Outstanding Strengthening and Toughening Behavior of 3D-Printed Fiber-Reinforced Composites Designed by Biomimetic Interfacial Heterogeneity

Abstract

3D printing of fiber-reinforced composites is expected to be the forefront technology for the next-generation high-strength, high-toughness, and lightweight structural materials. The intrinsic architecture of 3D-printed composites closely represents biomimetic micro/macrofibril-like hierarchical structure composed of intermediate filament assembly among the micron-sized reinforcing fibers, and thus contributes to a novel mechanism to simultaneously improve mechanical properties and structural features. Notably, it is found that an interfacial heterogeneity between numerous inner interfaces in the hierarchical structure enables an exceptional increase in the toughness of composites. The strong interfacial adhesion between the fibers and matrix, with accompanying the inherently weak interfacial adhesion between intermediate filaments and the resultant interfacial voids, provide a close representation of the toughness behavior of natural architectures relying on the localized heterogeneity. Given the critical embedment length of fiber reinforcement, extraordinary improvement has been attained not only in the strength but also in toughness taking advantage of the synergy effect from the aforementioned nature-inspired features. Indeed, the addition of a small amount of short fiber to the brittle bio-filaments results in a noticeable increase of more than 200% in the tensile strength and modulus with further elongation increment. This article highlights the inherent structural hierarchy of 3D-printed composites and the relevant sophisticated mechanism for anomalous mechanical reinforcement.

Keywords: 3D printing; composites; fiber alignment; hierarchical structures; interfacial heterogeneity.

Figure 1

Research strategies for simultaneous improvements of strength, stiffness, and toughness from distinctive architecture by analogy with hierarchically structured nature materials. a) Schematic illustration of 3D‐printed SFRP composites showing fibrous assemblies hierarchically built with individual intermediate filaments, with embedded micron‐sized fibers. b) Detailed illustration of the natural micro/macrofibril hierarchical structure of biological materials. c) Scanning electron microscopy images of the sequential additive architecture of 3D‐printed composites showing structural hierarchy: i) embedded micron fibers, ii) the intermediate filament matrix, and iii) the hierarchical structure. d) Process‐induced spontaneous fiber alignment. 3D printing leads to a high degree of material orientation in the printing direction, especially the alignment of embedded fibers with a high degree of orientation reaching a _zz = 0.9 (Z direction orientation value). e) Contribution of a heterogeneous interfacial adhesion system to biomimetic toughness behaviors. f) The interfacial adhesion of composites is highly dependent on the critical embedment fiber length; the fiber pull‐out mechanism, which occurs primarily for l < l _c, and the fiber breakage mechanism, which occurs primarily for l > l _c. Stress‐position profiles show the representative curves when the fiber is ① greater than l _c and ② less than l _c. σ _f is the ultimate tensile strength of the fiber. g) Strong interfacial adhesion between the fiber and the matrix was achieved with effective interfacial treatments. h) Weak interfacial adhesion between inter‐filaments spontaneously derived from low inter‐diffusion of polymer chains at the interface.

Figure 2

Material designs for strong interfacial adhesion between the fiber and the matrix. a) Formation of O₂/Ar plasma induced‐polar groups at the fiber surface. The inset describes the change in chemical compositions on the fiber surface before and after plasma treatment. b) Formation of hydrophilic grafting functional molecules in the polymer matrix. The inset shows the mechanism of ST‐assisted grafting polymerization of MAH to PLA. (See Figure S1, Supporting Information, for detailed mechanism.) c) Complex viscosity (η*, Pa*s) of the molten composites with respect to the frequency. d) Storage modulus (G′) and loss modulus (G″) curves with respect to the frequency. e) Representative 3D tomographic images of hierarchical SFRP composites inherently induced by the 3D printing process. f, g) Resultant tomographic images comparing the effects of rheological modification on the internal microstructure of the composites according to the contents of 10, 20, 30, and 40 wt% of f) PLA‐g‐MAH and g) PLA‐g‐STMAH. (The formation of a high number of isolated inner voids is attributed to poor processing conditions and is accompanied by a significant change in the viscosity.)

