Full Debonding Research Articles

Modeling and Laboratory Investigation of Tack Coats as Bituminous Pavement Interlayer

The adhesive properties of tack coats between asphalt pavement layers are crucial for the pavement’s structural behavior. This study first involved numerical analyses to compare stress patterns, deformations, and displacements in the pavement structure under various geometric and mechanical conditions. A rational calculation method based on the theory of elastic multilayer systems was used to quantify the impact of layer properties such as thickness, stiffness modulus, and Poisson’s ratio on interlayer bonding. Three bonding conditions—Full Friction, Partial Bonding, and Full Debonding—were analyzed to understand the tack coat’s effect between the top two layers. The second phase involved characterizing the mechanical behavior of the interface through shear strength tests (Leutner shear test) on both laboratory-prepared specimens and samples from a 10-year-old highway. Specimens were prepared using a Roller Compactor and tested under different interface conditions: hot-on-hot (H/H), residual bitumen 200 g/m2 (RB 200), and residual bitumen 400 g/m2 (RB 400). The tests examined the bonding effects in terms of tangential force and shear displacement at failure, as well as the impact of vehicular traffic on rutting and fatigue failure. Finally, this study investigated the long-term aging effects of the binder on interlayer bonding and sought to correlate the results of numerical calculations with those of the laboratory tests.

Relationship between electrical resistance and single CNT fiber displacement relative to cement matrix – new insights on the electric polarization and spreading at debonded/slipped interface

New relationships between electrical resistance (r) at the CNT fiber-to-cement matrix interface and fiber-to-matrix displacement (u) are established, enhancing the accuracy of self-sensing technology for post-cracking concrete. In the experiment, micromechanical behavior and electrical response (steady and sweeping direct current, DC) were simultaneously monitored through a single-fiber pullout test. It revealed that significant polarization exists at the interface, leading to incorrect resistance measurement. The polarization is eliminated by adding carbon fibers to percolation; after that, an exponential increase of r with u was then observed in both fiber debonding and slippage stages, while r dropped right after full debonding. In the simulation, the spreading of electric current from a partially debonded/slipped interface to distant matrix bulk was computed with a multi-physics FEM model, to correlate resistance change with the interfacial damage level – debonding crack length and fiber slippage. The model-based parametric study showed that the r-u relationship is more sensitive to fiber conductivity than fiber dimensions or matrix conductivity.

Guided Wave-Based Early-Stage Debonding Detection and Assessment in Stiffened Panel Using Machine Learning With Deep Auto-Encoded Features

Abstract Debonding between stiffener and base plate is a very common type of damage in stiffened panels. Numerous efforts have been made for debonding assessment in the stiffened panel structure using guided wave-based techniques. However, these studies are limited to the detection of through-the-flange-width debonding (i.e., full debonding). This paper attempts to develop a methodology for the detection and assessment of early-stage debonding (i.e., partial debonding) in the stiffened panel using machine learning (ML) algorithms. An experimentally validated finite element (FE) simulation model is used to create an initial guided wave dataset containing several debonding scenarios. This dataset is processed through a data augmentation process, followed by feature extraction involving higher harmonics of guided waves. Thereafter, the extracted feature is compressed using a deep autoencoder model. The compressed feature is used for hyperparameter tuning, training, and testing of several supervised ML algorithms, and their performance in the identification of debonding zone and prediction of its size is analyzed. Finally, the trained ML algorithms are tested with experimental data showing that the ML algorithms closely predict the zones of debonding and their sizes. The proposed methodology is an advancement in debonding assessment, specifically addressing early-stage debonding in stiffened panels.

