Carbon fiber reinforced polymer (CFRP) laminates can effectively enhance the fatigue life of steel structures. However, few studies have investigated the influence of crack-induced debonding in the CFRP-steel interface on the CFRP strengthening efficiency and the relationship of crack propagation and debonding development. This study experimentally and numerically – with the boundary element method (BEM) – investigated crack propagation and debonding development in CFRP-strengthened cracked steel (Q345, Q460, and Q690) plates. The fatigue test specimens were subjected to a maximum stress of 50% steel yield stress and a stress ratio of 0.1, during which real-time changes in CFRP strain distribution were recorded by digital image correlation (DIC). The CFRP strain gradient calculations showed that crack-induced debonding was crack length-dependent. The numerical fatigue life results were in good agreement with the experimental data when considering debonding and overestimated the experimental data without considering debonding, demonstrating the necessity of considering crack-induced debonding in calculation and design. Furthermore, a relationship was obtained between crack propagation and debonding development.
农用电泵产品是一种以潜水电机为动力，电机与水泵组装成一体潜入水下运转的提水机械，主要有小型潜水电泵、污水潜水电泵、井用潜水电泵和潜水螺杆电泵等几种类型。农用潜水泵小巧轻便，价格低廉、容易操作，在农村生产生活中起着重要作用。
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Carbon fiber reinforced polymer (CFRP) sheets have been used to strengthen steel structures to enhance fatigue life. However there is very limited information on the influence of temperatures on the fatigue strengthening efficiency. This paper describes an investigation on the behavior of cracked steel plates strengthened with CFRP at different temperatures. Firstly, through cylinder coupon tests, mechanical properties of the resin were found to vary at different temperatures, especially when the temperature exceeds the glass transition temperature T g . Secondly, three cracked bare steel plates and five cracked steel plates strengthened with high modulus CFRP sheets were tested under fatigue loading at different temperatures. The results prove the effectiveness of CFRP strengthening technology to increase fatigue life when the temperature ranges from −40°C up to 60°C. It is concluded that the variation of temperature has an obvious influence on properties of the resin, leading to influence on fatigue life. Finally, the existing analytical method to predict fatigue lives of CFRP strengthened steel plates at ambient temperature was modified by considering the temperature effect. Experimental and theoretical results are compared and reasonable agreement is achieved. The influence of the number of CFRP layers and CFRP modulus is also found based on the proposed analytical method.
Various analysis-oriented models have been developed over the past decades to predict the stress-strain relationship of fibre-reinforced polymer (FRP) confined concrete. Most of these models are built based on the path-independent assumption, and the mechanism of this kind of model has been revealed by the authors based on a 3D geometrical approach. However, it is widely recognized that FRP confined high-strength concrete (HSC) is path dependent, which means that the stress of passively confined HSC deviates from the actively confined test results. Most of the existing solutions for this problem employ an alternative stress-strain relationship instead of the original actively confined HSC model for FRP confined HSC. In this paper, the mechanism of the path-dependency of confined HSC is revealed based on the 3D geometrical method. In addition, a corresponding analysis-oriented model for confined HSC considering path dependency is proposed. This model includes an actively confined concrete model, a damage criterion and post-damaged HSC behaviours. The proposed model can identify the confining mode from the load path and can select a proper stress-strain relationship for confined HSC without artificial intervention. Finally, the proposed model is calibrated and verified by test data collected from the literature. The results show that the proposed model has better accuracy than existing path-dependent models.
