CFRP-steel Interface Research Articles

Analytical solution of the full-range behavior of adhesively bonded FRP-steel joints made with toughened adhesives

Fiber-reinforced polymer (FRP) composites represent an effective solution to strengthen and retrofit existing steel members. Namely, bonded or unbonded carbon FRP (CFRP) plates have been employed to improve the strength, fatigue behavior, and durability of steel bridges. In bonded solutions, the effectiveness of the CFRP reinforcement strongly depends on the adhesive employed to bond the plate, as failure usually occurs due to debonding. Within this framework, the use of toughened adhesives is particularly attractive since they may improve the load carrying capacity of the CFRP-steel interface, also providing a certain ductility. Debonding in CFRP-steel joints was previously studied using a cohesive approach. However, solutions able to describe the full-range behavior of joints with toughened adhesives and finite bonded length are not available in the literature. In this paper, a trapezoidal (trilinear) cohesive material law (CML) is employed to model the bond behavior of pultruded carbon FRP-steel joints made with a rubber-toughened epoxy adhesive, which showed cohesive debonding within the adhesive layer. The analytical solution provided is employed to study the experimental response of nine CFRP-steel joints tested using a single-lap direct shear set-up. Comparisons of analytical and experimental results of joints with three different bonded lengths confirm the effectiveness of the solution proposed.

Effects of the liquid rubber modified adhesive on the bond-slip response of the CFRP-steel interface

This paper presents an experimental and analytical study, purposely exploring the effects of the liquid rubber incorporation into epoxy resin on the bond performances between the carbon fiber reinforced polymer (CFRP) plate and steel. The experiment consisted of 36 double-shear bonded joints, and the investigated variables were the liquid rubber contents (i.e., 0, 5%, 10%, and 15% by weight) and adhesive thickness (i.e., 0.5 mm, 1.0 mm, and 2.0 mm). The failure mode, ultimate load, strain distribution, as well as the load-slip response were compared and discussed between specimens using the non-modified and liquid rubber modified epoxy adhesives, respectively. On the basis of the experimental results, a bi-linear bond-slip model considering the influence of liquid rubber content was proposed, and the critical parameters including the maximum interfacial shear stress, initial slip, fracture energy, and the maximum slip were identified, respectively. The present study lays a foundation for the subsequent research on the long-term bond enhancement for the CFRP-steel interface under freeze-thaw environment.

Experimental study on mixed mode-I & II bond behavior of CFRP-to-steel joints with a ductile adhesive

Utilizing ductile adhesive on CFRP-reinforced steel structures makes the bonded interface with good resistance to dynamic and fatigue loading, which is suitable for the rehabilitation of steel bridges. Currently, considerable studies on the aspect of the CFRP–steel interface are focused only on mode-II loading, ignoring the effects of mode-I loading which could lead to a catastrophic collapse of the retrofitted structures. In the present paper, modified CFRP-to-steel bonded double overlapped joints with various initial peeling angles (θ) were manufactured in order to consider mode-I loading and study the bond behavior of CFRP-to-steel joints under mixed mode-I&II loading. The results showed that the bonded side with weaker adhesion between CFRP and adhesive debonded first and presented a CFRP-adhesive interfacial debonding mode, while the debonding of the other side was delayed and showed a cohesive failure mode in the adhesive. The bond shear stress vs. slip relationship of the debonded side showed to have a bi-linear trend in which the bond strength and interfacial fracture energy were low whilst the other side showed a trapezoidal relationship with a constant plastic plateau and with higher interfacial fracture energy. It was found that sudden debonding caused a quick propagation in the CFRP strain as well as on the global interfacial slip of the joints. It was also found that the peeling angle had significantly deteriorated the local bond response in mode-II, i.e. the trapezoidal bond shear stress vs. slip relationship in the joints with θ=0° shifted to a bi-linear relationship in the joints with θ=1°, θ=2° and with θ=3° in which the bond strength, slip and fracture energy, all decreased very much. Moreover, with θ=4° was experimentally determined as the threshold angle from which the associated mode-I loading can, per se, separate the CFRP from steel.

