Delamination Resistance Research Articles

An experimental analysis of the end-notched flexure composite laminates beams with elastic couplings

This paper presents an experimental study of the composite beams with elastic couplings subjected to the end-notched flexure (ENF) test. The object of research were carbon/epoxy composite laminates with specific stacking sequences exhibiting the bending–extension and the bending–twisting couplings. In addition, multidirectional laminates with different delamination interfaces were tested. Influence of elastic coupling phenomena on behavior of laminates subjected to the bending moment as well as on fracture toughness and delamination surfaces after the ENF tests was discussed. All experiments were performed according to the ASTM D7905 Standard. Determination of crack initiation was supported by the acoustic emission technique. Data analysis of the ENF test was performed by using both the direct beam theory and the corrected beam theory. Results obtained by using the classical data reduction schemes were compared with the compliance-based beam method (CBBM). Greater values of the fracture toughness obtained by using the non-standardized methods can be explained by the effect of the fracture process zone which was taken into account in the CBBM calculation scheme. Delamination surfaces of coupled laminates after the ENF tests were investigated by using the scanning electron microscopy. Results revealed that the effect of elastic couplings might influenced on intense bridging phenomena, more extensive fracture process zone and contribution of the mode III in total delamination resistance. Nevertheless, application of the standardized ENF procedure to determination of the fracture toughness of laminates exhibiting elastic couplings is still possible, provided assistance of additional data reduction schemes and techniques or numerical analysis.

Enhancement of fracture toughness in secondary bonded CFRP using hybrid thermoplastic/thermoset bondline architecture

Structures made of carbon fiber-reinforced polymer (CFRP) can be assembled using adhesive bonding. However, such bonding is prone to brittle delamination, and a method to improve delamination resistance is desirable. Here, we propose a technique to introduce crack-arrest features that increase the R-curve response by engineering the adhesive bondline/interface. We specifically designed a wavy net-like thermoplastic insert that was embedded into the thermoset adhesive bondline where the new mechanisms of energy dissipation were generated. We demonstrate that the technique is effective at improving mode I fracture toughness of secondary bonded carbon/epoxy by more than 400%. The hybrid thermoset/thermoplastic bondline architecture was carefully tailored to achieve its best performance. We demonstrate that introducing porosities in the adhesive bondline (by adding a limited amount of thermoset adhesive) further improves the fracture toughness. This toughness improvement originates from the extrinsic toughening of the crack-arrest feature, which is enabled by the insert ductility and microstructures (via strand formation, anchoring and stretching).

Damage monitoring of glass/epoxy laminates with different interply fiber orientation using acoustic emission

This article investigates the effects of interface fiber orientation on the mode-I interlaminar fracture toughness, and the evolution of damage was monitored using acoustic emission technique. The results show that changing the interply fiber orientation improved the fracture toughness and delamination resistance significantly. The GIC initiation fracture toughness was found to increase by 87%, 71%, and 69% for 0°/45°, 0°/90°, and ±45° interfaces, respectively. The improvement in fracture toughness was attributed to the crack path diversion along the interply fiber orientation which offered more resistance to crack propagation. In addition, sentry function was also computed based on the correlation between the mechanical strain energy accumulated in the laminates, and the acoustic energy that propagates by fracture events made it possible to evaluate the delamination resistance. The results show that 0°/45° has high resistance to crack propagation followed by 0°/90°, ±45°, and 0°/0°. Finally, scanning electron microscopic analysis was used to determine the fracture surface in different interply fiber orientation of glass/epoxy laminates.

Interlaminar Fracture Toughness of Carbon-Fiber-Reinforced Epoxy Composites Toughened by Poly(phenylene oxide) Particles

In this study, carbon fiber reinforced epoxy composites were interleaved by polyphenylene oxide (PPO) particles to improve their delamination resistance. The morphology of PPO particles at the inte...

