Mixed-mode Debonding Research Articles

Mixed-Mode Debonding in CFRP-to-Steel Fiber–Reinforced Concrete Joints

Mixed-Mode Debonding in CFRP-to-Steel Fiber–Reinforced Concrete Joints

Experimental study on the mixed mode debonding by using a modified CFRP-to-steel double strap joint

The bond response of CFRP-to-steel joints in mixed mode-I/ΙΙ is a big concern since the loads consistent with mode-I may decrease the shear load capacity of the joints. For this motive, the current work introduces a modified CFRP-to-steel double strap bonded joint in which different initial peeling angles can be created. The specimens were monotonically loaded with a tensile machine until failure that subjected the CFRP-to-steel joint to a pull-pull condition. Although the specimens were geometrically symmetric throughout their length, the results showed that the initial debonding only occurred on one side of the two bonded CFRP strips. The side that initially debonded showed an interfacial failure between the CFRP strip and the adhesive, while the other side showed a CFRP delamination. The increase of the peeling angle significantly affected the initial shear load capacity, which decreased from the baseline 0° by 50% and 70% when 2° and 3° were adopted, respectively. Moreover, a peeling angle of 4º was found to be the critical peeling angle for the adopted CFRP-to-steel joints, i.e. the angle beyond which the specimens debond when subjected to pure mode-I loading even before any loads consistent with pure mode-II were applied to the bonded joints. The initial debonding induced a quick increase in the global slip and lower stiffness in the load-slip relationship. For the bonded region near the CFRP-loaded end, the maximum bond shear stress decreased from 20 to 7.5 MPa when the initial peeling angle varied from 0º-3º. To represent these effects on the local bond behavior, a new nonlinear bond-slip relationship is proposed and adjusted with the Integrate Absolute Errors (IAE) to the experimental results, which was achieved with IAE values lower than 4%. The results show, therefore, a high precision and versatility of the model to locally represent the bond behavior of the present CFRP-to-steel joints with different initial peeling angles.

A robust and efficient 3D mixed-mode cohesive interface model for predicting the debonding failure in FRP strengthened concrete structures

A new and practical cohesive zone model (CZM) considering the coupling between damage mechanics and plasticity is proposed to facilitate structural-scale finite-element (FE) simulations for concrete members strengthened with fiber-reinforced polymer (FRP). The model dwells on an adaptive sub-stepping technique to overcome common convergence difficulties during mixed-mode debonding between FRP and concrete. Additionally, it automatically adjusts the computational time increment based on the convergence performance of Newton-Raphson iterations, ensuring efficiency by applying time stepping selectively to critical integration points with diverging problems. Moreover, the model introduces a transformation between effective stress and nominal stress to enhance numerical stability when dealing with complex interfacial behaviors like stiffness degradation, plastic deformation, dilation effects, and mixed-mode failure. Thus, the paper’s novelty lies in the development of a stable and versatile 3D interface model achieved through advanced numerical optimization techniques. The resulting FE models have accurately reproduced various failure modes, such as FRP-concrete debonding and local buckling of FRP plates, demonstrating a high level of reliability and computational efficiency.

Three-dimensional numerical investigation of mixed-mode debonding of FRP-concrete interface using a cohesive zone model

• A 3D mixed-mode cohesive zone model of the FRP-concrete interface is proposed. • The mixed-mode damage factor based on the exponential softening relationship is clearly expressed. • The friction in the interface is considered independently by considering the interface sliding. • The simulation results fit well with the experimental ones in different validation cases. • The shear-peeling debonding behavior of the interface with different peeling angles is analyzed particularly. As a low-cost high-performance method for concrete structural reinforcement and repair, the external bonding (EB) of fiber-reinforced polymer (FRP) laminates has gradually become popular. But these FRP-reinforced structures usually fail prematurely due to the interfacial debonding. Thus, the mixed-mode debonding behavior of the FRP-concrete interface by a 3D cohesive zone model (CZM) will be analyzed in this study. The 3D CZM provided a detailed description of interfacial damage propagation and friction-sliding evolution during the whole debonding process. Based on the program coding which implements the proposed CZM, numerical models will be built according to the previous experiments. By comparison, the numerical and experimental results showed a good agreement. Besides, the key parameters, which include peeling angle, strength ratio, energy ratio, and friction coefficient, will also be investigated to assess their effect on the interfacial bond capacity.

