Strengthening Of Steel Structures Research Articles

Effect of adhesive ductility on the bond behavior of CFRP-steel cohesive interfaces made with toughened epoxy resin

The strengthening of steel structures can be effectively performed using carbon fiber-reinforced polymers (CFRP) plates. Specifically, the behavior of corroded and fatigue damaged steel bridges can be significantly improved using externally bonded (EB) or unbonded composite reinforcements. For bonded composites, the bond between the reinforcement and the steel substrate is the weak link of the system. Indeed, cohesive failure within the adhesive layer represents the main failure mode, which makes the selection of a proper adhesive fundamental to achieve an improvement of the system load carrying capacity and fatigue resistance. Within this context, the use of toughened adhesives represents a promising solution. However, research regarding these adhesives is still limited. In this paper, 18 single-lap direct shear tests are performed to evaluate the cohesive behavior of toughened and traditional adhesives. For some specimens, the full-range bond behavior, including the load response snap-back, was experimentally captured. The digital image correlation (DIC) technique is adopted to evaluate the adhesives cohesive material law (CML). The effect of adhesive ductility, plate stiffness, and bonded length on the joint load response is investigated. Finally, experimental results are validated and discussed against the analytical predictions of a cohesive model based on a trapezoidal CML.

Study on CFRP-Strengthened Welded Steel Plates with Inclined Welds Considering Welding Residual Stress.

Welded steel plates are widely used in various structural applications, and the presence of inclined welds is often encountered in practical scenarios. Carbon fiber reinforced polymer (CFRP) has been proven to be effective for strengthening steel structures. However, the behavior of CFRP-strengthened welded steel plates with inclined welds, particularly considering the influence of welding residual stress, is limited. This paper aims to investigate the tensile behavior of CFRP-strengthened welded Q355 steel plates with inclined welds considering welding residual stress (WRS). First, WRS data were obtained by the X-ray diffraction (XRD) method at different locations. The maximum tensile and compressive residual stresses are 0.39 and 0.14 times the yield strength of the steel, respectively. Then, finite element models were established to investigate the effects of weld angles, weld width, and height on the WRS distribution of welded steel plates. Finally, the tensile performance of CFRP-strengthened welded plates with WRS was studied by numerical simulation. The results showed that the weld angles have little effect on the distribution pattern of residual stress but significantly affect the peak tensile WRS. When the weld angle changes from 0° to 60°, the peak tensile WRS decreases significantly from 0.32 to 0.06 times the yield strength of steel; furthermore, the influence of weld width and height on WRS is relatively limited. Under tension loading, the maximum stress occurs near the weld. The ends of the weld enter the yielding state later than the middle part of the weld due to the distribution of the WRS. As the weld angle increases and the length of the weld increases, the stress in the weld zone decreases, while the stress in the base material zone correspondingly increases. In addition, CFRP strengthening can reduce the magnitude of stress. This study provides preliminary references for understanding the tensile behavior of CFRP-strengthened welded steel plates with inclined welds.

Numerical Investigation on Strengthening of Steel Beams for Corrosion Damage or Web Openings Using Carbon Fiber Reinforced Polymer Sheets

Fiber-reinforced polymers (FRPs) have been widely used to strengthen steel structures, which could suffer from corrosion or the introduction of web openings, for utilities such as ductwork, plumbing, electrical conduits, and HVAC systems. The present numerical study involves the application of unidirectional carbon FRP (CFRP) sheets to steel I-beams, damaged due to corrosion or web openings, to regain their lost load-carrying capacity. Finite element analysis (FEA) was utilized to develop and validate three beam models against existing experimentally tested specimens. Subsequently, a parametric study was conducted investigating the effect of various corrosion levels and the number of circular web openings on the yield and ultimate load capacities of the beams. The optimum number of CFRP layers needed to strengthen corroded beams was determined and six CFRP strengthening scenarios were adopted to determine the best configurations to retrofit steel beams with openings (SBWOs). The results revealed that corrosion, introduced by thinning the bottom flange, reduced both yield and ultimate load capacities, with a nearly perfect linear reduction in ultimate load for each 2.5% thickness loss. The optimum number of CFRP layers depended on the level of corrosion damage. Furthermore, while maintaining a constant total opening area, beams with a greater number of smaller circular web openings demonstrated higher yield and ultimate load capacities than those with fewer larger openings. Out of the six adopted CFRP strengthening scenarios, three configurations that involved applying CFRP sheets to both flanges and the web effectively restored the strength of SBWOs, when adequate CFRP layers were used.

