Delamination Behavior Research Articles

Characterisation of near pure mode II quasi-static and fatigue delamination behaviour of a carbon-glass interface using a pure-moment-loaded DCB test rig

This work covers the characterisation of delamination growth in a carbon-glass interface, representative of the one in a wind turbine blade. A pure-moment-loaded Double Cantilever Beam (DCB) test rig is used under quasi-static and fatigue loading. A new fatigue characterisation procedure is adopted by real-time controlling the prescribed energy release rate G. This procedure enables sweeping any desired G range at a user-defined rate independently of the compliance of the specimen. A finite element simulation using a cohesive zone model confirms the testing was performed at near pure mode II conditions. From the quasi-static tests, the onset and steady-state fracture toughnesses are characterised. Fibre bridging is observed, which is believed to explain the characterised R-curve effect. From the fatigue tests, Paris’ law is characterised in a wide Gmax range in each test. The characterisation of the quasi-static and fatigue fracture behaviour of the carbon-glass interface is thus successfully achieved.

Performance of innovative adhesive-free connections for glued-laminated timber under flexural load

Timber, a renewable resource with a low carbon footprint, has a giant potential to replace reinforced concrete (RC) structures in housing, which can decrease the environmental impact and lead to a healthier construction work environment. However, connections, as part of timber frames, are majorly made of steel and adhesives, which emit harmful pollution and negatively impact the timber structures. This study focuses on enhancing sustainability in construction by proposing adhesive-free timber connections for glued-laminated timber (glulam) panels. The study aims to contribute toward sustainable construction practices by reducing the reliance on adhesives and exploring alternative connection methods for glulam panels. This article presents four-point out-of-plane bending tests on glulam panels with innovative adhesive-free timber connections. The studied specimens compromised fabricated glulam panels and densified wood connectors made of pine and beech, respectively. Six different adhesive-free wood connections were designed and applied independently. Each connection was connected to two glulam panels by their end-grain sides. Therefore, twelve glulam panels, connected using these six connections, were tested. The panels had identical dimensions and materials. The connections were applied at the mid-span of the two connected panels. The experimental results on the flexural behavior, ultimate load, strength, and displacement of the six specimens are presented. The obtained mean load-carrying capacity of the specimens in the current research was greatly higher than that of the other specimens with different timber connections, such as timber-timber connections using compressed wood connectors. Additionally, the failure modes of the specimens were analyzed, which mostly exhibited the shear failure and delamination behavior. Most of the tested specimens failed in a ductile manner with a high ductility, which is suitable for the earthquake regions. The findings demonstrated the potential of using adhesive-free timber connections in glulam panels and contributing to the development of zero-energy buildings and sustainable construction practices while maintaining the structural integrity.

A concurrent multiscale model of stress-induced delamination behaviours of epoxy-impregnated rare-earth barium copper oxide superconducting pancake winding

Abstract Rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes are promising for high-energy and high-electromagnetic field applications. In epoxy-impregnated REBCO superconducting coils, due to the weak c-axial strength of REBCO CC tapes, delamination induced by thermal-mismatch stress and Lorentz force significantly threatens stable operation. The commonly used numerical modelling methods mainly include homogenized model and refined model. The former can realize the efficient calculation of the overall physical field but lose the local details including interfacial delamination behaviour, while the latter can realize the refined calculation but cost massive calculation. In this study, combining with bilinear cohesive zone model (CZM), a two-dimensional axisymmetric concurrent multiscale model was developed to balance the efficiency-accuracy trade-off for numerically investigating the mechanical properties and interface failure behaviours of REBCO superconducting coils under cryogenics and high-electromagnetic field. In the presented model, the homogenized superconducting coil was firstly constructed based on composites homogenized approach to estimate the electromagnetic-thermal-mechanical properties at macro scale. Then, the ‘dangerous regions’ in the macro scale was recognized based on quadratic failure criterion and replaced with a refined model considering delamination failure defined by CZM. Finally, the two scales are linked through the coupling interface. The accuracy and efficiency of the concurrent multiscale model were validated by refined model for the cases of delamination behaviour during cooling and excitation.

