Interfacial Debonding Failure Research Articles

Shear behavior of multi-row perforated GFRP rib shear connectors in GFRP-concrete hybrid beams/desks

Perforated GFRP ribs (PFRs) and epoxy bonding are commonly used shear connections ensuring a full composite action in FRP-concrete hybrid beam/deck. This study demonstrates test results of 27 push-out specimens to compare the failure modes, shear load-slip (P-S) curves, and shear performance of PFRs and epoxy bonding. The variables included the sanding depth on GFRP profiles (0.5 mm/1.0 mm) and the rows of PFRs (1/2/3/5). The push-out tests highlighted that the PFR specimens with 0.5 mm or 1.0 mm sanding depth experienced interface debonding failure and shearing failure of PFR, respectively. The epoxy bonding specimens with 0.5 mm or 1.0 mm sanding depth suffered adhesive failure and cohesive failure, respectively. Increasing the sanding depth from 0.5 mm to 1.0 mm is an effective approach to prevent the debonding failure. The typical shear load-slip (P-S) curves for PFRs exhibit the micro-slipping stage (slip≤0.2 mm) and the slipping stage (slip>0.2 mm). The P-S curves of epoxy bonding showed only the elastic stage. The shear capacity of the double-row PFR connector was close to epoxy bonding. The shear capacity and stiffness of the PFRs increased 37.2∼47.3 % and 99.7∼119.7 % with the sanding depth improved from 0.5 mm to 1.0 mm, respectively. Increasing the sanding depth would improve the shear stiffness of epoxy bonding. The shear capacity and stiffness of PFRs decreased with the rows of PFRs increased. Calculation equations for the shear capacity of epoxy bonding and PFRs were evaluated by comparing the test results collected from the literature and calculated results. Based on the elastic foundation beam theory, a calculation equation for the shear stiffness of PFRs was proposed and a theoretical bi-linear model was suggested to predict the load-slip curves.

A multiscale experimental study of CFRP-UHPFRC interfacial bond behaviour: Failure mechanisms and a fibre volume dependent bond-slip law

Carbon fiber reinforced polymer (CFRP) materials are being increasingly used with ultra high performance fiber reinforced concrete (UHPFRC) to construct composite structural members making use of their unique properties. As the interfacial bond behaviour between CFRP and UHPFRC plays a key role in the design of such members, it was investigated in detail by multiscale experiments of single shear pull-off tests of 26 CFRP-UHPFRC bonded joints with different bond lengths and widths of CFRP laminates and 0.5–4% volume fraction (Vsf) of straight high-strength steel fibers (length = 13 mm, diameter = 0.2 mm). 12 CFRP-normal strength concrete (NSC) bonded joints were also tested for comparison. The surface strain evolution and the interfacial debonding process were captured and analysed by the digital image correlation (DIC). The complex microscale failure mechanisms, such as the mortar cracking, the CFRP-UHPFRC interfacial debonding, and the steel fibers’ slipping and pulling out of the mortar, were visualised and analysed by the 3D micro X-ray Computed Tomography (μXCT) scanning. It was found that the debonded layer of mortar under the laminates was much thinner than that in the CFRP-NSC bonded joints, due to the lack of coarse aggregates in UHPFRC. The peak pulling forces for the specimens with Vsf = 0.5%, 1%, 2%, and 4% were 9%, 21%, 28%, and 29% higher than those without steel fibers, respectively. An empirical bond strength model and an interfacial bond-slip model were proposed based on the test results, accounting for Vsf, the material properties, and the laminate to UHPFRC width ratio. Finally, the proposed bond-slip model was applied in finite element modelling to simulate the flexural behaviour of an FRP-UHPFRC composite deck in the literature, with the interfacial debonding failure and the ultimate loads successfully predicted.