Strip-based numerical analysis of CFRP-reinforced steel plates with multiple debonding defects using the Runge-Kutta method

In recent years, carbon fiber-reinforced polymers (CFRP) have earned extensive attention in the field of structural rehabilitation. Adhesive bonding, as a prevalent technique, plays a crucial role in utilizing CFRP for strengthening steel structures. Nevertheless, debonding defects, commonly arising within the bond region between CFRP and host structures, hold the potential to induce the premature debonding failure of adhesively bonded joints. As a result, this research attempts to propose a strip-based numerical method using the explicit 4th-order Runge-Kutta technique (strip-based RK4) to investigate the full-range bond behavior of CFRP-to-steel single-lap joints with multiple arbitrary debonding defects. Also, three-dimensional (3D) finite element models (FEMs) of fully bonded single-lap joints were established in conjunction with cohesive zone modeling (CZM) to simulate the single-lap shear test reported in literature, based on which joints containing debonding defects were further modeled. Predictions from the proposed methodology were subsequently compared with test results and FE analysis to confirm its accuracy. Thereafter, the parametric analysis was carried out to investigate the effects of parameters, including bond length and defect position, on the bond behavior of the single-lap joint. The results indicate that the proposed strip-based RK4 method is accurate, with errors within 2% compared to FE analysis, and computationally efficient, with runtime being less than 50% of FE analysis. The findings of this study can be used as a guide for the design or damage assessment of CFRP-repaired structures in the future.

Feasibility of suppressing debonding failure for CFRP-hollow section steel tube composite member with a thick-walled section under tensile loading

Carbon fiber reinforced polymer (CFRP) is extensively utilized to strengthen steel structures. However, the premature debonding failure of CFRP inhibits its validity in strengthening tensile steel structures. Accordingly, the introduction of a thick-walled hollow section steel tube located at both ends of the CFRP-hollow section steel tube (CFRP-HSST) composite member was developed for the strengthening of tensile members. The steel tube in the novel composite member is constituted of a middle thin-walled section and a pair of thick-walled sections at each end providing anti-debonding resistance. The strengthening efficiency of the novel composite member under axial tensile loading was experimentally investigated by 8 specimens, comprising 4 circular cross section and 4 square cross section composite members. It was found that the presence of thickened section could achieve the stress gradient of CFRP and distribute the adhesive's interfacial shear stress. In addition, the finite element model (FEM) of the proposed CFRP-HSST composite member was developed and examined the adhesive damage distribution and evolution, and subsequently, a parametric analysis was carried out. The results indicate that the novel CFRP-HSST composite member can be regarded as an efficient strengthening solution for suppressing the premature debonding failure as well as increasing the strengthening efficiency.

Enhancing fatigue life of steel plates with CFRP laminate: Effects of curing temperature, stress range, and adhesive thickness

Steel bridges often require rehabilitation and retrofitting to address cracks and deteriorations caused by age, environmental factors, and increased loads. Carbon fibre reinforced polymers (CFRP) strengthening is a reliable and efficient method used for strengthening steel structures, particularly in steel joints subjected to cyclic loading from traffic. This study explores the impact of adhesive curing temperature on the fatigue behaviour of CFRP-steel joints under various stress ratios and ranges, it also evaluates the effect of adhesive thickness on fatigue life of the joint. Results indicate that curing temperature has no effect on fatigue behaviour, and the stress range-fatigue limit is constant at 30 MPa. Strain readings were taken to monitor debonding initiation, and the failure mode was predominantly steel-adhesive debonding. Adhesive thickness <2 mm has no significant effect on fatigue life of steel joints strengthened with CFRP. This research provides valuable insights for improving durability of steel bridges and structures.