Low-Velocity Impact Analysis in Composite Plates Incorporating Experimental Interlaminar Fracture Toughness

Reliable performance of composite adhesive joints under low-velocity impact is essential for ensuring the structural durability of composite materials in demanding applications. To address this, the study examines the effects of temperature, surface treatment techniques, and bonding processes on interlaminar fracture toughness, aiming to identify optimal conditions that enhance impact resistance. A Taguchi experimental design and analysis of variance (ANOVA) were used to analyze these factors, and experimentally derived toughness values were applied to low-velocity impact simulations to assess delamination behavior. Sanding and co-bonding were identified as the most effective methods for improving fracture toughness. Under the identified optimal conditions, the low-velocity impact analysis showed a delamination area of 319.0 mm2. These findings highlight the importance of parameter optimization in enhancing the structural reliability of composite adhesive joints and provide valuable insights for improving the performance and durability of composite materials, particularly in aerospace and automotive applications.

A slope-based J-integral approach and advanced image processing for assessment of the cyclic fatigue delamination behavior of adhesive joints

A fatigue fracture mechanics methodology was developed and established, employing a slope-based J-integral approach combined with advanced image processing techniques. Adhesively bonded double cantilever beam (DCB) specimens were tested under constant displacement amplitude loading. The beam rotation was tracked by affixing a repetitive pattern on the DCB specimens and capturing images at the maximum displacement amplitude. Using a custom-developed image processing procedure, the beam rotation was deduced. To validate the methodology, DCB fatigue experiments were conducted at 23, 60 and 75 °C on aluminum adherends bonded with a structural 2-K epoxy adhesive. The J-based approach was compared with a conventional, compliance-based linear elastic fracture mechanics (LEFM) method. The epoxy was a rather brittle, high-modulus adhesive with a bond line thickness of 0.25 mm, resulting in predominantly linear elastic material behavior. By analyzing the images taken during fatigue testing, a stiffening effect of the steel load blocks was observed. Excluding pattern elements directly below the load block yielded the best agreement between J-integral and LEFM data. Both approaches were in excellent agreement within the investigated temperature range. The investigated adhesive exhibited a highly temperature-dependent behavior, which was associated with higher crack propagation rates and a lower fatigue threshold at 60 and 75 °C.

Comparison of mode II delamination behaviours in multidirectional and unidirectional composite laminates

Multidirectional (MD) composite laminates are extensively employed in structural applications owing to their superior mechanical characteristics. Nevertheless, the evaluation of the fracture toughness of composite laminates primarily relies on tests using unidirectional (UD) specimens. This study evaluates the reliability of characterizing mode II delamination behaviour in MD laminates by using UD specimens. The quantification of delamination area through Digital Image Correlation (DIC) analysis is integrated with a physical Energy Release Rate (ERR) method to ascertain the fracture resistance, which is compared with the ERR derived via a modified J-integral method and the standardized compliance methods. Fractographic analysis reveals similar fracture mechanisms in specimens with identical interfaces. The physical ERR increases notably due to large-scale fibre bridging induced by fibre nesting at 0∘//0∘ interfaces. Conversely, in 0∘//90∘ interfaces, large-area matrix cracking enhances the intrinsic fracture resistance, excluding the extrinsic toughening provided by fibre bridging.

Fracture analysis under modes I and II of adhesive joints on CFRP in saline environment

This study analyzes the delamination behavior of adhesive joints after exposure to a saline environment for zero, one, and twelve weeks. Delamination was assessed under static and fatigue loading conditions in fracture Modes I and II, with a detailed analysis of fracture surfaces using Scanning Electron Microscopy (SEM) and Backscattered Electron (BSE) detection. The 3D images reveal significant morphological differences in fracture surfaces, showing variations in fatigue lines and the presence of impurities depending on the fracture mode. A probabilistic fatigue life analysis was performed using a Weibull regression model, showing notable changes, especially in Mode I at a high number of cycles. A chemical analysis using EDX and FTIR-ATR complemented the mechanical study, revealing an increase in sodium and chlorine concentrations with prolonged saline exposure. Oxidative degradation was also observed, with carbonyl groups increasing significantly over time, particularly in areas most exposed to the saline mist.

