Debonding Resistance Research Articles

Statistical assessment of a design formula for the debonding resistance of FRP reinforcements externally glued on masonry units

The present work aims to statistically assess a new design procedure to estimate the bond resistance of composite fabrics externally glued on masonry substrates. A database of experimental campaigns of bond test results between fiber reinforced polymer and masonry is collected and, after an overview of the design rules at disposal, an alternative formula is proposed starting from theoretical and experimental evidences. The main parameters influencing the ultimate load are defined through a correlation analysis and different assessment approaches are accounted for. Finally, the capabilities of the proposed approach are statistically evaluated and the advantages with respect to existing formulas are discussed.

Bond slip model of FRP-to-concrete surfaces

The bond between fibre reinforced polymer (FRP) plate and concrete surface has shown significant effect on the behaviour of strengthened reinforced concrete (RC) beam. Therefore, an intermediate crack (IC) debonding resistance which depends on the interface between FRP plate and the concrete surface remains as the main concern in designing RC beam strengthened with FRP plate. This research is to develop the bond slip model between the interface of FRP plate and concrete surface and to validate the model by carrying out experimental pull out test. Comparison was made between the new bond slip model and the existing bond slip model. It was found that the new bond slip model achieved very close results as compared to the published bond slip model.

Investigation on Effect of the Pitch of Shear Keys on the In-Plane Shear Performance of Sandwich Panels with PU Foam Core: FE Study

The main problem associated with using composite sandwich panels with foam core is their ability to withstand shear load. The techniques currently available to improve the shear performance and the skin-core debonding resistance of the sandwich structure, necessary for efficient and reliable design, have the drawback of significantly reducing the in-plane mechanical properties of the whole structure. The aim of the present work is to investigate the effects induced by inserting a semi-circular shear keys between the skin and the foam core to improve the in-plane shear response of the sandwich structure. Tension, compression and shear tests were performed for a complete static mechanical characterisation of the constituents of the sandwich. The mechanical properties of each component were used as input data in the finite element (FE) analysis using ABAQUS software. Parametric investigation using the FE analysis to select the most suitable pitch of the shear keys taking into consideration the material nonlinearity and the complex interaction between all the mating surfaces is performed. Five different pitches have been investigated namely 15, 25, 35, 45 and 55 mm. The results showed that the model with 45 mm pitch represents the most sustainable model among the others.

Effect of shear keys diameter on the shear performance of composite sandwich panel with PVC and PU foam core: FE study

The present paper addresses the most dominant failure mode of composite sandwich panels with foam core, where the prevention and limitation of debonding failure between skin and core is highly important design issues. This paper presents a semi-circular shear keys inserting between the skin and the foam core to improve the shear performance and skin–core debonding resistance for sandwich panels with foam core. Polyvinylchloride (PVC) and polyurethane (PU) foam core are chosen for the current study and sandwiched between glass fibre reinforced polymer (GFRP) skins, while the chopped strand (CS) glass fibre impregnated with epoxy resin is used for the shear key. A parametric investigation on the effect of the shear keys diameter on the in-plane shear performance has been performed numerically. In order that, a comprehensive experimental testing program was performed on the constituents to obtain the basic parameters used in the finite element (FE) model. The FE model of the ordinary sandwich panel was validated experimentally and proved its capability to represent the shear response of the sandwich panel. The suggested design was numerically investigated to select the most effective diameter for the shear key under in-plane shear load. For all investigated sandwich model configurations the suggested shear keys were able to stop skin–core debonding and to redirect it to diagonal orientation with a significant improvement in the overall shear performance.

