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Carbon fiber reinforced polymer composites have the advantages of being lightweight, having high strength and designability, and having been extensively used. However, the interlaminar toughness and delamination resistance of these composites are relatively poor due to their laminated structure and intrinsic brittleness of resin matrix. In this paper, commercialized free-standing carbon nanotube (CNT) films, drawn from CNT forests, were used to toughen the interlaminar interfaces of the composites. The effects of resin infiltration state and thickness of CNT films on the interlaminar toughening effect were systematically investigated. The results show that the pre-infiltration treatment of CNT films with acetone diluted epoxy resin solution can effectively improve the degree of resin infiltration. Compared with the samples containing untreated CNT film, the Mode I and Mode II interlaminar fracture toughness of the treated samples were significantly improved. The GIC reached a maximum of 1412.42 J/m2 at a CNT film thickness of 5 µm, which was about 61.38% higher than that of the baseline. At a CNT film thickness of 15 µm, the GIIC reached a maximum value of 983.73 J/m2, approximately 67.58% higher than that of the baseline. The corresponding toughening mechanisms were also systematically analyzed. [ABSTRACT FROM AUTHOR]

"Interleaving" is widely used for interlaminar toughening of fiber-reinforced composites, and the structure of interleaving is one of the important factors affecting the toughening efficiency of laminates. Several experiments have demonstrated that compared to continuous and dense structures, toughening layers with structural heterogeneity can trigger multiple toughening mechanisms and have better toughening effects. On this basis, this work further investigates the application of heterogeneous toughening phases in interlaminar toughening of bidirectional GFRP. CNT was selected to construct toughening phases, which was introduced into the interlaminar of composites through efficient spraying methods. By controlling the amount of CNT, various structures of CNT toughening layers were obtained. The fracture toughness of modified laminates was tested, and their toughening mechanism was analyzed based on fracture surface observation. The results indicate that the optimal CNT usage (0.5 gsm) can increase the initial and extended values of interlayer fracture toughness by 136.0% and 82.0%, respectively. The solvent acetone sprayed with CNT can dissolve and re-precipitate a portion of the sizing agent on the surface of the fibers, which improves the bonding of the fibers to the resin. More importantly, larger discrete particles are formed between the layers, guiding the cracks to deflect in the orientation of the toughened layer. This generates additional energy dissipation and ultimately presents an optimal toughening effect. [ABSTRACT FROM AUTHOR]

Ultrathin CNT fiber veils (100–300 nm thick) were used as interleaves to improve the interlaminar fracture toughness of unidirectional carbon fiber reinforced polymer composites. These veils, drawn from vertically aligned CNT forests, were directly deposited at the mid-plane of CFRPs either longitudinally or transversely to the carbon fiber. The results revealed that triggering multiple toughening mechanisms is the key to achieve high toughness FRP composites. When the veils were oriented parallelly to the carbon fiber, the crack propagates in a "zig-zag" manner, not only interacting with more CNTs, but also triggering more carbon fiber bridgings, leading to a steeply "rising" R-curve. The superiority and effectiveness of this technique is exemplified by the extremely thin toughening layers (about 300 nm thick) needed to remarkably improve the propagation value of Mode I interlaminar fracture toughness of the CFRP by approximately 107%. The Mode I toughening factor (η I), defined as the change in Mode I interlaminar fracture toughness per interleaf-to-ply thickness ratio, comes out as high as 713, which is far outweigh the state-of-the-art. However, transversely-placed CNT veils will seriously block the formation of carbon fiber bridgings, remarkably degrading the Mode I interlaminar toughness of the laminates. [Display omitted] [ABSTRACT FROM AUTHOR]

Unidirectional carbon fiber reinforced polymer (CFRP) composites fabricated by Vacuum Assisted Resin Transfer Molding (VARTM) are tested at four initial strain rates (25, 50, 100 and 200 s −1 ) and six temperatures (−25, 0, 25, 50, 75 and 100 °C) on a servo-hydraulic high-rate testing system to investigate the effects of strain rate and temperature on their tensile properties and failure behaviors. A high speed digital camera is used to capture the deformation and failure behavior of the CFRP composites at a sampling rate of 20,000 fps with resolution of 256 × 256 pixels. In order to illuminate corresponding strain rate effect mechanism, carbon yarn samples are complementally tested under dynamic tensile loading at four different strain rates (40, 80, 120 and 160 s −1 ) utilizing an Instron drop-weight impact system. The stress–strain responses at these strain rates are discussed. A Weibull statistics model is used to quantify the degree of variability in tensile strength and to obtain Weibull parameters for engineering applications. The non-uniform strain and displacement fields as well as the damage process are characterized by digital image correlation (DIC) method. [ABSTRACT FROM AUTHOR]