A multi-linear constitutive relation considering the temperature effect on quasi-static mode I delamination in UD/MD laminates

In this study, a multi-linear constitutive relation taking into account temperature and fiber bridging is proposed for characterizing delamination behavior in composite laminates under various temperature conditions. An approach combining analytical solution and J-integral is also established for determining the cohesive parameters in the multi-linear constitutive relation. To validate the proposed constitutive relation, mode I quasi-static delamination experiments of unidirectional (UD) and multidirectional (MD) carbon/bismaleimide laminates are carried out at 25 ℃ (room temperature), 80 °C and 130 ℃. The experimental results show that the increasing temperature resulted in a monotonic increase in the fracture toughness of the UD laminates while affect the fracture toughness of MD laminates slightly. A FE model is established with the implementation of the proposed multi-linear constitutive relation using UMAT subroutine. Good agreements between the experimental and simulated results demonstrate the validity of the proposed constitutive relation, with the relative difference of peak load between predicted and experimental values less than 8.2 % and the relative difference of initial and steady-state fracture toughness between predicted and tested results less than 15 %. This study provides the possibility to numerically study the temperature effect on the delamination behavior of laminates and has promising applications in the damage tolerance design of composite structures.

Mode I fatigue delamination growth behavior and model for multidirectional laminates with the same overall stiffness

In order to isolate the influence of ply orientation on the mode I fatigue delamination propagation behavior of carbon fiber reinforced composite laminates, multidirectional laminates with the same overall stiffness are designed while three different interfaces (0//0, 45//45, and 90//90), which can avoid the coupling effect of remote plies. Test results show that the fatigue delamination behavior is obviously affected by the interface angle. In addition, a novel and simple method for determining the fatigue delamination resistance is proposed. The measured fatigue delamination resistance is lower than the quasi-static one for all interfaces. The specimen with 45//45 interface exhibits more significant delamination resistance, whereas the delamination resistances of specimens with 0/0 and 90//90 interfaces are similar, which is consistent with the phenomenon of more bridging fibers accompanying in the delamination process of 45//45 interface. In order to consider the effect of fiber bridging and reduce the dispersion of the data, experimental data are further processed using a normalized model considering the effect of fiber bridging. The normalized results can be characterized by a single curve, suggesting that the normalized model is effective in describing the fatigue delamination behavior in the presence of fiber bridging.

Understanding delamination behavior of air electrode in solid oxide electrolysis cells through in situ monitoring of internal oxygen partial pressure

In this study, we monitored the development of internal pO2 in solid oxide electrolysis cells (SOECs) in situ using an embedded probe and a reference electrode. Three types of air electrode cells were compared: LSM (La0.8Sr0.2MnO3−δ) + YSZ (Y0.08Zr0.92O2−δ), LSCF (La0.6Sr0.4Co0.2Fe0.8O3−δ) + GDC (Gd-doped ceria) and LSM + GdCeScSZ ((Gd2O3)0.005(CeO2)0.005(Sc2O3)0.1(ZrO2)0.89). The rate of pO2 increase with the increase in current density was highest in the LSM + YSZ cell, reaching ∼32201 atm at 0.6 A cm−2, resulting in air electrode delamination and intergranular fractures. This physically developed delamination with high pO2 is akin to catastrophic failure, accelerating degradation. The LSCF + GDC cells exhibited a maximum pO2 of ∼12.25 atm and operated stably without increasing pO2 or delamination. In the LSM + GdCeScSZ cells, internal pO2 was substantially suppressed to ∼10−3 atm, with a degradation rate comparable to that of the LSCF + GDC cell. However, the air electrode delaminated without intergranular fractures. This chemically developed delamination without high internal pO2 does not significantly accelerate degradation. These results indicate that the delamination mechanism may vary even with the same LSM electrode, depending on its ionic conduction characteristics.