Failure mechanisms of sandwich composites with orthotropic integrated woven corrugated cores: Experiments

To enhance the skin–core debonding resistance of sandwich composites, a new integrated woven corrugated sandwich composite (IWCSC) was designed and fabricated. Quasi-static compression, shearing and three-point bending tests were carried out to reveal the mechanical property and the failure mechanism of the IWCSC. In compression, gradual core crushing and contacting with the skin induced ductile load–displacement curves featured by a long deformation plateau. The anisotropic core structure leads to orthotropic anti-shearing resistances in the warp and weft directions. The difference is even at the level of a magnitude. IWCSC is an anisotropic bending dominated light weight material. Anisotropy of the core structure leads to different failure mechanisms of IWCSC panels in bending. Bended in the warp direction failure mode of the beam is core shearing while in the weft direction is indentation. High skin–core debonding resistance has been testified by the experiments. Behaviors of IWCSC panels were predicted and the predictions are close to the tested data.

Debonding resistance of FRP-to-clay brick masonry joints

Debonding at the FRP-to-masonry interface has been identified as the preferred failure mechanism in fibre-reinforced polymer (FRP) retrofitted masonry as it allows for some redistribution of forces. The results of 14 FRP-to-masonry bond tests are presented, where the FRP was near surface mounted (NSM) to stack-bonded clay brick masonry. These tests were conducted to investigate the effect that variables such as cyclic loading and FRP strip dimensions have on the debonding resistance of a NSM FRP-to-masonry joint. These results were then incorporated into a large database of FRP retrofitted masonry pull test results by various researchers over the past 10years. The database includes results for both externally bonded (EB) and NSM retrofitting techniques. From this database, local bond–slip parameters such as the maximum interface shear stress, τmax, and the maximum slip, δmax, were investigated to determine correlations between these values and masonry material properties. Further, 15 existing concrete and masonry bond strength (maximum load at the FRP-to-substrate interface) models in the literature were assessed for their use with masonry by comparing these models against the results in the pull test database. Based on the comparative statistics of the test-to-predicted bond strength it is concluded that a new FRP-to-masonry bond model is required which gives more accurate predictions. Results include a discussion on the global load–slip response and FRP-to-masonry interface behaviour.

Mechanical Reinforcement of Three-Dimensional Spacer Fabric Composites

Three-dimensional (3-D) spacer fabric composite is a novel lightweight sandwich structure, the reinforcement of which is integrally woven with two facesheets connected by continuous fibers (named piles) in the core. Usually the 3-D spacer fabric composite without extra reinforcement is called mono-spacer fabric composite, which provides outstanding facesheet / core debonding resistance. However, its mechanical properties cannot meet the demand of structure application because of the thin facesheet and low load-bearing capacity of high piles. Hence, two reinforcement methods were developed by laminating additional weaves at the facesheet and filling foam materials in the core to strengthen the facesheet and piles, respectively. This paper aims to investigate the influences of reinforcement methods on the mechanical behaviors and damage modes of 3-D spacer fabric composites under flatwise compressive, shear, edgewise compressive and three-point bending loads, by comparing with mono-spacer fabric composites. The results indicate that additional weaves reinforcement can enhance edgewise compressive and flexural properties effectively. Foam filling is one of the best options to improve the flatwise compressive and shear properties, and especially, there are synergistic effects between piles and foam under flatwise compressive load. Besides, the failure modes of reinforced and mono-spacer fabric composites are different.

Strengthening of a model composite restoration using shape optimization: A numerical and experimental study

ObjectiveThis study aims to validate a cavity shape optimization approach for improving the debonding resistance of dental restorations by carrying out fracture tests on restored model teeth with standard and optimized cavity designs. MethodThe bio-mimetic stress-induced material transformation (SMT) optimization method was incorporated into the finite element (FE) program ABAQUS as a user material (UMAT) subroutine. The method uses stress minimization to optimize the cavity shape of a MOD restoration in an artificial premolar with special reference to the tooth–restoration interface under occlusal loads. The mechanical performance of the optimized design was first verified through FE analysis and then compared with that of the conventional design using fracture tests on model teeth. ResultsThe SMT optimization process indicated a T-shape cavity as a more favorable design for the MOD restoration in the artificial premolar. Compared with the conventional parallel wall, or undercut design, the T-shape cavity was shown numerically to reduce the interfacial stresses by up to 69%, and experimentally to increase the mean debonding resistance of the model teeth by 23% (p<0.05). SignificanceCavity shape optimization can help increase the debonding resistance of restored teeth by reducing the interfacial stresses between tooth and restoration under occlusal loads.

