Progressive Debonding Research Articles

Decreasing propagation rate of interfacial debonding between a single carbon fiber and epoxy matrix under cyclic loading

The interfacial debonding of a single carbon fiber transversely embedded in a dumbbell-shaped epoxy sample was generated under cyclic loading, and images were captured using synchrotron radiation X-ray computed tomography. A fatigue testing machine driven by a piezoelectric actuator placed along the beamline for in situ observation was developed for precise alignment. Interfacial debonding was initially observed under a static tensile load and was confirmed to be almost of the same length at both ends of the carbon fiber, implying negligible bending deformation due to inclination. Cyclic loads were then applied to the sample to capture the progressive debonding. The propagation rate of the interfacial debonding decreased as the number of cycles increased. Another sample with a single carbon fiber aligned parallel to the loading direction was prepared following a single-fiber fragmentation test. Interfacial debonding was clearly observed around the fiber breakage. Cyclic loads were also applied to this sample; however, no progression of the interfacial debonding was evident. Degradation of the interfacial strength between the carbon fiber and epoxy matrix was not confirmed under cyclic loading within the elastic deformation range.

The Mechanism of Rockbolts in Jointed Rock under Uniaxial Tension

In this research, the rockbolt mechanism of a jointed rock mass under uniaxial tension was systematically revealed with a laboratory test and a numerical simulation. It was found that the rockbolt rock mass experienced five stages under uniaxial tension, the densification stage, elastic stage, plastic deformation stage, progressive debonding stage, and complete debonding stage. The stress-strain curve and ultimate tensile strength of a rockbolt rock mass were analyzed by taking the rockbolt spacing and rockbolt angle as variables. It was found that the improvement effect of the reduction of the rockbolt spacing on the ultimate tensile strength was limited. When the rockbolt spacing was reduced to a certain limit, the stress concentration area between adjacent rockbolts was connected and destroyed, resulting in the increase of the rockbolt rock strength becoming smaller, and even having a downward trend. The increase of the rockbolt angle led to the change of the stress mode and failure mode of the whole structure, and the ultimate tensile strength first increased and then decreased. The optimal rockbolt angle was between 60° and 70°. It is worth noting that there was an obvious mechanical occlusion between the thread on the rockbolt surface and the rock mass, resulting in the multi-stage step-down characteristic of the stress-strain curve in the complete debonding stage. The results of this study can provide a reference for the design and construction of similar projects.

Liquid metal-filled phase change composites with tunable stiffness: Computational modeling and experiment

Liquid metal-filled phase change composites have promising applications as tunable stiffness components and reconfigurable structures in soft robotics, actuators, and metamaterials. At present, there is a lack of mechanistic understanding on the microscale-macroscale correlation of their mechanical behaviors. To fill this knowledge gap, we employ computational micromechanics modeling to simulate and reproduce the macroscopic mechanical behaviors of these liquid metal composites. Specifically, the liquid metal composites are simulated using representative volume elements and the particle-matrix interfaces are modeled as cohesive surfaces. The model is able to predict not only the stress-strain curves of the composites with temperature-induced phase transition but also progressive debonding between the particles and the polymer matrix. Moreover, the computational model also reproduces the debonding-induced Mullins effect of the liquid metal composites. The simulation results agree well with our experimental data by calibrating the modeling parameters carefully.

Development of a GPU parallel computational framework for impact debonding of coating–substrate interfaces

Single-particle tests are normally used to evaluate the impact resistance performance of automotive coatings. Impact-induced debonding is one of the major failure patterns for coating–substrate structures of a vehicle. Currently, it remains a challenging task to accurately and efficiently simulate progressive debonding of automotive coating–substrate interfaces under single particle impact. The main purpose of this work is to develop a graphics processing unit (GPU)-based parallel computational framework to achieve this end. An efficient coating finite element model in three dimensions is established, where solid elements with good aspect ratios and solid-shell elements are respectively used to discretize the impact contact region and the rest region of a coating. An intrinsic cohesive zone model coupled with a mortar-based contact algorithm is used to accurately describe the progressive debonding behavior of coating–substrate interfaces. The computational efficiency of the developed method is dramatically enhanced by recourse to the GPU parallel computing technique. Three benchmark tests are carried out to validate the effectiveness and efficiency of the developed computational framework. Finally, the novel computational framework is successfully applied to the debonding analysis of an organic coating bonded with an aluminum substrate under single-particle impact. Results show that the GPU parallel computational framework can achieve a total speedup of 136.5, which provides a powerful tool for coating debonding analysis.