Exploring delamination behavior in carbon nanofiber-reinforced sandwich structures with various corrugated cores: An experimental study on mixed-mode crack growth

In this paper, the delamination behavior between the face sheet and core of sandwich panels under a mixed mode of failure during TPSB (Three-Point Sandwich Beam) testing is investigated. The experiments were conducted on two sets of specimens: one reinforced with 0.25% carbon nanofiber weight and the other consisting of samples without carbon nanofibers. The core of the sandwich structures was made of PVC foam, and the corrugated cores and face sheets were made of glass-epoxy fibers. The investigated specimens had corrugated cores with different geometries, such as trapezoidal, square, and triangular. The Force-Displacement and R-CURVE diagrams of sandwich panels reinforced with and without carbon nanofibers were extracted. The strain energy release rate increased with increasing crack length, and this trend in the mixed mode of failure was generally upward. The strain energy release rate in the sandwich panel reinforced with carbon nanofibers with trapezoidal, square, and triangular corrugated cores increased by 20.7%, 39.8%, and 44.6%, respectively. These findings underscore the potential of carbon nanofiber-reinforced sandwich structures for diverse applications requiring enhanced fracture resistance under varying loading conditions. These structures have many applications in wind turbine blades and aerospace industry.

Effect of fiber orientation of adjacent plies on the mode I delamination fracture of carbon fiber reinforced polymer multidirectional laminates

AbstractThe extensive use of multidirectional composite laminates requires to understand the delamination behavior at interfaces between plies with different fiber orientations. In this study, double cantilever beam testing was performed to investigate the mode I interlaminar fracture of carbon fiber reinforced polymer laminates with different central interfaces (0//0, 0//+45 and +45//−45). X‐ray microtomography revealed the curved crack front shape that was incorporated to calculate fracture toughness. The fracture toughness varies with the crack length in a typical delamination resistance curve, which can be quantified by an empirical equation. Incorporation of variable fracture toughness into a cohesive zone model can better predict the delamination fracture of the laminate, compared to constant toughness. It was found that the delamination mechanism is independent of central interfaces. However, compared to unidirectional laminates, multidirectional laminates are less resistant to crack initiation, but more resistant to propagation with higher toughness and shorter fiber bridging zone.Highlights Mode I interlaminar fracture of CFRP laminates with different central interfaces was investigated experimentally and numerically. Curved crack front shape was incorporated to calculate fracture toughness. Incorporation of variable fracture toughness into a cohesive zone model can better predict the delamination fracture of the laminate. Multidirectional laminates are less resistant to crack initiation but more resistant to propagation with higher toughness and shorter fiber bridging zone.

Effect of crossing warp arrangements on delamination resistance of 3D woven composite T-joints under in-plane tensile loading

An experimental study for investigating the delamination behaviour of 3D woven composite T-joints with weave variations and optimising weave architectures is carried out. This study involves 10 types of crossing warp architectures at the junction. Quasi-static tensile load is applied to two flanges of 3D woven composite T-joints to evaluate the in-plane mechanical performance. The crossing warp architecture effectively improves the in-plane mechanical performance. Results indicate a significant influence of crossing warp arrangements on failure modes of the 3D woven composite T-joints. The use of internal crossing warp architectures leads to severe delamination in the 3D woven composite T-joints while the composite T-joints with 3D woven external crossing warps primarily fail due to the debonding of fibres and matrix and fibre breakage. The optimal weave architecture for 3D woven composite T-joints is confirmed by analysing the in-plane mechanical behaviour with different crossing warp arrangements and proportions. Regardless of the crossing warp proportions, the external crossing warp architectures outperformed their internal counterparts in resisting delamination, resulting in a maximum increase of 68.75 %, 30.04 % and 116.81 % in modulus, strength and failure strain respectively.