Experimental study on bonding properties of Fe-SMA-to-steel bonded interface

The interfacial bonding performance between Fe-based shape memory alloys (Fe-SMA) and steel is of utmost importance for the reinforcement of steel structures using Fe-SMA. In order to better reveal the bonding performance and ensure the reliability of Fe-SMA-to-steel bonded interfaces, the single-lap-shear tests of 27 adhesive joints were conducted. The experimental results indicate that the bearing capacity increases with the bonding length until reaching the effective bonding length (Le), with SA, Sika, and LC adhesive joints possessing Le of around 90 mm, 90 mm, and 100 mm, respectively. Better bonding performance can be achieved with 1–1.5 mm thickness for the 3 types of adhesive joints, while the bearing capacity of LC adhesive joints is less affected by the adhesive thickness. The SA joints experienced interface debonding failure, the Sika joints experienced the mixed failure mode of interface debonding and cohesive failure, and the LC joints exhibited cohesive failure. For the bonded interfaces, cohesive failure is the preferred mode, evidenced by Fe-SMA plates in LC joints reaching 60% of their tensile strength at failure. Furthermore, utilizing the established constitutive model of Fe-SMA and the strain propagation from experiments, the shear stress evolution and interfacial bond-slip relationship of Fe-SMA-to-steel bonded interfaces were analyzed, demonstrating that the nonlinear behavior and high ductility of Fe-SMA significantly affect the load-bearing performance and failure evolution. This paper verifies the feasibility of reinforcing steel structures with Fe-SMA through adhesive bonding connection, providing an experimental basis for the theoretical analysis and practical application of Fe-SMA-to-steel bonded interfaces.

Fatigue delamination behavior in composite laminates at different stress ratios and temperatures

This study provides an investigation on mode I fatigue delamination growth (FDG) with fibre bridging at different R-ratios and temperatures in carbon-fibre reinforced polymer composites. FDG experiments were first conducted at different temperatures of R-ratios 0.1 and 0.5 via unidirectional double cantilever beam (DCB) specimens. A fatigue model, employing both the strain energy release rate (SERR) range and the maximum SERR around crack front as similitude parameter, was proposed to interpret FDG behavior. The use of this model can collapse FDG data with fibre bridging at different R-ratios into one master curve, obeying well with the similitude principles. Accordingly, it was found that FDG can accelerate with elevated temperature, but decrease at sub-zero temperature. Furthermore, there are strong correlations between the fatigue model parameters and temperature using this model in FDG interpretations. Taking these correlations into account can extend the model to accurately predict FDG behavior of other temperatures. Fractographic examinations demonstrated that temperature has effects on the FDG damage mechanisms. Both fibre/matrix interfacial debonding and matrix brittle failure were observed in FDG of −40℃. Fibre/matrix interfacial debonding becomes the dominant failure in FDG of RT and 80℃. No obvious difference on the fracture morphology was identified for FDG at different R-ratios of a given temperature.

A progressive damage model for ceramic matrix mini-composites with thick interphases

Toughness enhancement in ceramic matrix composites (CMCs) with brittle matrix and fiber phases is often accomplished by introducing a weak finite-thickness interphase between the fiber and matrix. The current work presents a progressive damage model to predict the tensile response of single tow CMCs (mini-composite) representative of a unidirectional composite at the microscale. Implementation of a 3-phase shear-lag model for a geometrically accurate representation of the underlying microstructure in CMCs with finite thickness interphase has been highlighted. A probabilistic progressive modeling approach has been adopted, accounting for multiple matrix cracking, interfacial debonding, and fiber failure in 3-phase mini-composites. The predicted tensile response of CMCs from the progressive damage modeling approach agrees with experimental results obtained for C/BN/SiC mini-composites validating the approach.