Experimental and numerical studies on progressive debonding of grouted rock bolts

Understanding the mechanism of progressive debonding of bolts is of great significance for underground safety. In this paper, both laboratory experiment and numerical simulation of the pull-out tests were performed. The experimental pull-out test specimens were prepared using cement mortar material, and a relationship between the pull-out strength of the bolt and the uniaxial compressive strength (UCS) of cement mortar material specimen was established. The locations of crack developed in the pull-out process were identified using the acoustic emission (AE) technique. The pull-out test was reproduced using 2D Particle Flow Code (PFC2D) with calibrated parameters. The experimental results show that the axial displacement of the cement mortar material at the peak load during the test was approximately 5 mm for cement-based grout of all strength. In contrast, the peak load of the bolt increased with the UCS of the confining medium. Under peak load, cracks propagated to less than one half of the anchorage length, indicating a lag between crack propagation and axial bolt load transmission. The simulation results show that the dilatation between the bolt and the rock induced cracks and extended the force field along the anchorage direction; and, it was identified as the major contributing factor for the pull-out failure of rock bolt.

Towards progressive debonding in composite RC beams subjected to thermo-mechanical bending with boundary constraints – A new analytical solution

Interfacial debonding is a critical problem in beams strengthened externally by sticking a plate at its soffit. This is mainly due to uncertainty of failure mode when the beam is subjected to bending and boundary constraints. Many analytical solutions are available for pure-shear tests when beams are subjected to thermo-mechanical loads; however, only few studies are available when plated beams are subjected to bending. This paper presents a simple approach to study the combined effects of temperature dependent parameters and thermo-mechanical loads on debonding mechanism, interfacial stress-distribution, development of bond-length and load of debonding. First-order closed-form solutions are proposed to address two possible cases of simply supported beam: roller-ended or pin-ended. Proposed solutions also introduce temperature effects (as thermal strains) within the interfacial adhesive to fill the gap in literature; this is useful to relate the effect of glass-transition point of adhesive with the composite action. The analytical observations are validated with focussed experiments and literature, and verified with continuum-discrete FE model; leading to parametric investigations and key observations. Effects of temperature are defined in terms of reverse- and inverse-actions.

Cohesive Zone Modeling of the Elastoplastic and Failure Behavior of Polymer Nanoclay Composites

Cohesive zone model (CZM) is commonly used to deal with the nonlinear zone ahead of crack tips in materials with elastoplastic deformation behavior. This model is capable of predicting the behavior of crack initiation and growth. In this paper, CZM-based finite element analysis (FEA) is performed to examine the effects of processing parameters (i.e., the clay nanoparticle volume fraction and aspect ratio) in the mechanical behaviors of a polymeric matrix reinforced with aligned clay nanoparticles. The polymeric matrix is treated as an ideal elastoplastic solid with isotropic hardening behavior, whereas the clay nanoparticles are simplified as stiff, linearly elastic platelets. Representative volume elements (RVEs) of the resulting polymer nanoclay composites (PNCs) are adopted for FEA with the clay nanoparticle distributions to follow both stack and stagger configurations, respectively. In the study, four volume fractions (Vf = 2.5%, 5%, 7.5% and 10%) and four aspect ratios (ρ = 5, 7.5, 10, and 20) of the clay nanoparticles in the PNCs are considered. Detailed computational results show that either increasing volume fraction or aspect ratio of the clay nanoparticles enhances the effective tensile strength and stiffness of the PNCs. The progressive debonding process of the clay nanoparticles in the polymeric resin was predicted, and the debonding was initiated in the linearly elastic loading range. The numerical results also show that PNCs with stagger nanoparticle configuration demonstrate slightly higher values of the engineering stress than those based on the stack nanoparticle configuration at both varying volume fractions and aspect ratios of the clay nanoparticles. In addition, CZM-based FEA predicts a slightly lower stress field around the clay particles in PNCs than that without integration of CZM. The present computational studies are applicable for processing PNCs with controllable mechanical properties, especially the control of the key processing parameters of PNCs, i.e., the volume fraction and aspect ratio of the clay nanoparticles.

