Double Cantilever Beam Experiments Research Articles

Toughening of thick bonded interfaces through architected crack-arresting features

This research investigates the application of additively manufactured crack-arresting features (CAFs), designed from tough and soft polymers, in enhancing the performance of thick glass fiber-reinforced polymer composite-epoxy adhesive joints found in wind turbine rotor blades. Mode I fracture and fatigue behaviors of these joints are assessed through double-cantilever beam experiments and compared against pristine joints. In pristine joints, cracks consistently deviate from the adhesive bondline into the composite adherend, causing a sudden increase in strain energy release rate. Finite element models based on linear elastic fracture mechanics are employed to provide insight into this behavior. Experimental results demonstrate that joints with architected CAFs achieved higher strain energy release rates, more stable failure mechanisms, and minimized damage to the composite adherend. Under fatigue loading, joints featuring tough CAF material exhibit slower fatigue crack growth compared to both pristine joints and those with soft CAF material.

Introducing a nano-scale surface morphology parameter affecting fracture properties of CNT nanocomposites

Carbon Nanotube (CNT) particles and membranes improve interlaminar fracture toughness of nanocomposites. The novelty of this study is to introduce the effect of a new nanoscale parameter - CNT network (bundle) size to interphase thickness ratio - on modes I and II interlaminar fracture toughness of CNT Polymer Nanocomposites (PNCs). This investigation includes the stochastic distribution function of CNT bundle size, interphase thickness, and modes I and II fracture toughness of PNCs based on an extensive experimental study at the nano- and macro-scales. The Atomic Force Microscopy PeakForce Quantitative Nanomechanics Mapping technique was used to collect 500 datasets for a low-weight percentage of CNT PNC and 180 datasets for a high-weight percentage of CNT PNC. Each dataset included CNT bundle size and interphase thickness at the nanoscale. Double cantilever beam and end-notched flexure experiments were conducted to collect 600 modes I and II fracture toughness data. Two-, three-, and four-parameter Weibull models were used to simulate the nano- and macro-scale material properties of CNT PNCs. Results indicate that (i) a lower CNT bundle size to interphase thickness ratio improves fracture toughness, and (ii) a four-parameter Weibull model is the most efficient model for simulated data.

Mechanics of inner core debonding of composite sandwich beam with CFRP hexagonal honeycomb

The inner core debonding of composite sandwich beams with carbon fiber reinforced polymer (CFRP) hexagonal honeycomb core is investigated theoretically and experimentally. Based on the principle of minimum potential energy, a theoretical model is established to reveal the core debonding mechanism with Timoshenko’s beam theory and Extended High-Order Sandwich Panel Theory (EHSAPT) for composite face sheets and CFRP hexagonal honeycomb core, respectively. The theoretical approach of core debonding simulation is based on the framework of the cohesive zone model (CZM). By constructing the constitutive relation of the core interface as the discontinuous stress and displacement boundary conditions, the vertical core debonding can be simulated overcoming the singularity of equilibrium equations. The crack propagation process is decomposed into three stages with the consideration of the double crack propagation mechanism. In experiments, the regular and reinforced hexagonal honeycomb sandwich beams are designed and fabricated, and the quantification of the defect degree of the core bonding interface is achieved by equivalent energy release rate obtained from the homogenized hexagonal honeycomb. The double cantilever beam (DCB) experiments are performed to determine the equivalent energy release rate and reveal the core debonding mechanism of CFRP hexagonal honeycomb sandwich beams. Comparing the load–displacement curves and debonding deformation patterns indicates that the established theoretical model can effectively predict the core debonding mechanical response. Finally, parametric analysis is carried out to discuss the impact of structural dimension parameters, defect location and defect degree on the debonding modes of CFRP hexagonal honeycomb sandwich beams.