Interfacial behavior of externally bonded BFRP-to-concrete joints using different epoxy adhesives

The purpose of this study is to investigate the influence of adhesive types on the interfacial behavior of externally bonded basalt fiber reinforced polymer (BFRP) sheet-concrete substrate. Twelve manufactured double-lap shear samples were tested using four bonding adhesives. The strains on the surface of BFRP‐concrete joint were determined using electrical resistance strain gauge together with two‐dimensional digital image correlation technique (2D‐DIC). Experimental results show that properties of different bonding adhesives i.e., stiff (linear elastic, higher strength) and soft (nonlinear elastic, lower modulus, higher ultimate strain) has a significant impact on the failure mode, stiffness, ultimate load, strain and shear stress distribution nonetheless not so much on bond-slip relationship. The failure modes of specimens changed from interfacial debonding or cohesive failure in concrete substrate for stiff adhesive specimens to debonding in adhesive layer when soft adhesives are used. Stiff adhesive interfaces experienced higher ultimate loads from 1.1 to 1.3 times higher than soft adhesive interface. Meanwhile soft adhesives are shown to possess a lower maximum shear stress, higher interfacial fracture energy and longer stress transfer length.

Mechanical properties of adhesive interface for the expandable packer rubber with nylon 66 cord under high pressure and high temperature

In this paper, the expandable packer rubber with nylon 66 cord was selected as research object. By carrying out a single pull-out experiment of nylon 66 cord rubber, it was gotten that the critical failure energy of the adhesive interface between the cord and the rubber matrix was 3.21 kJ/m2. The Cohesive Zone Model (CZM) was used to describe the bonding state of interface between nylon cord and rubber matrix and the debonding failure criterion was then established for the adhesive interface between nylon 66 cord and rubber matrix. The mechanical properties and working performance were analyzed systematically for the expandable packer rubber under different pressures and well temperatures. The results showed that:1) When the fracturing pump pressure was 50 MPa∼80 MPa, the shear stress of the packer rubber matrix was less than the allowable shear stress of 17.5 MPa, and the matrix didn't appear shear tearing damage. So the packer rubber filled with nylon cord could effectively protect the rubber matrix and avoid shear tear damage. 2) When the fracturing pump pressure was 80 MPa, the interface failure energy of the lower rubber was greater than the critical failure energy of 3.21 kJ/m2, and the interface debonding failure occurred between cord and matrix. 3) When the well temperature was 25 °C–175 °C, the shear stress of the rubber matrix increased with the increase of well temperature, but he maximum shear stress of the matrix was less than the allowable shear stress of 17.5 MPa. The failure energy of the adhesive interface was less than the critical failure energy of 3.21 kJ/m2. So there was no shear tear on the matrix of the rubber and the adhesive interface was well bonded. Therefore, the expandable packer rubber with nylon 66 cord could work well in a higher well temperature environment. It is concluded that the research results can provide theoretical guidance for exploring the performance of the expandable packer rubber with nylon cord.

Using end buckles to improve debonding resistance in FRP-bonded precracked RC beams

The strengthening effect of reinforced concrete (RC) members externally bonded (EB) with fiber reinforced polymer (FRP) laminates is significantly affected by interfacial debonding failure. To improve the connection between FRP sheets and concrete, a novel type of anchor device for FRP sheets called buckle was proposed in this study. This device can effectively lock the end of the sheet using a patented method of wrapping. In addition to conventional external bonding, bolts were installed on the buckles to form hybrid anchored (HA) FRP sheet. To verify the effectiveness of this new method, a precracked RC beam was prepared in comparison with two undamaged beams strengthened with EB or HA FRP sheets. As observed, end buckles can limit the development of cracks regardless of initial cracking. Despite the occurrence of intermediate debonding due to initial cracks, both the ultimate capacity and failure ductility were significantly improved. Owing to the end buckles, the debonding resistance was excellent, and the initial cracking of the beam hardly affected the strengthening effect. In addition, existing formulas were used to calculate the lower limit of the ultimate load of the specimen strengthened with HA FRP, which was still higher than that of the specimen strengthened by EB FRP. Therefore, the results confirmed that the debonding resistance was greatly enhanced due to the locking of the FRP sheet via end buckles, and the proposed method is useful to guarantee the strengthening effect.