Research on the fiber reinforced polymer in new construction and in strengthening of steel structures

Due to their distinct advantages, fiber-reinforced polymer (FRP) composites gained popularity during the past 200 years in civil engineering area, including high durability, corrosion resistance, light weight, high modulus strength, and high quality of corrosion resistance. These ranges of features led FRP composites find their application in mechanical, structure, civil engineering, and other industries areas. Traditional construction materials such as steel cannot feed the desirable expectation, and these high-tech materials are becoming more and more popular to replace the traditional materials. By analyzing experiments conducted by other scholars, the following essay will provide a critical review of FRP’s use in strengthening steel structures, not only for its advantages but also for its disadvantages and material properties. After the research, more experiments and theoretical research should be done to let FRP strengthen-materials be accepted and widely used in industries, as FRP materials still have some drawbacks.

Review on Detection of Internal Defects in CFRP Strengthened Steel Structures Using Eddy Current Pulsed Thermography

Abstract: This study investigates the effectiveness of the current pulse thermal technique (ESRT) in detecting internal defects in СFRР reinforced metal structures. This study covers various internal defects of FRР reinforced steel structures, including cracks, small tracks in СFRР, metal cracks / inter-steel cracks. We theoretically analyze the effect of these problems on the magnetic and thermal response of structures. To confirm the results of the theater, statistical and diagnostic tests were performed on steel plates reinforced with СFRР with various defects. Numerical and diagnostic results show that EСРТ can estimate observed deformations and show low temperature regions in EСРТ thermograms. In addition to the thermal characteristics of the thermograms, the appearance of the temperature profile and changes in temperature are also examined for reanalysis of the thermal analysis. In addition, based on the test results, the effectiveness of EСRT to restore the size and scope of errors was demonstrated. Overall, theoretical, numerical and experimental results show that EСRT is effective in diagnosing internal defects in CFRP-reinforced steel structures

Гібридні конструкції балок на транспорті із застосуванням металу

To analyze the future use of hybrid and combined structures for floors of buildings and structures for civil and industrial purposes, including in transport, to expand the range of beam structures, to develop an innovative design solution for a hybrid beam with low material consumption with increased reliability and durability of the structure. Analysis of the advantages of hybrid structures, materials used for their design, calculation of a metal-composite beam using existing engineering methods according to STO “Strengthening steel structures with composite materials. Design and production technology of works ”and numerical methods of calculation with the involvement of calculation programs. The advantages of hybrid and combined structures were revealed, a decrease in the material consumption of a metal-composite beam reinforced with boroplastic was revealed in comparison with a steel one. The proposed constructive solution of the beam can be used in floors and roofs of buildings and structures, transport facilities, especially in aggressive environments, as well as in the overhaul and reconstruction of industrial and civil facilities. New developments of beams using steel concrete and composite materials for designed and reinforced structures are presented. An innovative design of a metal-composite beam was developed - a steel beam reinforced with an external reinforcement system made of a composite material, protected by a utility model patent, its static and structural calculation was performed using the SCAD computer complex, the stress-strain state was analyzed, an analysis was made of reducing the consumption of steel of a metal-composite beam and others. its advantages over traditional steel and reinforced concrete beam solutions.

An Experimental Study on Shear Performance of Adhesive Interface between Steel Plates and CFRP

CFRP (Carbon Fiber Reinforced Polymer) are widely used in steel structural reinforcement. For steel structures strengthened with CFRP, except the cases the structures have defects before strengthening, the adhesive interface is the weakest part and CFRP debonding is the most common failure mode. In order to investigate the failure mechanism of CFRP strengthened steel structures, this paper presents an experimental study on shear performance of adhesive interface between steel plate and CFRP by twin shear model. Six steel plates strengthened with CFRP are divided into two groups, one has no damage, another has a gap at the mid. The specimens are tested under tensile loadings. The study results show that, the plates with a gap failed for CFRPs debonding, the cracking loading and breaking loading are 14.85kN, and 17.88kN respectively; the strain-loading curves had long linear stages, two strains decrease and other strains of another side increased rapidly at the cracking loading, then they both rose until the plates failed.