Characterization of mechanical and temperature effects on delamination and basic strength behaviors of PVC coated PUR foam substrate used in bus dashboard

This study focuses on the delamination and characterization of different loadings and temperature conditions of polyvinyl chloride (PVC)-coated polyurethane (PUR) foam component layers commonly used in bus dashboards. The method was developed through mechanical and thermal tests.The study involves the examination of primer and no primer PVC-coated PUR components using SEM (Scanning Electron Microscope), tensile-compression, three-point bending, primer characterization, peel, PUR material thermal expansion, and thermal tests. Physical and material tests were conducted to determine the mechanical properties of PVC and PUR foam and the material properties of the primer. Fourier transform infrared spectroscopy (FTIR) analysis was also conducted to identify the functional groups present in the PVC, PUR, and primer materials, helping to understand the material interactions.Some tests produced the expected results, while others did not. The primer characterization and three-point bending tests were unsuccessful due to sample foam breakage. However, tensile-compression tests, peel tests, thermal expansion tests, and thermal tests yielded the desired outcomes. During thermal testing of primed samples, it was observed that the samples shortened and bent as the primer material melted and began to separate from the PVC and PUR layers. This bending and swelling were attributed to the difference in thermal expansion coefficients between the melting primer material and the PVC and PUR layers. The results demonstrate that PVC-coated PUR components can be effectively separated, and they also highlight opportunities for design modifications to improve the durability of bus dashboards.

Mechanism based four-linear cohesive zone model for mode I fracture of different stacking sequence CFRP laminates

Delamination, a prevalent failure mode observed in laminated composites, exerts a significant impact on structural integrity and performance. The occurrence of fiber bridging during the fracture process adds complexity and elevates the research challenges associated with this phenomenon. Existing models exhibit limitations in accurately capturing bridging behavior and discerning its underlying mechanical mechanisms. This study addresses these limitations by analyzing experimental results, employing the J-integral, and analyzing R-curve behavior, proposing a mechanism-based four-linear cohesive zone model along with a new finite element implementation method. Comprising three overlapping bi-linear CZMs, this model effectively simulates mode I fracture behavior in laminates with different stacking sequences. Moreover, it intuitively illustrates the mechanical mechanisms during crack propagation and offers simplicity in implementation. This research contributes to a deeper understanding of composite fracture mechanics and provides a practical model for predicting delamination behavior in laminated structures.

Systematic Study on Swelling/Delamination of Layered Metal Oxides with Quaternary Ammonium Ions: Production of Well-Shaped/Oversized Unilamellar Nanosheets.

Enormous swelling of layered host compounds in an aqueous solution of various amines has been investigated as an important step in the synthesis of molecularly thin 2D nanosheets. However, a complete understanding of the reaction process has not been attained, which is the barrier for producing high-quality unilamellar nanosheets. Here, the swelling and delamination behaviors of platelet single crystals of protonated layered metal oxides are systematically examined with a series of tetraalkylammonium (TAA) hydroxide solutions. Upon contact with the solutions, the crystals immediately underwent massive expansion by several tens to hundreds of times. The swollen crystals can be delaminated into elementary layers by the application of external shear force. The exfoliation behavior is dependent on TAA ions, especially in terms of yield and lateral size/shape of the delaminated nanosheets. The swollen crystals with TAA ions with longer alkyl chains are delaminated almost completely, but irregular and fractured small sheets are yielded. Such long alkyl chains become entangled on the oxide layer and resulting hydrophobic interactions may be responsible for the lateral fragmentation. It is found that replacement of aqueous solutions with organic solvents to suppress the hydrophobic interactions is effective to produce oversized nanosheets in rectangular shape with sharp edges.

Delamination analysis of the epoxy impregnated REBCO racetrack coil under thermal stress based on a 3D model

Superconducting coils made of Rare-Earth-Barium-Copper-Oxide (REBCO) coated conductor (CC) exhibit superior electromagnetic performance. Employing epoxy impregnation can improve the structural integrity of the superconducting coils. However, the delamination behavior is observed in the epoxy impregnated REBCO coil when the environment temperature cool from the room temperature to 77 K. In previous studies, there is a few researches on the delamination and mechanical behavior of the epoxy impregnated racetrack coil. Therefore, this study proposes a three-dimensional (3D) mechanical-thermal model which incorporates the cohesive zone material (CZM) to investigate the delamination mechanisms in epoxy impregnated REBCO racetrack coils during cooling. We found that the coil experienced a higher tensile radial stress at the semicircular part than the straight part during the cooling process. This leads to that the delamination area tends to appear initially in the semicircular part with large tensile radial stress. And the stress concentration generated at the edge of the delamination area in the semicircular part can cause the extension of the edge of the delamination area to the straight part. In addition, the influences of the thermal expansion coefficient (CTE) of the mandrel and overband on the coil delamination behavior are studied in this paper. It is found that the radial stress, the initial position of the delamination, and the degree of delamination are affected by the CTE of the mandrel and overband. And the delamination of the coil can be avoided by reducing the tensile radial stress of the coil through reducing the CTE of the mandrel or increasing the CTE of the overband. And the prevention of the delamination in the semicircular part can obviously avoid the occurrence of the delamination in the straight part of the racetrack coil.