A simplified reinforcement and fracture mechanism analysis model of epoxy nanocomposites based on finite element simulation

Understanding the reinforcement and fracture mechanism of polymer matrix nanocomposites is crucial for the design of high-performance composite materials. By taking boron nitride@boronate polymer core-shell nanoparticles modified epoxy resin nanocomposites (EPNCs) as an example, a reinforcement and fracture mechanism analysis model of nanocomposites was established through the combination of experiments and simulations. Instead of the traditional programming method by using the external language, a new modeling method was introduced with the help of Digimat. The results indicated that the interface force between (within) EP matrix and nanoparticles, the shell thickness and nanoparticle content play important roles in the reinforcement and fracture of EPNCs. The bending strength increases with the raising of particle content, but decreases with the increasing of shell thickness. The simulated results were consistent with the experiments. The interface force evolves according to the order of interfaces within particles and their shell > matrix-shell > matrix interior. A proper interfacial force can make interfacial debonding and matrix failure occur at the same time, thus improving both the strength and toughness of nanocomposites. This model construction will provide theoretical guidance for nanocomposites design and the clarification of reinforcement mechanism. • Cross-examination of analytical, numerical and experimental results is conducted. • Appropriate interface, thin shell and particle content play important roles in composites reinforcement mechanism. • Finite element evaluation of core-shell nanoparticles reinforced epoxy resin composites wais established.

Fatigue behavior of notched steel beams strengthened by a prestressed CFRP plate subjected to wetting/drying cycles

Prestressed carbon fiber reinforced polymer (CFRP) strengthening is effective in suppressing crack propagation of steel beams. However, the fatigue behavior of notched steel beams with prestressed CFRP under a hygrothermal environment is not fully understood. Generally, notch machining on the steel beam is an ordinary method to simulate a fatigue defect for laboratory tests. In this study, the interfacial energy release rate at the notch location was first derived for strengthened steel beams. The theoretical results show that the interfacial energy release rate can be significantly reduced by prestressed CFRP strengthening, contributing to improving the interfacial fatigue performance. In addition, notched steel beams strengthened by prestressed CFRP (with a prestress level of 25% of tensile strength of CFRP plate) were exposed to wetting/drying cycles (WDCs) and then tested under fatigue loading. The experimental results show that prestressed CFRP strengthening increases the fatigue resistance of interfacial debonding initiation and failure 3.47 times and 7.83 times, respectively, which agrees with the tendency revealed by the theoretical results. In contrast, WDC exposure reduced the fatigue lives of interfacial debonding initiation and failure by 34.6% and 27.2%, respectively. An S – N curve based on the maximum principal interfacial stress at the notch location was proposed to predict the tendency of fatigue life of debonding initiation for strengthened steel beams. Moreover, the tendency prediction for fatigue life of interfacial debonding development for the strengthened steel beams with/without WDC exposure was developed based on Paris’ law. • Theoretically reveals that the prestressed CFRP reduces the interfacial energy release rate. • Prestressed CFRP strengthening significantly improves interfacial fatigue life. • Hygrothermal exposure leads to a significant reduction of interfacial fatigue life.

Interfacial debonding failure of CFRP-strengthened steel structures

Mode II debonding is one of the most typical failure modes on carbon fiber reinforce polymer (CFRP) plate-to-steel interface. In order to elucidate the interfacial debonding mechanism of CFRP-strengthened steel structures, single- and double-lap shear tests were conducted to investigate the mechanical behavior and debonding failure process at the FRP-to-steel plate interface. The technique of digital image correlation (DIC) was applied to measure the normal strain distribution on the surface of CFRP. Comparing the failure modes between these two joints, it can be found that a mode II fracture occurs on the interface of double-lap joint, which is caused by shear stresses. While the interface of single-lap joint subjected to eccentric loading is governed by mixed-mode I/II behavior, with the failure attributed to the combination of shear stress and bending moment. Additionally, the ultimate load and bond slip of the double-lap joints in mode II increase with the increment of bond length until an effective bond length is reached, beyond which the ultimate load remains unchanged. However, for single-lap joints subjected to eccentric loading, the ultimate loads with different bond length are almost the same. Based on bilinear bond-slip law obtained from experiment results, finite element model can be established to analysis the mode II interfacial debonding process of CFRP-strengthened steel structures, which shows an excellent agreement with the experimental results.

