Debond Length Research Articles

Theoretical study about influence of geometry and mechanical load on the delamination in tungsten disulfide/poly(methyl methacrylate) nanocomposite structure under axial load

The influence of the geometry and the magnitude of axially applied mechanical load on the delamination in three-layer tungsten disulphide (WS2)/SU-8/poly(methyl methacrylate) (PMMA) nanocomposite, is investigated theoretically. First, for considered nanostructures with thinner and thicker PMMA layer two different analytical solutions (Case 1 and Case 2) for the interface shear stress (ISS) in the middle layer of the structure are obtained, based on the application of two-dimensional stress-function method and minimization of the strain energy. Second, the theoretical criterion for delamination in the interface layer, based on the model ISS, is formulated and the obtained non-linear equation in respect to debond length is solved numerically, for both solutions, at different values of mechanical load and geometry of the structure layers. It was found that delamination doesn't appear at fixed WS2 length of 10 μm, if the loading is up to 5 GPa for Case 1 and up to 1.175 GPa for Case 2, respectively. With increasing WS2 length, the delamination occurs at increasingly higher values of the applied external load, for Case 2. For Case 1 a delamination is not occurs. At fixed applied load, it was found for Case 2, that as the length of WS2 increases, the debonding length increases, too. The obtained results could be used for fast prediction of delamination in similar nanostructured devices.

Failure mode of interlayer connection of longitudinally-connected ballastless track-bridge system under uneven pier settlement

• Proposing the characterization model of uneven pier settlement and interlayer connection failure in LBTBS. • Analysing the generation mechanism and development law of interlayer connection failure caused by uneven pier settlement. • Describing the evolution law of interlayer contact behavior of each layer of track caused by uneven pier settlement considering the existing interlayer connection failure. The generation and mechanical balance mechanisms and the deformation compatibility relationship of interlayer connection failure (ICF) of a longitudinally-connected ballastless track-bridge system (LBTBS) caused by uneven pier settlement is analyzed in this paper. The characterization model of the uneven pier settlement and the ICF of the LBTBS was derived based on Castigliano's Second Theorem, Linear Superposition principle, Heaviside function, and Tension-free Winkler beam. The characterization model was further verified by the field measured data and finite element model results. Finally, the development law of the ICF of the LBTBS caused by uneven pier settlement based on the characterization model was analyzed. The results show that the calculated results of the characterization model agree well with the field measured data and finite element model results, which verifies the correctness of the characterization model. Under uneven pier settlement, a void beneath the slab first appeared between the base slab and bridge, and then mortar debonding occurred between the CA mortar and track/base slab when settlement value is 193.48 mm. The maximum height of void beneath the slab developed away from the beam gap, and when the settlement value was less than 8.434 mm, the void beneath the slab length in the left area of the settlement pier was greater than that of the adjacent pier. With the increase in mileage, the height of the void beneath the slab and mortar debonding increased first and then decreased. In practical engineering, mortar debonding is not easy to notice, and the void beneath the slab between the base slab and the bridge should be inspected regularly. The existing fastener failure had little influence on developing a void beneath the slab caused by the uneven pier settlement. The existing mortar void aggravated the occurrence of mortar debonding. When the existing mortar void length exceeded 5 m, the mortar debonding length at both ends of the adjacent pier was about 1/2 of the existing mortar void length. When the length of the existing mortar void was less than 1 m, there was little effect on the length of the void beneath the slab.

Study on Bond Defect Detection in Grouted Rock Bolt Systems under Pullout Loads

In grouted rock bolt systems, bond defects often occur, which seriously affects the safety of rock mass structures. Therefore, in this study, based on the existence of bond defects, laboratory tests were conducted to detect the location and length of bond defects and study the guided wave propagation in grouted rock bolt systems under different pullout loads. The guided wave signal was analysed in the time domain and frequency domain. In addition to the laboratory test, a numerical simulation of the effect of different bond defect locations on ultrasonic guided wave propagation in rock bolts was conducted using a damage-based model. The influence mechanism of bond defects on guided wave propagation under different pullout loads was explored. The study confirmed that there existed a stress platform in the rock bolt at the bond defect under a pullout load. The location and length of the bond defect could be detected by the stress platform and guided wave. The debonding length increased exponentially with the amplitude ratio (Q) of low frequency to high frequency, and the Q value could be used as the quantitative index of debonding length. As the pullout load increased, the impedance mismatch between the rock bolt and cement mortar (defect) increased, and the guided wave propagation in grouted rock bolt systems was irregular. The pullout load weakened the guided wave propagation law. The larger the pullout load is, the greater the weakening effect is.

