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Graphite carbon nitride (g-C

The bridging N atom in g-C3N4 structure plays a decisive role in photo-generated charge transfer because it usually confines photo-generated electrons and holes in each heptazine, thus leading to severe recombination. In this work, a kind of 2-aminoterephthalic acid-derived benzene ring group with rich π-electrons was considered to integrate with bridging N to break the above-mentioned confining effect. On the basis of density-functional theory calculations and experimental analysis, this 2-aminoterephthalic acid-derived bridging structure facilitated to draw photo-generated charge out of heptazine unit, and its polarized asymmetric structure promoted the directional transfer of photo-generated charge carriers across adjacent heptazines, thus efficiently reducing the recombination. Meanwhile, the 2-aminoterephthalic acid-derived bridging structure also reinforced the connectivity of heptazine units in g-C3N4 framework and led to high degree of polymerization, which thus extended the π-conjugated electronic system of g-C3N4 and modulated the band structure favoring photocatalytic hydrogen production. Consequently, a high photocatalytic H2-production activity of 24,595 μmol h−1 gcat-1 was achieved on the bridging regulated g-C3N4 under visible light, with an apparent quantum yield of 48.7% at 425 nm.

Photocatalytic O2-generation reaction is recognized as a crucial step in water splitting and has drawn great attention of researchers. In this work, a hetero-layered composite photocatalyst was successfully prepared by a facile self-assembly method based on electrostatic interaction between oppositely charged Zn/Cr-layered double hydroxide (Zn/Cr-LDH) and lead niobate nanosheets. The layer-by-layer stacking of Zn/Cr-LDH and HPb2Nb3O10 nanosheets was beneficial for rapid migration of photo-induced charge carriers inside the photocatalyst because of large contact area. In the meantime, Zn/Cr-LDH and HPb2Nb3O10 components exhibited suitable energy-band alignment, which led to efficient separation of photo-induced charge carriers. The composite photocatalyst showed enhanced photocatalytic O2-generation activity under visible-light irradiation without loading cocatalyst. Briefly, this work expanded the applications of AB2Nb3O10-based materials in photocatalytic energy conversion and proved that constructing composites based on electrostatic self-assembly of complementary 2D materials is a promising strategy for development of more efficient photocatalysts.

A 2D/2D coupled NiCo2O4/g-C3N4 nanostructure was fabricated through an in situ growth strategy. NiCo2O4 worked as a novel oxygen-evolution-reaction cocatalyst, and the intimate interface combination by surface bonding and the related cooperative effect between NiCo2O4 and g-C3N4 were comprehensively explored. Consequently, the separation and migration of photo-generated charge carriers for g-C3N4 were efficiently promoted, as well as the energy barriers of the oxygen evolution reaction were reduced. NiCo2O4/g-C3N4 showed obviously enhanced photocatalytic O2-evolution activity (4.6 times that of bare g-C3N4), and the corresponding AQY achieved 4.9% at 380 nm. Then, Pt as a hydrogen-evolution-reaction cocatalyst was employed to cooperate with NiCo2O4 to construct dual cocatalyst loaded g-C3N4 for photocatalytic overall water splitting, in which H2 and O2 simultaneously evolved in a stoichiometric ratio of 2 : 1, and the AQY for H2 evolution reached 2.8% at 380 nm. Consequently, this work demonstrates a reliable strategy for g-C3N4 to construct a photocatalytic system for efficient overall water splitting.

Design of 2D/2D photocatalysts with strong interfacial interaction via abundant 2D coupling interfaces has been proved to be an efficient strategy to promote photocatalytic performance. Herein, colloidal suspensions containing HCa2Nb3O10 nanosheets (HCNO) and dicyandiamide were adopted as feed reagents, based on which 2D/2D HCa2Nb3O10 nanosheet/g-C3N4 composites (HCNO/CN) with efficient photocatalytic activities were synthesized by a facile one-pot method. By this way, the separation process of HCa2Nb3O10 nanosheets was eliminated and the effective heterojunction formed simultaneously. The optimal visible light-driven photocatalytic H2-evolution activity of the composite photocatalysts was 4.5 times that of pristine g-C3N4 (CN), and the enhanced photocatalytic activities were attributed to the following aspects. On the one hand, the large 2D contact interfaces and the matched energy bands resulted in an efficient type II heterojunction and it thus could accelerate the separation and transfer of photo-generated electron–hole pairs. On the other hand, the residual tetrabutylammonium hydroxide on HCNO obviously changed the structure of g-C3N4 by the in situ pyrolysis strategy, thus enhancing the light absorption, narrowing the bandgap and enlarging the specific surface area and pore volume of g-C3N4. This work presents an effective and convenient route to construct 2D/2D composite photocatalysts based on layered perovskite nanosheets and/or g-C3N4 nanosheets, by which the compositions and microstructures of the composites were optimized simultaneously during the pyrolysis process.