Enhancing CF/PEEK interfacial adhesion by modified PEEK grafted with carbon nanotubes

The weak interfacial adhesion caused by the low wettability of carbon fiber (CF) and inert polyetheretherketone (PEEK) hinders the great application potential of CF/PEEK composites. In this study, the interface between CF and PEEK was modified by coating hydroxylated PEEK grafted onto multi-walled carbon nanotubes (M-HPEEK). Modified CFs by M-HPEEK sizing had rougher surfaces with more polar groups, showing decreased contact angle and increased surface energy. The wettability of these CFs was enhanced, beneficial to PEEK infiltration and physical adhesion improvement. As a result, the interlaminar shear strength, flexural strength, and flexural modulus of modified CF/PEEK composites were increased by 73.0% (84.7 MPa), 163.2% (906.2 MPa), and 84.8% (58.4 GPa), respectively. An improved interface with stress transferring was proved by the interface debonding failure changed to fiber crack and resin yield. M-HPEEK sizing is considered as a promising modification method to optimize the interface performance of composites.

Manufacturing and characterization of continuous fiber-reinforced thermoplastic tape overmolded long fiber thermoplastic

Light-weight construction, design freedom, integration of functions, and compelling cost element are desired by Original Equipment Manufacturers (OEMs) and their suppliers. The emergence of overmolding of continuous-discontinuous reinforcement enables design freedom and ability to tailor stiffness, strength, and damage tolerance for structural applications. In this work, long fiber thermoplastics (LFT) are overmolded with continuous fiber-reinforced thermoplastic (CFRTP) tape are combined using extrusion compression molding process to evaluate the structural performance. The CFRTP tape overmolded LFT samples were characterized using nondestructive and destructive techniques to track fiber alignment, fiber distribution, manufacturing defects, interfacial bonding of the tape – LFT and effect of CFRTP tape on LFT. Mode 1 fracture toughness, G1c for tape overmolded LFT was higher by 25–30% compared to literature reported G1c. This response was attributed to excellent fiber distribution, good fiber wetting, and absence of voids at the interface. Three-point bend test indicated that CFRTP tape overmolded LFT composites were better able to resist damage under the bending load compared to constituent LFT composite. Flexural strength of the overmolded composite was higher by 119–142%, and modulus higher by 77–65% compared to constituent LFT composite. Simulated flexural results accurately represents the mechanical behavior of composites. The penetration energy of tape overmolded LFT composites determined by LVI test was in the range of 27.66–30.15 J, which is significantly higher than constituent LFT composite, 7.76 J. CFRTP tape overmolded LFT composite exhibits progressive fiber fracture, matrix cracking, and interfacial debonding failure, whereas, constituent LFT composite showed catastrophic fiber fracture.

Mechanical properties of steel-UHPC-steel slabs under concentrated loads considering composite action

Steel-concrete-steel (SCS) composite structure is widely used as the protective structure in the containment of nuclear power plants, offshore platforms, subsea tunnels, and so on. This study investigates the mechanical properties of SCS composite slabs with ultra-high performance concrete (UHPC) as the core material under concentrated loads. Five S-UHPC-S composite slabs were tested with different parameters of the spacing of headed studs and the thickness of the concrete core. Besides, one S-C-S composite slab with the normal concrete as the core material was tested for comparison. Two failure modes, i.e. punching shear failure and interfacial debonding failure, were defined considering different composite action between the concrete core and bottom steel plate. A punching shear cone was formed in punching shear failure, while interfacial debonding failure had an obvious interfacial slippage between the concrete core and bottom steel plate. The failure mechanism of these two failure modes was analyzed. Moreover, a design method to predict bearing capacities was developed, which could take the effect of the concrete core, steel fibers in UHPC, steel plates, and tie bars into consideration. In particular, a reduction coefficient was introduced to reflect the effect of composite action between the concrete core and bottom steel plate. The design method was suitable with six specimens in this study and other specimens in literature, which could provide theoretical guidance for the application of SCS composite structure in practical engineering.