Enabling energy-storing functions in structural composites is an appealing strategy to reduce the weight of systems in electric transport. Structural power composites (SPC) can be made by introducing energy-storing layers in traditional composite laminates, either as embedded devices or as thin integrated interleaves simultaneously performing load-bearing and electrochemical energy storage functions. However, the solid electrolyte in these layered architectures is typically a soft polymer, thus making the SPC very prone to delamination. Mechanical interconnection using structural matrix bridges between plies, analogous to metal connectors, provides a solution to alleviate delamination risks while maintaining the structural integrity of the SPC even under the most unfavourable combinations of loads. This paper reports on the use of experimental tests and FEM simulations to study the micromechanics of interlaminar polymer connectors in SPCs. It explores the effect of the connector area on the shear strength and energy density. The results provide a validated methodology to design laminated structural composites with a tailored balance of multi-functional properties of wide application to SPC of different constituents, with electrochemical energy storage either as capacitors or batteries. [Display omitted] [ABSTRACT FROM AUTHOR]

The introduction of nanostructured interlayers is one of the most promising strategies for interlaminar reinforcement in structural composites. In this work, we study the failure mechanism and interlayer microstructure of aerospace-grade structural composites reinforced with thin veils of carbon nanotube produced using an industrialised spinning process. Samples of unidirectional carbon fiber/epoxy matrix composites interleaved with different composition CNT veils were prepared using hot press method and tested for interlaminar fracture toughness (IFT), measured in Mode-I (opening) and Mode-II (in-plane shear), and for interlaminar shear strength (ILSS), evaluated by the short beam shear (SBS) test. The crack propagation mode could be directly determined through fractography analysis by electron microscopy and resin/CNT spatial discrimination by Raman spectroscopy, showing a clear correlation between interlaminar reinforcement and the balance between cohesive/adhesive failure mode at the interlayer region. Composites with full resin infiltration of the CNT veils give a large increase of Mode II IFT (88%) to 1500 J/m2 and a slight enhancement of apparent interlaminar shear strength (6.5%), but a decrease of Mode I IFT (−21%). A toughening factor, defined as the relative increase in toughness per interleaf-to-ply thickness ratio, give a record-high value of 15.8 for Mode II IFT. The results help establish the role of interlayer infiltration, interlaminar crossings and formation of a carbon fiber bridgings, for interlaminar reinforcement with interleaves. [ABSTRACT FROM AUTHOR]

Textile-reinforced concrete (TRC) is used widely in thin-walled structures due to its unique construction characteristics and superior mechanical properties. However, previous studies of the dynamic flexural properties of alkali resistant (AR)-glass TRC thin plates did not consider the influence of exposure to high temperature. In this study, the effect of impact velocity (range from 1.29 to 4.04 m/s) on AR-glass TRC after exposure to elevated temperatures up to 400°C was investigated experimentally using a drop-tower impact system. AR-glass TRC specimens were heated to target temperatures (100°C, 200°C, 300°C, and 400°C) in a furnace and then tested at room temperature. The effects of exposure to high temperature and the strain rate of loading (impact velocity) on the flexural mechanical properties were measured, and the microstructure and damage evolution of the composites was evaluated. Results show that the strain rate had a direct effect on the flexural properties of AR-glass TRC composite under impact after exposure to high temperatures. Furthermore, the weakening effect of high temperature in AR-glass TRC material could equal or even exceed the hardening effect due to the strain rate. The results were evaluated by comparing dynamic flexural stress and toughness features. Finally, the high-temperature deterioration mechanism of AR-glass TRC material was determined by comparing the changes in the matrix–textile interface and the textile fracture after exposure to different temperatures. [ABSTRACT FROM AUTHOR]