Inverse parameter identification framework for cohesive zone models based on multi-island genetic algorithm

Composite interfaces are commonly simulated by cohesive zone models with the key challenge being the calibration of interfacial parameters. A novel inverse identification framework is presented in this paper to determine the interface parameters of cohesive zone models. This approach employs the multi-island genetic algorithm to obtain the key parameters of cohesive zone model which can reproduce the experimental observations. Utilizing two independent metrics, namely the load-displacement response and the debonding length history, an objective function is formulated to give the framework inherent robustness. The interface debonding length is used to uniquely determine the debonding properties of the specimen. To demonstrate the feasibility of the proposed framework, the strength-based cohesive zone model is taken as an example. The inverse algorithm is adopted to identify the interface parameters of both the double cantilever beam (DCB) experiments and the fixed-ratio mixed-mode (FRMM) tests. The robustness, accuracy and sensitivity of the framework are validated through the double cantilever beam test. The findings indicate that the numerical results align closely with the experimental data, confirming that the interface parameters identified by the proposed framework can reproduce the experimental results.

An enhanced surface treatment for effective bonding of 7075-T6 aluminium alloys

ABSTRACT This study research focuses on optimising adhesion through surface treatments on 7075-T6 aluminium alloy substrates using a chromium-free protocol. The treatment involves acetone degreasing, NaOH etching, and sulphuric acid-based solution etching. Wettability analysis and scanning electron microscopy were used to characterise the treated surfaces, highlighting the effectiveness of the proposed protocol. The proposed etching protocol was then applied to aluminium alloy substrates, and fracture surfaces corresponding to each treatment were compared. Mechanical testing, specifically double cantilever beam (DCB) experiments, assesses the fracture energy of bonded assemblies in mode I. The results demonstrate that the sulphuric acid-based treatment significantly improves the surface characteristics, leading to enhanced adhesion and optimal fracture energy in bonded aluminium assemblies.

A numerical modelling framework for simulating rate-dependent delamination along polyurea/steel interfaces under Mode–I loading conditions

Polyurea coated structures have drawn significant attention from researchers across the globe for various applications such as waterproof roofing, vibration control, and protection from impact and blast threats. Despite widespread studies on the rate-dependent nature of polyurea and its bulk deformation response, there are very few works which study the delamination in polyurea coated structures and the associated rate sensitivity. The present work is aimed at two objectives: (i) to develop a suitable rate-dependent traction–separation model that fits an existing experimental dataset from literature related to rate-dependent delamination of polyurea/steel interface and, (ii) to develop a simple, robust numerical modelling framework to simulate the rate-dependent delamination process in a double–cantilever beam experiment found in the same literature source. Fulfilling the first objective resulted in identifying the important parameters in a traction–separation relation and their rate-dependency. A non-zero traction at zero crack opening displacement is one of the striking features of the experimental data and the proposed model takes that into account. As part of the second objective, a numerical modelling involving separate constitutive equations for the bulk and interface regions of the polyurea adhesive has been proposed. The interface is modelled as a rate-dependent linear elastic plastic material while the bulk is considered as hyperelastic material using two parameter Mooney–Rivlin strain energy function. This model was implemented in LS-DYNA. The proposed approach could reproduce the fracture mechanisms of the dynamic delamination process reported in the literature and helped in understanding the interaction between the bulk and the interface during the delamination at different strain rates. This approach could be a potential alternative to the cohesive zone method, easing the complexity of simulations involving dynamic delamination in commercial packages.

Thermal cycling effects on the mode-I delamination of multilayer laminated composites: An experimental-numerical approach

In this research, the influence of thermal cycling on the fracture toughness and maximum load in the mode-I delamination of polymer matrix composites (PMCs) was investigated. To reduce the effect of the remote ply orientation on the fracture toughness during initiation and propagation delamination, double cantilever beam (DCB) specimens with a stacking sequence of unidirectional [0]16 were considered. The specimens were subjected to thermal-cycled loading between 15°C and 65°C for 50-150 number cycles. One group of un-cycled specimens was examined at the commencement of the research as a control group to determine the effects of thermal cycling loading on mechanical characteristics of the second group specimens. During the DCB experiment, the specimens were inspected by 2 real-time cameras to record the delamination length and initial crack tip opening displacement (ICTOD). The strain energy release rate (SERR) approach was used for obtaining the critical fracture toughness from the experimental data. By employing the digital image correlation (DIC) method, the initial crack tip separation profile was obtained. The measured bridging laws with the cohesive zone elements model are used to accurately simulate the delamination propagation in DCB specimens. Investigation of load variations revealed that critical fracture toughness in the mode-I of delamination is firmly affected by the thermal cycling process.