Three-Dimensional Numerical Investigation of Mixed-Mode Debonding of Frp-Concrete Interface Using a Cohesive Zone Model

Three-Dimensional Numerical Investigation of Mixed-Mode Debonding of Frp-Concrete Interface Using a Cohesive Zone Model

Mixed-Mode Debonding Behavior between CFRP Plates and Concrete under Fatigue Loading

The cohesive zone model (CZM), which is capable of simulating the behavior of adhesively bonded joints subjected to static and fatigue loading, is widely accepted. In this study, fatigue peeling/shear of RC with externally bonded (EB) carbon fiber–reinforced polymer (CFRP) laminates under the mixed-mode implementation of the CZM is proposed. The procedure has been implemented in commercial software by programming the appropriate software-embedded subroutines. The main advantage of the proposed CZM is the use of coupled mixed-mode damage (shear-peeling), accounting for cumulative damage resulting from fatigue loading. Two-dimensional numerical simulations of the innovative double-lap-shear bonding test were performed by the proposed CZM. The excellent agreement between the simulated results and the experimental results are verified, and the good performance of the method as a predictive tool is demonstrated. Moreover, the peeling/shear propagation during loading can be effectively predicted.

A thermodynamically consistent cohesive-frictional interface model for mixed mode delamination

A new interface constitutive model based on damage mechanics and frictional plasticity is presented. The model is thermodynamically consistent, it is able to accurately reproduce arbitrary mixed mode debonding conditions and it is proved that the separation work is always bounded between the fracture energy in mode I and the fracture energy in mode II. Analytical results are given for proportional loading paths and for two non-proportional loading paths, confirming the correct behavior of the model for complex loading histories. Numerical and analytical solutions are compared for three classical delamination tests and frictional effects on 4ENF are also considered.

Three-dimensional isogeometrically enriched finite elements for frictional contact and mixed-mode debonding

We present an isogeometric enrichment technique for three-dimensional finite element computations applied to frictional contact and mixed-mode debonding. This is an extension of previous work that focused on two-dimensional and frictionless problems. To offer a more complete view of the enriched element’s performance, a comparison of the results to tri-variate isogeometric discretizations and standard Lagrangian elements is also included here. The enrichment is applied by discretizing parts of the surface that require higher accuracy with isogeometric basis functions, while the rest of the body uses Lagrangian shape functions. By using an isogeometric surface representation, the higher continuity across element boundaries and higher order of interpolation can be exploited. At the same time, the generation of tri-variate isogeometric meshes is avoided.A convergence study without any surface effects, involving only volume integrals, shows that the enriched elements can also be beneficial for these problems. The major advantage of the isogeometric element enrichment over standard tri-linear elements is demonstrated in contact problems including normal and tangential tractions. For both, mixed-mode cohesive debonding and frictional contact, the enrichment increases robustness and leads to more accurate results than standard linear Lagrangian elements. All computations are also compared to results using tri-variate isogeometric discretizations to give a complete picture of the element’s performance. It is also shown that the proposed enrichment formulation has advantages in mesh generation.

Extended Identification of Mechanical Parameters and Boundary Conditions by Digital Image Correlation

This paper represents a further contribution to the study of identification procedures for material mechanics resting on kinematic measurements provided by 2D Digital Image Correlation (DIC) at the microscale. Reference is made to non-conventional experiments on adhesively bonded assemblies industrially manufactured for aerospace applications. For calibration purposes a local approach is considered under plane stress conditions, focusing on a small sub-domain on the sample surface, in which mixed mode debonding is monitored. As a novelty, both the (cohesive) mechanical parameters of the interface and the actual boundary conditions prescribed at different time instants during the test are considered as unknowns to be estimated on the basis of full-field data. In this way, data smoothing and parameter identification procedures, so far usually performed in a sequence, are tackled simultaneously in a coupled framework. Since the inverse problem generalized as mentioned above turns out to be severely ill-posed, suitable regularizing provisions are applied, concerning the a priori regularity of (kinematic) displacement fields, from which boundary data are sampled, and the equilibrium (Neumann) conditions along the cracked part of the interface.

