Double Cantilever Beam Research Articles

Carbon nanotube-induced localized laser heating toward simultaneously enhanced laydown efficiency and fracture toughness of in-situ consolidated glass fiber thermoplastic composites

Thermoplastic composites suffer from weak inter-tape bonding during high-rate laser-assisted automated fiber placement (L-AFP). Here, we propose a strategy to simultaneously enhance the laser heating efficiency and fracture toughness of glass fiber-reinforced polypropylene prepreg tape for L-AFP process by depositing carbon nanotubes (CNTs) on the tape surface by a low-cost bath coating process. CNTs function as efficient light absorbing and photothermal coating to promote molecular inter-diffusion and interfacial healing between successive tapes, as well as toughen the weak interface by CNT pull-out and rupture. A 56 % increase in the near-infrared laser absorption of tapes results in a 946 % improvement in the laydown rate. CNTs also remarkably improve transverse temperature uniformity. Furthermore, double cantilever beam and pants tearing tests show that 0.5 wt% CNTs improve the mode I interlaminar fracture toughness and mode III fracture strength by 17 % and 52 %, respectively, attributed to the enhanced molecular inter-diffusion and CNT bridging effect.

Repair of acrylic/glass composites by liquid resin injection and press moulding

This paper presents repair methods for in-situ polymerised acrylic (Elium®)/glass fibre (GF) composites focusing on mode-I fracture toughness recovery. GF/acrylic composites were first subjected to double cantilever beam (DCB) tests to measure their Mode-I fracture toughness. The delaminated samples after DCB tests were repaired and rejoined. Two repair methods were performed: liquid resin injection and press moulding at two different temperatures (130 °C and 160 °C). The repaired samples were subjected to a second set of DCB tests. The fracture behaviours of the four specimen groups (virgin, resin-injected, pressed at 130 °C, and pressed at 160 °C) were evaluated in terms of strain energy release rates (GIC) during crack initiation and propagation. The results showed that specimens repaired by resin injection exhibited the highest GIC values, about 30 % higher than the virgin state, due to the formation of a semi-interpenetrating polymer network (semi-IPN) at the joining interface. Scanning electron microscopy images provided insight into distinctive fracture behaviours for each test group.

The effect of inhomogeneous microstructures on strength and stress corrosion cracking of 7085 aluminum alloy thick plate

Inhomogeneous microstructures and alloy chemistry along the thickness direction can greatly affect the mechanical and corrosion properties of new generation 7xxx aluminum alloy thick plates. The inhomogeneous microstructure, stress corrosion cracking (SCC) resistance and strength of AA7085 thick plate (160 mm) at T/2, T/4, and surface position using mechanical tests, double cantilever beam (DCB) crack extension tests and electron microscopy experiments. The results show that the ultimate tensile strength, yield strength and elongation of the T/2 position are slightly lower than those of the surface position, but the SCC resistance of the T/2 position is significantly higher than that of the surface position. The reason is that larger discontinuously quench-induced grain boundary precipitates (Q-GBPs), and lower Zn and Mg elemental content of GBPs (including Q-GBPs and age-induced GBPs (A-GBPs)) and grain boundaries (GBs). Furthermore, it is newly found that the mechanical effects of larger-sized low aspect ratio unrecrystallized grains contribute significantly to the excellent SCC resistance of the T/2 position. The large size grain structure makes short-range SCC cracks along the large-angle GB more tortuous at T/2, improving the stress intensity required for sustained crack extension. In addition, the high GB triple junction deflection angle resulting from a low aspect ratio grain structure at T/2 improves the average driving force required for crack extension and drive force reduction after localized crack growth.