Hierarchical Flax Fibers by ZnO Electroless Deposition: Tailoring the Natural Fibers/Synthetic Matrix Interphase in Composites.

Hierarchical functionalization of flax fibers with ZnO nanostructures was achieved by electroless deposition to improve the interfacial adhesion between the natural fibers and synthetic matrix in composite materials. The structural, morphological, thermal and wetting properties of the pristine and ZnO-coated flax fibers were investigated. Thus, the ZnO-coated flax fabric discloses an apparent contact angle of ~140° immediately after the placement of a water droplet on its surface. An assessment of the interfacial adhesion at the yarn scale was also carried out on the flax yarns coated with ZnO nanostructures. Thus, after the ZnO functionalization process, no significant degradation of the tensile properties of the flax yarns occurs. Furthermore, the single yarn fragmentation tests revealed a notable increase in the interfacial adhesion with an epoxy matrix, reductions of 36% and 9% in debonding and critical length values being measured compared to those of the pristine flax yarns, respectively. The analysis of the fracture morphology by scanning electron microscopy and X-ray microtomography highlighted the positive role of ZnO nanostructures in restraining debonding phenomena at the flax fibers/epoxy resin matrix interphase.

Scattering Analysis and Detection of Layered Plate Debonding Using Guided SH Waves with Boundary Element Method

This paper investigates the scattering behavior of guided shear horizontal (SH) waves in a two-dimensional, isotropic, and linear elastic layered plate with partially debonded interface by analyzing the reflection and transmission coefficients of scattered waves. The partial wave technique is established to form the displacement and stress of guided wave functions, and the boundary element method (BEM) is utilized to handle the numerical calculation with elastodynamic fundamental solutions in the frequency domain. After applying proper boundary conditions including continuity condition on the interface with traction-free debonding, the scattering coefficients can be obtained in terms of boundary element solutions. Two different materials (steel and aluminum) with various debonding lengths and locations in a 1 mm double-layered plate are considered. With several modes of the incident wave over a frequency range up to 4.5 MHz, the variations of scattering coefficients and scattering phenomena are numerically investigated as several parameters such as mode of the incident wave, materials, locations, and length of debonding are changed. The numerical results also suggest the potential of the suitable wave mode for the debonding detection, which can be useful for non-destructive inspection.

Finite element analysis of the effect of longitudinal debonding on stress redistributions around fibre breaks in randomly packed fibres

One of the challenges in longitudinal strength models of unidirectional composites is to comprehensively simulate the stress redistribution around a broken fibre. This redistribution is affected by the interfacial debonding of the broken fibre from the matrix. In the present work, a novel approach using finite element modelling is developed that considers all primary contributors to the longitudinal debonding in randomly packed fibre configurations. The parametric study revealed that a larger friction coefficient, interfacial fracture toughness and interfacial shear strength shorten the debond length and increase the stress concentration factor (SCF) on the neighbouring intact fibres. The magnitude of the SCFs strongly depends on the local fibre volume fraction, the radial distance between the intact fibres and the broken fibre, and the debond length. Incorporating interfacial debonding considerably reduces the SCF overpredictions by the well-bonded models. The presence of a matrix crack, encircling the broken fibre, locally increases the SCFs in the adjacent intact fibres and marginally shortens the debond length of the broken fibre. By incorporating these results into a strength model, the influence of the longitudinal debonding and co-existing matrix cracks on failure strain of unidirectional composites can be further tuned.

