Onset Of Debonding Research Articles

Simplified Procedure to Determine the Cohesive Material Law of Fiber-Reinforced Cementitious Matrix (FRCM)-Substrate Joints.

Fiber-reinforced cementitious matrix (FRCM) composites have been largely used to strengthen existing concrete and masonry structures in the last decade. To design FRCM-strengthened members, the provisions of the Italian CNR-DT 215 (2018) or the American ACI 549.4R and 6R (2020) guidelines can be adopted. According to the former, the FRCM effective strain, i.e., the composite strain associated with the loss of composite action, can be obtained by combining the results of direct shear tests on FRCM-substrate joints and of tensile tests on the bare reinforcing textile. According to the latter, the effective strain can be obtained by testing FRCM coupons in tension, using the so-called clevis-grip test set-up. However, the complex bond behavior of the FRCM cannot be fully captured by considering only the effective strain. Thus, a cohesive approach has been used to describe the stress transfer between the composite and the substrate and cohesive material laws (CMLs) with different shapes have been proposed. The determination of the CML associated with a specific FRCM-substrate joint is fundamental to capture the behavior of the FRCM-strengthened member and should be determined based on the results of experimental bond tests. In this paper, a procedure previously proposed by the authors to calibrate the CML from the load response obtained by direct shear tests of FRCM-substrate joints is applied to different FRCM composites. Namely, carbon, AR glass, and PBO FRCMs are considered. The results obtained prove that the procedure allows to estimate the CML and to associate the idealized load response of a specific type of FRCM to the corresponding CML. The estimated CML can be used to determine the onset of debonding in FRCM-substrate joints, the crack number and spacing in FRCM coupons, and the locations where debonding occurs in FRCM-strengthened members.

Performance of hybrid Innegra-carbon fiber composites

In this work material synergy with high stiffness carbon fiber with ductile high strength polypropylene fiber (Innegra S), (referred to as Innegra, herein) have been evaluated in a range of laminate designs. Both woven and discontinuous carbon fiber have been considered. The discontinuous fibers are based on three-dimensional deposition (3DEP) (referred to as 3DEP, herein) carbon fiber preform process. Eleven (11) variants of Innegra-carbon fiber hybrid laminates were investigated for tensile, flexure, compression, in-plane shear and low velocity impact response. The effect of position of Innegra within the laminate was studied and found to influence strength and stiffness properties. In terms of overall trends, Innegra provides upward of 18% improvement in strain (ductility) to the composite and eliminates brittle fracture of carbon fiber. The moduli trends follow the proportionality of Innegra fiber to carbon fiber plies. However, the strength is controlled by the interface between Innegra and carbon fiber. The primary failure mode in tension and compression is via onset of debonding between Innegra and carbon fiber. The 3DEP carbon fiber constituents provided highest values of in-plane shear indicating that three-dimensional (3D) network of carbon fiber provides higher shear resistance. The Innegra intensive variants exhibited superior energy absorption under low velocity impact, at energy levels 15–60 J. This work provides insight for designers to incorporate Innegra and carbon fiber hybrids in composite structures.

Adhesion performance tests and analysis of interface damage for CRTS III prefabricated slab tracks

Debonding between the track slab and self-compacting concrete (SCC) is a common form of damage in China railway track system (CRTS) III prefabricated slab tracks. To investigate the influence of interlayer cohesive parameters on the interface connection, composite specimens of track slabs and SCC were made to be used for splitting tensile and shear tests. The variation characteristics between the interface’s tension and displacement were obtained using digital image correlation (DIC) techniques, and the key parameters of a cohesive zone model (CZM) were determined and used to simulate the cracking behavior. Also analyzed was the interlayer damage behavior of the composite specimen using ABAQUS, the results of which were compared with those of the tests. Furthermore, a finite element simulation model of the CRTS III prefabricated slab track with CZM was used to analyze the impact of different cohesive parameters on interlayer damage under train loads and varying temperature gradient loads. The results show that: I). The DIC technique accurately describes the entire process from the damage initiation and propagation, to complete failure of the interface-bonded components; II). The finite element simulation results of the concrete composite specimens agree with the experimental results, indicating the CZM accurately reflects the interlayer cracking behaviors; III). Reducing the interface stiffness and increasing the critical fracture energy can slightly delay the onset of interface damage and debonding, respectively; IV). An increase in the cohesive strength can effectively inhibit the development of interface damage, for example, increasing the normal and tangential cohesive strength can improve the starting temperature gradient for interlayer damage initiation by 150 % and 45 %, respectively; and V). Under large positive temperature gradients (>70 °C/m), the interlayer debonding rate corresponding to most cohesive parameters reaches 100 % (complete failure of adhesion), making extreme temperature areas a necessary focus on the interlayer debonding of slab tracks. The research results of this paper can be used to analyze the damage behavior between the track slab and SCC layer, and provide a theoretical basis for the design and maintenance of CRTS III prefabricated slab track.