Controlled reconstruction and application research of lateral overlying strata structure in high-gas mines with thick-hard roofs for a fully-mechanized top-caving face

Due to the large mining area, the fully-mechanized top-caving mining with thick-hard roof is easy to form cantilever structure on the lateral roof of the working face, which on the one hand causes high stress level of adjacent roadway and serious deformation of roadway, on the other hand causes gas accumulation in corners, which brings severe challenges to safe and efficient mining of the mine. In this study, a mine facing such problems in Jincheng, China was taken as the research object. Based on the mechanical characteristics of coal and rock, the characteristics of overlying strata activity in the mining process of working face are mastered, and the dual effects of controlled transformation of lateral overlying strata structure on stress field and gas field were revealed. On this basis, roadway reinforcement and gas drainage schemes were put forward and applied. The results showed that the strength of the hard rock stratum was high in the triaxial stress environment, and it was not easily destroyed. However, once the strata exceed their strength threshold, they break down. In addition, the strength of coal is relatively low, and it is continuously deformed when the force exceeds its strength. The overlying strata structure after thick-hard roof fully-mechanized top-caving mining evolves in the following manner: “long cantilever length formed by the main roof being broken in the initial stage, voussoir beam formed by the upper hard roof being broken in the middle stage, and double cantilever beam formed by overlying strata compaction.” The stress carried by upper hard rock stratum is transferred to coal pillars, which is the main reason for the high stress environment of multi-purpose roadway with large coal pillars. The controlled transformation of lateral overlying strata structure by pre-splitting and roof cutting can realize the "transfer-unloading" of coal pillar stress and the "plugging and driving" of corner gas. Based on the hydraulic fracturing reconstruction of lateral overburden structure, the grouting reinforcement scheme of roadway and dynamic gas drainage scheme were put forward. The results demonstrated that after roof cutting, the maximum deformation of the surrounding rock in the multi-purpose roadway was reduced by approximately 90 %, and the maximum concentration of corner gas was decreased by 15.28 %. This approach successfully achieved a collaborative control effect on roadway surrounding rock stability and gas emission well within the safety limits.

Measuring traction–separation relationships of bonded Fe-SMA double cantilever beam joints

Adhesively bonded strengthening systems using self-prestressing iron-based shape memory alloys (Fe-SMA) have demonstrated significant potential. However, the premature nonlinear behavior of Fe-SMA during debonding failure presents challenges in ensuring the integrity of the adhesive joint. A recent study indicated that this behavior may lead to premature joint failure, and a similar conclusion can be drawn concerning plasticity in thin adherends. To characterize the failure process of Fe-SMA bonded joints, a method capable of measuring traction–separation relationships of thin adherends bonded with tough adhesives is essential. An algorithmic method, supported by an analytical nonlinear beam-on-foundation model and a minimization scheme, is shown to offer a solution. To benchmark the method against the J-integral approach, experiments are conducted using two types of adherends: thick steel with thickness-limiting plasticity and Fe-SMA with material behavior that inevitably leads to nonlinear deformation. Comparison of methods shows both measurement method are valid and allow for proper predictive modeling using the finite element method. The proposed method can be readily employed to characterize various thin adherends with elasto-plastic material behavior.

Assessing measurement uncertainties on high-speed DCB testing of composites

This study investigates dynamic behavior in Double Cantilever Beam (DCB) tests across loading rates from 6 mm/min to 3 m/s. Comprehensive instrumentation has been used to investigate the sources of measurement errors (load, opening and crack length) or biases (asymmetry) and to quantify their effects on the fracture toughness determination using three variations of the Corrected Beam Theory. In particular, challenges in establishing zero load and displacement references were revealed through monitoring initial test phases. The resulting uncertainties in the fracture toughness have been estimated to be between 7 and 15%. Arms asymmetry during higher loading rates (1 m/s and 3 m/s) has been recorded using Digital Images Correlation. Using this data and a simple beam model, mode mixity (GII/GI from Williams approach) computed is in the order of 10%–20%.

