Edge Debonding Research Articles

The biomechanical role of the functionally graded microfibrils in the wood cell wall

The cellulose microfibril in the wood cell wall possesses a functionally graded structure, the biomechanical contribution of which is unclear. At the molecular scale, we analyze the heterogeneous crystallinity of the microfibril and characterize its stiffness gradient with the help of molecular dynamics. The determined stiffness profile is then upscaled to the cell wall level, based on which a shear-lag analytical model is developed to reveal the load transfer mechanism in the cell wall. We find that the presence of the stiffness gradient can significantly alleviate the stress concentration on the interface by reducing the interfacial shear stress up to 70%. The analysis on the debonding procedure shows profoundly enhanced debonding resistance of the cell wall due to the graded structure. Moreover, we identify a novel “dual debonding” mode, meaning that the debonding initiates and propagates in both the edge and inner regions of the interface until the two debonded regions join each other, which is much more complex than the edge debonding mode exhibited by the conventional fiber-reinforced composites. The stress alleviating effect, together with the special debonding mode, profoundly postpones the interfacial debonding of the material, leading to a significantly enhanced debonding resistance. A discussion on the structural profile of the microfibril shows that the special “dual debonding” mode can only be achieved with specific stiffness gradients. In all, we believe our work helps to unravel the biomechanical role of the microfibril in the wood cell wall, and builds the theoretical foundation for tailoring the bio-inspired composites reinforced by functionally graded fibers.

Fatigue life assessment and damage evolution in Z-pinned laminates

An experimental investigation has been carried out to elucidate the damage evolution and failure mechanisms in unpinned and Z-pinned quasi-isotropic carbon fibre/epoxy laminates manufactured via a pre-hole pin insertion technique and subjected to in-plane tension-tension fatigue loading. X-ray computed tomography was used to analyze the internal configuration of samples subjected to several stress levels of the ultimate tensile stress (UTS) in order to identify and understand the internal damage evolution. The obtained results indicate that, at low cycles, the Z-pinned laminates exhibited poor fatigue behavior owing to the low ultimate tensile strength as compared to the unpinned laminates. The stiffness degradation plots and Micro CT images suggest that the Z-pin bridging inhibited extensive edge debonding which resulted in higher normalized stiffness for the Z-pinned laminates. Matrix microcracking initiated around the Z-pins and propagated through resin rich zones leading to poor load bearing ability and subsequent fibre splitting and rupture. It is worth noting that the failure modes of the quasi-isotropic laminates evolved from delamination dominated failure to the on-axial fibre failure.

Edge and Crack-Induced Debonding Analysis of FRP-Strengthened Concrete Beams Using Innovative Beam Finite Elements

In order to analyze the edge debonding and intermediate flexural crack–induced debonding of fiber-reinforced polymer (FRP)-strengthened concrete beams, several innovative beam finite elements were developed in this paper. The proposed elements were capable of modeling the concrete, adhesive layer, and FRP sheet in one macroscopic element. The explicit formulations of the proposed element stiffness matrices were given to simulate the adhesive layer in different loading stages. Comparisons against several numerical examples available in the literature were given to validate the proposed elements. The effects of the FRP tapering ends, load, and crack location on the interfacial shear stress within adhesive layers were effectively investigated using the proposed elements. It was found that the proposed elements could conveniently predict the edge and flexural crack–induced debonding behavior of FRP-strengthened beams with accuracy and computational efficiency.

Damage evolution behavior of bi-adhesive repaired composites under bending load by acoustic emission and micro-CT

In the field of composites patch repair, it is necessary to improve the repair performance of adhesive due to its important role in repair efficiency and durability. A bi-adhesive repair construction with two adhesives was designed and the carbon nanotubes were firstly applied in this bi-adhesive to carry out composites patch repair in this paper. Mechanical behaviors and deformation damages of different repair configurations under three-point bending loads were investigated using acoustic emission and X-ray micro-computed tomography. Failure mechanisms were deeply discussed through the analyses of characteristic signals and three-dimensional visualization of interior damage. The damage evolution behaviors were divided into three stages patch damage and edge debonding, further damage accumulation in adhesive layer and matrix, completely patch debonding and delamination by the clustering analysis and characteristic response of acoustic emission signals. Failure characteristics of scanning images on the adhesive interface and delamination interface revealed the changes of damage modes within different repaired specimens. The damage visualization analyses of X-ray micro-computed tomography was corresponding to the acoustic emission response of interior damage within repaired composites. A combination of these two technologies successfully realized the damage evolution in repaired composites, helping understand the evolvement rule of damage on the adhesive interface.