A microscopic elasto-plastic damage model for characterizing transverse responses of unidirectional fiber-reinforced polymer composites

Abstract A phenomenological-based micromechanical method, comprising the coupling of the matrix constitutive model and the cohesive zone (CZM) model at fiber-matrix interfaces, is presented to investigate the mechanical behaviors of unidirectional (UD) fiber-reinforced polymer (FRP) composites subjected to transverse tension and compression. The key point of this method is the establishment of a novel elasto-plastic damage constitutive model using a fracture plane based yield criterion. Both plastic deformation and progressive failure procedure is incorporated in the implicit simulation. Special focus is given to the determination of the input values of constituent material properties, suggesting that directly using the macroscopic matrix properties to characterize the mechanical responses at the microscale level may bring large discrepancies in homogenized stress-strain responses of UD composites. A parametrical study is also carried out to calibrate cohesive parameters. The numerical results, in good agreement with experimental measurements, clearly reveal the damage scenarios and the dominant failure mechanisms. It can be concluded that the debonding of the fiber-matrix interfaces is responsible for the ultimate transverse tensile strength, while compression failure is governed by two possible modes, i.e. matrix compressive failure if matrix failure firstly occurs, and matrix tensile and interface debonding failure when debonding initiates first. Besides, the distribution of fracture plane angles in matrix is examined to further validate the predictive capacity of the proposed model.

Dynamic interfacial debonding in sandwich panels

A nonlinear dynamic analytical approach is formulated aiming to investigate the dynamic interfacial debonding mechanism in sandwich panels. The approach is derived using the variational principle of virtual work and it incorporates the Extended High-Order Sandwich Panel Theory with a cohesive interface modeling for the interfaces between the face sheets and the core. The model adopts the first-order shear deformation theory kinematic assumptions for the face sheets along with a geometrically nonlinear behavior. The high-order small deformations kinematic assumptions that account for out-of-plane compressibility are considered for the core layer. The two interfaces link the three components of the panel together and the nonlinear traction-displacement laws introduce the interfacial nonlinearity into the model. The model considers dynamic effects in order to assess the influence of the inertial terms on the interfacial debonding mechanism and to examine the coupling between the two. The results of the dynamic analysis are compared with results of a static analysis for two cases: a sandwich panel with a pre-existing delamination subjected to an end-shortening loading and an intact sandwich panel subjected to three-point-bending. In the second case, the interfacial debonding triggers a dynamic response that substantially affects the structural behavior. The dynamic results, the assessment of the time scales of the process, and the comparison with the static results shed light on the significance of the dynamic nature of the interfacial debonding failure mechanism.

Interfacial debonding detection of strengthened steel structures by using smart CFRP-FBG composites

The interfacial debonding detection of multi-layered strengthened structures (i.e. carbon fiber reinforced polymer (CFRP) strengthened steel/concrete beams) has always been an important problem urgent to be solved. Fiber Bragg grating (FBG) sensor is limited to directly perceive the shear strain. How to monitor the interfacial bonding state based on optical fiber sensing technology thus attracts high attention. A smart CFRP-FBG composite has then been developed in this paper. The structural integrity of the composites has been non-destructively checked by ultrasonic testing technique. Theoretical study on the strengthened steel beam has been conducted to establish a relationship between the interfacial shear stress and the normal stress of the CFRP-FBG composites. Loading tests have been performed to check the measurement accuracy of the FBGs in series and the effectiveness of the composites to perceive the interfacial debonding failure. Results indicate that the proposed smart CFRP-FBG composites can accurately identify the interfacial debonding of the multi-layered structures. The degradation process of the interfacial bonding state can be favorably reflected by the variations of strain profiles measured by the FBGs in series in the composites. The proposed method can be used to instruct the concept design of damage detection. The developed CFRP-FBG composites can be adopted to identify the damage and make in-time maintenance in practical engineering.