Dual-scale numerical analysis of delamination in composite materials with varied fiber orientations at the interface

What is the relationship between the fiber orientation at delamination interface and the interlaminar fracture toughness of the composite? The answer is inconclusive based on experimental results. The conflicting observations stem from inconsistencies and inaccuracies in characterization techniques. As an alternative approach to circumvent the experimental challenges, the present study proposes a numerical dual-scale model for delamination, incorporating both micro- and meso-scale features. The model investigates two different fiber orientations at the mid-thickness interface: 0°/0° and 90°/90°. Comparative analyses demonstrate the importance of including a refined region with micromechanics to accurately predict the crack path during delamination. The results exhibit agreement with the literature models and enable estimation of early-initiation fracture toughness, considering delamination interacting with the microstructure while simulating a double cantilever beam specimen with reduced width. The approach provides a deeper understanding of delamination behavior and fracture toughness estimation, contributing to improving composite characterization techniques and design strategies.

Fracture behavior of composite piezoelectric intelligent structures formed by pasting MFC based on serial multi-scale method

The composite piezoelectric intelligent structures formed by combining piezoelectric materials with composite materials can achieve state perception and deformation control of the structure. Based on the molecular dynamics method, the continuum finite element method and the cohesive force model, the serial multi-scale research on the bonded composite piezoelectric intelligent structure was carried out from the micro, meso and macro scales. Molecular models of adhesive, macro fiber composite (MFC) surface and composite surface were established. The effects of temperature and pressure on the surface interaction energy of adhesive/MFC and adhesive/composite materials were analyzed. The traction-separation (T-S) relationship of cohesive region was obtained by virtual stretching method. Combined with the microscopic observation of the mesoscopic adhesive layer, the representative volume element (RVE) model of the mesoscopic adhesive layer considering the bubble defect was established, and the cohesive relationship curve of the adhesive layer was obtained. A double cantilever beam (DCB) macro finite element model was established to simulate the crack propagation behavior of the adhesive layer, and compared with the experimental results.

Enhancing CFRP laminates with plasma jet arrays: A study of interlaminar mechanical properties

AbstractThis study delves into the efficacy of plasma jet array treatment in bolstering the interlaminar toughness of Carbon Fiber Reinforced Polymer (CFRP) prepregs, a crucial parameter for their deployment in high‐performance domains. Employing high‐frequency pulsed microsecond plasma jet arrays for surface modification, this research not only demonstrated a 19.49% upsurge in peak load capacity but also a remarkable 29.94% enhancement in Mode I interlaminar fracture toughness, as evident from Double Cantilever Beam (DCB) tests. Furthermore, the surface wettability improvements were underscored by a substantial decrease in water contact angle from 108.33° to 31.45° and a total surface energy augmentation from 12.20 to 64.16 mN/m, post a 1‐min plasma treatment. X‐ray Photoelectron Spectroscopy (XPS) results indicated an appreciable rise in oxygen functionalities, suggesting enhanced resin‐fiber bonding. Additionally, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) analyses revealed notable decreases in surface roughness and microscale defects, contributing to smoother surfaces conducive to better adhesive bonding and reduced manufacturing defects. These multifaceted improvements, stemming from alterations in surface chemistry and topography, underpin the significant potential of plasma treatment in advancing CFRP laminate applications requiring superior durability and mechanical resilience.Highlights A plasma jet arrays method to enhance CFRP's interlaminar properties was studied. The influence mechanism of plasma modification was revealed in detail. The interlayer toughening mechanism of CFRP after plasma modification is proposed.

Effect of laser treatment on strength of moisture-degraded epoxy adhesive joints and microscopic observation of treated surface

ABSTRACT We examined the effect of laser processing surface treatment on the mechanical strength of adhesion. In addition to laser processing, double cantilever beam (DCB) and lap joint (LJ) specimens were prepared using an epoxy adhesive on aluminum plates that underwent acid etching, sandblasting, and acetone wiping. The open face method was employed to accelerate moisture degradation, and the tensile-shear strength and energy release rate of the specimens were measured. Laser processing resulted in remarkably high values of the measured energy release rate, even when the adhesive deteriorated. To examine the reason for the high strength, we performed a detailed fracture surface analysis and electron microscopic observation of the adhesive interface. When the interface of the fractured surface was observed, the laser treated surface was densely formed with fine filamentous protrusions that held the adhesive well. These filaments exhibited high oxidation resistance and achieved strong adhesion even when the adhesive and adherend deteriorated because of moisture. The laser treated adherend surface exhibited two strong anchoring effects owing to a significant increase in the surface area and the microfilamentous structure on the surface. However, the type of surface treatment did not have a significant effect on the LJ adhesion strength.