Acoustic emission cluster analysis of composite laminates with different interfacial fiber orientations based on gaussian mixture model

The present study aimed to find a method with higher efficiency to cluster acoustic emission events. The acoustic emission signals of composite laminates with different interfacial fiber orientations were captured during the Double cantilever beam (DCB) experiments. Through feature selection of the preprocessed data using the Relief F algorithm, it was found that most of the information in the acoustic emission signals can be represented by the amplitude, inverse frequency, center frequency, peak frequency and so on. Subsequently, principal component analysis and expectation-maximization and Gaussian mixture models were adopted to cluster the data with reduced dimensions. This method can not only distinguish among different damage mechanisms but also show the level of damage concentration according to their types. Furthermore, once the clusters were assigned to different damage mechanisms, they could be identified precisely using both the amplitude and peak frequency. Finally, based on the loading curves of the four types of specimens, the cumulative events and energies of the acoustic emission were compared and analyzed, and different damage mechanisms were identified.

Experimental study on interface failure behavior of 3D printed continuous fiber reinforced composites

Additive manufacturing (or 3D printing) for continuous fiber composite exhibits great potential for fabrication of next-generation lightweight sophisticated structural components. Nevertheless, understanding of interface characteristics of printed parts has remained an open research question for broader applications of this new technology in engineering practice. This study aimed to evaluate the interfacial quality of 3D-printed carbon fiber reinforced plastic (CFRP) composites from two aspects: interlaminar fracture toughness in a pure mode and interfacial failure mechanism in a mixed mode. Prior to the experimental tests, the microstructural quality of the 3D printed specimens was characterized, which showed that the void content and shape of different materials are different. There is a clear boundary line where the short fiber matrix layer and the continuous fiber reinforced layer are in contact, while the interface formed between the same material layers has no evident connection traces. The standard specimens were printed for testing pure mode I and mode II interlaminar fracture toughness, through which the G Ι C and G IIC of interface between continuous carbon fiber layers was characterized. It was found that the fiber bridging phenomenon presented in the double cantilever beam (DCB) experiments, resulting in a higher initial mode I fracture toughness. The end notched flexure (ENF) tests found that the fracture toughness of initial mode II in between the 0° continuous carbon fiber layers was relatively low. The scanning electron microscope (SEM) analysis revealed that the separated surface had only a small area of the matrix being sheared. The single lap specimen printed directly was proposed as a simplified model for simulating complex structural parts subjected to mixed stress of peel and shear. The mechanical responses and failure characteristics of the interfaces composed of different materials and different fiber angles under mixed stress were studied. It is found that each specimen presented some mixed failure modes, and the interface characteristics of different materials and different fiber orientations were completely different. The results gained in-depth understanding on the interfacial properties of 3D printing fiber reinforced structures, thereby providing key data and knowledge for practical applications. • Different layering methods with different interlayer morphology and void shape. • The critical strain energy release rates of type I and typeⅡwere obtained. • Continuous fiber bridging enhances type I fracture toughness. • The shear resistance of the interface between continuous fibers and matrix is very weak. • Reinforcement materials and fiber angles affect the interfacial bond strength and failure.

Mode I interlaminar fracture behavior of none-felt needled composites

None-felt needling technology is a novel 3D preform preparation technology, which improved the mechanical properties of needled preforms effectively. However, the mode I interlaminar fracture behavior of none-felt needled composite has not yet been clarified. In this paper, none-felt needled fabrics and their composites were prepared, the structure of the fabrics was characterized, and mode I interlaminar fracture performance of the composites were carried out through experiments and simulation method. It was found that none-felt needled composites showed effect on improving mode I interlaminar fracture property compared with the traditional needled composites, the maximum failure load and interlaminar fracture toughness (GIC) increased by up to 107% and 129% respectively. The double cantilever beam (DCB) experiment revealed that the failure mode of the none-felt needled composites were manifested as cracking of the matrix and brittle fracture of the needled fiber bundles. More content and thicker needled fiber bundles of none-felt needled fabric are important factors to improve the mode I interlaminar performance. Moreover, based on the failure mechanism and the structural features, mode I simulation model of none-felt needled composites was developed. The load–displacement curves obtained from the simulations were well closed to the experiment, and the error range was between 2.0% and 7.5%. None-felt needled composite structure with high interlaminar properties is hopeful to be applied in composite thin-walled components such as radomes and cabin sections of high-speed aircraft.

