Lecture 5

Question 1

Stent Mechanical Analysis

Answer 1

• stents develop plastic strains from 20-50% during deployment
• analyzing stent geometries requires finite element method (FDA requirement in addition to physical testing)

Question 2

Principle of stent design

Answer 2

• creation of plastic hinge at the junction of interconnected struts
• radial force during deployment increases the angle between adjacent struts, increasing overall device diameter
• minimal elastic recoil
• region connecting both struts must undergo extensive plastic deformation to maintain the stent diameter

Question 3

Uniaxial plastic deformation test characteristics

Answer 3

• necking
• strain hardening
• failure

Question 4

Engineering quantities to true quantities

Answer 4

• stress = F/A –>
  = S(1 + e)
• TRUE stress from current area
• strain = ln (L/L0) –>
  = ln(1 + e)
  = TRUE strain

Question 5

Abaqus requirements

Answer 5

• for plastic material, Abaqus requires Young’s Modulus (E) and Poisson’s Ratio (v)
• true stress vs plastic strain curve

Question 6

Slip dislocations

Answer 6

• plastic deformation results from gliding of dislocations (atomic bonds breaking and reforming)
• driven by shear stress, not dependent on volumetric stress/pressure

Question 7

Mohr’s Circle

Answer 7

• expressing the stress state of a material point without shear stress can be done by rotating reference axes
• remaining normal/direct components are the principal stresses (sigma1, sigma2)

Question 8

Von Mises yield theory

Answer 8

• plastic deformation in metals is volume preserving, occurs due to gliding of dislocations
• form a deviatoric stress tensor (sigma with hat) by subtracting volumetric stress components from total stress
• Von Mises stress (sigma v) is a variant of this tensor
• review equations!!!

Question 9

Unit cell analysis of stent

Answer 9

• stents are comprised of repeating/periodic units
• only one unit cell needs to be modeled if correct boundary conditions are applied at the symmetry boundaries
• use 2D plane strain model to reduce computational time (then convert diameter change to circumferential displacements)