Page 52 - Advances in Biomechanics and Tissue Regeneration

46 3. DESIGN, SIMULATION, AND EXPERIMENTATION OF COLONIC STENTS

FIG. 3.14 Strain-stress curves of the used Nitinol alloy [52]. Experimental curve obtained from mechanical test at 22°C, adjusted curve in Abaqus
6.11 using the parameters in Table 3.2, and extrapolated curve at body temperature (37°C).

TABLE 3.2 NiTi Material Parameters Defining the Constitutive Stress-Strain Curve of the Alloy Used in the
Present Study
Parameter Description Value

E A Austenite elasticity module 52,650MPa
Austenite Poisson’s ratio 0.33
ν A
Martensite elasticity module 38,250MPa
E M
Martensite Poisson’s ratio 0.33
ν M
Maximum transformation strain 6%
ε L
AM
σ s Start of transformation austenite martensite stress 385MPa
AM
σ f End of transformation austenite martensite stress 460MPa
MA
σ s Start of transformation martensite austenite stress 200MPa
MA
σ f End of transformation martensite austenite stress 180MPa
T 0 Reference temperature 22°C
C AM ∂σ AM 6.1MPa/°C
S
∂T
C MA ∂σ MA 6.1MPa/°C
S
∂T
Volumetric transformation strain 0.06
ε L_Vol
The loading and unloading rate of change of plateau with the temperature C AM and C MA were obtained from the literature [51].

3.3.2 Stent Geometry
Considering habitual diameters for colonic stenting of the order of 30mm, resistance to radial compression and
buckling are determining factors, and stents that show better behavior in this sense are those of rigid knots. Therefore,
initially, a configuration of Z-shaped radial spring with rigid knots may be suitable for the development of a new
design.

I. BIOMECHANICS