A Moving Interface Finite Element Formulation to Predict Dynamic Edge Debonding in FRP-Strengthened Concrete Beams in Service Conditions

A new methodology to predict interfacial debonding phenomena in fibre-reinforced polymer (FRP) concrete beams in the serviceability load condition is proposed. The numerical model, formulated in a bi-dimensional context, incorporates moving mesh modelling of cohesive interfaces in order to simulate crack initiation and propagation between concrete and FRP strengthening. Interface elements are used to predict debonding mechanisms. The concrete beams, as well as the FRP strengthening, follow a one-dimensional model based on Timoshenko beam kinematics theory, whereas the adhesive layer is simulated by using a 2D plane stress formulation. The implementation, which is developed in the framework of a finite element (FE) formulation, as well as the solution scheme and a numerical case study are presented.

Detection of edge debonding in composite patch using novel post processing method of thermography

In the present work, the step heating thermography technique has been used to detect disbond defects at the edge of the adhered carbon/epoxy patch on the aluminum plate. The four common post-processing algorithms including principle component analysis (PCA), thermal signal reconstruction (TSR), Fourier transform (FT), and one-dimensional wavelet transform (1D WT) have been applied to the infrared thermal images obtained from the composite patch thermography inspection. A novel post processing method based on the area under temperature-time curve (Thermal Signal Area: TSA) has been proposed for the infrared thermal image analysis. The proposed post-processing method is fast and non-sensitive to noise. A comparison of the new proposed method with the four common post-processing methods has been carried out on disbond detection in a composite patch in a quantitative and qualitative manner. Furthermore, a data fusion of the proposed method with the 1D wavelets transform has been applied to disbond defect characterization which improves the accuracy by decreasing noise level.

On preferential debonding during demolding of a sandwiched elastomeric layer

Separation of an elastomeric layer sandwiched between two flexible molds is a common processing step in the fabrication of soft articles such as ophthalmic lenses. The demolding process is typically engineered for the interfacial separation to preferentially occur at a desired interface by applying displacements at suitably selected locations. This study addresses the potential to predispose debonding along the desired interface by exploring roles of (i) the relative flexibilities of the two molds, (ii) differential preheating of the two molds and the interlayer, and (iii) the interlayer curvature. These objectives are accomplished by numerically analyzing the axisymmetric deformations of an elastomeric layer of uniform thickness sandwiched between the flexible molds, the flanges of which are pried apart by applying displacements along the axis of symmetry. The debonding at each interface between the molds and the interlayer is modeled by using a bilinear traction-separation (TS) relation. It is found that (i) debonding initiates preferentially from the edge of the interface between the elastomer and the mold of lower bending rigidity; (ii) preferential debonding can be engineered by differentially preheating the assembly; (iii) edge debonding is inherently biased to the interface with the smaller radius (inner) mold interface, and (iv) the occurrence of the preferential edge debonding at an interface can be attributed to a bias in the opening shear towards that interface. However, when the overall confinement of the system is large enough as dictated by the flexural rigidities of the molds for an elastomeric layer of given modulus and thickness, debonding may nucleate internally and complicate the progression of the separation process. Internal debonding may also occur when the temperature differential between the two molds is large enough at a given absolute level of temperature increment. Sensitivities of the predicted debonding mechanisms to prescribed biases in the TS parameters are assessed. These results could prove useful in design of demolding processes to achieve desired separation characteristics.