Deep-green inversion to extract traction-separation relations at material interfaces

The traction-separation relationship of an interface is a critical component to understand and model the delamination behavior of multi-layer composites in situations where large scale bridging is active. Limited by current experimental techniques, the extraction of traction-separation relationships often relies on inverse approaches, where far field measurements are used as input data. In this article, a data-driven model based on Green’s function embedded neural networks is proposed (namely Deep Green Inversion, or DGI), where the input consists of far field displacement fields while the output is the desired but initially unknown tractions and separations on the interface. Specifically, Green’s functions are embedded as loss function terms along with other terms based on mean squared error and the field equations associated with loaded elastic bodies. The approach is first verified via analytical solutions to a simply supported beam subject to end moments and a non-uniform traction applied to a portion of one boundary of the beam. Hyperparameter training is included in order to elucidate the influence of weight factors that are applied to the different constraints that are considered. The developed approach is then successfully validated for mode-I and mixed-mode cohesive zone extraction problems using only far-field displacement synthetic data that are generated from numerical solutions to the problems. As part of the validation process and consideration of any limitations of the approach, the amount of displacement data required to produce robust traction-separation relations is deliberated. The tractions and separations within the cohesive zone are extracted with an average global error of 9.06 % and local error of 9.24%, using only 55–60% of the available experimental data. The proposed approach is then applied to double cantilever beam experiments with six different types of interaction between the beams, where the input displacement fields are obtained using digital image correlation. The traction-separation relationships extracted via the DGI neural network developed here agree very well with the results obtained via a direct extraction approach. These results suggest that the proposed approach is quite general, considering the range of traction-separation relations that were explored.

Enhanced SCC resistance of 7056 aluminium alloy by Y and Si additions

The effect of Y and Si additions on the microstructure and stress corrosion cracking (SCC) of the high Zn-containing 7056 aluminium alloy has been investigated. The morphology and composition of dispersions were analysed by transmission electron microscope equipped with super-X energy disperse spectroscopy. The SCC resistance was measured by a double-cantilever beam experiment. Results indicated that the SCC resistance was significantly improved by the mixed Y and Si additions, which can mainly be attributed to the reduction in the recrystallisation fraction and the increase in the sub-grain boundaries fraction caused by the high coherent (Al,Si)3(Zr,Y) dispersions and another important reason was the formation of the Y–Si-containing oxide film with the higher compactness and electrochemical impedance.

I-fiber implantation robot for composite parts

I-fiber implantation is a new stitching technology that can effectively enhance the interlayer performance of laminated composites. This paper presents and evaluates the design and implementation of the I-fiber robot implantation system integrated for producing high-performance fiber preforms for advanced composites. The system was constructed and validated through I-fiber robot implantation experimentation. It was demonstrated that automated I-fiber implantation could be achieved by use of an industrial robot. The programming method and computer-aided manufacturing software of the I-fiber implantation robot were feasible and effective. The double-cantilever-beam (DCB) experiments showed that the implantation of I-fiber significantly improved the interlaminar fracture toughness of the laminated composite, where the maximum load value increased by up to 106%. The DCB load–displacement curve presented a zigzag shape, where the peaks and valleys were the location points of the I-fiber break. It was also found that for the reinforced laminated composite without an I-fiber head, the delamination failure was manifested as resin cracking and I-fiber pullout, while for the I-fiber with a certain head length, the I-fiber failure mechanism was brittle fracture. I-fiber with a certain head length could significantly improve the interlayer performance of the composite. In addition, DCB experiments also revealed that the implantation matrix had little effect on the interlayer performance of I-fiber reinforced composites, and the failure load value and the I-fiber implantation volume showed an obvious proportional relationship.