Experimental and theoretical study on mechanical behaviors of CFRP–steel interface

The performance of the CFRP–steel interface is crucial to the reinforcement efficiency of the steel structures strengthened by using externally pasted CFRP sheet method; while it is influenced significantly by adhesive properties and geometrical characteristics of joints. The present study aims to optimize the adhesive layer of the CFRP–steel interface and establish a theoretical model of predicting the interfacial behaviors of the optimized CFRP–steel joints. A commercially available adhesive was modified by addition of Nano-SiO2 ranged from 0 to 7wt% to ascertain the optimal content of Nano-SiO2. A series of tensile tests were carried out on dog-bone adhesive specimens, single-lap shear specimens and CFRP–steel double-lap joints to obtain the mechanical properties of the adhesive at different content of nanofiller and the corresponding bond–slip laws in the unmodified and modified CFRP–steel interfaces. Accordingly, theoretical models were derived to predict the load capacity of the joints and the bond stress distribution along the CFRP–steel interface during different loading phases. The results show that the mechanical properties of the adhesive are improved with the addition of nano-SiO2 up to 1.4 wt%. When the content of nano-SiO2 rises from 1.4 to 7 wt%, the tensile and shear strengths as well as the tensile modulus decline gradually whereas the ductility keeps growing with a lower growth rate. These indicate the nano-SiO2 content of 1.4 wt% is favorable. Both the peak stress and the load capacity of the CFRP–steel joint increase with the adhesive thickness varying from 0.5 to 1 mm. The bond–slip law of the unmodified CFRP–steel interface can be simplified to be bilinear, regardless of the adhesive thickness and curing duration. In contrast, the modified interface presents a trilinear curve due to the special plastic plateau caused by the filling of nano-SiO2. Consequently, a larger fracture toughness and a higher load capacity can be reached by the optimized joints. The theoretical results are in agreement with the experimental phenomena and results. The modified adhesive is beneficial to utilize the CFRP material better and improve the reinforcement efficiency.

A Study on the Bond-Slip Relationship of the CFRP-Steel Interface of CFRP Strengthened Steel.

The bonding interface between the CFRP and the steel plate is the weak link of CFRP-strengthened steel structures. This paper studies the bond–slip relationship of the CFRP–steel interface by experiments and numerical tests. First, a series of double-strap experiments on a CFRP-strengthened steel plate are carried out. The results show that the maximum shear stress of the bonding interface of the Q345B specimen is larger than that of the X100 specimen. The initial slip and maximum slip become larger as the thickness of the bonding interface becomes larger. Finite element analysis of the above tests is carried out; we introduce the maximum stress criterion to simulate the bonding interface, which assumes that when the nominal stress of the material reaches the maximum nominal stress of damage, the material begins to damage. The FE model established has proved very effective for analyzing the bond characteristics of CFRP-strengthened steel plates. Finally, a verification test was carried out, using an FE analysis to verify the accuracy of the modified equations; the results prove that the results of the modified equations are in good agreement with the numerical results and experiment results, which verifies the effectiveness of the equations.

Bonding Performance of CFRP-Steel Interface after Continuous High Temperature

The bonded shear performance of CFRP-reinforced steel structures following continuous high-temperature natural cooling was investigated experimentally, and 30 CFRP-steel specimens were subjected to single-shear tensile testing. In analyzing the shear stress-displacement relationship in each working condition, the equation for the coupling effect of the heating temperature and duration on the ultimate shear stress was proposed. The results show that the ultimate shear stress at the CFRP-steel interface tends to decrease with the increase of heating temperature, and the ultimate shear stress at the heating temperature of 300°C is 32.37% of that at 25°C. The high-temperature duration has an obvious influence on the bonding performance of the CFRP-steel interface, and the CFRP-steel specimens fail when the heating temperature is 300°C and lasts for 120 min.

Environmental durability of epoxy-bonded CFRP-to-steel joints in mode I fracture

Exposure to environmental stresses can weaken the connection of epoxy-bonded CFRP-strengthened steel elements. As a result, designers often express concern about possible premature delamination arising from combined peel and shear effects at the CFRP-steel interface. This study was undertaken to further knowledge in relation to the peel failure of epoxy-bonded CFRP-to-steel joints subjected to hygrothermal conditioning. The effect of the bond-line thickness on the bond strength was also examined in this study. A total of 54 specimens were tested including 30 adhesive CFRP-to-steel specimens and 24 adhesive coupons. Two commercially available structural epoxy adhesives were examined. Hygrothermal conditioning may cause the adhesion failure near to the edges of the adhesive joints. The load-carrying capacity and strain energy release rate of adhesive joints depend on the adhesive and hygrothermal environment, and can be degraded up to 15%. The statistical analysis using two-way ANOVA method was applied and showed that the wet conditioning has a significant effect on the maximum failure load of CFRP-to-steel bonded specimens. Reducing the bond-line thickness to less than 1 mm can result in a major reduction in load-carrying capacity and strain energy release rate of CFRP-to-steel bonded specimens. Humid and wet conditioning cause up to 44% and 63% drop in elastic modulus, 31% and 55% reduction in tensile strength, and 32% and 44% increase in average maximum strain of adhesive materials, respectively.