Influence of uniform temperature variations on hybrid bonded joints with a circular or tubular cross-sectional area

The use of lightweight structures is a current concern in several engineering domains. To obtain such types of structures, the bonding technique using Carbon Fibre-Reinforced Polymers (CFRP) has been most recently considered a primary option. If CFRP is known to have a high strength-to-weight ratio or high corrosion resistance, the bonding technique does not need to add other fixation components and it also prevents stress concentrations. However, when combined with, e.g. a metallic surface, the high difference between the thermal expansion coefficient of the CFRP composite and the metallic material may raise some issues when the adhesively bonded structure is subjected to thermal loading. Therefore, the present work presents an analytical model that facilitates the comprehension of the impact of temperature on a hybrid bonded joint with a circular or tubular cross-sectional area. The full debonding process of a double but bonded joint with a regular curvature is discussed thoroughly. Due to the susceptibility of current adhesives to lose their mechanical properties for relatively high temperatures, the vitreous transition temperature of the adhesives and their influence on the local adhesive model is considered in a deeper analysis. The Finite Element Method (FEM) was used to validate all the derived analytical equations, which were achieved due to the close predictions obtained from both ways, i.e. from the numerical simulations and the proposed closed-form solutions.

Multispeckle diffusing wave spectroscopy as a tool to study heterogeneous mechanical behavior in soft solids

Multiple speckle diffusing wave spectroscopy (MSDWS) can be applied to measure spatially heterogeneous mechanical behavior in soft solids, with high sensitivity to deformation and both spatial and temporal resolution. In this paper, we discuss the mathematical approach behind the quantification of the deformation rate from MSDWS data and provide guidelines for optimizing the selection of experimental parameters in measurements. After validating the method in extensional tests on an elastomer, we provide an example of the potentiality of MSDWS by measuring the spatial distribution of the deformation rate during shear debonding of adhesive tapes. We quantitatively characterize the deformation rate distribution related to shearing and peeling under loading. A highly heterogeneous deformation rate distribution is observed, and time-dependent measurements reveal an increase in deformation localization hundreds of seconds before full debonding. This behavior, previously predicted by theory and simulation, is demonstrated here experimentally for the first time.

Debonding characterization of stiff film/compliant substrate systems based on the bilinear cohesive zone model

Interfacial debonding is one of the main failure modes of stiff film/compliant substrate systems, which are common in flexible electronics. In order to characterize their debonding process, we propose a theoretical model based on the bilinear cohesive zone model, where the stiff film acts like a membrane and the substrate is modeled by an elastic half-plane. The interfacial shear stress appears as a singular integral in the governing equations, and this term can be solved by expanding into the first-kind Chebyshev polynomials and further adopting the collocation method. The solutions of the present model are verified by the previous experimental data and the finite element analysis. The results indicate that the present model can not only capture the non-uniform deformation of the compliant substrate, but also describe the full debonding process including the elastic, elastic-softening and elastic-softening-debonding stages. Besides, the influences of film modulus, film thickness and interfacial properties on the debonding process are discussed. The present study can help us predict the debonding process of the stiff film/compliant substrate systems such as flexible electronics and guide the optimal design to prevent premature interfacial debonding.

Effect of thermally expandable particle additives on the mechanical and reversibility performance of adhesive joints

This paper provides an experimental investigation of the effect of Thermally Expandable Particle (TEP) adhesive additive on the mechanical and reversibility performance of epoxy-bonded load single lap joints (SLJs). Joint substrates are made of aluminum 6061 and woven carbon fiber reinforced plastic 0/90/0 substrates. TEP additive is used for modifying the structural epoxy adhesive used in this study. An electrically charged RF coil is placed around the joint bond area; generated electromagnetic field would heat the aluminum substrate which would in turn heat the modified adhesive. Very large volumetric expansion of the hollow spherical Thermally Expandable Particles (TEPs) would significantly reduce adhesive strength and lead to joint debonding under a minimal load of 100 N. Various weight concentrations of the TEP in the range of 5% to 25% with particulate size from 6 to 16 µm are used for investigating the effect on joint static and fatigue strength as well as the reversibility performance. Measured time to achieve full debonding of the joint (under charged RF coil), is used for assessing the reversibility performance.

