Mechanical anisotropy of 3D-printed digital materials at large strains

Abstract

3D-printed digital materials whose mechanical behavior travels between those from thermoplastic to rubbery polymers have become increasingly important. However, their mechanical functionalities have not been fully exploited due to intrinsic mechanical anisotropy resulting from microstructural heterogeneity. Here, we combine mechanical testing, microscopy analysis and micromechanical modeling for a comprehensive understanding of complex deformation mechanisms responsible for the printing-orientation-dependent nonlinear mechanical behavior of digital materials at small to large strains. Towards this end, we construct representative volume elements that account for highly anisotropic microstructural features resulting from the printing-orientation-dependent diffusion and mixing between photocurable base resins. We then demonstrate, through micromechanical analysis, that stable compressive deformation of well-aligned elliptical hard thermoplastic inclusions embedded within the surrounding soft rubbery matrix gives rise to initial elastic anisotropy. Our experimental and micromechanical modeling results also show that the interplay between buckling instability and plastic deformation of the high-aspect-ratio hard domains governs mechanical anisotropy at large strains as well as the printing-orientation-dependent resilience and energy dissipation capabilities in these digital materials.

Figures (11)

• Figure 1: Printing-orientation-dependent mechanical behavior of 3D-printed materials. Stress-strain curves of (a1) and (a2) base materials (VeroBlackPlus and TangoPlus) and (a3) and (a4) digital materials (DM1 and DM2) printed in the x-, y- and z-directions during loading and unloading at a strain rate of 0.01 $\mathrm{s}^{-1}$ (insets: printing-orientation-dependent stress-strain curves of the DM1 and DM2 materials at small strains). (b1) Ratios of elastic stiffnesses and (b2) ratios of energy dissipations and residual strains in the digital materials printed in the z- and x-directions.
• Figure 2: Microstructures of representative 3D-printed digital materials. Optical microscopy images of zx-plane cross-sections and the corresponding magnified views for (a1) and (a2) DM2 and (b1) and (b2) DM1 materials. (a3) and (b3) show the optical microscopy images of xy-plane cross-sections for the DM2 and DM1 materials, respectively.
• Figure 3: Construction of representative volume elements (RVEs) for 3D-printed digital materials. (a1) Schematic of the deposition pattern of the photocurable base resins (VeroBlackPlus and TangoPlus) on the zx-plane together with (a2) the corresponding RVE comprising well-aligned, high-aspect-ratio elliptical hard domains embedded within the surrounding soft matrix. (b1) Schematic illustration of cylindrical specimens comprising $n$ alternating VeroBlackPlus/TangoPlus layers along the x-direction with a layer thickness $t$; (b2) as the number of VeroBlackPlus/TangoPlus layers ($n$) increases (or the layer thickness $t$ decreases), a macroscopic response close to that of the homogeneous blend can be obtained due to diffusion and mixing between the photocurable base resins. (c1) Stress-strain curves at a strain rate of 0.01 $\mathrm{s}^{-1}$ for the cylindrical specimens with varying layer thicknesses ($t$ = 40, 80, 120, 240 and 360 $\upmu$m). (c2) Comparison of the macroscopic stress-strain responses measured experimentally (solid line) and predicted numerically (dashed line) for cylindrical specimens with $t$ = 40 $\upmu$m. (d) A broad, blurred interfacial region across VeroBlackPlus and TangoPlus layers along the x-direction.
• Figure 4: Experiments vs. micromechanical modeling results for the DM1 materials. (a) Representative volume elements for the DM1 materials ($\mathrm{v}_{\mathrm{hard}}$$\sim$ 72%) comprising elliptical hard domains with $k$ = 9.3, 6.7, 5.0 and 3.9. (b) Stress-strain curves (experiments) of the DM1 materials subjected to plane-strain compression along the global z- and x-axes at a strain rate of 0.01 $\mathrm{s}^{-1}$. Orientation-dependent mechanical behavior of RVEs (numerical simulations) with (c1) $k$ = 9.3 and (c2) $k$ = 6.7, 5.0 and 3.9. (d) Stress ratios, defined as the stress along the z-direction relative to that along the x-direction in experiments (open symbols) and numerical simulations (solid lines). Contours of the plastic shear strain rate $\dot{\gamma}_{p}$ in the deformed configurations of hard domains within RVEs with (e1) $k$ = 9.3 and (e2) $k$ = 3.9 loaded in the z- (upper insets) and x-directions (lower insets) at a macroscopic strain level of 0.15; here, only elliptical hard domains are displayed.
• Figure 5: Experiments vs. micromechanical modeling results for the DM2 materials. (a) Representative volume elements for the DM2 materials ($\mathrm{v}_{\mathrm{hard}}$$\sim$ 40%) comprising elliptical hard domains with $k$ = 8.0, 5.5, 4.1 and 3.2. (b) Stress-strain curves (experiments) of the DM2 materials subjected to plane-strain compression along the global z- and x-axes at a strain rate of 0.01 $\mathrm{s}^{-1}$. Orientation-dependent mechanical behavior of RVEs (numerical simulations) with (c1) $k$ = 8.0 and (c2) $k$ = 5.5, 4.1 and 3.2. (d) Stress ratios, defined as the stress along the z-direction relative to that along the x-direction in experiments (open symbols) and numerical simulations (solid lines). Contours of the plastic shear strain rate $\dot{\gamma}_{p}$ in the deformed configurations of hard domains within RVEs with (e1) $k$ = 8.0 and (e2) $k$ = 3.2 loaded in the z- (upper insets) and x-directions (lower insets) at a macroscopic strain level of 0.15; here, only elliptical hard domains are displayed.