Failure behaviors of cord-rubber composite materials under mixed mode I/II loading: Experimental and numerical simulation

Cord-rubber composites are widely used in automation with their flexibility and high strength. Interlaminar delamination damage is one of the main failure modes of this component during service. Therefore, we investigated the interlaminar delamination behavior of cord-rubber composites under different loading modes. Herein, an analytical method was proposed for determining the mode I and mode II fracture toughness of cord-rubber composites through T-peel and 0-degree peel tests. The interlaminar delamination mechanism of mixed mode I/II for cord-rubber composites was investigated using the cohesive zone model (CZM). Based on the analytical method, the interlaminar delamination failure behavior under mixed mode I/II loading conditions is accurately prediction, and the error is below 14 %. Meanwhile, cohesive failure within the cord fabric layer occurs first, followed by interfacial failure at the interface between the cord fabric and rubber layers.

A new cohesive-based modelling of moisture-dependent mode II delamination of carbon/epoxy composites

The cohesive zone model (CZM) is widely employed for simulating delamination and debonding behaviour in engineering structures. However, the choice of CZM shape impacts the accuracy of simulation results. Therefore, the selection of a suitable Traction-Separation Law (TSL) with appropriate cohesive parameters is crucial. This study focuses on simulating the mode II delamination of unidirectional carbon/epoxy composites under varying moisture content levels (0%, 2.2%, 3.8%, and 5.3%) using the End Notched Flexure (ENF) test. Trapezoidal TSL (TTSL) with varying pseudo-plasticity parameter (Γ) was implemented and compared. A guideline was proposed in this study to aid in the selection of cohesive parameters, and a relationship between these parameters and moisture content was established. The results indicate that Γ = 0.99 yields a more accurate force-displacement response compared to Γ = 0. The estimated cohesive zone length falls within the range of 2.0–2.4 mm. During crack growth, the analysis reveals that, at the peak force, a region approximately 0.5–0.6 mm ahead of the crack tip undergoes total failure. Simultaneously, damage initiation occurs in the region 2.3–2.4 mm beyond the complete failure zone.

Thermo-mechanical coupled three-dimensional finite element simulation analysis of drilling thermoplastic braided carbon fiber composite and optimization of process parameters

Optimizing the process parameters of thermoplastic composites during drilling is paramount for mitigating tearing, delamination, and other forms of damage, while simultaneously enhancing the tensile strength and extending the long-term service life of the overall structure. This study focuses on exploring a thermal-mechanical coupling method for predicting dynamic mechanical progressive failure and determining optimal process parameters during the drilling of Braided Carbon Fiber Reinforced Polyether Ether Ketone (BCF/PEEK). Initially, a thermal conduction constitutive model of BCF/PEEK was developed based on the proposed thermal distribution ratio calculation method. Concurrently, a user-defined material subroutine VUMAT and a bilinear cohesive element model, were implemented on the ABAQUS/Explicit platform to simulate the delamination behaviors of drilling BCF/PEEK using a tapered drill-reamer. Subsequently, a comprehensive BCF/PEEK drilling experiment platform was constructed, and the simulation accuracy of the drilling finite element (FE) model was verified through temperature, thrust force, and hole-wall morphology analyses. Finally, response surface regression models were established for process parameters, and the optimal parameters were predicted and validated. The results indicate that minimum thrust force and temperature can be achieved using the tapered drill-reamer with a spindle speed set at 4878.79/3000 r/min and a feed speed of 34.04/30 mm/min, respectively, with a maximum error of only 8.78 %.