Analytical Studies on Mixed-Mode Debonding along the FRP–Concrete Interface in the Peeling Test

The bonding of fiber reinforced polymer (FRP) strips and plates to the concrete structures has been found to be an effective technique for flexural strengthening. The FRP is then under both pulling and peeling forces, resulting in a combination of shear sliding and opening displacement along the FRP/concrete interface. A novel experimental set-up is studied that a peeling load is applied on the FRP sheet by a circular rod placed into the central notch of the beam. Based on the linear-elastic fracture mechanics approach, a theoretical analysis is conducted on specimens representing the peeling behavior. From the numerical analysis, the load–displacement curves, load–stiffness of FRP sheet curves, and load–fracture energy curves affected by different variables are discussed. The peel load is related to the FRP sheet stiffness and to the interfacial fracture energy. Therefore, only two material parameters, the interfacial fracture energy of FRP–concrete interface and stiffness of FRP sheets, are necessary to represent the interfacial fracture behavior. The theoretical load–deflection curves of specimens agree well with the corresponding experimental results in the literatures.

Effect of interfacial debonding on the failure behavior in a fiber-reinforced composite subjected to transverse tension

Abstract By using computational micromechanics, macroscopic stress–strain curves for a glass/epoxy lamina subjected to transverse tension were determined in this paper. To compute stress for given strain, a finite element model of a three-phased unit cell with the hexagonal symmetry was employed. Mixed mode debonding conditions between reinforcement and matrix were modeled by a bilinear cohesive law. A stress transfer between matrix and fiber was simulated by a inhomogeneous interphase. A detailed analysis of a debonding growth was also presented. Parametrical studies showed that a choice of values for cohesive parameters as well as debonding locations do have an influence on the macroscopic response of material. An ability of the proposed model to simulate the softening behavior of the material under transverse tension and to predict final failure was demonstrated.

Modeling of mixed-mode debonding in the peel test applied to superficial reinforcements

This paper focuses on modeling of the interface between a rigid substrate and a thin elastic adherend subjected to mixed-mode loading in the peel test configuration. The context in which the investigation is situated is the study of bond between fiber-reinforced polymer (FRP) sheets and quasi-brittle substrates, where FRP sheets are used as a strengthening system for existing structures. The problem is approached both analytically and numerically. The analytical model is based on the linear-elastic fracture mechanics energy approach. In the numerical model, the interface is discretized with zero-thickness contact elements which account for both debonding and contact within a unified framework, using the node-to-segment contact strategy. Uncoupled cohesive interface constitutive laws are adopted in the normal and tangential directions. The formulation is implemented and tested using the finite element code FEAP. The models are able to predict the response of the bonded joint as a function of the main parameters, which are identified through dimensional analysis. The main objective is to compute the debonding load and the effective bond length of the adherend, i.e., the value of bond length beyond which a further increase has no effect on the debonding load, as functions of the peel angle. The detailed distributions of interfacial shear and normal stresses are also found. Numerical results and analytical predictions are shown to be in excellent agreement.

Nonlinear fracture mechanics of flexural–shear crack induced debonding of FRP strengthened concrete beams

External bonding of FRP plates or sheets has emerged as a popular method for strengthening reinforced concrete structures. Debonding along the FRP–concrete interface can lead to premature failure of the structures. In this study, the FRP–concrete interface debonding induced by a flexural–shear crack is analyzed using a nonlinear fracture mechanics approach. A bilinear bond-slip model is used to simulate the shear traction-separation law of the FRP–concrete interface; while the normal traction-separation law of the interface is approximated by a triangular model. Closed-form solutions of interfacial stresses, the axial force of the FRP plate are obtained for the whole debonding process and verified with numerical analysis using finite element analysis. The peeling effect induced by the flexural–shear crack is accounted for and its effect on the interface debonding is examined in detail in this study. Parametric studies are conducted to provide a better understanding of the mode-dependent debonding process induced by the flexural–shear crack. The present model provides a unified description of the debonding initiation and propagation, which can be used to analyze mixed-mode debonding of the FRP–concrete interface efficiently and effectively.

A Three-Dimensional Viscoelastic Analysis of Thin Film Delamination in a Peninsula Blister Specimen

The objective of this paper is to examine three-dimensional and viscoelastic effects in the peninsula blister test for thin film adhesion, which is considered an effective way to measure the interfacial fracture toughness of bonded thin film structures. As will be demonstrated in this paper, analytical solutions are sometimes inadequate for accurately simulating peninsula blister experiments because linear elastic behavior is assumed in the thin film as well as in the debonding process zone at the interface. For this purpose, a three-dimensional cohesive layer model and corresponding liquid loading simulation algorithm were developed and implemented into an in-house test-bed finite element analysis (FEA) code (NOVA-3D). Features such as three-dimensional mixed-mode debonding, large displacements and rotations, residual stresses in the thin film, and time-dependent (viscoelastic) effects in the thin film were considered. Numerical convergence and a stable debond growth were obtained over a fairly large debond length when suitably refined FEA mesh and liquid volume increment were employed. Detailed benchmark comparisons of the finite element predictions with analytical solutions and experimental results were performed, and good agreement was obtained.