Experimental and Numerical Study on the Mode I Delamination Toughness of Z-Pinned Composite Laminates

An experimental investigation was performed on mode I delamination of z-pinned double-cantilever-beams (DCB) and associate z-pin bridging mechanisms. Tests were performed with ten types of samples: (1) big-pin reinforced DCB (double-cantilever-beams) with three areal densities D=2.01%, 5.15%, 8.04%, respectively; (2) median-pin reinforced DCB with three areal densities D=0.85%, 2.17%, 3.40%; (3) small-pin reinforced DCB with three areal densities D=0.25%, 0.63%, 0.90% and (4) without pin reinforced DCB specimens. Delamination tests samples were prepared from unidirectional continuous carbon fibre/epoxy prepreg (T300/TDE86), made into 3 mm thick unidirectional laminates with and without a block of Z-pins in the crack path. Fracture testing was carried out under Mode I (standard DCB test). Experiments have shown that increases in debond resistance and ultimate strength depend on the material, size, density, location of the pins and the mechanisms of pin deformation. A finite element (FE) model is developed to investigate mode I delamination toughness of z-pin reinforced composite laminates. The z-pin pullout process is simulated by the deformation of a set of non-linear springs. A critical crack opening displacement (COD) criterion is used to simulate crack growth in a DCB made of z-pinned laminates. The toughness of the structure is quantified by the energy release rate, which is calculated using the contour integral method. The FE model is verified for both unpinned and z-pinned laminates. Predicted loading forces from FE analysis are compared to available test data. Good agreement is achieved. The numerical results indicate that z-pins can greatly increase the mode I delamination toughness of the composite laminates.

Bond behaviour of FRP-to-clay brick masonry joints

The out-of-plane bending and in-plane shear response of unreinforced modern clay brick masonry walls retrofitted with fibre-reinforced polymer (FRP) strips is often governed by debonding failure mechanisms. Hence, it is necessary to quantify the fundamental interface bond–slip model, which describes the debonding behaviour of the FRP-to-masonry interface. This paper presents the results of a series of 29 pull tests investigating the use of externally bonded (EB) and near surface mounted (NSM) retrofitting techniques. Test variables included: surface preparation; geometric properties; location of FRP (relative to perpend joints and cores); bonding agent of bed joints (mortar and quick drying paste); bonding method for glass fibre sheets (plate bonding and dry lay-up); and FRP material. A discussion of the test results and preliminary practical recommendations are also provided. A model used to predict the intermediate crack debonding resistance was verified against the test data. The model is generic in that it is applicable to both the EB and NSM retrofitting techniques. This generic model was shown to give very good ultimate strength predictions for the series of 29 pull tests conducted as part of this research.

Phenolic rigid organic filler/isotactic polypropylene composites. III. Impact resistance property

A novel phenolic rigid organic filler (KT) was used to modify isotactic polypropylene (iPP). The influence of KT particles on the impact resistance property of PP/KT specimens (with similar interparticles distance, 1.8 μm) was studied by notched izod impact tests. It was found that the brittle-ductile transition (BDT) of the PP/KT microcomposites took place at the filler content of about 4%, and the impact strength attains the maximum at 5% (with filler particles size of 1.5 μm), which is about 2.5 times that of unfilled iPP specimens. The impact fracture morphology was investigated by scanning electron microscopy (SEM). For the PP/KT specimens and the highdensity polyethylene/KT (HDPE/KT) specimens in ductile fracture mode, many microfibers could be found on the whole impact fracture surface. It was the filler particles that induced the plastic deformation of interparticles ligament and hence improved the capability of iPP matrix on absorbing impact energy dramatically. The determinants on the BDT were further discussed on the basis of stress concentration and debonding resistance. It can be concluded that aside from the interparticle distance, the filler particles size also plays an important role in semicrystalline polymer toughening. Keywords rigid organic filler, polypropylene, impact