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.

Development of data-driven prediction model for CFRP-steel bond strength by implementing ensemble learning algorithms

Bonding carbon fiber reinforced polymer (CFRP) laminates has been broadly utilized in steel structure rehabilitation. As for the final strengthened capacity, the bond strength between CFRP and steel usually plays a dominant role instead of the CFRP’s mechanical property. However, the bond behavior of the CFRP-steel (CS) interface is very complicated with various failure modes and consequently the bond strength is hard to estimate leading to the CFRP strengthened steel structure insecure. Under this circumstance, in order to accurately predict the bond strength of CS, efficient data-driven models were developed through implementing ensemble learning (EL) algorithms named by “gradient boosting decision tree (GBDT)” and “random forest (RF)” as two representative ones on a collected CS single-shear test database. These models’ performances on bond strength prediction were compared and also three representative machine learning algorithms “artificial neural network (ANN)”, “support vector machine (SVM)” and “classification and regression tree (CART)” are utilized for validating the necessity. The comparison results indicate that the model generated by the GBDT algorithm attains the best accuracy for CS interfacial bond strength prediction (R2 = 0.98) among the ensemble and machine learning algorithms. Through model explaning analysis, the mechanism behind the GBDT based prediction model was also carefully verified. After these tests and analyses, the GBDT based model was proved to have the potential to facilitate the design and evaluation of CFRP strengthened steel structures.

Debonding development in cracked steel plates strengthened by CFRP laminates under fatigue loading: Experimental and boundary element method analysis

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.

Experimental study on the bond behavior of the CFRP-steel interface under rapid loading

In this paper, a series of the carbon fiber reinforced polymer (CFRP)-steel single-shear specimens was manufactured to investigate the effects of the adhesive types and loading rates on the bond behavior of CFRP-steel interfaces. The main properties of the bonded joints were examined under loading rates of 0.01 mm/s, 10 mm/s and 100 mm/s, including the failure mode, the ultimate load, the load-slip curve, the CFRP strain distribution and the bond-slip relationship. The experimental results showed that the main mechanical properties of CFRP plate and adhesives increased with increases in the loading rate. The ultimate load of the CFRP-steel joint with adhesive sikadur-30 (CN) increased with increasing loading rate, while the ultimate load of the interface with adhesive sikadur-30 substantially decreased as the loading rate increases from 0.01 mm/s to 10 mm/s. Finally, the bilinear bond-slip parameter model was proposed by analyzing the influence of loading rate on key parameters.

Bond-slip model of the CFRP-steel interface with the CFRP delamination failure

The bond-slip model of a carbon fiber reinforced polymer (CFRP)-steel bonded interface is crucial for predicting the bond behavior of CFRP-strengthened steel structures. CFRP delamination failure is often encountered in existing studies on the bond behavior between CFRP plates and steel plates. However, this failure mode has been investigated by few scholars, especially regarding the interfacial bond-slip model based on CFRP delamination failure. In this paper, a total of 16 single-shear joints were manufactured to investigate the effects of the adhesive types, the adhesive thicknesses and the CFRP types with different inter-laminar shear properties on CFRP-steel bonded interfaces. Strains and displacements on the surface of the CFRP-steel joint were measured by three-dimensional digital image correlation (3D-DIC) technology. The bond-slip model based on CFRP delamination failure was established for the first time by introducing the inter-laminar shear strength, the inter-laminar shear modulus and the inter-laminar shear energy dissipation of the CFRP plate. The results showed that the adhesive thickness and the inter-laminar shear properties of the CFRP plate had a significant effect on the bond-slip model based on CFRP delamination failure. Finally, the accuracy of the model was verified by comparison with experimental data.

Study on the Interface Constitutive Relation between Carbon Fiber Fabric and Steel.