Modeling of progressive debonding of interphase- matrix interface cracks in particle reinforced composites using VCFEM

The Voronoi cell finite element model (VCFEM) containing three phases of inclusion, interphase and matrix in an element has been proven to be very accurate and efficient in the analysis of inclusion reinforced composites. In this paper, we extended our previous study to include the matrix-interphase interface debonding problems. The traction-free at the debonded matrix-interphase interface and the traction reciprocity at the bonded matrix-interphase interface were introduced into the functional by displacement as Lagrange multiplier. A modified complementary energy functional was constructed. When the matrix-interphase interface was debonded according to critical normal stress law, additional nodes are added as node pairs with existing nodes whose stress exceeds the critical value. This process was interated until the stress of all nodes did not exceed the critical value. Thus, the mesh re-division technology was realized for the interface debonding progressively. The proposed VCFEM is validated by results from MARC. The results show that the proposed VCFEM was more efficient. Based on the developed model, the influence of interphase properties on stress-strain cure was studied during the debonding process. This showed that the interphase modulus and thickness had great influence on the overall mechanical behavior of composites.

Visualizing pectin polymer-polymer entanglement produced by interfacial water movement

In this report, we investigated the physical conditions for creating pectin polymer-polymer (homopolymer) entanglement. The potential role of water movement in creating pectin entanglement was investigated by placing water droplets—equivalent to the water content of two gel phase films—between two glass phase films and compressing the films at variable probe velocities. Slow probe velocity (0.5 mm/sec) demonstrated no significant debonding. Corresponding videomicroscopy demonstrated an occasional water bridge, but no evidence of stranding or polymer entanglement. In contrast, fast probe velocity (5 mm/sec) resulted in 1) an increase in peak adhesion strength, 2) a progressive debonding curve, and 3) increased work of cohesion (p < .001). Corresponding videomicroscopy demonstrated pectin stranding and delamination between pectin films. Scanning electron microscopy images obtained during pectin debonding provided additional evidence of both stranding and delamination. We conclude that water movement can supply the motive force for the rapid chain entanglement between pectin films.

A three-scale micro-mechanical model for elastic–plastic damage modeling of shale rocks

This paper is devoted to study the elastic–plastic damage behavior of heterogeneous shale rocks. The representative microstructure of this kind of rocks is first studied in order to define the representative elementary volume for the implementation of homogenization procedure. Three relevant material scales are considered. Inter-particle pores are distributed at the nanoscopic scale. Fine grains of calcite and kerogen are immersed at the microscopic scale. Large grains of minerals are embedded at the mesoscopic scale. Effective elastic properties of shale rocks are first determined by using a three-step linear homogenization procedure. The plastic damage behavior is estimated by developing a three-step nonlinear homogenization method. The effective plastic behavior of porous clay matrix with nanoscopic pores is described by an analytical model. The effects of small and large grains of various mineral inclusions are investigated by using a two-step incremental model. The damage due to progressive debonding of mineral inclusions is taken into account. After the implementation of the proposed model, comparisons between numerical results and experimental data are presented.