Figure 3

Effective fiber architecture designs for simultaneous improvements of strength, stiffness, toughness in composites. a) Segmentation images of individual fibers to quantify the fiber length, position, and orientation. b) Localized fiber orientation with representative directional arrows in each region block. c) Fiber orientation mapping image in the spherical coordinate system. d) Color‐coded tomography images to verify the fiber length distribution with respect to l _c; red‐color fibers (l > $l_{\mathrm{c}}^{40}$), yellow‐color fibers ($l_{\mathrm{c}}^0$> l > $l_{\mathrm{c}}^{40}$), and green‐color fibers (l < $l_{\mathrm{c}}^0$). e) IFSS (τ) between fiber and matrix was evaluated with push‐in tests using nanoindentation. Load‐displacement curves compose of four segments: the initial load segment (A) that represents incomplete contact between the indenter tip and the fibers, the linear region (B) where the fibers and matrix deform elastically under a load, the nonlinear region (C) where the slope starts to decrease following the onset of interfacial failure under load, and (D) the unloading curve. f) Correlation between l _c and τ. Stress‐position profiles show correlation between l _c and τ. g) Representative fiber length distribution (l). $l_{\mathrm{c}}^{10}$–$l_{\mathrm{c}}^{40}$ denote each boundary line axis of l _c in composites with PLA‐g‐STMAH of 10–40 wt%.

Figure 4

In situ tomographic observation on 3D‐printed SFRP composites under tensile loading condition. a–c) Representative 3D tomographic images showing final failures of 3D‐printed hierarchical BF/PLA/PLA‐g‐STMAH composites. Various deformation and fracture modes based on fiber length and interfacial conditions; a) ① l < l _c, l ≈ 50 µm, IFSS ≈ 30 MPa; b) ② l > l _c, l ≈ 700 µm, IFSS ≈ 30 MPa; and c) ③ l > l _c, l ≈ 700 µm, IFSS = 70 MPa. Color‐coded tomography images showing fiber length distribution inside. Given as red‐color fibers (l > 600 µm), yellow‐color fibers (300 µm > l > 600 µm), and green‐color fibers (l < 300 µm). d–f) Detailed illustration of the progressive failure process at each tensile loading sequence for 3D‐printed hierarchical BF/PLA/PLA‐g‐STMAH composites. Progression of the deformation and fracture modes of specimens based on fiber length and interfacial conditions; d) ① l < l _c, l ≈ 50 µm, IFSS ≈ 30 MPa; e) ② l > l _c, l ≈ 700 µm, IFSS ≈ 30 MPa; and f) ③ l > l _c, l ≈ 700 µm, IFSS = 70 MPa. 2D extended images A–F correspond to the marked regions for A–F in (d–f), respectively. g) Representative stress–strain curves of 3D‐printed hierarchical SFRP composites showing each tensile loading sequence at which a fractographic image was acquired with X‐ray microscopy. Circle points represent the strains corresponding to the fractographic images in (d–f), respectively.

Figure 5

Detailed illustration of representative failure mechanism of 3D‐printed hierarchical BF/PLA/PLA‐g‐STMAH composites. a) 3D tomographic images showing progressive failure process at each tensile loading sequence; (0) initial state, (i) first load sequence, (ii) second load sequence, (iii) third load sequence, and (iv) final failure. b) Comprehensive description of deformation and fracture progression.

Figure 6

Comparative results of tensile strength and its stress–strain curve of 3D‐printed composites according to the increment of interfacial strength. a) Representative stress–strain curve of 3D‐printed single matrix BF/PLA/PLA‐g‐MAH composites showing typical mechanical behavior of engineering materials that exhibit stiff but brittle nature. b) Resultant failure mechanism transition from fiber pull‐out to fiber bridging to achieve stiffness reinforcement as interfacial strength increases. c) Representative stress–strain curve of 3D‐printed hierarchical BF/PLA/PLA‐g‐STMAH composites showing a simultaneous improvement in strength, stiffness, and toughness. d) Resultant failure mechanism showing multiple necking behavior to achieve toughness enhancement as interfacial strength increases. e–g) Comparative results of the specific tensile strength ((σ/ρ)/(σ ₀/ρ ₀)), specific tensile modulus ((E/ρ)/(E ₀/ρ ₀)), and maximum strain at break (ɛ/ɛ ₀) of the two composite systems.