Failures of laminates under nonpenetrative impacts

Various failures of laminates under low-velocity impacts without penetration are simulated systematically with a limited number of inputs, all measurable following existing standards without any data calibration. No iteration is required to determine any of the failures investigated in this work. Any two adjacent lamina (primary) layers are inserted with a matrix (secondary) layer, whose stresses are modified through two modification coefficients (MCs). The MCs are determined through peak loads of double cantilever beam and end-notched flexure tests on unidirectional laminates, and their weak sensitivity to sample dimensions is shown. Delamination is reproduced by deleting failed secondary-layer elements. The homogenized stresses of fiber and matrix obtained by Bridging Model are converted into true values through the author’s true stress theory to estimate constituent-induced intralaminar failures, such as fiber breakage, matrix crack and interface debonding, against the monolithic fiber or matrix strengths measured independently. Primary- layer elements attaining a fatal failure (fiber breakage or matrix crack accompanied with a critical strain condition) are deleted before an impact termination corresponding to separation of the impactor from target. The predicted delamination areas and impact force, displacement and energy histories for the laminates of three lamination angles under different impact energies agree well with our measured counterparts, validating the efficiency of the simulation.

A finite crack growth energy release rate for interlaminar fracture analysis of composite material at different temperatures

The interlaminar fracture toughness of unidirectional fiber reinforced polymer composite material is temperature and crack growth history dependence. The non-constant interlaminar fracture toughness challenges the wide application of the linear elastic fracture mechanics-based approach for analyzing the interlaminar crack growth in composite material and structures. In addition, a quantitative relationship between the ductility of the matrix and interlaminar fracture toughness of the fiber reinforced composite is still highly desirable. In this paper, both pure resin and composite double cantilever beam (DCB) are tested at different temperatures. A finite crack growth energy release rate is used to analyze the interlaminar crack growth behavior of composite material at different temperatures. The plasticity of the composite DCB is modeled by Hill’s anisotropic plasticity, with the properties determined from multiscale analyses. It is found that a single constant finite growth energy release rate can well predict the interlaminar mode I crack growth of composite material at different temperatures. The roles of the ductility of matrix, thickness of the DCB, and crack growth history on the interlaminar mode I fracture toughness of composite material are quantitatively determined by using the present method.

Steady-state crack propagation during DCB test with viscoelastic-viscoplastic interface – Correlation between rheological behavior and effect of crack propagation rate

Adhesively bonded joints are known to suffer from creep and relaxation phenomena due to the viscous nature of the adhesive. These effects are detrimental to the durability of structural joints as they can lead to progressive and slow crack propagation rate phenomena and possible delayed failure. Master curve approaches are often used to assess the durability of structural assemblies, but the physical justification for such approaches requires additional investigation to more accurately assess the coupling between structural effects and the possible complex long-term rheological behaviour of the adhesive layer. Following similar approaches developed to study the steady-state crack propagation regime in bulk materials, an Eulerian description of crack propagation along a viscous adhesive layer during a double cantilever beam test is implemented. Taking into account viscoelastic-viscoplastic and various hardening rules, some master curves can be obtained describing the evolution of the crack propagation rate as a function of the stationary loading conditions. With such a model, the role of adhesive layer behaviour and joint geometry on crack propagation conditions can be discussed, as well as the applicability of such a master curve approach.

Degradation of an underwater epoxy adhesive and its bonding to steel subjected to water or seawater immersion

Underwater adhesive is used to bond carbon fiber reinforced polymer (CFRP) composites to offshore or underwater structures. However, the long-term moisture exposure poses a challenge to the reliability and service life of the underwater adhesives. Therefore, the present article aims to provide a deep understanding of the durability of the underwater adhesive and the long-term bond performance between the CFRP and steel. The hygrothermal resistances of an underwater adhesive and CFRP-adhesive-steel double cantilever beam (DCB) immersed in deionized water and seawater at 25 °C, 40 °C, and 60 °C were investigated. The degradation mechanism was determined by the chemical characterization of the adhesive. Dynamic thermomechanical properties and tensile strength degraded significantly (≥62 %) before water uptake saturation due to adhesive plasticization. A high temperature (60 °C) can improve Mode I fracture toughness by at least 68 % due to an increase in the toughness of the adhesive.