Mechanical behavior of timber-concrete composite members under cyclic loading and creep

Composite structures have emerged over the past several years, and are being increasingly studied and applied in civil engineering projects. In this study, a composite structure was combined with timber under tension and concrete to resist compression. Although several connection systems to bind wood and concrete exist, an innovative system was applied in the present study based on a specific wood treatment and the use of adhesives. Eight hybrid timber-concrete composite (TCC) members were fabricated and tested under static and cyclic loading, and creep. The results indicate that the minimum and maximum loads (which represent dead and live loads, respectively) were constant during a cyclic bending test. The mid-span deflection evolved during the test. An analytical model based on the compatibility of the strains was developed to predict the evolution of the displacement at the mid-span and integrate the effect of creep phenomena. Although the TCC members were cyclically tested under the maximum nominal load (live load), the evolution of the mid-span deflection was governed by the creep phenomena. Finally, the composite TCC members were subjected to a residual bending test until failure. TCC members with ordinary concrete showed a progressive loss of bending stiffness during a four-point bending test. This was caused by progressive debonding of the concrete slab and a diminution of the load capacity compared with TCC members that were not cyclically loaded. Additionally, a TCC member with ultra-high performance fiber-reinforced concrete (UHPFRC) achieved similar mechanical behavior as the TCC member tested under a static load.

A mean-field homogenisation scheme with CZM-based interfaces describing progressive inclusions debonding

The objective of the present study is to describe the progressive debonding of inclusions in particle or fibre reinforced composites. To do so, the mean-field homogenisation scheme of Mori-Tanaka is enriched to take into account imperfect interfaces. The interfaces are modelled by a bilinear Cohesive Zone Model (CZM) taking into account normal and tangential effects. Results obtained with this new mean-field homogenisation scheme are compared to 2D FE-based numerical simulations that are used as reference results. The effects of inclusions volume fraction and size are also observed.

Analysis of interfacial interaction in UHPFRC-strengthened reinforced concrete beams

The behaviour and failure mode of reinforced concrete elements strengthened with a thin tensile layer of ultra-high performance fiber-reinforced concrete (UHPFRC) is governed by the interface interaction between the UHPFRC and conventional concrete. In order to analyze the relative displacements at the interface and its influence on the crack pattern evolution and monolithic response, digital image correlation (DIC) has been used in this contribution. With the help of DIC, the distinct stages of the behaviour of UHPFRC-strengthened beams have been correlated with the crack formation and development. Special attention has been paid to the progressive debonding due to the strain incompatibility between UHPFRC and conventional concrete when cracks develop. It is shown that the utilization of the capacity of the UHPFRC can be achieved from a certain thickness of the strengthening layer for shear-critical elements, thereby providing a gain of shear strength and ductility. In contrast, a thinner UHPFRC layer has been more beneficial for flexure-critical elements.

DEM simulation of mortar-bolt interface behaviour subjected to shearing

In this study, a 3D DEM model containing a mortar-bolt interface subjected to shearing was established in the context of the simplified rock bolt model (SRBM) proposed in a companion paper. The DEM model was calibrated against a series of laboratory experiments to reproduce the mechanical characteristics of a cement mortar with a uniaxial compressive strength of 30 MPa. The DEM simulation has led to a detailed observation and an in-depth understanding of the mode II progressive debonding of the mortar-bolt interface and subsequent mortar rupture (due to mechanical interlocking). In addition, the effects of particle size of mortar and profile configurations of rebar bolts (i.e., different rib spacings and rib heights) on simulation results were discussed. The numerical findings in the study were validated against laboratory measurements and a broad agreement was observed.

A numerical insight into elastomer normally closed micro valve actuation with cohesive interfacial cracking modelling

Pneumatic NC (normally closed) valves are widely used in high density microfluidics systems. To improve actuation reliability, the actuation pressure needs to be reduced. In this work, we utilize 3D FEM (finite element method) modelling to get an insight into the valve actuation process numerically. Specifically, the progressive debonding process at the elastomer interface is simulated with CZM (cohesive zone model) method. To minimize the actuation pressure, the V-shape design has been investigated and compared with a normal straight design. The geometrical effects of valve shape has been elaborated, in terms of valve actuation pressure. Based on our simulated results, we formulate the main concerns for micro valve design and fabrication, which is significant for minimizing actuation pressures and ensuring reliable operation.