Peeling failure at the interface is one of the main failure modes for CFRP (Carbon Fiber-Reinforced Polymer)-reinforced steel structures. However, there are very few reported studies on the bond-slip relationship at the CFRP-steel interface. A series of simple shear tests were carried out in the present paper. The influence of the fiber fabric’s width and thickness, the surface roughness of the steel sheet, and the thickness of the adhesive layer on the bonding performance of the CFRP fabric to steel interface was considered. The interface constitutive model and bonding strength model were further established under multiple factors. The results show that with the decrease of the surface roughness, the interface’ ultimate peeling load increases gradually, and the failure has a tendency to develop from a glue-steel interface to a glue-CFRP interface. The test pieces that were subjected to sand blasting obtained the peak value of the ultimate peeling load. This indicates that sand blasting can effectively enhance the interface bonding strength. The theoretical values obtained via the interface prediction model are consistent with the experimental values. This proves that the newly developed interface prediction model can effectively predict the local bonding slip and bonding strength of the interface.

Experimental study on the bond behavior of the CFRP-steel interface under the freeze–thaw cycles

The bond–slip degradation relationship between carbon fiber-reinforced polymer and steel in a freeze–thaw environment is crucial to evaluate the long-term service performance of steel structures strengthened with carbon fiber-reinforced polymer plates. However, limited studies on the durability and long-term performance of the carbon fiber-reinforced polymer-steel-bonded interface are the major obstacle for the application of carbon fiber-reinforced polymer plates in strengthening steel structures. This paper reports an experimental study to investigate the effects of the carbon fiber-reinforced polymer bond length and the freeze–thaw cycles on the bond behavior of the carbon fiber-reinforced polymer-steel-bonded interface. The three-dimensional digital image correlation technique is applied to obtain displacements and strains on the surface of the single-shear specimen. The experimental results present herein include the failure mode, the ultimate load, the carbon fiber-reinforced polymer strain distribution, the displacement distribution, and the bond–slip relationship. The results show that the ultimate load increases with increasing bond length until a certain bond length value is reached, after which the ultimate load remained approximately constant, and the ultimate loads of carbon fiber-reinforced polymer-steel interface decrease gradually under freeze–thaw cycles. The bond–slip parameters degradation models are proposed, and the bond–slip degradation relationship under the freeze–thaw cycles is established. Finally, the bond–slip degradation relationship is confirmed through comparisons with the experimental results.

Corrosion mitigation of CFRP-steel interface with sacrificial anodes

ABSTRACTThis paper presents a corrosion mitigation scheme for steel members strengthened with carbon fiber reinforced polymer (CFRP) sheets. Of interest is the behavior of CFRP-steel interface with sacrificial anodes (zinc coins) that are in direct contact with the steel substrate. In accordance with electro-galvanism, accelerated corrosion testing is conducted to examine the effects of the sacrificial anodes on interfacial capacity, friction between the CFRP and corroded steel surface, and strain development along the interface. Ancillary tests indicate that the strength of the bonding agent and CFRP is not influenced by corrosion-conditioning, whereas the coefficient of friction increases with exposure time. Although the average corrosion rate of the interface slows down owing to the presence of the sacrificial anodes, this benefit diminishes as the anodes are consumed by free energy reductions in the zinc-steel galvanic couple. Under the progression of corrosion damage, the cases with the sacrificial anodes better preserve the interfacial capacity and demonstrate more stable responses than those without the anodes until CFRP-debonding failure takes place. An analytical model consisting of Markovian theory and Monte-Carlo simulation is developed to predict the capacity transition of the interface from one state to another. The concept of information entropy is taken to assess the resilience of the corrosion-damaged interface with the sacrificial anodes.

Effect of Temperature Variation on Bond Characteristics between CFRP and Steel Plate

In recent years, application of carbon fiber reinforced polymer (CFRP) composite materials in the strengthening of existing reinforced concrete structures has gained widespread attention, but the retrofitting of metallic buildings and bridges with CFRP is still in its early stages. In real life, these structures are possibly subjected to dry and hot climate. Therefore, it is necessary to understand the bond behavior between CFRP and steel at different temperatures. To examine the bond between CFRP and steel under hot climate, a total of twenty-one double strap joints divided into 7 groups were tested to failure at constant temperatures from 27°C to 120°C in this paper. The results showed that the joint failure mode changed from debonding along between steel and adhesive interface failure to debonding along between CFRP and adhesive interface failure as the temperature increased beyond the glass transition temperature (Tg) of the adhesive. The load carrying capacity decreased significantly at temperatures approaching or exceedingTg. The interfacial fracture energy showed a similar degradation trend. Analytical models of the ultimate bearing capacity, interfacial fracture energy, and bond-slip relationship of CFRP-steel interface at elevated temperatures were presented.