This paper describes an investigation carried out to explore the relationship between the morphological characteristics, mechanical properties, and failure modes of legacy butt welds. A total of 31 tension coupons were cut out of butt-welded tubular structures which had been in service for over 50 years. Monotonic tension tests were conducted to investigate the mechanical behavior and the failure modes of the welds. A digital image correlation (DIC) system was used to monitor the strain distribution on the cut-out surface of the coupons. Results showed that morphological imperfections such as misalignment and plastically deformed regions near the welding zone reduced the capacity and ductility of the weld. Data from the DIC system revealed that fracture at the welding zone initiated from pre-existing imperfections.

Structural members with confined concrete are becoming increasingly popular in civil engineering applications because of their superior strength and ductility. In these structural members, the concrete is subjected to dilation-induced (passive) lateral compressive stresses from the confining device (e.g., a steel tube). Existing research has led to theoretical models that predict closely the stress–strain behavior of concrete under uniform confinement (e.g., concrete in circular steel tubes under concentric axial compression), but theoretical models with a similar capability have not been achieved for the more common situation of concrete under non-uniform confinement (e.g., concrete in rectangular steel tubes). This paper presents a three-dimensional (3D) plasticity constitutive model that is accurate in predicting the stress–strain behavior of concrete in various scenarios of confinement. In the proposed model, a well-established open strength surface with associated open yield surfaces is combined with a hardening/softening rule compatible with both plastic volumetric compaction and dilation. In addition, a novel potential surface with a triangle-like deviatoric trace is proposed and calibrated with available experimental data of non-uniformly confined concrete. The implementation of the constitutive model in finite element analysis with an enhanced stress-return algorithm suitable for the novel potential surface is explained. While the focus of the present work is on monotonic compression-dominated loading, the model can be combined with fracture and damage theories to depict the behavior of concrete under tension-dominated and cyclic loading conditions. The performance of the proposed model is evaluated by comparing its predictions with a wide range of experimental data covering uniform active, uniform passive, and non-uniform passive confinement conditions, which demonstrates the capability and high accuracy of the proposed model.

This paper provides an overview of the basic elements, course of development, experimental evaluation, and numerical simulation of a thermally activated shape memory alloy (SMA) and carbon fiber reinforced polymer (CFRP) composite system for fatigue repair or retrofit of metallic structures. Nickel titanium niobium (NiTiNb) SMA wires, which are able to generate 400 MPa recovery stress upon thermal loading and maintain that stress level at a wide range of temperatures, was adopted to apply compressive stresses near the crack. Monotonic bond behavior of single and multiple SMA wires to CFRP was investigated; the debonding onset load and maximum capacity were quantified. The fatigue behavior of patches consisting of multiple wires bonded to CFRP was studied. Results indicated that the system could maintain 80% of the recovery stress, after up to 2 million load cycles, so long as the maximum applied stress was below the debonding onset level. A fatigue strengthening system, using such multiple SMA wire system as underlay and CFRP patch as overlay, was applied to fatigue sensitive steel plates for fatigue life improvement evaluation. The average fatigue life of the patched steel plates was over 26 times longer than that of the unpatched plates tested at the same load range. Finally, a numerical framework was developed to simulate the fatigue crack growth in steel plates patched with such strengthening system and was validated by the experimental data. The findings suggest that the proposed system could be a promising alternative to traditional techniques for fatigue crack repair.

This paper describes a numerical framework to simulate the fatigue crack growth of single edge-notched steel elements patched with shape memory alloy (SMA)/fiber reinforced polymer (FRP) composite patches. The framework includes the small crack propagation. The small crack propagation stage was simulated using the analytical model based on the equivalent initial flaw size method. The bridging effects of the SMA/FRP composite patch were simulated by two consecutive finite element simulations considering the interfacial debonding during fatigue loading. Numerical predictions were validated by comparison with experimental results. A parametric study was conducted using the validated model. The considered parameters were the number of SMA wires used in the patch, the applied stress range, and the length of the crack in the steel substrate at the time of installing the patch. The numerical results indicated that the SMA wires increased the critical crack length at which fracture occurred and that the patch was more effective at lower stress ranges. Further, the SMA/FRP patch can substantially delay the failure of a member with very little remaining fatigue life. The results demonstrate the potential of this technique and highlight the importance of early intervention when repairing fatigue sensitive details.

Considering the fact of the cost control in photocatalysis, it is significant to develop the noble-metal-free photocatalysis for H2 evolution. Herein, CoS2 in-situ loading and S doping for g-C3N4 were synchronously constructed by hydrothermal and sulfidation processes. With systematical physicochemical characterizations, it was found that partial N atoms in g-C3N4 were replaced by S atoms, improving the visible-light absorption ability for more generation of photo-generated carriers. CoS2 as an effective noble-metal-free H2-evolution cocatalyst induced and captured photo-generated electrons for promoting separation of photo-generated carriers synergistically with S doping. More contact between CoS2 and g-C3N4 were formed by the in-situ growth of CoS2 nanosheets on the surface of g-C3N4 nanosheets. It was important that Co-S bonding between CoS2 and S-doped g-C3N4 was built by the synchronous construction of CoS2 in-situ loading and S doping, strengthening the directed transfer of photo-generated electrons from S-doped g-C3N4 to CoS2. Accordingly, the synergetic function of CoS2 in-situ loading and S doping effectively elevated the noble-metal-free photocatalytic activity of g-C3N4 for H2 evolution under visible-light irradiation.