Industrial manufacturing and characterization of multiscale CFRP laminates made from prepregs containing graphene-related materials

The introduction of graphene-related materials (GRMs) in carbon fibre-reinforced polymers (CFRP) has been proved to enhance their mechanical and electrical properties. However, methodologies to produce the 3-phase materials (multiscale composites) at an industrial scale and in an efficient manner are still lacking. In this paper, multiscale CFRP composites containing different GRMs have been manufactured following standard procedures currently used in the aerospace industry with the aim to evaluate its potential application. Graphite nanoplateletelets (GNPs), in situ exfoliated graphene oxide (GO) and reduced graphene oxide (rGO) have been dispersed into an epoxy resin to subsequently impregnate aeronautical grade carbon fibre tape. The resulting prepregs have been used for manufacturing laminates by hand lay-up and autoclave curing at 180 °C. A broad characterization campaign has been carried out to understand the behaviour of the different multiscale laminates manufactured. The degree of cure, glass transition temperature and degradation temperature have been evaluated by thermal evolution techniques. Similarly, their mechanical properties (tensile, flexural, in-plane shear, interlaminar shear and mode I interlaminar fracture toughness) have been analysed together with their electrical conductivity. The manufacturing process resulted appropriated for producing three-phase laminates and their quality was as good as in conventional CFRPs. The addition of GO and rGO resulted in an enhancement of the in-plane shear properties and delamination resistance while the addition of GNP improved the electrical conductivity.

Experimental Investigation into the Mechanical Behavior of Textile Composites with Various Fiber Reinforcement Architectures

The role of fiber reinforcement architecture in adjusting the mechanical properties of glass-fiber-reinforced epoxy composite materials is studied. Fibrous E-glass unidirectional (UD) and 3-dimensional (3D) samples with an identical fiber volume fraction were prepared. The UD composites displayed excellent properties along the fiber direction, but the 3D ones showed excellent properties in all three directions. The high delamination and impact resistances were additional advantages of 3D-reinforced composites to choose them as reliable materials for structural applications.

Effects of Graphene-Based Fillers on Cathodic Delamination and Abrasion Resistance of Cataphoretic Organic Coatings

This study aims to demonstrate the excellent protective performance of functionalized graphene oxide (fGO) flakes in acrylic cataphoretic coatings. The filler content provides an important contribution in improving the chemical and mechanical resistance of the acrylic matrix. The morphology of the fillers was first investigated by optical and electron microscopy, analysing the distribution of the fGO flakes within the polymer matrix. After that, the flakes were added to the cataphoretic bath in different concentrations, resulting in four series of samples. The cathodic delamination of the coatings was assessed with cathodic polarization cycles and with measurements carried out with a scanning Kelvin probe. Finally, the abrasion resistance at the macroscopic and microscopic level was studied by scrub testing and scratching atomic force microscopy analysis, respectively. The incorporation of fGO at the optimized concentration of 0.2 wt.% greatly increases the cathodic delamination resistance of the acrylic matrix, resulting in an effective barrier against the effects of absorbed aggressive substances. Graphene-based fillers also enhance abrasion resistance, thanks to their high mechanical strength. Thus, this work demonstrates the great protective benefits that can be obtained when using fGO flakes as reinforcing fillers in cataphoretic coatings.

Improving the delamination resistance of fibre reinforced polymer composites using 3D woven metal Z-Filaments

This paper presents a new approach for greatly increasing the delamination resistance of fibre reinforced polymer composites via through-the-thickness weaving of high-strength, high-toughness metal z-filaments. Dry carbon fabric preforms were woven in the through-thickness direction with thin and continuous stainless steel or copper filaments, and then infused with epoxy resin to create 3D woven composites. The modes I and II interlaminar fracture toughness properties of the resultant composites are compared with those of a 3D woven composite reinforced with z-filament made of continuous carbon fibre tows. Experimental testing and finite element modelling of delamination crack growth reveals that metal z-filaments can promote much greater improvements to the mode I interlaminar fracture toughness compared to carbon z-filament. The steel z-filament increases the mode I delamination resistance by over 50 times, with the fracture toughness value (~28 kJ/m2) being the highest reported value for composite laminates. The metal z-filaments are less effective than the carbon z-filament at increasing the mode II delamination resistance, although the improvements are still large. The magnitude of improvements in modes I and II delamination toughness by metal z-filaments depends on several factors, including the alloy type, mechanical properties (e.g. tensile strength and ductility), volume fractions, and crack bridging toughening mechanisms.