Assessment of mechanical, thermal and morphological behavior of nano-Al2O3 embedded glass fiber/epoxy composites at in-situ elevated temperatures

Owing to the outstanding properties of nano-Al2O3 particles have incited material researchers to have a promising invasion in the field of fibrous polymeric composites. There is an uncertainty of nano-Al2O3/polymer interfacial stability at high temperature engineering applications. Current investigations explicate the effects of nano-Al2O3 content on the mechanical behavior of glass/epoxy (GE) composites at various in-situ elevated temperatures. The flexural properties and viscoelastic behavior of the composites have been investigated and evaluated. The Weibull design parameters were analysed as a function of nano-Al2O3 content and different test temperatures. Scanning electron microscopy analyses were carried out to understand various interfacial strengthening mechanism and micro-mechanism failures. These results indicated that incorporation of 0.1 wt% of nano-Al2O3 in GE composites at room temperature could be considered an optimal value in flexural strength enhancement. The fracture surfaces demonstrated a combination of fiber pullout, interfacial debonding, matrix drainage and fiber imprints failure morphologies. Weibull analyses responded a reasonable agreement with the experimental results.

Full-range mechanical behavior study of FRP-to-concrete interface for pull-pull bonded joints

External bonding of fiber reinforced polymer (FRP) has become a popular technique for strengthening concrete structures all over the world. The mechanical performance of the interface between FRP and concrete is one of the key factors affecting the behavior of the strengthened structure. The debonding behavior of FRP-to-concrete interface is related to specific bonded joint. Compared with the pull-push bonded joint, there are few studies revealing the interface mechanical behavior of pull-pull bonded joint so far, and in-depth theoretical study is not profound enough either. This paper presents an analytical solution for the debonding process in such an FRP-to-concrete bonded joint in single-shear model. Base on the realistic bi-linear local bond-slip law, two critical lengths are obtained, and the interface debonding failure follows different regimes. Closed form expressions for the interfacial relative slip, shear stress distribution, normal stress in the FRP plate and the load-displacement response at the loaded end are derived for different loading stages, whose accuracy is verified by finite element numerical simulation. While the solution is developed with particular reference to FRP-to-concrete bonded joints, it can be equally applicable to similar bonded joints made of other materials (e.g. FRP-to-steel).

Resistance of interfacial debonding failure of GFRP bars embedded in concrete reinforced with structural fibers under cycling loads

GFRP rebars with different surface treatments such as helical wrapping and sand coating were evaluated under cyclic loads. The pull-out specimens were made from the addition of structural fibers (hooked end steel, PP, PVA fibers) with 1% of fiber volume fraction in a concrete matrix with expected change in interfacial bonding property as well as ductility improvement after first cracking. The experimental results were analyzed in terms of bond stress-slip curve, bond strength ratio, and energy dissipation. Severe failures in the interfacial layers of the GFRP rebars at the resin-bar fiber interface was observed as cycling loads increased. The addition of structural fibers to concrete generally showed significant changes for debonding failure mechanism since the strong bonding resulted in reduced energy dissipation as well as a sharp increase of load in the bond stress-slip curve. For the GFRP rebars, wedge effects from crushed or cracked particles due to repeated damage in the interfacial debonding zone affected the low dissipated energy. Abrupt pull-out bar failure was represented. The effect of fibers in the interfacial bonding layer confirmed that resistance of steel rebar to cycling debonding failures from rebar types showed the best performance. In particular, the addition of hooked steel fibers indicated the best performance regardless of rebar types.