Peeling of Finite-Length Plates From an Elastomeric Foundation: A 1D Cylindrical Bending Solution

Abstract Quasi-static peeling of a finite-length, flexible, horizontal, one-dimensional (1D) plate (strip, thin film) from a horizontal, thin, elastomeric layer (foundation) is considered. The displaced end of the plate is subjected to an upward deflection or to a rotation. The top of the interlayer is perfectly bonded to the plate, and its lower surface is bonded to a rigid, flat substrate. A transversality (debonding) condition is derived for peeling, based on the common fracture mechanics approach. Whereas debonding from a Winkler foundation can be expressed in terms of the displacement (or equivalently the foundation stress2) at the bond termination, the sixth-order formulation required for elastomeric foundations involves a more complex debonding criterion. Transversality relationships are used to describe this limit state (here the onset of debonding) in terms of co-state variables, herein the deflection and slope at the peel front. In the analysis, bending is assumed to be paramount, linear Kirchhoff–Love (classical) plate theory is used to model the deformation, and therefore displacements are assumed to be small. The foundation is linearly elastic and incompressible. The effects of the work of adhesion, the length of the plate, and the initial nonbonded length of the plate are investigated. The results are compared to those for a Winkler foundation. By replacing the shear modulus of the interlayer by viscosity, and displacements by their time derivatives, the results are expected to apply to viscous liquid interlayers as well.

A Finite Element Model for Predicting the Static Strength of a Composite Hybrid Joint with Reinforcement Pins.

This paper presents a finite element model for predicting the performance and failure behaviour of a hybrid joint assembling fibrous composites to a metal part with reinforcement micro pins for enhancing the damage tolerance performance. A unit-strip model using the cohesive elements at the bond interface is employed to simulate the onset and propagation of debonding cracks. Two different traction-separation laws for the interface cohesive elements are employed, representing the fracture toughness properties of the plain adhesive bond and a pin-reinforced interface, respectively. This approach can account for the large-scale crack-bridging effect of the pins. It avoids using concentrated pin forces in the numerical model, thus removing mesh-size dependency, and permitting more accurate and robust computational analysis. Lap joints reinforced with various pin arrays were tested under quasi-static load. Predicted load versus applied displacement relations are in good agreement with the test results, especially for the debonding onset and early stage of crack propagation.

Predictive abilities of pseudodiscretization and pseudograin discretization schemes of the Mori–Tanaka homogenization, benchmarked against real and virtual RVEs

In this article, the predictive abilities of two prevalent homogenization formulations, two-step Mori–Tanaka (MT–MT) and MT–Voigt are benchmarked using virtual representative volume element (RVE) involving full FE models and real RVE using prior published experimental data. MT–MT and MT–Voigt are interpreted as involving “pseudodiscretization” and “pseudograin discretization,” respectively. A comprehensive test campaign involving all possible permutations and combinations of modulus of inclusions and matrix (inclusions stiffer than the matrix and vice versa) and different loading types are conducted. The comparisons are done at three length scales: effective modulus, stresses in individual inclusions, stress distribution in the interphase, and strain at the onset of interphase debonding. Overall, both schemes have similar predictive capabilities for the effective modulus and phase average stress. The predictions of the stresses in the individual inclusions for complex RVEs with inclusions length and orientation distribution are correct as average trends but do not represent the full details of the FE-revealed strain scatter. Depending on the modulus mismatch, or loading direction, better predictions can be achieved by either MT–MT or MT–Voigt, even if the microstructure considered remains the same. The relative difference in the strain at the onset of debonding, predicted by MT–MT and MT–Voigt formulations, is about 10%. HIGHLIGHTS Two-step MT–MT and MT–Voigt are benchmarked against virtual and real RVEs Comparisons are done for all permutations of modulus mismatch and loading types Predictions are compared at three length scales: RVE, inclusions, and interphase

Multiscale study on the mechanical behaviour of catalyst layer under tensile and humidity cyclic loading

The durability and life of proton exchange membrane fuel cells (PEMFCs) decrease as the mechanical damage of the catalyst layer (CL) is aggravated due to mass and electronic transport constraints. Owing to the challenge of real-time monitoring of internal stress and microstructure changes through experiments, developing an effective modelling technique to capture the onset of cracks and delamination is extremely urgent. For this purpose, this work focused on probing the effect of stress concentration regions on the mechanical behaviour of CL under tensile and humidity cyclic loading. Based on the measurement of adhesive force between Pt/C and Nafion, we constructed an interfacial model through molecular dynamics simulation and correlated it with the cohesive zone model in the micro-mechanical model. Finite element simulation was employed to predict the stress–strain response and onset of interface debonding. Some results have shown good agreement with experimental data from the literature. This model induced by interfacial strength, porosity, and ionomer expansion curve can be used to examine the damage degree of CL during the PEMFC durability testing process.