Embedding NSM FRP Plates for Improved IC Debonding Resistance

The use of near surface mounted (NSM) fiber reinforced polymer (FRP) strips for strengthening reinforced concrete structures shows great promise as the strains at intermediate crack (IC) debonding are generally much greater than for externally bonded FRP strips. In this research, the NSM technique is taken a step further by embedding the NSM strip, i.e., is by providing cover to the strip. It is shown that embedment can increase the IC debonding resistance by up to three times. Importantly, embedment allows: substantially larger strains at IC debonding and, hence, greater ductility; the use of larger cross sections of FRP plate; and, through providing cover to the NSM, may be the first step in enhancing the fire resistance. In this paper, 20 new pull tests are described from which mathematical expressions are developed for the effect of embedment on both the IC debonding resistance and its associated local bond stress–slip (τ–δ) relationship.

Shear Transfer across Cracks in FRP Strengthened RC Members

The shear capacity of unplated reinforced concrete (RC) beams depends on the transverse shear to form the critical diagonal crack (CDC) as well as the transverse shear capacity across the CDC. The latter depends on the reinforcing bars crossing the CDC as they provide forces normal to the CDC that allow the shear to be transferred by aggregate interlock. For steel reinforcing bars, these normal forces can be assumed to depend on the ductile yield capacity of the reinforcing bar. However, the problem is more complicated when dealing with fiber reinforced polymer (FRP) plated RC beams, as the normal force now depends on the brittle intermediate crack debonding resistance of the plate as well as the brittle nature of the FRP material. In this paper, eight push tests have been used to directly determine the contribution of externally bonded (EB) and near surface mounted (NSM) FRP plates to the shear capacity, and these are compared with further six EB and NSM steel plated members. It is shown that plate reinf...

IC Debonding of FRP NSM and EB Retrofitted Concrete: Plate and Cover Interaction Tests

The use of fiber-reinforced polymer (FRP) externally bonded (EB) plates is widely accepted as an efficient and unobtrusive retrofitting technique. FRP near-surface mounted plates are now also gradually gaining acceptance due to their substantial increase in debonding strains over EB plates. However tests have shown that the intermediate crack (IC) debonding resistances of FRP plates can be reduced by their interaction with adjacent parallel plates and with parallel free surfaces, that is the cover; this is often reflected in design rules where the IC debonding resistance of individual plates depends on the width of the plate as a proportion of the width of the concrete specimen and on the cover. In this paper, 22 new pull tests are reported that study the IC debonding interaction with adjacent plates and cover. The results are encouraging as they show that there is little reduction in the IC debonding resistance until the lateral cover or gap between plates is relatively small.

IC debonding resistance of groups of FRP NSM strips in reinforced concrete beams

The use of fibre reinforced polymer (FRP) externally bonded (EB) plates in the form of pultruded and wet lay-up plates is now generally accepted as an efficient and unobtrusive technique for retrofitting reinforced concrete structures and is applied worldwide. However, EB plates, and in particular EB pultruded plates, tend to debond at strains much lower than their fracture strains. An alternative technique of adhesively bonding pultruded plates or strips in narrow grooves sawn into the concrete cover, that is near surface mounted (NSM) plates or strips, is now gradually gaining acceptance as tests have shown that the debonding strains can be much higher than that for EB plates. However, tests have also shown that NSM plates can interact with adjacent parallel NSM plates to cause intermediate crack (IC) debonding of groups of NSM strips at reduced strengths. This paper develops a mathematical model for the IC debonding resistance of groups of NSM plates for use in the flexural and shear strengthening of reinforced concrete beams.

Measurement of the Debonding Resistance of Strongly Adherent Thick Coatings on Metals via In-plane Tensile Straining

When the ratio hc/hs of coating and substrate thicknesses is large enough, interfacial debonding can be induced to propagate from the root of a transverse crack under in-plane loading. An energy balance analysis accounting for the flow rule of the substrate allows translating the load for steady state debonding into an upper bound for the debonding toughness. The method is validated by FEM simulations using a cohesive zone model.