Environmental fatigue crack growth of PV glass/EVA laminates in the melting range

AbstractThe delamination of encapsulants in photovoltaic (PV) modules is a common issue that leads to power loss due to optical losses. Encapsulant debonding is usually examined under monotonic loading conditions subsequent to environmental exposure such as damp heat. Service‐relevant, superimposed environmental‐mechanical fatigue loads are not considered adequately. Hence, the environmental fatigue delamination resistance of thermally toughened double glass laminates with an ethylene vinyl acetate copolymer (EVA) adhesive layer was investigated in this study. Focus was given to the melting range of EVA, in which the non‐crosslinked crystalline phase fraction is already in the partly molten state. Double cantilever beam specimens were tested on an electrodynamic test machine at temperatures of 60, 70, 80, and 90°C and relative humidity (rh) levels of 2%, 30%, 50%, and 80%. The fractured surfaces were characterized by digital microscopy, Fourier transform infrared spectroscopy (FTIR), X‐ray photoelectron spectroscopy (XPS), and differential scanning calorimetry (DSC). The cyclic fatigue tests revealed a decay in delamination resistance at elevated temperature and humidity levels. At 70°C, the delamination resistance was low, regardless of the relative humidity. Most of the laminates failed by debonding. XPS analysis showed a reduction of the C=O and C–O content, along with an increase in Si–O content with increasing relative humidity. For laminates tested at 60 and 70°C, an EVA recrystallization peak was observed in DSC experiments. This peak was shifted to significantly higher temperatures at 80% rh. XPS and DSC indicated local hydrolysis within the porous fracture process zone ahead of the crack tip. Consequently, acetic acid formation led to a decrease in delamination resistance, resulting in lower fatigue threshold values. The investigations confirmed the significant impact of environmental conditions on the fatigue delamination resistance within glass/encapsulant laminates. Notably, acetic acid formation and a significant reduction in delamination properties were observed after around 100 h of environmental fatigue exposure.

A finite-thickness rate-dependent cohesive zone model

This paper presents the formulation of an enriched cohesive element (CE) accounting for the confinement and the viscoelasticity of the adhesive layer under pure mode I solicitation. This formulation follows the structure of the hybrid approach, which combines the use of solid bulk elements (BEs) and CEs. This leads to define the enriched CE as a zero-thickness interface core embedded within two solid bulk adhesive regions. The interface is modelled with a spring whose stiffness is degraded by a damage variable. The bulk adhesive regions are modelled with a standard linear solid (SLS) model with one Maxwell arm. This formulation is implemented using the ABAQUS finite element (FE) software through a custom UMAT material routine, and applied to a single enriched CE as well as to a double cantilever beam (DCB) bonded joint. It is also evaluated numerically against the respective equivalent hybrid models.

A Numerical Method for Unstable Propagation of Damage in Fiber-Reinforced Plastics with an Implicit Static FE Solver

Finite element analyses of the propagation of damage such as fiber compressive failure and delamination have greatly contributed to the understanding of failure mechanisms of fiber-reinforced plastics owing to extensive studies on methodologies using Continuum Damage Mechanics and Fracture Mechanics. Problems without the need for consideration of inertia, such as Double-Cantilever Beam tests, are usually solved by implicit FE solvers, and explicit FE solvers are appropriate for phenomena that progress with very high velocity such as impact problems. However, quasi-static problems with unstable damage propagation observed in experiments such as Open-Hole Compression tests are still not easy to solve for both types of solvers. We propose a method to enable the static FE solver to solve problems with unstable propagation of damage. In the present method, an additional process of convergence checks on the averaged energy release rate of damaged elements is incorporated in a conventional Newton–Raphson scheme. The feasibility of the present method was validated by two numerical examples consisting of analyses of Open-Hole Compression tests and Double-Cantilever Beam tests. The results of the analyses of OHC tests showed that the present method was applicable to problems with unstable damage propagation. In addition, the results from the analyses of DCB tests with the present method indicated that mesh density and loading history are not significantly influential to the solution.