Experimental and theoretical investigation of FRP-steel composite plate under cyclic tensile loading

As the basic element of fiber-reinforced polymer (FRP) strengthened steel structures, FRP-steel composite plate (FSP) is a type of superior composite plate that combines the advantages of steel and FRP. This study combines different types of FRP and steel plates to develop an FSP element. Cyclic tensile tests are conducted to investigate the hysteretic behavior of the FSP, and the yield strength, peak strength, initial stiffness, post-yield stiffness, and residual deformation are analyzed. The experimental results indicate that the stress–strain curves of the FSP are bilinear prior to the fracture of the FRP, and the FSP exhibits a more stable post-yield stiffness and smaller residual deformation than a steel plate, which endows structures with better repairability after an earthquake. A finite element model (FEM) is established to investigate the mechanical behavior and failure process and is validated using experimental results. In addition, a theoretical model based on the composite material superposition theory is proposed and verified to simulate the stress–strain restoring force curves of FSP under cyclic tensile loading. The FEM and theoretical models can guide the practical design of FRP strengthened steel structures.

Bond behavior of CFRP-to-steel bonded joints at different service temperatures: Experimental study and FE modeling

The bond behavior of carbon fiber-reinforced polymer (CFRP)-to-steel interface in strengthened steel structures is likely to be significantly affected by service temperature variations, mainly because of interfacial thermal stresses and the changes in the local bond-slip behavior. However, there is lack of comprehensive study on the temperature effects on the bond behavior and the debonding load of the CFRP-to-steel interface at different service temperatures. This paper presents the results from the pull-out tests on CFRP-to-steel double-lap bonded joints tested at service temperatures ranging from −20 °C to 60 °C. The test results including failure modes, debonding loads, load–displacement curves, CFRP strain distributions and interfacial shear stress and slip responses are investigated and compared. Also, a bilinear local bond-slip model is developed to describe the relationships between the shear stresses and the interfacial slips at different service temperatures. A finite element (FE) model is proposed to predict the bond behavior and the debonding loads of the bonded joints at different service temperatures and validated through the comparisons between experimental and FE results. The validated FE model is then used to further investigate the temperature effects on the CFRP strain distributions, interfacial shear stresses and the debonding loads of the bonded joints at different service temperatures. The experimental and FE results have indicated that the debonding loads of the CFRP-to-steel interface are significantly reduced at both low and high service temperatures. The elastic stiffness and the peak shear stress of the local bond-slip model are generally reduced with the service temperature increase, while the interfacial fracture energy decreases at both low and high service temperatures.

Bond Performance of CFRP/Steel Double Strap Joint at Elevated Temperatures

Carbon fiber-reinforced polymer (CFRP) has been used widely in the strengthening of steel structures. Steel/CFRP systems being subjected to elevated temperatures is realistic in summer climate events in many countries, which leads to the degradation of the bond performance between CFRP and steel. Therefore, it is critical to study the bond behavior of the CFRP/steel system under elevated temperature. This paper investigates the mechanical performance of CFRP/steel adhesively bonded double strap joints under different temperatures. Thirty CFRP/steel double strap joints were tested to failure under temperatures between 10 °C and 90 °C. It was found that the joint failure mode changed from adherend failure to debonding failure as the temperature was approaching glass transition temperatures. In addition, the ultimate load and joint stiffness decreased significantly under temperatures near to and higher than glass transition temperatures. Based on the experimental results, a model is proposed to predict the bond stress of the CFRP/steel under different temperatures.

