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To increase the service life of composite materials, self-healing, and damage detection are essential. Although a lot of research has been done on self-healing and damage-reporting materials, it is still difficult to combine self-healing with in-situ and real-time damage detection in bulk resin and composites. By integrating extrinsic self-healing based on microcapsules and internal self-healing based on coordination interaction, the simultaneous self-healing of matrix and interface damage of fiber-reinforced composites was achieved in this study. Specifically, two-component microcapsules filled with epoxy/mercaptan repair agent were inserted into the matrix and Ag nanoparticles (AgNPs) were introduced into the surface of carbon fiber by electroless plating. Upon the rupture of microcapsules, matrix self-healing was used to reach a desirable level of synchronous healing efficiency. In the meantime, the excess sulfhydryl reacted with AgNPs on the fibers to establish a coordination bond for interface self-healing. More intriguingly, the high exothermic action of epoxy resin and mercaptan repair agent in the self-healing process was observed when using infrared thermal imaging technology for in-situ and real-time damage detection. [Display omitted] [ABSTRACT FROM AUTHOR]

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]

Continuous carbon fiber-reinforced polymer (CCFRP) composites are lightweight and strong materials that have been used in a wide range of applications in automotive and aerospace industries. Traditional manufacturing processes (e.g., lay-up and out of autoclave techniques) are not capable of fabricating complex composites. Additive manufacturing (AM) has been increasingly used to fabricate composites with complex geometries. In this study, CCFRP with concentric and isotropic carbon fiber infill patterns were additively manufactured using fused deposition modeling (FDM). The microstructures of CCFRP composites with both infill patterns were characterized by an optical microscope and a scanning electron microscope (SEM). The void density was characterized for infill, solid and carbon fiber regions. Four-point flexural tests and tensile tests were conducted to evaluate the effects of carbon fiber concentration and infill patterns on both tensile and flexural behaviors. A stiffness averaging method was used to model the elastic behavior of CCFRP samples with different microstructures by taking into account porosity in infill, solid and carbon fiber bundles. Experimental results have shown that the specimens with the concentric carbon fiber infill pattern exhibit better flexural strength and energy absorption capability than those with the isotropic carbon fiber infill pattern. As carbon fiber concentration and the number of fiber rings increase, the flexural strength of the CCFRP composite increases. The specimen with 48.72 wt% carbon fibers showed the highest flexural strength of 270.63 MPa. In comparison with chopped carbon fiber-reinforced polymer composites, the flexural strength of the CCFRP specimen was increased by 40%. The predicted elastic moduli are in good agreement with experimental data. [ABSTRACT FROM AUTHOR]