Experimental approach for measuring the peeling strength of a sheet bonded on a curved adherend with pressure sensitive adhesive and predicting the peeling resistance by finite element analysis

ABSTRACT To evaluate the peel resistance of a device wherein a polyimide (PI) sheet is bonded to a curved surface with a pressure-sensitive adhesive (PSA) experimentally and analytically, we applied a mechanical simulation considering the viscoelasticity of the pressure-sensitive adhesive (PSA) and the fracture energy of the bonding interface. The mechanical behavior against the local load applied on the PI sheet surface was evaluated using this simulation. To verify the accuracy of this simulation, we fabricated a local load tester that could apply a load to a local area of 8 mm in diameter and compared the mechanical behavior with the simulation results. Dynamic mechanical analysis of the PSA with a film thickness of 19 μm was performed using the nanoindentation technique. The generalized Maxwell model parameters were estimated by fitting the relaxation elastic modulus E(t) to conduct a mechanical simulation. For the double cantilever beam test to derive the interfacial fracture energy, soft PI sheets were reinforced with Al substrates. As a result of these analyses, it was confirmed that the relationship between the displacement and load obtained by this simulation agrees well with the actual local load test. This technique is useful for predicting the bonding strength of complex-shaped devices fabricated using soft adherends and adhesives.

Fracture toughness of aged oil paints

The fracture toughness in opening mode GIc for selected oil paints from Mecklenburg's Paint Reference Collection after approximately 30 years of natural ageing was determined using the procedure adopted from the ASTM D5528–13 standard in which double cantilever beam specimens are split in tensile tests. The careful reglueing procedure allowed multiple fracturing tests to be carried out that not only improved statistics of the measurements but also provided insight into the variation of the fracture toughness across the paint film observed for some paints. The latter was due to pigment sedimentation or chemical change of the oil binder as a result of the easier access of oxygen from the side open to the air. For the lead white paint, used over centuries both in paint films and oil grounds, the GIc values more than doubled from 18 to 39 J/m2 for three consecutive cracks formed at an increasing distance from one side of the paint film. The study demonstrated that fracture toughness corresponding to the first crack formation in aged oil paints ranged between 10 and 40 J/m2, relatively low values compared to the animal glue-based ground in paintings. In consequence, oil paints are more vulnerable to crack initiation and growth induced by tensile stress than the ground layer. The measurements filled the gap in the knowledge required for the analysis of fracturing or delamination processes in paintings.

Mitigating Crack Propagation in Hybrid Composites: An Experimental and Computational Study

The exceptional properties of carbon nanotubes (CNTs) make them ideal nanofillers for various composite materials. In carbon fiber-reinforced polymer (CFRP) composites. CNTs can be grown on the carbon fiber surface to act as a third interface between the fiber and the matrix. However, it was established that the uncontrolled random growth of CNTs could exacerbate delamination in composite structures. Thick nanofiller films could hinder the epoxy from seeping into the carbon fiber, resulting in insufficient interlaminar strength. Hence, the density and distribution of nanofillers play a crucial role in determining the hybrid composite fracture mechanisms. In this investigation, CNTs were grown using the low-temperature technique into specific patterns over carbon fibers to discern their derived composites’ fracture properties. The composite fracture energy release was probed using a double cantilever beam (DCB) test setup and digital image correlation (DIC) to monitor interlaminar crack propagation. A standard finite element simulation model based on the cohesive zone method (CZM) was also utilized to delineate fracture behaviors of the various composite configurations. Results conclude that a coarser pattern of CNT growth enhances resistance to crack propagation, thus improving the interlaminar fracture toughness of a composite structure.