Effect of adhesive failure on measurement of concrete cracks using fiber Bragg grating sensors

Systematic deviations in the sensing process of optical fiber sensors (OFSs), which are induced by interfacial failures (concrete/adhesive (C/A)), are unavoidable. Thus, it is important to evaluate the interfacial failure of heterogeneous OFSs to correct their systematic deviations. In this study, experimental investigations were conducted on the interfacial failure of adhesive-packed fiber Bragg grating sensors (FBGs) attached to a concrete matrix. Further, an inverse analytical method for crack-induced strain distribution at the C/A interface was proposed. Finally, the strain of the initial debonding was determined using an energy-based criterion, as well as the debonding length. The results indicate that the fracture propagation of notched concrete beams can be quantitatively analyzed by combining FBGs and the DIC technology. Theoretical values of the debonding length and fitted strain of the adhesive subjected to digital imaging conformed with the test results. The plastic failure of adhesive materials was related to the performance of C/A interfacial bonding.

Study on propagation characteristics of ultrasonic guided wave and detection of the defect in resin bolts

The nondestructive testing of the bolt anchoring quality based on ultrasonic guided wave technology is significant to ensure project security. In this paper, an improved Legendre polynomial method (ILPM) is presented to study the longitudinal axisymmetric guided waves in a full-length bonding resin bolt which is a typical bilayer structure. The ILPM employs analytical integral calculation instead of numerical integral in the traditional Legendre polynomial method (TLPM), leading to superior computational efficiency. The effectiveness of the ILPM is validated, and the convergence and computational efficiency are also discussed. The attenuation and group velocity curves of guided waves are solved for the resin bolt. Through the theoretical analysis of dispersion characteristics, the optimal excitation frequencies for experimental detection can be found. Meanwhile, the guided wave detection system of a resin bolt is established to verify the theoretical results. Based on the Self-Sending and Self-Receiving experiment, the resin bolts with different anchoring defects (debonding length and incision size) are studied. The influences of L-mode defect detection ability and defect size parameters on the amplitude of the guided wave signal are analyzed. The experimental results show that L-mode can be used to detect the defects in the bolt, and the defect size can be characterized by the defect echo amplitude.

Effect of crack and fiber length on mode I fracture toughness of matrix-cracked FRC beams

Pre-cracking fiber-reinforced concrete (FRC) is the most challenging aspect of determining the fracture toughness of such materials. In this paper, a matrix-crack (MC) approach suggested recently by the authors was adopted to estimate the real mode I fracture toughness (KIC) of FRC beams. The effect of notch depth to beam depth ratios (a/d) of 0.1, 0.3, and 0.5 and fiber lengths of 35 mm, 50 mm, and a hybrid fiber composed of 50% from each length, on KIC was investigated. Similarly, traditional through-thickness cracked (TTC) beams were cast for comparison. Hooked-end steel fibers with 1% fiber volume fraction were used to produce FRC. A three-point bending test was performed on thirteen sets of FRC beams. The span/depth ratio was kept equal to four. The results indicated that the MC approach with different a/d successfully predicted KIC of FRC differently than the TTC approach. For MC beams, KIC of FRC decreased by increasing a/d. Furthermore, the length of debonding fibers has a detrimental impact on KIC. The beam with long fibers showed higher fracture energy than the short fibers based on the work of fracture method recommended by RILEM. However, increasing the length of the fibers reduced KIC because of increasing their debonding length and subsequently decreasing their efficiency.