Investigation into Z-Pin Reinforced Composite Skin/Stiffener Debond under Monotonic and Cyclic Bending

Skin/stiffener debonding has been a longstanding concern for the users of stiffened composite panels in long-term service. Z-pinning technology is an emerging solution to reinforce the composite assembly joints. This work experimentally characterizes the progressive debonding of Z-pinned skin/stiffener interface with the skin under static bend loading. The three-stage failure process is identified as: flange edge debonding, pin/laminate debonding, and ultimate structural failure. Three different distribution patterns were compared in terms of the static debonding properties revealed the affirmative fact that locating pins in high normal stress regions, that is close to the flange edges in skin/stiffener structures, is more beneficial to utilize the full potential of Z-pinning reinforcement. The unit strip FE model was developed and demonstrated effective to analysis the effect of Z-pin distribution on the ultimate debond load. On the other hand, the evolution of fatigue cracks at Z-pinned skin/flange interface was investigated with a series of displacement-controlled fatigue bending tests and microscopic observations. Results show that Z-pinning postpones crack initiations at low displacement levels, and the remarkable crack-arresting function of pins enables the structure a prolonged fatigue life. However, pins become less effective when the maximum displacement exceeds the crack initiation level due to gradually pullout of pins.

Operational modal analysis and wavelet transformation for damage identification in wind turbine blades

This study demonstrates an application of a previously proposed modal and wavelet analysis-based damage identification method to a wind turbine blade. A trailing edge debonding was introduced to an SSP 34-m blade mounted on a test rig. Operational modal analysis was conducted to obtain mode shapes for undamaged and damaged states of the blade. Subsequently, the mode shapes were analyzed with one-dimensional continuous wavelet transformations for damage identification. The basic idea of the method is that structural damage will introduce local mode shape irregularities which are captured in the continuous wavelet transformation by significantly magnified transform coefficients, thus providing combined damage detection, localization, and size assessment. It was found that due to the nature of the proposed method, the value of the identification results highly depends on the number of employed measurement points. Since a limited number of measurement points were utilized in the experiments, only certain damage-sensitive modes, in which pronounced damage-induced mode shape changes occur, are applicable for valid identification of the damage.

Initiation of edge debonding: coupled criterion versus cohesive zone model

Cohesive zone models and criteria based on finite fracture mechanics are two alternatives to analyze edge debonding. A comparison between the two approaches is presented in this paper. The coupled criterion which combines a stress and an energy conditions to estimate crack initiation is used and compared with a bilinear cohesive law. Predictions of the debonding onset are in good agreement provided the characteristic fracture length of the interface remains smaller than the characteristic dimension of the specimen.

Multi-layer modeling of edge debonding in strengthened beams using interface stresses and fracture energies

The onset and propagation of edge debonding in beams strengthened with externally bonded fiber-reinforced composite plates, is here examined using an innovative multi-layer formulation. A novel mixed mode coupled criterion is adopted to predict debonding initiation accounting for both fracture energies and interfacial stresses and considering different debonding locations across the adhesive layer. The subsequent debonding propagation is studied by using a mixed mode fracture criterion. The beam system is assumed to be composed of three physical components, namely the beam, the adhesive layer and the bonded plate, each one being modeled by one or several first-order shear deformable layers. Both strong and weak interface constitutive relations are introduced to model the two physical interfaces (i.e. beam/adhesive and adhesive/plate), whereas a strong formulation is used to model the mathematical interfaces between layers inside each physical components. Results show that the proposed multilayer formulation, implemented by using a multivariable 1D finite element technique, enables to calculate interlaminar stresses and fracture energies in reasonable agreement with results obtained by using a continuum FE model, but with a significantly reduced computational cost. The numerical analyses, carried out also in a parametric form by varying both interfacial fracture toughness and strength, show that although the accuracy can be improved by using more layers for each components and adopting coupled strong/weak interface formulations, the use of few layers for each components of the strengthened system suffices to obtain an accurate prediction of debonding initiation and propagation.

Improved finite element modeling of piezoelectric beam with edge debonded actuator for actuation authority and vibration behaviour

The main emphasis of the present work is to model a piezoelectric beam with edge debonded actuator by employing finite element method based layerwise shear deformation theory, to improve the accuracy with which the actuation authority and natural frequencies are computed. The surface-bonded piezoelectric actuators and the host beam are considered to rotate individually due to shear deformation, while they are assumed to undergo the same flexural deflection and slope. These modeling aspects have improved the accuracy of the computed results. Edge debonding of actuator is accounted in the model by modeling healthy and debonded regions of the beam individually and subsequently applying the displacement continuity conditions at the interfaces of different regions. The investigations are carried out to find the effect of different extents of edge debonding on the actuation authority and natural frequencies of the debonded piezoelectric beam. It is found from the numerical results that the behaviour of the beam with edge debonded actuator is affected considerably as regards to the actuation authority and marginally with respect to natural frequencies. Further it has been shown that, when an actuator is edge debonded it introduces the local modes besides displaying significant reduction in its actuation authority.