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

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

Programmable Nanopatterns by Controlled Debonding of Soft Elastic Films.

We report a facile patterning technique capable of creating nanostructures with different feature heights (hS), periodicities (λS), aspect ratios (AR), and duty ratios (DR), using a single grating stamp with fixed feature height hP and periodicity λP. The proposed method relies on controlling the extent of debonding and morphology of the contact instability features, when a rigid patterned stamp is gradually debonded from a soft elastic film to which it was in initial conformal contact. Depending on whether the instability wavelength (λF scales with the film thickness hF as λF ≈ 3hF) and the periodicity of the stamp feature (λP) are commensurate or not, it is possible to obtain features along each stamp protrusion when λF ≈ λP or patterns that span several stripes of the stamp when λF > λP. In both cases, the patterns fabricated during debonding are taller than the original stamp features (hS > hP). We show that hS can be modulated by controlling the extent of debonding as well as the shear modulus of the film (μ). Additionally, when λF > λP, progressive debonding leads to the gradual peeling of replicated features, which, in turn, allows possible tuning of the duty ratio (DR) of the patterns. Finally we show that by the simultaneous modulation of AR, DR, and hS, it becomes possible to create surfaces with controlled wettability.

Exploring the influence of micro-structure on the mechanical properties and crack bridging mechanisms of fibrous tufts

A constitutive model for tufts bridging a mode I delamination is presented. The tuft is modelled as a rod, laterally supported by an elastic medium and clamped at both ends. A fracture mechanics approach is introduced to describe the progressive debonding of the tuft from the embedding laminate. The debonding model requires the identification of stiffness, strength and toughness properties, which depend both on the laminate/tuft architecture and the constituent materials. Such identification is carried out via experimental data obtained from tensile tests on single tufts inserted in a pre-delaminated non-crimp fabric composite. The experimental results are complemented by micro-scale finite element analysis. The mode I bridging law obtained from the constitutive model is implemented into a meso-scale cohesive zone formulation. This formulation is applied to predict the response to delamination of tufted Double Cantilever Beam (DCB) coupons. The cohesive zone approach is validated by means of experimental data from DCB tests. It is shown that the proposed micro- to meso-scale modelling approach yields results in good agreement with the experiments.

Analysis of FRP-to-concrete interfaces using a displacement driven partial interaction model

Fibre-reinforced polymer (FRP)-to-concrete bonded interfaces are susceptible to bond failure in the concrete at the interface. This paper presents the details of a partial interaction model that is capable of analysing such bond failure in FRP-to-concrete joints. The model is driven by displacing the slip between the FRP and concrete at the loaded end of the joint and is thus able to capture unloading of the bonded interface. Such unloading is due to a reduction in the available length of the bonded plate on account of progressive plate debonding. A bond stress versus slip model is calibrated from test results and incorporated into the partial interaction model. Predictions of strain, bond stress and slip along the length of the bonded plate are produced and the results are found to correlate with test measurements. Finally, parametric studies on a typical joint enable insights to be gained on the parameters used to define the bond-slip model.

Three-Dimensional Modeling of Short Fiber-Reinforced Composites with Extended Finite-Element Method

AbstractThis manuscript presents a modeling approach based on the extended finite-element method (XFEM) for modeling the mechanical behavior of three-dimensional short fiber composites including interface debonding. Short fibers are incorporated into the XFEM framework as deformable elastic two-dimensional rectangular planar inclusions. Enrichment functions account for both the presence of axial deformable fibers within the composite domain and the progressive debonding along the fiber matrix interfaces. A modeling strategy is provided that is particularly suitable for failure analysis of composites with high-aspect-ratio inclusions, in which direct numerical analysis is computationally intractable. The performance of the proposed XFEM model is numerically assessed by comparing model predictions to the direct finite-element method.