Influence of Milled Glass Fiber Fillers on Mode I & Mode II Interlaminar Fracture Toughness of Epoxy Resin for Fabrication of Glass/Epoxy Composites

The present work is focused on improving mode I and mode II delamination resistance of glass/epoxy composite laminates (50 wt.% of glass fibers) with milled glass fibers, added in various amounts (2.5, 5, 7.5 and 10% of the epoxy weight). Including fillers in the interlayer enhances the delamination resistance by providing a bridging effect, therefore demanding additional energy to initiate the crack in the interlaminar domain, which results in turn in enhanced fracture toughness. The maximal increase of mode I and mode II fracture toughness and of flexural strength was obtained by the addition of 5% milled glass fiber. The mechanism observed suggests that crack propagation is stabilized even leading to its arrest/deflection, as a considerable amount of milled glass fiber filler was oriented transverse to the crack path. In contrast, at higher filler loading, tendency towards stress concentration grows due to local agglomeration and improper dispersion of excess fillers in inter/intralaminar resin channel, causing poor adhesion to the matrix, which leads to reduction in fracture toughness, strength and strain to failure. Fractured surfaces analyzed using scanning electron microscopy (SEM) revealed a number of mechanisms, such as crack deflection, individual debonding and filler/matrix interlocking, all contributing in various ways to improve fracture toughness.

In vitro estimation of reduction in strength and wear resistance of UHMWPE for joint prostheses due to lipid-induced degradation.

Ultra-high molecular weight polyethylene (UHMWPE) is used as a bearing surface of joint prostheses and has been reported to absorb lipids such as squalene (SQ) and cholesterol esters in vivo. These lipids have been suggested by in vitro studies using SQ as a model lipid to have the potential to induce polymer degradation. However, the impact of lipid-induced degradation on the strength and wear resistance of UHMWPE is unknown. In this study, lipid-induced degradation was simulated by SQ absorption and subsequent accelerated aging, and its influence on the strength and wear resistance of UHMWPE was investigated using wear, fatigue crack growth, and delamination testing. Lipid-induced degradation was found to have little impact on fatigue crack growth rates and delamination resistance. These results were consistent with previous reports that lipid-induced degradation is localized near the surface. However, we also found that lipid-induced degradation increased the wear rate of both non-crosslinked and crosslinked UHMWPE by a factor of 2.5 and 14, respectively. These results indicate that lipid-induced degradation may affect the durability and long-term clinical outcome of joint replacements due to increased wear of UHMWPE.

The temperature effect on the inter-laminar shear properties and failure mechanism of 3D orthogonal woven composites

Recent increases in the use of carbon fiber reinforced plastics, especially for high-temperature applications, has induced new challenges in evaluating their mechanical properties. The effects of temperature on the shear performance of 3-dimensional orthogonal and 2-dimensional plain woven composites were compared in this study through double-notch shear tests. A scanning electron microscope was employed to investigate the fiber/matrix interface properties to reveal the failure characteristics. The results showed that temperature had a visible impact on the inter-laminar shear strength (ILSS), deformation modes, and failure mechanism. The ILSS decreased as temperature increased, which was caused by the degradation of the matrix properties and fiber/matrix interface properties at high temperature. A finite element model was established to analyze the transient deformation process and the damage mechanism of the 3D orthogonal woven composite. This indicated that Z-binder yarns could improve the delamination resistance of 3D orthogonal woven composites, especially under high temperatures. The changes in failure modes of the 3D orthogonal woven composites was put down to thermal softening of the epoxy resin caused by high temperature and the undulation of the yarns.

Cure-dependent microstructures and their effect on elastic properties of interlayer toughened thermoset composites

To shorten the cure cycle for thermoset prepreg composites, we can either increase the heating rate or the curing temperature. However, this raises the question whether modifying the cure cycle affects the laminate microstructure and physical and mechanical properties. Current state-of-the-art aerospace prepreg composites incorporate thermoplastic particles into the interlayer to increase delamination and impact resistance, which creates a more complicated composition and laminate microstructure. This research investigates the effect of curing conditions on the microstructure and elastic properties of an interlayer toughened prepreg system, Toray's T800SC/3900-2B. Laminates were processed to the same degree of cure using cure cycles with different heating rates. Optical microscopy showed that microstructural characteristics such as the interlayer thickness, particle shape, particle volume fraction and inter-particle distance are dependent on curing conditions. It was found that the glass transition temperature of the toughening particle, the resin viscosity and the elastic deformation of the fibre bed are important for the cure-dependent microstructural evolution at the pre-gelation stage. The out-of-plane lamina shear modulus (G13) dependence on temperature of fully cured laminates was measured with Dynamic Mechanical Analysis (DMA). The lamina G13 decreases with increasing cure cycle heating rate at 150–200 °C, where toughening particles are rubbery and the matrix is glassy. A micromechanical model demonstrates that the reduction of lamina G13 at higher heating rates results from a decrease in the interlayer shear modulus. Interlayer microstructural features such as particle volume fraction, particle aspect ratio and inter-particle contact contribute to the cure path dependence of the interlayer shear modulus. This study provides insight into the fundamental processing behavior of interlayer toughened prepregs and the influence on elastic properties.