Crack propagation behavior of ultra-high-performance concrete (UHPC) reinforced with hybrid steel fibers under flexural loading

This study investigates the crack propagation behavior of UHPC blended with straight steel fibers of different lengths (6, 13, and 20 mm), at varying replacement ratios (0.5%, 1.0%, and 1.5%) and a constant total fiber volume fraction (Vf = 2.0%), under flexural loading. The results indicate that blending with 0.5% medium length fibers and 1.5% long fibers yields the best flexural behavior compared to other investigated specimens. Using digital image correlation (DIC) technique, the evolution of surface deformation and strain field was analyzed. Based on the recorded images and corresponding analyses, the full debonding strain between the fibers and the matrix was determined to be about 0.32%. Moreover, the maximum strain for generating cracks was approximately 3.25%, irrespective of the fiber length. When longer fibers were replaced with shorter fibers at 0.5% or 1.0% replacement ratios, Crack opening displacement (COD) variations along the beam presented a linear shape. However, at higher replacement ratios (1.5% and 2.0%), the profile showed an abrupt change, and COD variations were non-constant. The samples comprising hybrid fibers presented a faster decline in the crack initiation strain rate at higher replacement ratios of the longer fibers. In the initial stage of cracking, the crack propagated rapidly; however, the crack propagation speed dropped dramatically during crack evolution before it stabilizes.

Improvement of fiber corrosion resistance of ultra-high-performance concrete by means of crack width control and repair

This study aims to investigate the influences of the pullout state and pre-crack width on steel fiber corrosion in ultra-high-performance concrete (UHPC) and its implication on the interfacial bond and tensile performances. For this, two pullout states, such as partial and full debonding, and five pre-crack widths, ranging from 0.02 to 0.5 mm, were considered. An epoxy-based crack repair process was also proposed, and its benefits on limiting steel fiber corrosion were evaluated. The average bond strength of steel fiber from UHPC could be improved by 54%–59% after exposure to a corrosive environment for 4 weeks, mainly due to partial surface corrosion. The debonding region was the main passage of the NaCl solution and led to the growth of ferric oxide. The crack width of ultra-high-performance fiber-reinforced concrete (UHPFRC) clearly affected the degree of steel fiber corrosion and the tensile performance. The tensile behavior of the micro-cracked UHPFRC with a small crack width below 0.15 mm was insignificantly influenced by the 4 week corrosion; whereas, the 0.3-mm cracked UHPFRC provided 10%–14% higher tensile strength and maintained higher stress levels in the softening region because of the moderately corroded fiber surface. Given the wider pre-crack condition (0.5 mm), no increase in the tensile strength was detected by partial ruptures of steel fibers. The steel fiber corrosion in cracked UHPFRC could be effectively prevented by the crack repair process, and no change in tensile behavior was thus obtained after exposure to a corrosive environment for 4 weeks.

Parametric FEM modeling of multilayer castellated canted cosine theta (CCT) magnets

Canted cosine theta (CCT) magnets have received increased attention in the recent years due to their inherent characteristics of high field quality and low conductor stresses, as each turn is supported by the metallic former. CCT geometry is suitable for complex magnets—such as nested dipoles, which allow full 360° control of the magnetic field direction. Modeling and design of such magnets is the subject of this work. A parametric 3D modeling framework for multilayer CCT magnets has been developed and optimized allowing efficient model generation without a need for both: CAD software and time consuming meshing. The model has been created using the APDL scripting language and solved using the Ansys software. By direct generation of nodes and elements the time to generate a model with one million elements was of the order of 10 min. A framework for script generation was developed, as well as the relations between the mesh parameters and the total number of nodes and elements. Detailed mesh dependence study was carried-out with in-depth analysis of solver performance (solution time, amount of written data, RAM requirements). Global and local convergence criteria were introduced to find the optimal mesh size. The case of a four layer 2.5 Tm nested dipole was analyzed in details including the worst-case scenario with full debonding between the CCT layers. Thanks to the castellations the nested-dipole is safe to support 144 000 Nm of torque due to Lorentz forces even with full de-bonding between the layers.