Simulation and analysis on improving progressive collapse resistance of RC assemblies by embedding locally debonded rebars near beam-ends

A novel method has been experimentally validated using locally debonded short rebars embedded at the beams' half-height near beam-ends to improve the progressive collapse resistance of reinforced concrete (RC) beam-column assemblies. As an extended study, this paper numerically investigated the load transfer mechanism of RC beam-column assemblies, the principle of resistance improvement by the novel method, and the influence of critical design parameters on the effectiveness of the novel method. First, the finite element models were created, with emphasis on the modeling of the bond-slip behavior of reinforcing steel bars in pre- and post-yield ranges. This behavior was found to have significant impacts on the deformation and load-carrying capacities of the RC beam-column assemblies. Second, the finite element models were validated against test data and then employed to understand the load transfer mechanisms. The resistance of the RC beam-column assemblies at the stage of small deformation was found to primarily provided by flexural/arch mechanism, reaching a maximum at middle column displacement (MCD) ≈ 0.5 h (i.e., 150 mm). Once the value of MCD exceeded h (i.e., 300 mm), the catenary mechanism was mobilized and approximately 90% of the resistance at this stage was contributed by the axially tensile force of the beams. On this basis, the principle of resistance improvement using the novel method was discussed, i.e., the progressive collapse resistance of the RC beam-column assemblies was improved through fully mobilizing catenary action and maintaining the flexural mechanism in the beams. Finally, parametric studies were conducted with consideration of the debonded length (lpd) and the reinforcement ratio (ρs,mid) of locally debonded short rebars. Results indicated that the appropriate lpd ranged from 600 to 1000 mm, and the proper ρs, mid varied from 1.5% to 2.2% when used in engineering practice.

Implementation of multiscale mechanisms in finite element analysis of active composite structures

Interrogation of composite structures for inherent damage is investigated implementing three-tier analysis scheme. The analysis starts at the structure level where a general-purpose Finite Element code ABAQUS is employed to obtain the stress field in the second level of analysis that is the composite laminate. A special material routine is prepared to propagate the local fields to the individual plies and hence to the third level of analysis which is the microstructure modelling of the composite. Through the third level of analysis, interface damage between fiber and matrix is checked implementing a certain failure criteria. The interaction between the different length scales; that are, structure, macro, and micro scales, is implemented using the non-mechanical strains caused by damage. Embedment of electrically active fibers in the laminate serves as a tool to interrogate the structure for inherent damage through the electric displacement generated due to the electro-mechanical coupling. To verify the developed multi-scale model, a comparison with the experimental results of a tube problem under combined internal pressure and axial load is presented. The results show good agreement in both the undamaged and damaged state. As an application, a plate with a hole under biaxial load is solved. The output includes not only the structure global behavior during damage but also detailed information about the location of damage within the structure, the plies that exhibit damage, type of damage and the fiber/matrix debonded length. Comparison between the electric displacement of the active fibers in the undamaged and damaged states is presented as well.

Characterizing the interfacial fracture energy of stiff islands on stretchable films

Abstract This study characterized the interfacial fracture energy of stiff islands deposited on a thermoplastic polyurethane (TPU) film. The film can deform by >200%. The film was stretched using a designed fixture, and the fracture behaviors of the islands were observed using a microscope. The island–substrate interface debonding lengths associated with different levels of substrate strain were determined in the stretching tests. Because the stretchable film was a nonlinear material, the Ogden model was employed to characterize the nonlinear constitutive relation. Through the tensile tests, the material parameters in the Ogden model were determined using the reduced-gradient optimization method. On the basis of the measured debonding lengths, a finite element model was generated for the nonlinear properties of the film, and the energy release rates at the crack tip were calculated using the J-integral method. The energy release rates, representing the interfacial fracture energy, were calculated on the basis of the arrested crack associated with different crack lengths. Results reveal that the interfacial fracture energy increased from 0.14 to 0.91 kJ/m2 as the debonding length increased. The behavior is related to the rising resistance curve in TPU materials. In addition, the shearing-dominated mode slightly decreased as the debonded length increased in the stretching tests.

