Fig 1.
A: stent CAD model, B: flat projection of the stent CAD model and C: basic dimension of the intended stent illustrated within the magnified stent unit cell. The later built direction (BD) within the laser powder bed fusion process is indicated by the blue arrow.
Table 1.
Overview of the experimental framework.
Fig 2.
Experimental set up of the stent compression tests with parallel plates.
A: Initial positioning of the stent between the two plates. B: Stent after compression between the two plates.
Fig 3.
Stent configurations investigated in this study.
(from left to right): idealized CAD model, stentAB in the as-built (AB) condition, stentHT in the heat treated (HT) condition, and stentEP-HT in the electropolished and heat treated (EP-HT) condition. The laser powder bed fused stents are reconstructed based on X-ray computed tomography (CT) data. Pores within the struts are illustrated in magenta (bottom row). The build direction (BD) is indicated by an arrow.
Fig 4.
Illustration of the discretization of the stent models.
A: Magnified view of mesh for the idealized CAD model stentCAD. B: Magnified view of mesh for stentAB in the as-built condition (also representing stentHT due to similar morphology). C: Magnified view of mesh for stentEP-HT in the electropolished and heat treated condition.
Table 2.
Investigated stent models and their discretization.
Fig 5.
Flow curves derived from the results of the uniaxial tensile test for laser powder bed fused flat specimens.
The tensile specimens have a thickness of 1 mm. The flow curve representing the as-built condition is indicated by the black line and the flow curve representing the heat treated condition by the red line. The experimental data points are marked by spheres.
Fig 6.
Illustration of the set-up of the stent compression simulations.
Table 3.
Overview of the models used in the stent simulations.
Fig 7.
Deviation in strut shape between the as-designed and actual laser powder bed fused (L-PBF) stents.
StentCAD correspond to the morphology of the as-designed stent CAD model. StentAB, stentHT, and stentEP-HT correspond to L-PBF stents in the as-built (AB), heat treated (HT), and electropolished and heat treated (EP-HT) conditions, respectively.
Fig 8.
Scanning electron microscopy images highlighting the geometric irregularities related to the laser powder bed fusion process.
StentAB, stentHT and stentEP-HT correspond to laser powder bed fused stents in the as-built (AB), heat treated (HT), and electropolished and heat treated (EP-HT) conditions, respectively.
Table 4.
Analysis of the basic morphological parameters of the laser powder bed fused stents.
Fig 9.
Experimental determined mechanical response of laser powder bed fused stents under compression.
The shaded curve areas show the experimental results of the compression tests of the respective stent configuration (as-built stents (AB, gray area), heat treated stents (HT, light red area), electropolished and heat treated stents (EP-HT, light blue area). To indirectly consider the impact of the geometrically induced stiffness, the mean mass of the respective stent configuration is provided in the legend.
Table 5.
Experimentally determined radial force at 50% compression Frad.50% and the corresponding mass m of the respective stent configuration.
Fig 10.
Metallographic sections of a laser powder bed fused (L-PBF) stents.
A: Metallographic section of a L-PBF stent in the as-built condition highlighting the observed substructure. B: Metallographic section of a L-PBF stent in the heat treated condition.
Fig 11.
Mechanical response of laser powder bed fused (L-PBF) stents.
A: Modification of the initial flow curves which were derived from flat tensile test to allow for the description of the L-PBF 316L stent material in the as-built and heat treated condition. B: Comparison of the experimental determined global mechanical response of the stents under compression (shaded curve areas) with the numerical predicted response of the respective reconstructed stent model configuration (solid lines) using the modified flow curves (L-PBF 316L ABmod., HTmod.). StentAB, stentHT and stentEP-HT correspond to the reconstructed stents in the as-built (AB), heat treated (HT), and electropolished and HT (EP-HT) conditions and, respectively. StentCAD,AB and stentCAD,HT correspond to the as-designed stent CAD models with the as-built and heat treated material properties, respectively.
Fig 12.
Comparison of the determined flow curve of the laser powder bed fused (L-PBF) 316L stent material with the flow curves and corresponding hardening parameters obtained from experimental data of L-PBF 316L micro-specimens from the literature [53–55].
Fig 13.
Qualitative validation of the uniaxial stent compression simulations.
A: Comparison of the numerically predicted deformed stent shape (gray) with the X-ray CT reconstructions of the corresponding experimentally compressed laser powder bed fused (L-PBF) stent (blue). B: Comparison of scanning electron microscopy images with the equivalent plastic strain in areas of large deformations for stentEP-HT, near a surface crack (top row) and near strut nodes (bottom row). StentAB, stentHT and stentEP-HT correspond to the reconstructed L-PBF in the as-built (AB), heat treated (HT), and electropolished and HT (EP-HT) conditions, respectively.
Fig 14.
Contour plots of the von Mises stress σvM on the outer surface of the laser powder bed fused (L-PBF) stents under compression of 0.8 mm.
A: Stents in the as-built and B: in the heat treated material condition illustrated in the front (top row) and the side view (bottom row). StentAB, stentHT and stentEP-HT correspond to the reconstructed L-PBF stents in the as-built (AB), heat treated (HT), and electropolished and HT (EP-HT) conditions, respectively. StentCAD,AB and stentCAD,HT correspond to the as-designed stent CAD models with the as-built and heat treated material properties, respectively.
Fig 15.
Predicted global deformation behavior of laser powder bed fused (L-PBF) stents during expansion.
A: Predicted expansion behavior of L-PBF stent in the as-built material condition. B: Predicted expansion behavior of L-PBF stent in the heat treated material conditions. StentAB, stentHT and stentEP-HT correspond to the reconstructed L-PBF stents in the as-built (AB), heat treated (HT), and electropolished and HT (EP-HT) conditions, respectively.
Fig 16.
Influence of reduced strut diameters/notches and pores (section A-A, B-B) for the electropolished and heat treated stent (stentEP-HT) in the area of large plastic deformation using the contour plot of equivalent plastic strain distribution.