Galvanic Current Influencing Interface Deterioration of CFRP Bonded to a Steel Substrate

AbstractThis paper presents an experimental program investigating the effect of galvanic current on the physical and mechanical characteristics of carbon fiber reinforced polymer (CFRP) composite sheets bonded to a steel substrate. Electrochemical reaction is induced by galvanic interaction between anodes (CFRP-steel interface specimens) and cathodes (metallic strips) linked with an electrolyte (a 3.5% sodium chloride solution). Thirty-five test specimens are exposed to various periods of galvanic current from 0 to 72 h until their corrosion rate is converged. Hydrated ferric oxide forms along the CFRP-steel interface with some concentration in the vicinity of its edge, which accompanies a loss in surface area and mass. The electrochemical reaction imposed by the galvanic current exponentially decays with an increase in exposure time. The initiation of corrosion noticeably affects the load-carrying capacity of the CFRP-steel interface, whereas its propagation is not a critical attribute until substantial ...

Debonding Mitigation of CFRP-Strengthened Steel Beams with Silyl-Modified Polymer and CFRP Wrap Anchorage

AbstractThis paper presents an innovative strengthening methodology for damaged steel beams with carbon-fiber-reinforced polymer (CFRP) sheets. Emphasis is placed on controlling premature CFRP debonding using an emerging material called silyl-modified polymer (SMP) in conjunction with its hybrid bond application with a conventional epoxy adhesive. The hypothesis proposed is that the low-modulus SMP addresses stress singularity along the CFRP-steel interface, while the high-modulus epoxy preserves interfacial capacity. CFRP anchorage assists the efficacy of such a hybrid bond configuration. A two-phase experimental program is conducted to evaluate the performance of various CFRP-strengthening schemes, including element- and structural-level investigations (i.e., interface and beam tests, respectively). Test results reveal that the SMP bond mitigates CFRP debonding that is commonly observed in epoxy bond. The strengthening systems employed significantly improve the flexural behavior of the test beams in ter...

Residual performance of a silyl-modified polymer adhesive for CFRP-steel interface exposed to thermally-induced stress states

This paper presents an experimental program evaluating the residual behavior of carbon fiber reinforced polymer (CFRP)-steel interface bonded with a silyl-modified polymer (SMP) adhesive when subjected to thermally-induced stress states. A total of 40 specimens are prepared and exposed to elevated temperatures from 25°C to 200°C. According to test results, interfacial capacity is well maintained up to a temperature of 100°C; however, thermal hysteresis takes place beyond 100°C. Exposure temperatures higher than 175°C result in a phase-transition of the adhesive morphology during heating and influence the adhesion characteristics of the bonding agent. Geometric discontinuities along the interface affect CFRP-strain development and a shear-lag mechanism is noticed, which is virtually independent of the degree of thermal exposure. Fiber disintegration is the primary failure mode of the interface up to 150°C, including intricate local fiber dislocation and partial CFRP pull-out. The specimens exposed to temperatures higher than 175°C revealed a simple CFRP-debonding failure mode due to the thermal damage of the SMP. A probabilistic approach is taken to complement the experimental observation. The probability of the CFRP-steel interface tested is found to be normally distributed. The Bayesian updating method probabilistically infers the thermal characteristics of the interface with emphasis on temperature-dependent interfacial fracture energy. Design properties are proposed for practical application of the CFRP-steel interface with SMP.

PREDICTIVE RESPONSE OF NOTCHED STEEL BEAMS REPAIRED WITH CFRP STRIPS INCLUDING BOND-SLIP BEHAVIOR

This paper presents the flexural behavior of notched steel beams repaired with carbon fiber-reinforced polymer (CFRP) strips. A combined experimental and computational approaches are used to examine local plasticity near the damage and the effects of CFRP-repair. A modeling approach is proposed to take into account the bond-slip behavior of CFRP-steel interface. The experimentally validated models are further used to conduct a parametric study addressing various engineering properties of CFRP composites and adhesives. The CFRP-repair is shown to restore the strength of the damaged beam. The CFRP strip relieves the stress concentration resulting from the presence of the notch, reducing the high local plasticity. The parametric study confirms the improved effectiveness of high modulus CFRP (i.e., exceeding 150 GPa) in affecting repairs of steel members. Under static loading conditions, the stiffness of the adhesive bond line influences the local behavior of the CFRP-steel interface but has little effect on the overall member behavior.