Experimental and numerical evaluation on debonding of fully grouted rockbolt under pull-out loading

The axial loading in rockbolts changes due to stress redistribution and rheology in the country rock mass. Such a change may lead to debonding at rockbolt to grout interface or rupture of the rockbolt. In this study, based on laboratory experiments, ultrasonic guided wave propagation in fully grouted rockbolt under different pull-out loads was investigated in order to examine the resultant debonding of rockbolt. The signals obtained from the ultrasonic monitoring during the pull-out test were processed using wavelet multi-scale analysis and frequency spectrum analysis, the signal amplitude and the amplitude ratio (Q) of low frequency to high frequency were defined to quantify the debonding of rockbolt. In addition to the laboratory test, numerical simulation on the effect of the embedment lengths on ultrasonic guided wave propagation in rockbolt was conducted by using a damage-based model, and the debonding between rockbolt and cement mortar was numerically examined. It was confirmed that the ultrasonic guided wave propagation in rockbolt was very sensitive to the debonding because of pull-out load, therefore, the critical bond length could be calculated based on the propagation of guided wave in the grouted rockbolt. In time domain, the signal amplitude in rockbolt increased with pull-out load from 0 to 100 kN until the completely debonding, thus quantifying the debonding under the different pull-out loads. In the frequency domain, as the Q value increased, the debonding length of rockbolt decreased exponentially. The numerical results confirmed that the guided wave propagation in the fully grouted rockbolt was effective in detecting and quantifying the debonding of rockbolt under pull-out load.

Effect of fiber bending induced matrix shear behavior on machined surface quality in carbon fiber reinforced plastic milling

Fiber bending by the cutting tool is one of the dominant causes of fiber pull-out on machined surfaces during CFRP milling. During the bending, the polymer matrix between the fibers is subjected to shear stress resulting in fiber-matrix debonding. The increase in the debonding length causes an increase in the fiber pull-out length. In milling, not all the new surfaces created by tool-workpiece engagement become the machined surface. However, during this process, the strain energy absorbed by the deformed matrix is released into fiber-matrix debonding along the fibers which can cause fiber pull-out on the machined surface. In this study, shear strain energy of polymer matrix due to fiber bending during milling tool-workpiece engagement was calculated with the geometrical relationship between fibers and a cutting edge. We found that the increase in Ra and Rz of the machined surface had clear tendencies to the increase in the calculated energy.

Micromechanical experimental study on the inter-fibre failure under tension: Microscopical observations

The study of the matrix/inter-fibre failure at micromechanical level by means of numerical methods predicts the appearance of differents stages in the development of this mechanism of damage; these numerical studies have also allowed the main features of each stage (such as the debond length at the interface and the kinking angle) to be identified. The development of experimental studies aiming to check the relevance of the aforementioned numerical results, the validity of the hypotheses assumed, as well as the identification of additonal parameters, is crucial for the advance in the knowledge of this mechanism of damage. Based on this, the research shown in this work focused on the tensile test (under different loading levels) of specimens manufactured from carbon-epoxy cross-ply symmetrical laminates. The microscopical observation of the 90º laminae of the tested samples, first results in the analysis of the appearance of the transverse cracks as a function of the load. Second, the study concentrates in the identification of the previously numerically predicted stages of the mechanism of damage and the measurement of key  parameters such as the debond length, the kinking path and the influence of nearby fibres.

Effects of dynamic fibre failure in unidirectional composites

The stress redistribution around a broken fibre is a substantial issue in longitudinal strength analysis of unidirectional (UD) composites. Most investigated models are based on a regular fibre packings and classical shear lag theory (static failure) that considers pre-broken fibers, where the dynamic aspects of brittle fiber failure are not considered. In the present work, Representative Volume Elements with random fibre packings were modeled by means of 3D finite element analysis, and simulated considering the event of dynamic fibre failure. Significant effects were observed on the stress concentration factor (SCF) and ineffective load length which are related to the fibre debonding length and the volume of matrix damage-plasticity zones. Hence, it can be concluded that an accurate representation of the stress redistribution after a fibre breakage event requires consideration of dynamic effects.