Durability assessment of FRP-concrete bond after sustained load for up to thirteen years

Small-scale plain concrete notched beams strengthened with carbon and glass fiber reinforced polymer (CFRP and GFRP) sheets were tested in flexure to assess the change in debond onset strain following sustained flexural loading in indoor and outdoor environments for as long as 13 years. The bond tests were all conducted quasi-statically in a normal laboratory atmosphere. For indoor conditioning, debond onset strain was practically unchanged in the CFRP beams over 13 years, while a decrease was seen in the GFRP beams. For outdoor conditioning, which included freeze-thaw cycles, rain, hot weather, and ultraviolet exposure, clear reductions in debond onset strain were observed in both GFRP and CFRP strengthened beams.

Reveal of Internal, Early-Load Interfacial Debonding on Cement Textile-Reinforced Sandwich Insulated Panels

Internal interfacial debonding (IID) phenomena on sandwich façade insulated panels are detected and tracked by acoustic emission (AE). The panels are made of a thin and lightweight cementitious composite skin. In the lab, the panels are tested under incremental bending simulating service loads (i.e., wind). Local (up to 150 mm wide) skin-core detachments are reported in the early loading stage (at 5% of ultimate load) and are extensively investigated in this study, since IID can detrimentally affect the long-term durability of the structural element. A sudden rise in the AE hits rate and a shift in the wave features (i.e., absolute energy, amplitude, rise time) trends indicate the debonding onset. AE source localization, validated by digital image correlation (DIC) principal strains and out-of-plane full-field displacement mapping, proves that early debonding occurs instantly and leads to the onset of cracks in the cementitious skin. At higher load levels, cracking is accompanied by local debonding phenomena, as proven by RA value increases and average frequency drops, a result that extends the state-of-the-art in the fracture assessment of concrete structures (Rilem Technical Committee 212-ACD). Point (LVDT) and full-field (AE/DIC) measurements highlight the need for a continuous and full-field monitoring methodology in order to pinpoint the debonded zones, with the DIC technique accurately reporting surface phenomena while AE offers in-volume damage tracking.

Strengthening of reinforced concrete beams in shear using different steel reinforced grout techniques

AbstractIn this study, steel reinforced grout (SRG) is proposed for shear strengthening of reinforced concrete (RC) beams using the near‐surface embedded (NSE) technique. It is believed based on several research contributions in the literature that the NSE technique precludes or delays the onset of premature debonding and achieves higher strength increase in strengthened beams compared to the externally bonded (EB) counterpart. The tests conducted in this study used 13 RC beams to determine the shear behavior of RC beams strengthened in shear using SRG. The effect of the strengthening technique (NSE versus EB), SRG fabric density, strengthening scheme (side‐bonded vs. U‐wrapped), and the strengthening configuration (continuous vs. discontinuous) on RC shear enhancement was studied. The strengthening effectiveness of the SRG system was assessed in terms of the shear capacity, failure mechanism, load‐deflection response, and strain results. The NSE‐SRG increased the beam shear strength by an average of 100%, alleviated SRG debonding, and enhanced the deformation characteristics. The average increase in the shear strength of the EB‐SRG strengthened beam was 54% and 105% for the continuous and discontinuous SRG strips, respectively. An analytical model is proposed to predict the shear capacity of both the NSE‐SRG and EB‐SRG strengthened beams and give accurate and safe predictions.

Interfacial mechanical properties of carbon/glass hybrid thermoplastic epoxy composite rods

Interfacial mechanical properties of carbon/glass hybrid thermoplastic epoxy composite rods play a crucial role in the determination of the composite strength and fracture behavior. In this study, push-out and push-back tests using three hybrid rods with different carbon/glass ratios were performed to investigate the interfacial shear strength and fracture mechanism. Debonding occurred at the interface between the glass fiber bundles and thermoplastic epoxy. After the onset of debonding, sinusoidal behavior during sliding was observed in the load displacement curve, which was identical with the interface roughness in terms of wavelength. Debonding and sliding strength were considered to be related to the interfacial bonding region between the carbon fiber bundle core and surrounding braided glass fiber bundles.