Experimental and statistical analysis of carbon fiber/epoxy composites interleaved with nylon 6,6 nonwoven fabric interlayers

Thermoplastic interleaving is a promising technique to improve delamination resistance of laminated composites. In this study, plain-weave carbon fiber/epoxy composites were interleaved with nylon 6,6 nonwoven fabrics with an areal weight density of 17 gsm. The carbon fiber/epoxy composite laminates with/without nylon 6,6 nonwoven fabric interlayers were manufactured by VARTM technique. Double cantilever beam fracture toughness tests were carried out on the prepared composite test specimens in accordance with ASTM 5528 standard. The experimental test data were statistically analyzed by two-parameter Weibull distribution. The results showed that the initiation and propagation fracture toughness Mode-I fracture toughness of carbon fiber/epoxy composites could be improved by about 34 and 156% (corresponding to a reliability level of 0.50) with the incorporation of nylon 6,6 interlayers in the interlaminar region, respectively. The results also revealed that the percent increase in the propagation fracture toughness value was 67 and 41% at reliability levels of 0.90 and 0.95, respectively.

Improving delamination resistance through tailored defects

Delamination is a well-known damage mode exhibited by laminated composites which has been extensively studied in unidirectional laminates (UD). In this work, a novel strategy consisting of the introduction of a small interlaminar defect to enhance the fracture resistance to delamination of UD composites is presented. This toughening concept is based on the simultaneous propagation of multiple interlaminar cracks along different interfaces. A preliminary numerical study showed that the minimum size required by the initial defect to trigger dual propagation was 4 times the ply thickness for intermediate and thin plies for any applied mixed mode, except for pure mode II. An experimental campaign to prove the toughening concept with 1 and 3 defects at adjacent interfaces was carried out and compared against a reference configuration under three different applied mode mixities (ϕglob=0.4,0.2 and 0). The single defect configuration reached an improvement in the fracture resistance of +300% under pure applied mode I, and +80% and +100% for applied mixed modes of 0.2 and 0.4, respectively. The configuration with 3 defects reached even higher fracture toughness values: +430% in pure applied mode I, and +115% and +100% for applied mixed modes of 0.2 and 0.4, respectively. A finite element model employing cohesive zones successfully predicted both, the phenomenon of multiple crack propagation, and the effective fracture resistance of the process. Interestingly, among the dissipation mechanisms reported experimentally, the extensive promotion of fiber bridging under applied pure mode I was observed, which was not reported in the baseline configuration.

Multi-objective particle swarm optimisation of multilayer functionally graded coating systems for improved interfacial delamination resistance

In this paper a multilayer coating system was considered for a 316 L stainless steel substrate with potential application for hydrogen storage. The coating system was comprised of a 316 L stainless steel/Al2O3 functionally graded (FG) layer and a SiC top coat. The composition gradation and thicknesses of the FG layer and SiC top coat were considered as parameters to be optimized for higher edge interfacial delamination resistance within the system. A novel method of delamination energy variation was first used for finite element analysis (FEA) of the system and the best set of parameters were defined as well as an interfacial delamination criterion (maximum stable edge delamination crack length in the system). Analytical solutions of thermal residual stress, strain and strain energy density were also studied for the system. Two new multi-objective functions were derived based on the analytical equations and further investigated to maximize interfacial delamination resistance of the system using multi-objective particle swarm optimisation (MOPSO). The optimized conditions achieved by different methods were finally compared and concluded to be in good agreement. Overall, the use of the new objective functions for MOPSO, combined with FEA, increased the maximum stable delamination crack length from 7 μm (obtained by a thermal residual stress criterion used by previous researchers) to 50 μm (using the new method). Therefore, FEA based on energy variation and MOPSO with the new objective functions were found to be applicable for optimisation with respect to interfacial delamination resistance.