In this work, nanosized MoP was synthetized in a novel phosphorization process at a relatively low temperature under an ambient-air atmosphere and then assembled with g-C3N4 to form MoP/g-C3N4 coupled photocatalyst by a mixing and heat-treating method. Significantly, the obtained noble-metal-free MoP/g-C3N4 photocatalyst showed superior photocatalytic H2-production activity and excellent stability than most reported noble-metal-free cocatalysts modified g-C3N4. The optimal MoP/g-C3N4 photocatalyst possessed a H2-production activity of 3868 μmol h−1 gcat−1 and the corresponding AQY reached 21.6 % at 405 nm. The incorporation of nanosized MoP into g-C3N4 could broaden the absorption range of visible light, built a conducive highway (Mo(δ+)−N(δ−) bond) for electrons to transfer from g-C3N4 to MoP, which improved the separation and transportation efficiency of charge carriers, and reduce the energy barrier for hydrogen evolution reaction. This work provided a reliable guideline for designing novel and simple synthetic method for cocatalyst and then constructing cocatalyst/photocatalyst coupled structure.

This paper evaluates the effectiveness of a new type of shape memory alloy (SMA)/fiber-reinforced polymer (FRP) patch for repair of fatigue-sensitive steel elements. Twenty-seven single edg...

Sacrificial reagents (SRs) are widely used in photocatalytic H2-evolution systems, and the physical/chemical properties of SRs may be changed after mixing. In our study, triethanolamine, methanol, and lactic acid were selected as three kinds of typical organic SRs, which were of alkaline amine, neutral alcohol, and acidic carboxylic acids, respectively. Graphic carbon nitride (g-C3N4) and titanium dioxide (TiO2), which are of metal-free polymerized-organic and metal-oxide inorganic semiconductors, respectively, were selected as two kinds of typical photocatalysts. By measuring and comparing the photocatalytic H2-evolution performance of g-C3N4 and TiO2 in single-sacrificial-reagent systems, corresponding binary-sacrificial-reagent systems, and trinary-sacrificial-reagent (TSR) systems, the effects of mixed-sacrificial-reagents (MSRs) on H2 evolution over these two typical photocatalysts were investigated. It was found that the photocatalytic H2-evolution rates could be enhanced by mixing appropriate SRs to corresponding photocatalysts, for instance, TiO2 showed the best photocatalytic performance in 20% TSR system. However, g-C3N4 showed the best photocatalytic performance in 20% triethanolamine compared with MSR systems. The effects of MSRs on H2 evolution over typical photocatalysts could be explained by the adsorptivity between SRs and photocatalysts. Functional groups of SRs that are concerned with the adsorption between SRs and photocatalysts might change in the MSR systems. The weakening of adsorption between SRs and photocatalysts led to the decreased photocatalytic H2 evolution rates over photocatalysts. We provide useful guidance to the design of MSR systems in order to promote the photocatalytic H2 evolution.

This paper summarizes the details of a numerical and experimental research program that was conducted to study the debonding of carbon fiber–reinforced polymer (CFRP) patches from cracked steel members under fatigue loading. A preliminary numerical model was developed to investigate the influence of patch debonding on the fatigue life of cracked steel elements. Results indicated that altering the shape and increasing the size of the debonded region could change the calculated crack growth rate by up to 54 times. To validate the model, six steel edge–notched tension coupons were patched with CFRP materials and tested under fatigue loading, while full-field strain distributions were monitored using a digital image correlation (DIC)–based measurement system. Based on the experimental results the numerical model was refined to incorporate the interfacial traction-separation behavior. A parametric study was conducted using the refined numerical model. The results indicate that the size and shape of the...

This paper proposes a nondestructive method to evaluate the health status of resistance spot-welded (RSW) joint under service load using lead zirconate titanate (PZT) active sensing system, in which the PZT transducers were used as both actuator and sensor. The physical principle of the approach was validated through a numerical analysis showing that an opening between the faying faces at the welded joint occurred under tension load. The opening decreased the contact area hence reduced the amplitude of the stress wave received by the PZT sensor. Therefore, by comparing the energy index of the signals before and after the loading, the health condition of the joint can be evaluated. Five ST14 steel single lap joint specimens were tested under tension load while being monitored by the PZT sensing system and digital image correlation (DIC) system in real time. The data obtained from the DIC system validated the numerical results. By comparing the energy index of the signal obtained from the PZT sensing system before and after unloading, it was concluded that the RSW joint was intact after being loaded to the service load. The proposed method is promising in evaluating the health condition of RSW joint nondestructively.