Prediction of interfacial debonding between stiff spherical particles and a soft matrix with the coupled criterion

The tensile load at debonding onset between stiff particles and a soft matrix is predicted with the coupled criterion which combines a stress and an energy conditions. A finite element model of a representative volume element is implemented in order to consider the influence of the size of the particles, the particle volume fraction, mixed mode conditions at the interface and the presence of residual stresses. It is also shown how this approach can be used in order to analyze experimental data for the identification of the interfacial fracture parameters.

Prediction of debonding growth in two-dimensional RVEs using an extended interface element based on continuum damage mechanics concept

In the present study, a new interface model based on continuum damage mechanics (CDM) is developed to investigate the fiber-matrix interfacial debonding in the composite material. This model differs from Voronoi cell finite element and cohesive zone element method. To this purpose, a 2D classical elasticity formulation with non-interactive strength-based criteria and complex variable method with non-interactive energy-based criteria are applied for the analysis of the debond onset and propagation in an RVE with a single fiber, respectively. Then, the CDM approach is used to bridge between analytical and numerical methods. The developed model based on CDM is implanted into ANSYS commercial software to compare the accuracy of the proposed method with the obtained results by the cohesive zone model in RVEs containing single and multiple fibers. The admissible matching between the obtained stress-strain responses reveals the capability of the proposed approach to predict the fiber-matrix debonding as a primary damage mode in composite materials.

Strengthening of RC beams with externally bonded and anchored thick CFRP laminate

Premature debonding of externally bonded FRP laminate from retrofitted reinforced concrete (RC) members can lead to inefficient use of FRP and can limit the level of strength increase that can be achieved. In this investigation, π-CFRP anchors are used in an attempt to delay the onset of premature debonding and to achieve higher strength in beams retrofitted with a 1.2 mm thick and 50 mm wide CFRP laminate. This investigation consisted of testing six large scale T-beams with a 4500 mm span, 400 mm height and 500 mm flange width under four-point bending. Two beams were tested without retrofit as control beams, one beam with the laminate epoxy bonded to the beam and the remaining three beams with the laminate epoxy bonded and anchored using CFRP π-anchors. One of the beams with 30 anchors exhibited a 46% increase in the debonding load over the beam without anchors while the laminate attained a maximum strain equal to 80% of its ultimate strain capacity, a 94% increase compared to the maximum strain reached in the companion beam strengthened with only the epoxy bonded laminate. The displacement ductility ratio of the latter beam at debonding exceeded 4. The results demonstrate the π-anchoring system effectiveness and a feasible way for efficiently utilizing strong and thick laminates in strengthening RC members.

Interface crack model using finite fracture mechanics applied to the double pull-push shear test

An analytical procedure predicting a debond (interface crack) onset and growth in an adhesive joint between two beams or plates is developed and applied to a specific configuration often used in reinforcement tests in civil engineering. The procedure is based on Timoshenko beam theory and Linear Elastic-(Perfectly) Brittle Interface Model (LEBIM) combined with the Coupled Criterion of the Finite Fracture Mechanics (CC-FFM) for mixed-mode fracture. First, a sixth order differential equation in the shear stresses along the adhesive layer is deduced and solved, leading to closed form expressions for both shear and normal stresses in the adhesive. Then, the critical value of the applied load necessary to produce debonding is predicted by coupling a stress and an energy condition based on: (i) the stress distribution produced in the interface before the debond onset and (ii) the energy released during the debonding process along the interface. Although the developed procedure can be applied to several types of joints with different geometries, materials and loads (e.g., double lap joint tests including adherents made of steel or composites), herein it is applied to the double pull-push shear test where the debond onset and growth between a Carbon Fibre Reinforced Polymer (CFRP) laminate and a concrete block occurs. For such a case, the debond is produced under predominant fracture mode II; nevertheless, it is shown that relevant normal (peeling) stresses associated to mode I may appear as well. A comparison of the present solution with a previous one by the shear-lag model is provided as well.