Cohesive zone model of FRP-concrete interface debonding under mixed-mode loading

External bonding of FRP plates or sheets has emerged as a popular method for strengthening reinforced concrete structures. Debonding along the FPR-concrete interface can lead to premature failure of the structures. In this study, the FRP-concrete interface debonding under general mixed-mode loading is analyzed using a cohesive zone model. A novel nonlinear bond-slip model is used to simulate the shear stress-separation law of the FRP-concrete interface; while the normal stress-separation law of the interface is approximated by a linear elastic model. Closed-form solutions of interfacial stresses, FRP stress are obtained for a typical single-lap specimen for the whole debonding process and verified with experimental results. This model provides a unified description of debonding initiation and progression of the FRP-concrete interface. The mode mixity of the debonding is also retrieved naturally in this model. Parametric studies are conducted to provide a better understanding of the mode-dependent debonding process of the FRP-concrete interface. The present model can be used to analyze mixed-mode debonding of the FRP-concrete interface efficiently and effectively.

Modelling of mixed mode debonding in externally FRP reinforced beams

In this paper a refined model able to analyze edge debonding problems in beams strengthened with externally bonded composite laminated plates, is presented. The structural system is viewed as composed by three physical different components: the base beam (made of steel or concrete), the adhesive layer and the bonded plate. Each component may be comprised by one or several mathematical layers which adopts the first-order shear deformation laminate theory. Bonding and continuity conditions between different layers are simulated by using the interface modelling technique. Strong and collapsed interface models are introduced in order to capture stress singularities and to reduce the complexity of the analysis, respectively. Governing equations for displacement fields complemented with boundary and continuity conditions, are obtained by a variational approach. According to a fracture mechanics approach, the analysis is carried out by evaluating the total and individual mode components of energy release rate (ERR). Applications for typical strengthened systems, carried out by numerical integration procedures, are proposed in which the energy release rates are evaluated by means of interface displacement jumps, leading to a very efficient numerical procedure. The approximations introduced in the model with respect to the adopted number of mathematical layers are analyzed and comparisons with existent models are given. For the simpler two-layer model of the structure, comparisons are given with the closed-form solutions obtained in [Greco F, Nevone Blasi P, Lonetti P. An analytical investigation of debonding problems in beams strengthened using composite plates. Eng Fract Mech 2006, in press]. The convergence to the results from continuum analysis is investigated when a refined assembly of layers is adopted, by means of comparisons with predictions from very careful FE solutions. Finally, the effect of different debonding modes on the overall behaviour of the structural system is analyzed. These results show the capability and the accuracy of the proposed approach to predict debonding failure behaviour in both steel and concrete strengthened beams. As a matter of fact, the proposed approach involves reduced computational cost with respect to FE solutions based on 2D continuum elements and the use of a multi-layer structural model leads to avoid some complexities related to the classical elasticity theory for bimaterial interface cracks.

A treatment of mixed mode fracture in debonding

A strength based fracture criterion is introduced to describe progressive cracking. Separate internal state variables for each principal failure mode ensure consistent treatment of Mode I, Mode II and mixed mode failure. In numerical examples mixed mode debonding is observed at a curved material interface whereas Mode I separation dominates the response in a homogeneous material.

Effect of Bondline Thickness on Mixed-Mode Debonding of Adhesive Joints to Electroprimed Steel Surfaces

Abstract Structural applications of adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for techniques to assess adhesive joint strength, particularly along bondline interfaces where compliant adhesives contact more rigid metallic surfaces. The present study investigates the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel adherend surfaces. Three bondline thicknesses, representative of structural automotive joints, were evaluated for unprimed and primed bondlines. Experimental results for static load versus debond extension were input to finite element analyses for computing debond parameters (strain energy release rates). The debonds always initiated at a through-the-thickness location that had the greatest peel component of strain energy release rate. The total strain energy release rate values correlated well with trends in joint strength as a function of bondline thickness.