Effectiveness of Fan Anchors in Preventing Debonding in FRP-Strengthened Steel Members

Fibre Reinforced Polymers (FRPs) are extensively employed to strengthen existing structures because of their several advantages over other strengthening techniques. On the other hand, the premature debonding of FRP reduces its effectiveness in strengthening steel structures. Anchoring FRP composites is an effective solution to delay or even prevent their debonding. Very limited anchorage methods, however, have been introduced for FRP-strengthened steel structures and the need for an effective anchorage system remains. Fan anchor has been validated as one of the remedies against debonding failure in FRP-strengthened concrete structures. Considering the advantages that fan anchors offer, the use of fan anchors for FRP-strengthened steel structures is proposed and evaluated in this paper. Since FRP-steel joints have a different bond-slip law than FRP-concrete joints and the strengthened steel members are prone to buckling-debonding interactions, this study focuses on the efficiency of fan anchors in delaying FRP debonding by assuming that an adequate mechanical connection between the dowel and the steel substrate is provided. Three experimental studies involving shear, flexural and buckling strengthening of steel components were simulated through finite element modelling, and fan anchors were added to the models after validation. The effect of fan anchors on strength, failure mode and FRP's strain distribution of the models was examined. The study showed that the fan anchor was successfully able to delay debonding mode, which increased the strength and ductility and exploited a higher strain capacity of FRP plates.

Гибридные конструкции балок на транспорте с применением металла и композитов

Purpose: To pursue the analysis of hybrid and combined frame possible usage for overlaps of buildings and structures for civil and industrial purposes, including on transport, to expand beam structure nomenclature, to develop innovative structural solution of hybrid beam with material low consumption but with frame increased reliability and durability. Methods: Analysis of the advantages of known hybrid frames, of materials, used for their projection; metal composite beam calculation using existing engineering techniques in accordance with STO 38276489.003-2017 “Strengthening of steel structures with composite materials. Projection and techniques of work production” and numerical calculation methods involving calculation software programs. Results: New workouts for beams with the use of steel concrete and composite materials for projected and strengthened structures are presented. Innovative frame of metal composite beam has been developed — steel beam strengthened with external reinforcement system of composite material, protected by a useful model patent, its static and structural calculation has been performed using SCAD computing complex; stress-strain state has been analyzed, the analysis of steel consumption reduction for a metal composite beam and of its other advantages in comparison with steel and reinforced concrete beam traditional solutions has been carried out. Practical significance: The advantages of hybrid and combined structures are revealed, the decrease in material capacity for a metal composite beam strengthened by boroplastics in comparison with a steel one is revealed. The proposed structural solution of a beam can be used in overlaps and coverings for buildings and structures, transport buildings, especially, in aggressive environment conditions as well as during major repairs and reconstruction of industrial and civil purpose facilities.

Degradation mechanism of steel/CFRP plate interface subjected to overloading fatigue and wetting/drying cycles

The bond strength of the interface between carbon fiber-reinforced polymer (CFRP) and steel is crucial for CFRP-reinforced steel structures. However, the influence of fatigue loading and the service environment on the bond durability of strengthened steel structures requires further investigation. This work focused on the bond degradation mechanism of the steel/CFRP interface subjected to overloading fatigue damage (OFD) and wetting/drying cycles (WDCs). A total of 12 CFRP/steel joints subjected to OFD and/or WDCs was tensioned to failure. Samples were collected from the debonding zone near the loading end for scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The SEM results revealed that cracks induced by OFD, and microcracks and hydrolysis caused by WDCs in the adhesive were the main reasons for interfacial bonding degradation. OFD introduced localized cracks in the adhesive, whereas WDCs caused widely distributed and staggered microcracks in the adhesive. OFD-induced cracks accelerated moisture intrusion and subsequently enhanced the generation of microcracks, leading to a further decrease in the interfacial bond strength. In addition, the EDS results demonstrated a content change in interfacial chemical elements and precipitates at different sampling locations, which also confirmed the above mechanical degradation mechanism. Furthermore, changes in the load capacity, interfacial stiffness, and failure mode were examined, and the aforementioned degradation mechanism was verified.