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A novel hybrid beam consisting of pultruded GFRP three-cell box-section and concrete slab was proposed for pedestrian bridges. The flexural behavior of the proposed beam was investigated through experimental and analytical studies. First, material characterization tests were conducted to evaluate the mechanical properties of the GFRP material. Second, four- and three-point bending tests were carried out on four beams with varying span lengths. The flexural and shear stiffness of the beams were calculated and evaluated based on the experimental results. Design equations were proposed to predict the ultimate flexural strength of the hybrid beam. Finite element (FE) models were constructed to validate the effectiveness of the proposed hybrid beam. A good correlation was found between the experimental results, the analytical predictions from the proposed design equation, and the numerical results from FE modeling. The combined bolted and bonded connection provided effective shear transfer between GFRP and concrete, ensuring a good composite action of the hybrid beam. Additionally, the proposed three-cell box-beam increased the web local buckling strength of GFRP beams subjected to a concentrated load, avoiding the premature buckling failure of the entire beam. The proposed hybrid GFRP-concrete beam provides an effective approach to increase the span length of GFRP structures.
随着国家农机购置补贴政策的大力实施,广大农民朋友的购机热情高涨。但由于缺乏足够的专业知识,在选购农机具的过程中,往往变得犹豫不决。特别是大中型农机具的选购,需要花费很大的一笔资金,千万不可以掉以轻心。因此，以下五点可供参考。
Using steel slag as concrete aggregate is an effective, eco-friendly method of steel slag disposal. However, the volume expansion of steel slag aggregates (SSAs) creates premature cracks and reduces the mechanical property and durability of concrete, limiting steel slag aggregate concrete (SSAC) applications. A concrete filled-in fiber-reinforced polymer (FRP) tube (CFFT) is a structural member that exploits the high tensile strength of FRP and the lateral expansion of concrete under compression. FRP tubes can be used for the external confinement of concrete to increase its strength and ductility. In this paper, a steel slag aggregate concrete filled-in FRP tube (SSACFFT) is proposed for steel slag disposal. The physical and mechanical properties of an SSACFFT were investigated experimentally. The volume changes in the SSAC and SSACFFT during high-temperature curing were measured using 3D laser scanning. The results showed that the SSAs in concrete expand and cause cracking/spalling during hydration, which is effectively restrained by the FRP tube. The strength, failure modes and stress–strain relationships of the SSACFFT were investigated via axial compression tests. Finally, the SSACFFT was proposed as a prefabricated structural component called FRP-tube-confined concrete core encased prestressed rebar (FCCC-PR) to capitalize on the volume expansion characteristic of SSAC.
A strengthening approach to improve the buckling resistance of steel members in compression is introduced in this paper, where a mortar-filled FRP tube is sleeved outside the steel member and wrapped with FRP fabrics at the ends of the tube. Through axial compression tests on 18 specimens with bi-axial symmetrical cross-section, the load–strain development and failure modes were obtained and improved load bearing capacity and ductility were evidenced. The effects of the cross-section of core steel, slenderness and FRP fabric layers wrapped at the ends of specimens were investigated. It was found that, after strengthening, the failure modes changed from steel yielding at the mid-height of the steel member due to global buckling to local damage at the steel end. As a result, the load bearing capacity increased by 44–215% and ductility increased by up to 877%. The segmentation model was developed for strengthened specimens, by which the calculated load bearing capacity agrees well with the experimental result.
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This paper studies the long-term behavior of carbon fiber–reinforced polymer (CFRP) plates under sustained loads. Two typical loading conditions, CFRP plate loading controlled by displacement constraints and force constraints, are studied. The first section investigates the prestress loss of the CFRP plate when it is loaded via displacement constraints, which serves as a prestressed system applied for prestressed structures. Short- and long-term measurements show that the prestress loss of CFRP is caused by material creep, temperature change, and CFRP and anchorage slippage. When the CFRP prestress level is 5.0%–12.3%, the average prestress loss after 42 days is 5.1%. The second section investigates the permanent deformation of the CFRP plate with curved anchorages caused by creep under a certain sustained load (i.e., controlled by force constraints), which serves as a suspension bar or cable applied for determinate structures. The creep strain-to-time relationship, creep rate, and residual strength were obtained. The creep strain-to-time relationship strongly depends on the stress level applied; the creep coefficient is less than 10.94% and the residual strength is greater than 80% of the initial tensile strength after 1000 h. The findings show the effectiveness and reliability of these two typical cases of CFRP plates under long-term sustained loads.