Using digital image correlation to evaluate the bond between carbon fibre-reinforced polymers and timber

The use of optic measurements such as digital image correlation to take strain measurements of fibre-reinforced polymers bonded to a substrate has been on the increase recently. This technique has proven to be useful to fully characterize the bond behaviour between two materials. Although modern digital cameras can take high-definition photos, this task is far from simple due to the tiny displacements that need to be measured. Consequently, digital image correlation measurements lead to relative errors that, at an initial stage of the debonding process, are higher than those calculated close to the debonding of the fibre-reinforced polymer from the substrate. This study aims to evaluate and analyse the use of the digital image correlation technique on the bond between carbon fibre-reinforced polymer laminates and timber when subjected to a pull-out load consistent with fracture Mode II. To allow the quantification of the relative errors obtained from the digital image correlation measurements during the full debonding process, several strain gauges were used to measure the strains in the carbon fibre-reinforced polymer composite. The accuracy of the digital image correlation measurements is analysed by comparing it with those obtained from the strain gauges, which is a very well-established measuring technique. Another contribution of this study is to check the versatility of the digital image correlation measurements on a broader range of situations. To that end, several timber prisms were bonded with seven different bonding techniques with and without the installation of a mechanical anchorage at the carbon fibre-reinforced polymer unpulled end. The results showed that the digital image correlation technique was able to track the slips calculated from the strain gauge measurements until the debonding load, but after that, some difficulties to measure the displacements of the anchored carbon fibre-reinforced polymer-to-timber joints were detected. The digital image correlation technique also over predicted bond stresses when compared with those taken from the strain gauges, which led to bond–slip relationships with higher bond stresses.

Experimental calibration of the bond-slip relationship of different CFRP-to-timber joints through digital image correlation measurements

Nowadays, the use of the Digital Image Correlation (DIC) technique has spread and it is being used in several engineering areas to measure displacements. The available data obtained from the DIC measurement to evaluate the bond performance between a Carbon fibre Reinforced Polymer (CFRP) externally bonded to a timber substrate is scarce. From the existing data obtained with other materials, this contactless technique revealed to be quite useful but its accuracy with other well-established techniques, such as the use of electric strain gauges is not well understood yet. Therefore, the current work aims to study the accuracy of 2D DIC measurements with the measurements obtained from the use of strain gauges within a low-cost perspective. To that end, several CFRP-to-timber bonded joints were tested under the single-lap shear test and different bonding techniques were considered as well. Some flaws intrinsically derived from the DIC measurements that complicate the bond assessment, such as the fluctuations in the generated displacements field, are identified, and to bypass this problem a new methodology is proposed. This new methodology is based on two different closed-form solutions that, after defining the local and global bond behaviours of different CFRP-to-timber bonded joints, allowed to eliminate the fluctuations found from the DIC measurements, facilitating the estimation and the comprehension of the full debonding process of the CFRP-to-timber joints, which was achieved with a good proximity to the homologous debonding process derived from the strain gauge measurements.

An MD study of the polymer–polymer adhesion via connector chains: some aspects of the competition between bulk dissipation in the interphase and pull-out of interface-connecting molecules

Atomistic simulation approach was used to analyze the separation behavior of polymer–polymer interfaces. This was achieved using coarse-grained molecular dynamics (MD) simulations of some connector chains embedded in two adjacent polymer entangled melts. To separate the contribution of polymer molecules pull-out from the micro-bulk dissipation in the viscoplastic interphase, surrounding the surface of separation, two kinds of simulations were considered: (a), simulations of the pull-out mechanism, where the extraction of the connectors out of the melts takes place through a complex mechanism of forced reptation only; (b), simulations of the full debonding process where the micro-bulk dissipation contributes to the overall crack opening and can skew the description of the interface separation by pure connector molecules pull-out. From the MD simulation results, some relevant parameters (e.g., a cohesive strength or an adhesion energy) of the separation behavior of the interface being modelized were extracted. In addition, one was able to separate quantitatively connector pull-out from micro-bulk dissipation (in the surrounding bulk region of the interface). This means that we were able to emphasize: (i) how the straining divides into interface opening and bulk deformation and (ii), how the macroscopic opening rate diffuses throughout the melts and the interface opening rate, since the mechanical behavior of polymers is strain-rate dependent. A comparison between two kinds of MD simulations, where a system is stretched along one direction, is shortly discussed as a necessary prelude to future investigations.