An analytical cohesive crack modeling approach to the edge debonding failure of FRP-plated beams

This paper focuses on the prediction of edge debonding for a beam retrofitted with a Fiber-Reinforced Polymer plate. This failure mechanism, also known as plate-end debonding, stems from the concentration of interfacial stresses arising at the termination of the strengthening plate. Early models of edge debonding adopted failure criteria based on interfacial stresses. However, due to the typically catastrophic nature of this failure mechanism, approaches based on Linear Elastic Fracture Mechanics (LEFM) became increasingly established. In this paper, the problem is addressed by means of the Cohesive Crack Model (CCM) and of the Finite Fracture Mechanics (FFM). These models are able to bridge the gap between the stress- and the energy-based approaches and nevertheless have been used in a limited number of analytical studies to date. Based on a cohesive interface law with linear softening, closed-form solutions are derived for the interfacial stresses and the load–displacement curves, as well as for the ultimate load. It is found that LEFM usually overestimates the debonding load, thus justifying the need for the proposed approach; on the other hand, debonding load estimates based on FFM are in excellent agreement with the CCM predictions. Finally, a parametric analysis highlights the effect of the geometry/material properties on the structural response, as well as the ductile-to-brittle transition and the possible occurrence of structural instabilities depending on the test controlling parameters.

Finite element modelling and vibration control study of active plate with debonded piezoelectric actuators

A finite element model for a piezoelectric plate with edge debonded actuators is presented. This model is employed to investigate the effect of edge debonding on actuation authority, natural frequencies and vibration control performance. The regions of the plate with the piezoelectric patches are modelled such that each layer undergoes rotation due to shear deformation independently. The necessary constraints for continuity of displacements at the interfaces of the layers are imposed. The plate with edge debonded actuators is idealized by dividing it into debonded regions and healthy regions. A finite element procedure for imposing the constraints regarding continuity of displacements at the interfaces of the adjacent regions is developed and is implemented using MATLAB. Experiments are conducted for finding the actuation authority and natural frequencies of the plate with debonded actuators. It has been found that the developed model has predicted the mechanics of actuator debonding properly. The investigations have revealed the fact that the edge debonding of actuators will result in considerable degradation in actuation authority and vibration control performance.

Strengthening of Reinforced Concrete Arches with Externally Bonded Composite Materials: Testing and Analysis

An experimental and analytical study that includes testing to failure of fiber-reinforced polymer (FRP) strengthened medium-scaled reinforced concrete shallow arches and the application of a specially tailored high-order finite element for their analysis is presented. Three arches are tested. One is used as a control arch, whereas the other two are strengthened with externally bonded FRP strips in different patterns. The loading system includes six nonsymmetric vertical point loads equally spaced along the arch. The theoretical study is based on a multilayered finite-element approach, which accounts for the deformability of the adhesive layer and its shear and radial normal (through the thickness) stiffness. The arch is modeled as a polygon that consists of inclined specially tailored high-order finite elements. The results show that applying the FRP strips leads to an increase of about 40% in the failure load of the arch, changes to the cracking pattern, and a significant increase in deflection capacity. Edge debonding of the FRP strip is observed during the test but without causing total failure of the arch. The ability of the theoretical model to describe the overall structural response, the stress transfer mechanism in the strengthened arch, and the interfacial stresses and the local stress concentrations near irregular points are also demonstrated. Together they throw light on the behavior of the FRP-strengthened arch and on the evolution of its failure mechanism.

Dynamic edge debonding in FRP strengthened beams

Abstract The debonding failure in beams strengthened with externally bonded composite materials, as well as in many other layered and adhesively bonded structural elements, is a dynamic process. This paper studies the dynamics of the edge debonding process in beams strengthened with Fiber Reinforced Polymer (FRP) composite materials and derives an analytical model for the dynamic debonding process. The model adopts an extended high order approach, incorporates the dynamic inertial effects, and considers the high order displacement, acceleration, and stress fields in the adhesive layer. The model uses two cohesive interfaces for the consideration of independent debonding mechanisms at the two physical interfaces of the adhesive layer. It also accounts for the inelastic accumulation of damage at the interfaces and for their degradation under the effect of the dynamic and cyclic response. Along with the derivation of the model and its validation through comparison with 2D dynamic finite element analysis, the paper studies the dynamics of the edge debonding mechanism under two loading conditions. In the first case, the load is applied statically but the debonding mechanism drives the structure into the dynamic regime. In the second case, the dynamic debonding mechanism is triggered by an impulse load. In both cases, the results reveal and quantify the critical dynamic features of the edge debonding mechanism, its time and velocity scales, and its impact on the performance and resilience of the strengthened beam.

