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This study examines the inter-layer and cross-layer fracture behavior of 3D printed short CFRP composites using both experimental and numerical methods. First, the orthotropic mechanical response of the 3D printed CFRP samples was characterized by tensile tests. The out-of-plane Young's modulus and yield strength were measured to be about 25% and 32% of the in-plane counterparts, respectively. Second, compact tension fracture tests were performed to characterize the fracture behavior of the CFRP composite samples. The crack resistance curves of inter-layer and cross-layer fracture were independently derived using the elastic-plastic fracture theory. Both cross-layer and inter-layer fracture exhibited stable crack growth with the rising crack resistance curves. Finite element simulations incorporating a cohesive zone method was used to numerically determine the critical cohesive strength and fracture toughness from the experimental results. The experimentally measured and numerically calibrated cross-layer fracture toughness was found to be three-fold of the inter-layer counterparts. In addition, fracture surface morphology and the crack-tip strain field evaluated by the digital image correlation technique revealed three important observations: (1) the cross-layer fracture resulted in a larger fracture process zone ahead of the crack tip compared to the inter-layer fracture, (2) the cross-layer fracture exhibited a fibril-bridging toughening mechanism along with fiber pull-out failure at the micro-scale, and (3) the inter-layer fracture showed a uncracked-ligament toughening mechanism arising from the locally enhanced inter-layer adhesion due to short fiber-bridging. The fracture mechanisms reported in this study provide practical strategies for designing and 3D printing CFRP composites with superior fracture-resistance. [Display omitted] [ABSTRACT FROM AUTHOR]

This study investigates the interlaminar shear behavior of 3D printed CFRP composites fabricated by fused deposition modeling (FDM) through a combined experimental and numerical study. Short beam shear (SBS) tests were performed to quantitatively characterize interlaminar shear strength (ILSS) of 3D printed continuous and short CFRP composites. Numerical simulations incorporating the extended finite element method (XFEM) and cohesive zone model (CZM) were conducted to rationalize the experiments. Two different combinations of continuous and short carbon fiber reinforcements were studied. The first composite layup made of 48 layers of continuous CFRP filaments was used to determine the intrinsic ILSS of the fully continuous CFRP composites. The second composite layup composed of 24 layers of continuous CFRP filaments and 24 layers of short CFRP filaments was used to determine the ILSS of the continuous and short CFRP composites. The average ILSS of these two composite layups were measured to be about 40.9 MPa and 24.4 MPa, respectively. Four distinct stacking sequences were implemented to examine the effect of stacking sequence of the continuous and short CFRP layers on the ILSS. The ILSS exhibited a significant sensitivity to the stacking sequence. The ILSS tends to increase with the number of the consecutive short CFRP layers. A failure mode transition from a brittle matrix cracking of short CFRP layers to cracking in the interface between the continuous and short CFRP layers was observed. The failure mode transition is attributed to the enhanced fracture strength and toughness of the short CFRP layers due to the increase in layer thickness. [ABSTRACT FROM AUTHOR]

[Display omitted] • The flexural modulus and strength of the multi-material struts decrease with the increasing core-to-strut thickness ratio. • The energy absorption capacity of the multi-material struts is greater than that of the individual constituents. • Multi-material lattices with tunable in-plane compression modulus and strength are achieved. • The multi-material lattices exhibit greater energy absorption capacity compared to those of the individual constituents. Modern engineering applications require advanced materials that are stronger, tougher, and lighter. Nature offers various structural architectures to achieve superior mechanical properties. Inspired by bone with a spongy soft-core and a compact hard-shell, this study introduces architected multi-material lattices with tunable mechanical properties such as stiffness, strength, and energy absorption. The architected lattices are fabricated via multi-material fused filament fabrication. The hard polylactic acid (PLA) shell of the lattice struts maintains the stiffness, while the soft thermoplastic polyurethane (TPU) core enhances toughness and energy absorption capacity. The microstructural and mixed-mode fracture characteristics of the PLA-TPU interfaces were examined since the interfacial microstructure and adhesion are crucial to stress transfer in multi-phase composite materials. This study explores the flexural behavior of the multi-material struts and reveals two distinct failure modes depending on the volume fraction of the soft-core through 3-point bend tests. This study demonstrates the bio-inspired design on a hexagonal planar lattice with a strut thickness-to-length ratio of 0.3. Tunable stiffness, strength, and energy absorption are achieved by varying the core-to-strut thickness ratio. Compression test results show that the architected lattices exhibit an energy absorption capacity that is about 2–3 times greater than that of the constituent single-phase lattices. [ABSTRACT FROM AUTHOR]

• Effective stress-strain relationship for the grouted sleeve connections is presented. • Transverse pressure influence coefficient exerted by the sleeve coupler was modeled. • The performance of prefabricated piers under cyclic loading was tested. • The proposed model was verified by using the connection test and pier test. • The proposed model can be implemented in 1D, 2D, and 3D finite element analysis. This paper describes the development of an effective stress-strain relationship model for grouted sleeve connections. The transverse pressure influence has been calibrated by using the local bond stress-slip experiment including the effect of the initial construction error of the reinforcing bar. Then, the proposed effective stress-strain model was presented by solving the governing equation of the bond stress-slip behavior for the long-bonded grouted sleeve connection. The grouted sleeve connection can be equivalent to a reinforcing bar with a higher elastic modulus and yielding strength. The equivalent elastic modulus decreases with the increase in the initial construction error, and can be calculated by using the proposed model. The experiments on the long-bonded grouted sleeve connection and prefabricated concrete piers were carried out to verify the proposed model. The tested effective stress-strain relationship of the grouted sleeve connection, the maximum slip of the reinforcing bar embedded in this connection, and the strain distribution of the sleeve coupler were successfully predicted. Moreover, the performance of prefabricated concrete piers under cyclic loading was successfully simulated by using the proposed model. [ABSTRACT FROM AUTHOR]