3D interfacial debonding during microbond testing: Advantages of local strain recording

The microbond (MB) test is the most widely adopted micromechanical test to characterize fibre matrix interfaces but typically lacks reliability and output for determining multi-parameter interface models. In the current research, the MB test is enhanced by incorporating Fibre Bragg Grating (FBG) sensors for local fibre strain monitoring. Strain-force data is used to analyse and validate the type and paramter values of a cohesive zone modelling (CZM) basis in the three-dimensional interface damage model. For the prepared epoxy resin droplets, that are used as a benchmark case, a bi-linear CZM traction-separation law is fitted for each droplet. The results confirm the selection of maximum FBG strain, force–strain profile with the two primary peaks in the force–strain derivative, and the peak force to be valid for proper interface characterization. The analysis of the performed tests clearly reveal the droplet fracture process to consist of four distinct stages. Only after the first stage, interfacial crack propagation independent of the point on perimeter is achieved. Full debonding occurs during the fourth stage.

Fiber push-in failure in carbon fiber epoxy composites

Micromechanical quasi-static and cyclic fiber push-in tests performed with an indenter in-situ in a scanning electron microscope on polished slices (thickness around 300 μm) of carbon-fiber epoxy composites provide information on the effect of fiber density on the fiber-matrix debonding and the fiber failure. Specifically, "close-packed", i.e., fibers surrounded by six nearest neighbor fibers (called "hexagonal" hereafter) versus "isolated" fibers, defined as fibers at least one fiber diameter distant from the next nearest fiber, were compared. In spite of the more compliant behavior of isolated fibers under quasi-static and cyclic indentation loads, initiation of fiber-matrix debonding and push-in failure (fiber splitting into two or three parts) occurred at roughly comparable loads for both, "hexagonal" and "isolated" fibers. Earlier tests on thinner slices of such composites (thickness around 30 μm) had resulted in full debonding and fiber push-out without fiber failure. The fibers in the thicker slices showed first indenter imprints on the surface and an increasing hysteretic behavior in the load-displacement curves before debonding. The analysis of the cyclic load-displacement curves yields the elastic-plastic and the hysteretic (debonding) energy contributions.

Development of the strain field along the crack in ultra-high-performance fiber-reinforced concrete (UHPFRC) under bending by digital image correlation technique

The digital image correlation (DIC) technique was applied to the analysis of displacement fields along the surface cracks of ultra-high-performance fiber-reinforced concrete (UHPFRC) under a bending load. Analysis was performed on the fiber bridging forces acting on a unit area of cracked sections belonging to different regions (perfect bonding region, partial debonding region, and full debonding region), according to the fiber distribution characterization and DIC data. By using the DIC technique, the visualization and quantification of the UHPFRC fracture properties could be determined. The length of the uniform zone increased, whereas the fracture process decreased under the external load of the limit of proportionality (LOP). With the appearance of macro-cracks under the external load of the modulus of rupture (MOR), the length of the uniform and fracture process zones increased as the fiber content increased. However, the localization zone decreased. The steel fiber volume fractions had limited influence on the tensile strain at cracking and the complete debonding of fibers.

Mechanical Behavior of Fiber-to-concrete Interface in Textile Reinforced Concrete: Theoretical Model

This paper presents a theoretical solution of a reinforcement-to-concrete interface model under pull-push loading. Expressions for the interfacial shear stress distribution and load-displacement history are derived for different loading stages. The full debonding propagation process is discussed in detail and the analytical solutions are verified by comparing with existing theoretical models. Results of the analytical solution are presented to illustrate how the bond length and local bond-slip law affect the interfacial bond behavior. While the case study in this paper is on textile reinforced concrete, the analytical solution is equally valid to similar mechanical cases such as rebar reinforced concretes.