A multiscale cohesive zone model for rate-dependent fracture of interfaces

Rate-dependent fracture has been observed for a silicon/epoxy interface as well as other polymer interfaces, where both the interfacial strength and toughness increase with the separation rate. Motivated by this observation, we propose a multiscale approach to modeling a polymer interface, from atomic bonds to the macroscopic specimen, considering the energetics of bond stretching, the entropic effect of long molecular chains, the kinetics of thermally activated chain scission, and statistical distributions of the chain lengths. These multiscale features are seamlessly assembled to formulate a rate-dependent cohesive zone model, which is then implemented within a standard finite element package for numerical simulations. This model relates the macroscopically measurable interfacial properties (toughness, strength, and traction-separation relations) to molecular structures of the interface, and the rate dependence results naturally from the kinetics of damage evolution as a thermally activated process. The finite element simulations with the cohesive zone model are directly compared to double cantilever beam experiments for the rate-dependent fracture of a silicon/epoxy interface, yielding reasonable agreement with just a few parameters for the molecular structures of the interface. Such a multiscale, mechanism-based cohesive zone model offers a promising approach for modeling and understanding the rate-dependent fracture of polymer interfaces.

Correlation between stress corrosion cracking resistance and grain-boundary precipitates of a new generation high Zn-containing 7056 aluminum alloy by non-isothermal aging and re-aging heat treatment

The new generation high Zn-containing 7xxx series aluminum alloys were more prone to stress corrosion cracking (SCC) compared with the traditional 7xxx series aluminum alloys. However, the effect of grain boundary precipitates (GBPs) characteristics, especially microchemistry, on SCC of new generation high Zn-containing 7xxx series aluminum alloys were not investigated in sufficient detail. In the paper, the GBPs characteristics of a new generation high Zn-containing 7056 aluminum alloy were prepared by different aging heat treatments. The morphology and microchemistry of GBPs were analyzed by transmission electron microscope equipped with super-X energy disperse spectroscopy. The stress intensity factor (KI) and SCC propagation velocity(v) were measured by double cantilever beam (DCB) experiment. The correlations between the GBPs microchemistry and SCC resistance had been discussed. The results showed that the SCC resistance of a new generation high Zn-containing 7056 aluminum alloy was significantly improved by high termination temperature non-isothermal aging and re-aging heat treatment compared with T6 and T77 aging heat treatment. In addition, it was indicated that the SCC resistance was positively with the Cu content of GBPs.

Mode I fracture toughness of hybrid co-cured Al-CFRP and NiTi-CFRP interfaces: An experimental and computational study

In the spirit of advancing joining concepts as well as adoption of hybrid composites, the interface of a metallic foil and carbon fabric reinforced polymer matrix composite was explored experimentally and computationally. An out-of-autoclave manufacturing method, double-infusion VARTM, was successfully employed to create a hybrid composite laminate that was composed of a layer of metal foil (Al or NiTi) placed at the center of sixteen layer plain weave T300 carbon fabric with epoxy Epikote-Epikure 04908 matrix. The fracture behavior was obtained via the Double Cantilever Beam (DCB) tests at room and elevated temperatures and were monitored on-line using Rayleigh backscattering fiber optics technique. Distributed strain profiles were measured on both at the top and bottom surfaces of the specimen to validate/assist computational models. This new technique to measure distributed strains in-situ during testing has introduced a new dimension to the visualization of strain energy release upon crack propagation.The virtual crack closure method was employed to simulate crack propagation at the hybrid interface. The experimental results revealed the dominant mode of failure as cohesive. Load-displacement and strain profiles measured from the experiments were in good agreement with the numerical results. The foremost research highlight is the novel integration of computational simulation with fiber optics Distributed Strain System (DSS) in DCB experiments to elucidate crack growth at a hybrid interface.