Experimental investigation on the effect of local debonding of longitudinal reinforcement on seismic performance of precast concrete columns

The influence of the local debonding of longitudinal reinforcement on the seismic performance of prefabricated column with the grouted longitudinal reinforcement in corrugated sleeves is investigated. A total of 10 precast concrete specimens and 1 cast-in-place concrete specimen were designed and tested. Important factors such as anchorage length, local debonding position, debonding length and axial compression ratio were considered. The seismic performance was evaluated under low cycle reversed loading tests, and the failure mode, hysteretic curve, skeleton curve, ductility performance, energy dissipation capacity and stiffness degradation of the specimens were obtained and analyzed. It is found that the hysteretic curve of the precast column with local debonded longitudinal reinforcement is fuller than that of the bonded precast column, and the ductility and energy dissipation capacity of the precast column with local debonded longitudinal reinforcements are significantly improved. The effects of the debonding length and position of the longitudinal reinforcements on the ductility, energy consumption and bending deformation of the precast columns are also discussed. This study is of great significance to improve the bearing capacity and seismic performance of prefabricated columns on the top of the independent foundation in practical engineering.

Longitudinal debonding in unidirectional fibre-reinforced composites: Numerical analysis of the effect of interfacial properties

Longitudinal fibre-matrix debonding is governed by interfacial strength, fracture toughness, thermal residual stresses, friction, and matrix plasticity. The proposed finite element model for fibre-matrix longitudinal debonding associated with fibre breakage accounts for these features, retrieving more realistic results for the stress redistribution around a fibre break. In contrast with the majority of the available finite element models, the current model does not impose the debond length and enables debond propagation based on the assigned interfacial properties. Several parametric studies have been performed to assess the effect of input parameters in two configurations: single- and multi-fibre packings. Higher values for interfacial friction coefficient, thermal residual stress and interfacial fracture toughness restrain the debond propagation and consequently accelerate the stress recovery. Conversely, including matrix plasticity facilitates the debond propagation. A prescribed matrix crack, concentric with the broken fibre and as large as thrice the fibre radius, has no significant effect on the extent of the debond but increases the stress concentration on the nearest intact fibres in the multi-fibre model. The results of the proposed finite element model match the reported laser Raman spectroscopy literature data [1]. The current study improves the prediction capability of models for the longitudinal tensile failure of unidirectional composites.

Modeling of micro-annulus propagation along cementing interface in stimulated horizontal wellbore

ABSTRACT Once there are micro-annulus at cementing interfaces in the process of reservoir stimulation, it is easy to form fluid channeling in the later development. Fluid channeling reduces the efficiency of reservoir development and even the service life of oil and gas wells. Based on the characteristics of ‘multi-stage’ in stimulated reservoir volume, a mathematical model was set up to calculate the interface debonding length driven by hydraulic pressure. The propagation process of micro-annulus in stimulated reservoir volume was also analyzed. The results show that the propagation speed of the micro-annulus gradually slows down with the progress of fracturing. Meanwhile, the debonding azimuth of the micro-annulus and the cement elastic modulus have dominant effects on the propagation length. It is recommended that the setting position of the packer be outranged from the debonding area of the cementing interface in the design of the stimulated reservoir volume scheme to avoid fluid channeling between the stages. This study can predict the unsealing length of cement sheath during hydraulic fracturing as a reference for the optimization of fracturing parameters.

Revised notched coating adhesion test to account for plasticity and 3D behaviour

Specialized polyamide-imide coating on a CuSn10Pb10 substrate, a material combination utilized often in modern bearing applications, is fabricated using the solvent casting method. The coating adhesion is studied using the well-known notched coating adhesion (NCA) test. Conventionally, the critical strain required to cause the debond propagation is determined by visual observation and indirectly linked to measured strain data to calculate fracture toughness. Here, digital image correlation (DIC) is used to systemically study the coating deformations during testing that enables quantitative determination of the instantaneous debond length. With the introduced method, the critical strain to induce the debond of the coating and the propagation of the debond can be determined for non-elastically behaving specimens reliably. The coating’s debond onset is studied with virtual crack closure technique (VCCT) here. The method can take 3D effects into account in detail and provides a sophisticated method to determine the critical energy release rate. Additionally, cohesive zone modelling (CZM) is used to simulate debond progression. Nonlinear stress–strain responses are observed taking place both with the coating and the substrate materials. The results emphasize that the coating plasticity has a remarkable role in the test behaviour which needs to be taken into account in the revised analysis.