Fatigue to Failure: FEA Predictions of Limit Conditions for Axial Fatigue Loading of Generic Coronary Stent Designs Prepared for ASTM F October 26, 2006 Rockville, MD

Objective Determine axial fatigue loading conditions to generate fracture in 1M cycles Evaluate the effect of residual stress and cold work due to balloon expansion process –Applied stress affected by underlying residual stress –Limit conditions affected by presence of cold work

Methodology / Assumptions Use FEA to determine axial extension corresponding to allowable alternating stress for four cases: –Expanded stent, with residual stress and cold work (typical protocol for balloon expanded stent) –Expanded stent, annealed, cold work considered –Expanded stent, annealed, cold work not considered –Unexpanded stent (no residual stress or cold work) Limiting alternating stress –Based on endurance limit (S e = 0.5S ult ), assumes 1M cycles –Extrapolated from typical S/N curve for 100k cycles

Fatigue Scenario Studies Summary CaseConfigurationStress StateStrain State 1ExpandedResidual Stress Cold Work 2Expanded (annealed) No Residual Stress Cold Work 3Expanded (annealed) No Residual Stress No Cold Work 4Not Expanded (As Cut) No Residual Stress No Cold Work Part_2_058OD_5L

Stent Expansion: Residual Stresses, Cold Work Plastic Strain due to Cold Work Stresses during Expansion and Recoil

Modified Goodman Approach to Endurance Limit Case 2 Mean Stress Stress Amplitude S e (cold work) S ult (cold work) S ult, S e Increase with Increasing Cold Work Mean Stress effected by Residual Stress State Cases 3,4 Case 1

Effect of Residual Stress CaseExtensionMax StressComments Residual Stress +0.1%290MPa -Large tensile and compressive stresses in the middle of struts and bridges -Yield stress increased due to load history No Residual Stress +0.1%200MPa -Uniform load path with unaltered yield stress Effective stress during extension with residual expansion stress Effective stress during extension without residual expansion stress

Handbook of Materials for Medical Devices, Editor J.R. Davis, MPa 17% 520MPa 0% Effect of Cold Work on the Tensile Strength % Cold Work Tensile Strength Endurance Limit 1M+ cycles 0520 MPa260 MPa MPa405 MPa Effective Plastic Strain S e ~ 0.5 S ult Typically Accepted Value for the Endurance (Shigley), assumes 1M cycles

Case 1 – Expansion, Residual Stress and Cold Work

Case 3 – Expanded, No Residual Stress, No Cold Work

Fatigue Scenario Studies Case Stress State Strain State Endurance Limit S e ~0.5 S ult Fatigue Conditions 1M+ cycles Failure Location (Bridges) Expanded Residual Stress ~120MPa Cold Work ~7% Point by Point ~320MPa ±0.21% Expanded (annealed) No Residual Stress Cold Work ~7% Point by Point ~320MPa ±0.19% Expanded (annealed) No Residual Stress No Cold Work260MPa±0.14% Not Expanded (As Cut) No Residual Stress No Cold Work260MPa±0.10%

Fatigue to Failure at 100k cycles Estimation of Fatigue Strain-Life Curves for Austenitic SS in Light Water Reactor Environments, Chopra and Smith, 1998 Case Fatigue Conditions 1M+ cycles Fatigue Conditions 100k cycles Expanded±0.21%±0.30% Expanded, annealed, cold work considered ±0.19%±0.27% Expanded, annealed, no cold work considered ±0.14%±0.21% Not Expanded (As Cut) ±0.10%±0.14%

Observations Effect of residual stress is small –0.21% versus 0.19% predicted extension amplitude Effect of cold work more significant –0.21% versus 0.14% predicted extension amplitude Sensitivity regarding 100k versus 1M cycles –0.21% versus 0.30% predicted extension amplitude Asymmetry due to bridge configuration

Recommendations Follow typical balloon expanded stent protocol: assume residual stress and cold work –Perform testing with ±0.21% predicted extension –Should produce failure at 1M cycles Consider also testing at higher levels: –For example: ±0.25 or ±0.30% –Should produce failure closer to 100k cycles Grip it and rip it!