Multi-layer interfacial fatigue and interlaminar fracture behaviors for sisal fiber reinforced composites with nano- and macro-scale analysis

In this study, fatigue performance of the multiple interfaces within sisal-fiber-reinforced composites (SFRCs) in the nanoscale was examined with nano-dynamic mechanical analysis technique. Cyclic loading with varying applied indentation loads at different frequencies was employed to study nanoscale interface failure. To correlate nanoscopic damage to macroscopic interfacial failure mechanisms, double-cantilever-beam (DCB) experiments were conducted to reveal the effects of hierarchical structure of sisal fibers on the interfacial failure behaviors of laminated SFRCs. Fiber bridging and fiber entanglement were observed during DCB test, which made the crack propagation path tortuous and introduced higher Mode I interlaminar fracture toughness compared with glass-fiber-reinforced composites. To model these actual multi-interfacial fracture behaviors, a numerical model subject to the DCB experiment was developed with designed cohesive-zone model in the pre-set crack front. Good consistency between numerical simulation and experimental results verified the efficiency of proposed model in simulating the multi-layer failure behaviors of laminated SFRCs.

A mode-dependent energy-based damage model for peridynamics and its implementation

The mathematical modeling of failure mechanisms in solid materials and structures is a long standing problem. In recent years, peridynamics has been used as a theoretical basis for numerical studies of fracture initiation, evolution and propagation. In order to investigate damage phenomena numerically, suitable material and damage models have to be implemented in an efficient numerical framework. This framework should be highly parallelizable in order to cope with the computational effort due to the high spatial and, depending on the problem, temporal resolution required for high accuracy. The open-source peridynamic framework Peridigm offers a computational platform upon which new developments of the peridynamic theory can be implemented. Today, isotropic material models and a very simple damage model are implemented in Peridigm. This paper proposes three energy-based damage criteria. The implementation approach as well as the extension of Peridigm with these physically motivated models is described. The original criterion of Foster et al. is adapted for ordinary state based material. The other two criteria utilize the decomposition of peridynamic states in isotropic and deviatoric parts to account for the failure-mode dependency. The original criterion is verified by the numerical simulation of two mechanical problems. At first, a virtual double cantilever beam (DCB) experiment is performed to determine the energy release rate. This value is the fundamental material property required for the proposed criteria. Additionally, the DCB problem is then used to investigate the convergence of the numerical scheme implemented in Peridigm. In a second step, a model of a plate with a cylindrical hole under tensile loading is compared with an extended finite element method solution. Results of both numerical solutions are in good agreement. Finally, a fiber reinforced micro structure model is used to analyze the effect of the different criteria to the damage initiation and crack propagation under a more complex loading condition.

Multiscale carbon fibre–reinforced polymer (CFRP) composites containing carbon nanotubes with tailored interfaces

AbstractPristine and functionalized multiwalled carbon nanotubes (MWCNTs) with tailored interfaces were efficiently dispersed in an epoxy matrix using a three‐roll mill and further reinforced with carbon fibres. 1.3‐Dipolar cycloaddition of azomethine ylides was used for the chemical modification of MWCNTs by a solvent‐free approach. The influence of different loadings and types of MWCNTs on the final properties of the epoxy matrix was studied. Moreover, the most promising formulations were selected for manufacturing of prepreg sheets. The transversal tensile properties and the interlaminar fracture toughness under mode I loading (GIC) of multiscale carbon fibre–reinforced polymer (CFRP) composites were characterized. The results point out that it is not straightforward to transfer the remarkable intrinsic properties of MWCNTs to the composite level, although an overall positive trend was found. Double cantilever beam experiments showed that GIC of CFRP composites was improved 44% at ultralow content of functionalized MWCNTs (0.043 wt%).

Propagation of damage in bolt jointed and hybrid jointed GLARE structures subjected to the quasi-static loading

GLARE laminate is a high-performance material used extensively in industry. This study investigates the progression of damage in bolt jointed and hybrid (bolt/bonded) jointed GLARE structures through a series of tests, including double cantilever beam experiments, end notched flexure experiments and tensile tests. Numerical simulations of tensile tests on bolt and bonded jointed GLARE structures are performed using finite element software ABAQUS. The experimental and numerical results provide evidence that delamination is the dominant failure mode as a result of the weak interlaminar strength between the metal and the glass fiber reinforced polymer (GFRP) layers and the secondary bending moment. Bearing damage occurs mainly around the bolt hole, and the matrix with 90°directional fibers is damaged more easily than the layer with 0°directional fibers. A suggestion is proposed to design the optimal hybrid jointed GLARE structures by adjusting the stiffness of the liquid shim.