Generic Debonding Resistance of EB and NSM Plate-to-Concrete Joints

It is well established that adhesively bonding plates to the surfaces of reinforced concrete members is an efficient retrofitting approach. Specifically, two techniques have emerged: Using thin externally bonded (EB) sheets/plates and near-surface mounted (NSM) strips/bars. A good amount of research has been undertaken worldwide to understand the fundamental behavior describing such adhesively bonded plate-to-concrete joints. Unfortunately, until now, no generic model exists to determine the debonding resistance of both retrofitting techniques. In this paper, a generic analytical model is derived to determine the debonding resistance of any adhesively bonded plate-to-concrete joint using an idealized linear-softening local interface bond-slip relationship. The model is derived using a unique definition of the debonding failure plane and confinement ratio such that it is suitable for both the externally bonded and near-surface mounted techniques. The model is validated by comparison with existing push-pull data as well as 14 new push-pull tests with varying plate cross-section aspect ratios. Comparison with an existing well-known model demonstrates the suitability of the proposed generic model. The model can be used to predict the intermediate crack debonding resistance of strengthened reinforced concrete members.

Simulation of pin-reinforced single-lap composite joints

A simple and efficient computational approach is presented for analyzing the benefits of through-thickness pins for restricting debond failure in joints. Experiments have shown that increases in debond resistance and ultimate strength depend on the material, size, density, location, and angle of deployment of the pins and the mechanisms of pin deformation, which are complex and strongly affected by the mode ratio of the debond crack. Here the mechanics problem is simplified by representing the effects of the pins by tractions acting on the fracture surfaces of the debond crack. The tractions are prescribed as functions of the crack displacement, which are available in simple forms that summarize the complex deformations to a reasonable accuracy. The resulting model can be used to track the evolution of competing failure mechanisms, including tensile or compressive failure of the adherends, joint debonding (creating leak, for example, if the joint is in a pipeline), and ultimate failure associated with pin rupture or pullout. Calculations illustrating complex mode ratio variations are presented for a lap joint specimen comprising curved laminate segments cut from pipes.

Helicoidal Composites

The behavior of a carbon/epoxy composite material is examined for improved mechanical properties using a design based on a helicoidal architecture. Helicoidal composites with 40 layers, [180/170/160/150/140/130/120/110/100/90/80/70/60/50/40/30/20/10/(0)/2] S are used. Through-thickness reinforcement was provided in some of the specimens by the stapling with stainless steel staples. For the helicoidal arrangement, the gradual change in in-plane properties in each layer results in lower interlaminar shear stresses. It is found that the specimen with helicoidal stacking sequence has better debonding resistance and improved damage tolerance. In a tensile test, it is shown that the helicoidal architecture results in higher energy absorption due to limits on the development of transverse cracks. In the circular plate test, the helicoidal specimen shows the highest load and energy absorption. The impact test shows the highest penetration resistance. Some specimens with through-thickness reinforcement were manufactured and characterized. The reinforced laminate presents higher load carrying capacity, better delamination resistance and a graceful failure.

Prestress model for shear deformation debonding of FRP- and steel-plated RC beams

Adhesive bonding plates to the surfaces of RC structures is a widely accepted form of strengthening or stiffening because of the ease of application and unobtrusive nature of this retrofitting procedure. Design guidelines have identified three major forms of plate debonding, one of which is debonding due to the shear deformation across a critical diagonal crack (CDC). CDC debonding is particularly important in RC beams with stirrups, as they are prone to debond before the shear capacity of the beam is attained. In this paper it is shown that the CDC debonding resistance of a plated RC beam is analogous to the shear resistance of a prestressed beam; a design procedure based on the shear resistance of prestressed beams is developed, and correlates well with 77 plated beam tests. In a companion paper the prestress analogy developed here is taken further: it is shown that code approaches for determining the shear resistance of prestressed beams can be used to predict CDC debonding. This has allowed the develo...