Sequential Linear Analysis for the Prediction of the Symmetrical or Non-Symmetrical Character of the Debond Onset and Propagation Along a Fiber-Matrix Interface

In this work, a recently proposed numerical tool is used to predict the onset and growth of debonds appearing along a single glass/carbon fiber embedded in an epoxy matrix subjected to transverse loads. The fiber-matrix system is modelled using the FEA commercial code ABAQUS, together with a solving algorithm programmed in Python and named Sequential Linear Analysis (SLA). Besides, the interface behavior is modeled using the Linear Elastic Brittle Interface Model (LEBIM) included in ABAQUS by means of a UMAT subroutine. The developed models are able to reproduce the non-symmetrical (one-side) debond at the fiber-matrix interface. Moreover, the results obtained show that the appearance of a unilateral debond may be affected both by the material employed and the size of the matrix cell where the fiber is embedded.

A Comparative Study of Cohesive Law Shapes in Analytical Modeling of Interfacial Debonding in Lithium-Ion Battery Electrodes

To determine the impact of cohesive law shapes on the modeling of interfacial debonding in lithium-ion battery electrodes, analytical methods based on different cohesive models for the debonding process have been developed individually. Three different cohesive laws, namely, triangular, trapezoidal, and rectangular laws, have been employed. To ensure comparability, the cohesive strength and the fracture toughness have been set to be identical for different cohesive laws. The evaluation of debonding onset has suggested that the cohesive law shape affects the modeling results only when the interface is ductile. The largest possible difference for the triangular law and the rectangular law on the debonding onset has been estimated. A discussion for specific electrodes has also been provided.

Debonding at the fiber/matrix interface in carbon nanotube reinforced composites: Modelling investigation

Debonding at the fiber/matrix interface is an important failure mechanism in composite materials. It controls, among others, the intralaminar cracking process. A large amount of experimental data exists indicating that the addition of carbon nanotubes (CNTs) in a composite can hinder onset and development of these cracks. Here we report results of a modelling study aiming to better understand the effect of carbon nanotubes on the fiber/matrix debonding. The study is performed on an example of a carbon fiber/epoxy unidirectional composite with two practically realizable CNT configurations: agglomerated CNTs in the matrix and CNTs grown on fibers. The composites are subjected to transverse tension. The stress distribution prior to debonding is predicted using a two-scale finite element model based on the embedded element technique. The onset and propagation of the debond are then analyzed using a surface-based cohesive zone model with a linear traction-separation law. Our results show that CNTs have a strong effect on the stress distribution in the matrix and the debonding process, although their effect on the latter is less pronounced. CNT agglomerates are found to act as stiff microscopic inclusions leading to magnification of stress concentrations and earlier initiation of the debonding process. The composites with CNTs grown on fibers, on the other hand, show suppression of stress concentrations at the interface and slight delay in the debonding onset compared to the composite without CNTs.

Ultrasonic monitoring of insulated block joints

Insulated block joints are essential components used in railway tracks. They are divided into circuits and are used for train detection and signalling. However, they also represent a weak point in the track system and have a finite life. Condition monitoring of these components for planning preventative maintenance is currently labour intensive, and can be significantly expensive for the rail operator. In this study, insulated block joints were fatigued via shear load, whilst being condition monitored for degradation using a normally incident ultrasonic technique. Tests were also initially performed on lap-joints and shear specimens to further understand the response of the ultrasonic signal to failure of the adhesive layer under controlled conditions. Dynamic reflection coefficients as well as the applied load were recorded in all tests, and results were compared to failure zones on the specimens. The results showed that the ultrasonic technique was able to determine the onset of failure and de-bonding of the adhesive layer in addition to degradation and wear. The technique was also able to highlight differences in performance between two different liners, pultruded glass reinforced polyester resin and a flexible glass fibre sheet, with the latter showing improved resistance. The outcomes of this study have highlighted the viability of condition monitoring insulated block joints using an ultrasonic approach and have provided a basis for a future field trial.

Convergence of the BEM Solution Applied to the CCFFM for LEBIM

A procedure based on the Linear Elastic Brittle Interface Model (LEBIM) combined with the Coupled Criterion of Finite Fracture Mechanics (CCFFM) is successfully implemented in a 2D Boundary Element Method (BEM) code. In the original LEBIM formulation, the values of the interface strength, fracture toughness and stiffness are dependent on each other. Therefore, for a large interface stiffness, when the elastic interface tends to a perfect (infinitely stiff) interface, LEBIM is not able to properly characterize the crack propagation. The use of the CCFFM applied to LEBIM, with both the stress and energy criteria imposed as independent fracture conditions, allows to uncouple the interface fracture toughness and strength, obtaining realistic predictions for crack propagation even for stiff interfaces. This code is successfully applied to the problem of debond onset and growth in the pull push test. A benchmark problem is solved, focusing on the convergence of the load-displacement curve and crack-tip solution for h-refinements of BE meshes.