Analysis of Pull-Out Behaviour of Straight and Hooked End Steel Fibres

The effective utilisation of steel fibre reinforced concrete (SFRC) requires in-depth and detailed understanding of bonding mechanisms governing the tensile behaviour. In response to this demand, this study presents an empirical description of the axial force required to pull out a hooked end steel fibre from a cementitious matrix. It is based upon consistent experimental tests that show the force versus displacement plot is formed from a sequence of events in which partial and full debonding at the interface is followed by bending of the hook knee to raise the force to its maximum. A loss of peak force occurs with the reversed plasticity involved in a full straightening of the fibre that precedes the rapid sliding to its full removal under a falling force. The stages are assembled in the said order with a piecewise connection between linear segments and a curve of exponential decay. A power law can be introduced to describe the knee bending stage if preferred. The normalised co-ordinates were adopted for the simple mathematical discontinuous function of the full pull-out process. Normalising force and displacement for hooked fibre is based upon the maximum values found for straight fibre pull out. This would apply to an overall embedded length of unity and a unit peak force for a full debonding of straight fibre in similar materials. For hooked fibre the normalised co-ordinates given refer to the initiation and duration of each event to be tabulated at fractions of the embedded length with multiples of that force. Such predictions are seen to be in good agreement with average pull-out response curves.

Impact of Multiple Debonding on Adhesion Energy

Direct bonding energy relates to the well-known “strength” or “toughness” of direct bonding interface. It is the energy needed to separate two bonded surfaces. In contrast, adhesion energy is the energy available to bring the two surfaces together. Due to hysteretic effects, the two energies may be different and, while the first energy has been largely measured, the second one is the subject of very few papers only [1, 2, 3]. In this study, a new phenomenon will be shown in the adhesion energy behaviour using two silicon wafers. As shown in figure 1b, several partial debondings and rebondings of the same silicon direct bonding structure result in an increase of the bonding wave speed. After reaching a maximum bonding wave speed, a full debonding brings the speed back to its initial value. As it has already been shown theoretically by Rieutord et al. [4] and verified experimentally by Larrey et al. [3], the bonding wave speed is linked to the adhesion energy of the system. Therefore, an increase of the bonding wave speed consequently indicates an increase of the adhesion energy. As shown in figure 1b, multiple partial debondings increase the bonding wave speed by more than 50%, which increases the adhesion energy by more than 40%. The relationship between adhesion energy and bonding wave speed is not linearly dependent, as defined in the equation of figure1a [4] and shown in figure1c. Different experiments have been done to find the cause of this peculiar behavior. A model will be proposed and discussed. REFERENCES [1] D. Grierson and K.T. Turner, ECS Trans. 33, 573 (2010). [2] D. Radisson, Direct Bonding of Patterned Surfaces, phdthesis, Université Grenoble Alpes, 2014. [3] V. Larrey, G. Mauguen, F. Fournel, D. Radisson, F. Rieutord, C. Morales, C. Bridoux, and H. Moriceau, ECS Trans. 75, 145 (2016). [4] F. Rieutord, B. Bataillou, and H. Moriceau, Physical Review Letters 94, (2005). Figure 1

Effect of matrix cracking on electrical resistivity of high performance fiber reinforced cementitious composites in tension

The effect of matrix cracking on the electrical resistivity of high performance fiber reinforced cementitious composites (HPFRCC) was investigated by performing multi-steel fiber pullout tests combined with electrical resistivity measurement. The electrical resistivity of the HPFRCCs fell immediately after the matrix cracking and then started to increase after full debonding of the fibers during fibers pullout processing. The specimens with the two pre-designated cracks exhibited a higher reduction in the electrical resistivity than those with the one pre-designated cracks or pre-crack. The source of the sensing damage of HPFRCC in tension significantly depends upon matrix cracking and fiber matrix debonding.