Enhancing buckling capacity of angle steel using unbonded CFRP laminates processed by Vacuum-assisted Resin Transfer Molding (VaRTM)

The carbon fiber reinforced polymer (CFRP) adhesive-bonding technique has been increasingly developed for strengthening steel structures. However, the application of this strengthening method is still considered ineffective since it requires complicated and burdensome steel surface treatments. The bonding strength of CFRP on steel, on which this method highly depends, is also prone to performance decreases due to inevitable environmental exposures. Alternatively, unbonded CFRP strengthening is proposed in this study. A total of twelve specimens are prepared and tested to evaluate the possibility of application of this strengthening method in enhancing buckling performance of angle steels. All CFRPs on the strengthened specimens are created through a process of Vacuum-assisted Resin Transfer Molding (VaRTM). It is concluded from the investigation results that the unbonded CFRP is effective in strengthening angle steels, where the buckling strength enhancements by 8.5%-54.3%. The strength increase is clearly affected by CFRP length, number of CFRP layers, and steel’s slenderness ratio. A change in the curvature of the specimens due to the presence of unbonded CFRPs can also be observed. The VaRTM process produces higher fiber content of laminates. Analytical studies carried out at the end of this paper provide a reasonable solution for strength design and a good correspond for post-buckling prediction.

Nondestructive evaluation of carbon fiber reinforced polymer (CFRP)-steel interfacial debonding using eddy current thermography

When carbon fiber reinforced polymer (CFRP) is used to strengthen steel structures, interfacial debonding between CFRP and steel can substantially compromise the composite action and trigger structural failure. This study investigates eddy current thermography (ECT) for nondestructive evaluation of CFRP-steel interface. Four CFRP-steel specimens with different shapes and sizes of interface defects were tested. With eddy current induced in steel plates, thermal profiles of CFRP were measured to detect and characterize interface defects. The results showed that the proposed ECT system was capable of detecting CFRP-steel interfacial defects within 80 s. ECT results were subjected to edge effects that compromised the evaluation resolution and precise determination of damage regions. The edge effect was prominent in the heating stage but alleviated in the cooling stage because of heat redistribution. The blurring effect caused by the heat diffusion from the internal defect to the external surface of CFRP also hindered precise determination of damage regions. It is recommended to perform ECT for shallow interfacial defect in the cooling stage of steel-CFRP structures to achieve desired results.

Effects of steel surface treatment with silanized carbon nanotubes on the bonding properties between steel and epoxy adhesive

ABSTRACT The interfacial bonding properties of steel and adhesives are the key factors affecting the application of external bonded fiber reinforced polymer composites to strengthen steel structures. The present study proposed a new method to treat steel surface with carbon nanotubes (CNTs) for modification of the interfacial bond strength between steel and an epoxy adhesive. CNTs were modified in ethanol-water mixture by silane and PVP (Polyvinyl Pyrrolidone) to improve the dispersity of CNTs. Steel plates treated through dip-coating or spray-coating with the modified CNTs suspensions were used for single lap joint tests. Compared to the control samples, the treated steel surfaces resulted in a significant increase in the interface shear strength (i.e., by 52.8%). The desirable enhancement is attributed to the existence of CNT between the steel and adhesives, which is connected to the steel surface through Si-O-Fe covalent.

Improvement in bond behavior and thermal properties of carbon fiber-reinforced polymer strengthened steel structures

This study focused on the bond strength and environmental durability of carbon fiber-reinforced polymer (CFRP)-strengthened steel structures, by investigating the improvements in the surface treatment of the steel substrate, thermal properties of the epoxy resin, and curing process of the composite. The results showed that the steel–adhesive interfacial bonding behavior can be improved by increasing the surface roughness and using a low-viscosity primer to enhance the wettability. During the initial curing, a lower ambient temperature during the gel time can reduce the porosity of the multi-CFRP laminate. However, raising the initial curing temperature is advantageous for the mechanical and thermal properties of the composite. In addition, the thermal properties of the epoxy matrix were clarified by differential scanning calorimetry tests, based on which isothermal curing degree curves of the system were drawn. The results demonstrated that post-curing at 80 °C is vital for increasing the glass transition temperature, which is also appropriated for the primer. Finally, the moisture and thermal effects on the deterioration of the adhesive joint were examined by conducting short-period aging tests in immersion and wet–dry cyclic environments, during which delamination strength reduction and failure mode transformation were observed.