Finite element modeling and bending analysis of piezoelectric sandwich beam with debonded actuators

The present work pays emphasis on investigating the effect of different types of debonding on the bending behaviour of active sandwich beam, consisting of both extension and shear actuators. An active sandwich beam finite element is formulated by using Timoshenko's beam theory, characterized by first order shear deformation for the core and Euler-Bernoulli's beam theory for the top and bottom faces. The problem of debondings of extension actuator and face are dealt with by employing four-region model for inner debonding and three-region model for the edge debonding respectively. Displacement based continuity conditions are enforced at the interfaces of different regions using penalty method. Firstly, piezoelectric actuation of healthy sandwich beam is assessed through deflection analysis. Then the effect of actuators' debondings with different boundary conditions on bending behavior is computationally evaluated and experimentally clamped-free case is validated. The results generated will be useful to address the damage tolerant design procedures for smart sandwich beam structures with structural control and health monitoring applications.

A Finite Fracture Approach for Determining the Fracture Onset of a Brazed SiC Specimen

The failure initiation of a brazed sample made of silicon carbide substrates and submitted to bending is analyzed with the help of a criterion which combines a maximum incremental energy release rate and a maximum tensile stress conditions. Two different modes of cracking are considered to develop in the vicinity of the free edge between the brazed layer and the ceramic substrate: edge debonding and substrate cracking. The comparison of the predictions with the experimental results allows estimating the fracture properties of the bonding.

Effect of creep on the edge debonding failure of FRP strengthened RC beams – A theoretical and experimental study

Abstract This paper experimentally and theoretically investigates the influence of creep of the concrete and the adhesive on the edge debonding failure load of reinforced concrete beams strengthened with externally bonded composite materials. The theoretical study includes a stress analysis phase and a fracture analysis phase. In the stress analysis phase, the strengthened beam is modeled as a layered structure and creep of the different materials involved, as well as cracking and material nonlinearity of the concrete are considered using rheological generalized Maxwell models. The theoretical model also accounts for the development of shear and vertical normal stresses through the thickness of the adhesive layer. The failure criterion for edge debonding is based on a fracture mechanics model that uses the concept of the energy release rate through the J -integral and accounts for the creep deformations. An experimental study is conducted, which includes testing of three strengthened beams and one control under short-term loading for evaluation of their failure loads, and long-term testing of another set of beams under different levels of sustained loads for a period of one year. After the sustained loading period, the beams are further loaded to failure without releasing the existing load, in order to estimate the influence of creep deformations on the residual load carrying capacity. In both sets of tests, the failure mode is characterized by interfacial edge debonding. The experimental and theoretical results show that, depending on the material properties, creep can increase or decrease the maximum load that leads to edge debonding failures in FRP strengthened members.

On the Interaction of Thermally Induced Buckling and Debond Propagation in Patched Structures

The coupling of edge debonding and thermal buckling of patched beam-plates possessing initial edge detachment is examined for the case when the structure is subjected to a uniform temperature change. The geometrically nonlinear analytical model employed is that established by the authors in a prior work. The problem is recast in a mixed formulation in terms of the transverse deflection and the membrane force to aid in the analysis and physical interpretation. The interaction of edge-debond propagation and thermal buckling is studied. The phenomenon of buckle-trapping, originally observed by the authors in a congruent study, as well as the phenomenon of sling-shot buckling, is seen to manifest itself in the debonding behavior. The evolution of the structure is predicted as a function of given material and geometric parameters from numerical simulations based on analytical solutions of the nonlinear problem. A (propagating) contact zone adjacent to the bonded region is accounted for, and its presence or absence, as well as its nature, is seen to be highly influential in the global as well as local behavior of the structure.