Structures

Pseudo-elastic behavior of SMATemperature induced phase transformation

Pseudo-elastic Stress-Strain Behavior

Material Testing: Shape Memory Alloy

• Transformation temperatures (DSC or other)• Stress/strain tensile curve with unloading• Application may require tensile data at additional

temperatures

Temperature Dependent Material Behavior of Shape Memory Alloys

NiTi Stent

Nickel-Titanium alloys show temperature dependent material behavior. Shape memory effect (that deformed specimens, regained their original shape after a loading cycle) is observed at a certain temperature.

Input:ASSσ

ASfσ

SASσ

SAfσ

−σ ,ASs

Lε To Cm Cs

ASfσ

SAfσ

−σ ,ASs

ASSσ

SASσ

Lε

To

Cm

Cs

ASSσ

ASfσ

SASσ

SAfσ

Input data for Mechanical SMA

Differential Scanning Calorimetry (DSC)

Shaw & Kyriakides, (1995), (courtesy of M.-H. Wu )

• DSC can be used to determine transformation temperatures of shape memory materials

• Heating curve: As,Af

• Cooling curve: Ms,Mf

• Austenite is Cubic (BCC)• Martensite is Monoclinic

Shape Memory Effect (SME)• Shape memory effect is a consequence of a crystallographically

reversible solid-solid phase transformation occurring in particular metal alloys (Ni – Ti, Cu based alloys).

• This transition occurs between a crystallographically more-ordered phase (called austenite) and a crystallographically less-ordered phase (martensite).

Stability for Martensite and Austenite Phases

Vulnerable Plaque

• Morphology• Tissue characteristics• Tissue properties and geometry become

important in evaluating device

Christensen, (2002)

Inverse Analysis Problem

• Correlate material properties to measured behavior

• Use to estimate ranges of properties for tissue• Example: estimation of vessel wall cyclic strains

from cine PC-MRI data (Draney, et.al., 2002)

Conclusions1. Testing and analysis are complementary;

both are essential2. Use together for maximum benefit

Reduce number of physical prototypesShorten development cycleAvoid surprises and delays

3. Applicable in all fields: ElectricalMechanicalBiomedical