Simulated and experimental demonstrations of interfacial adhesive strength for released layer utilized in flexible electronics

Interfacial adhesive strength is an important characteristic considered in multilayered active-matrix organic light-emitting diode (AMOLED) architecture as it has an impact on the mechanical reliability issue generated from complicated organic/inorganic hybrid lamination and final debonding procedure. Thus, an appropriate released layer design is needed to manage the capability and reliability of the encapsulated AMOLED structure de-bonded from the temporary carrier. Accordingly, this research utilizes the plasma-assisted surface modification to improve the interfacial adhesive strength of the polyimide (PI)/debonding layer (DBL) during mechanical debonding. In addition, a fracture-based finite element model of peeling testing architecture is constructed to analyze the adhesive behavior of the PI/DBL interface via the utilization of the modified virtual crack closure technique. The analytic results indicate that the suitable plasma-assisted surface modification significantly increased the nitrogen concentration and generated the high dissociation energy N–O bonds among the bonded interface between the PI/DBL thin films. The opening mode-fractured energy also takes the dominance proportion in the entire peeling load-induced interfacial energy release rate. The application of plasma-assisted surface modification enhanced the delamination resistance of the concerned interface and maintained the structural stability during the lamination and related release of a whole flexible AMOLED architecture.

Effects of inkjet printed toughener on delamination suppression in drilling of carbon fibre reinforced plastics (CFRPs)

Delamination has been recognised as the predominant damage induced during the drilling of carbon fibre reinforced plastics (CFRPs). It could significantly reduce the bearing capacity and shorten the service life of the designed component. To enhance the delamination resistance of CFRPs for different applications, great affords have been done to improve their interlaminar fracture toughness. However, due to the difficulty in accurately controlling the amount of the toughener applied in the interface, effect of the toughener content on the toughening efficiency is rarely studied. In this work, an experimental research was developed to investigate the performance of the toughener on the improvement of delamination resistance in the drilling of CFRPs and parametrically optimise the toughener content with the consideration of different feed rates. Specifically, poly(methyl methacrylate) (PMMA) solutions with various concentrations were selected to add on the CFRP prepreg, and co-cured together with layups. The inkjet printing technology was adopted to deposit the PMMA solutions for precisely controlled toughener contents. Through drilling experiments on the toughened CFRPs, it was found that the optimal content of the PMMA solution was 10 wt% to offer the least delamination, in particular, for the situation under the highest feed rate condition. The toughing mechanisms were also concluded by analysing the histories of the thrust force and torque in the drilling process. The results of this study is significantly contribute to the locally toughening of the composite interfaces and the improvement of the drilling quality, which is specifically helpful to strengthen the joint property for the structural design stage for the aircraft.

Improved Interlaminar Fracture Toughness and Electrical Conductivity of CFRPs with Non-Woven Carbon Tissue Interleaves Composed of Fibers with Different Lengths.

Non-woven carbon tissue (NWCT) with different fiber lengths was prepared with a simple surfactant-assistant dispersion and filtration method and used as interleaving to enhance both delamination resistance and electrical conductivity of carbon fiber reinforced plastics (CFRPs) laminates. The toughing effect of NWCT on both Mode I and Mode II interlaminar fracture of CFRPs laminate is dependent on length of fibers, where the shorter carbon fibers (0.8 mm) perform better on Mode I interlaminar fracture toughness improvement whereas longer carbon fibers (4.3 mm) give more contribution to the Mode II interlaminar fracture toughness increase, comparing with the baseline composites, and the toughness increase was achieved without compromising of flexural mechanical properties. More interestingly, comparing with the baseline composites, the electrical conductivity of the interleaved composites exhibited a significant enhancement with in-plane and through-the-thickness direction, respectively. Microscopy analysis of the carbon tissue interleaving area in the laminate indicated that carbon fibers with shorter length can form into a 3D network with more fibers aligned along through-the-thickness direction compared with longer ones. The shorter fibers thus potentially provide more effective fiber bridges, pull-out and matrix deformation during the crack propagation and improve the electric conductivity significantly in through-the-thickness direction.

Progressive multi-damage analysis of composite laminate using higher order zig-zag plate theory

In this article, a finite element model of multi-layer composite laminates with delamination based on higher order zig-zag theory is studied for the progressive delamination analysis. The degrees of freedom are simplified by the continuity conditions of shear stress between layers and the free-surface conditions at the bottom and top of laminates. The numbers of degrees of freedom are not reduced with this method while the model can take the initiation of delamination into consideration in return. Static loading analysis is implemented to simulate the performance in delamination resistance with two different plies and under two different boundary conditions. The present model can present good performance in the prediction of delamination phenomenon.

Delamination resistant composites by interleaving bio-based long-chain polyamide nanofibers through optimal control of fiber diameter and fiber morphology

In this work an innovative electrospinning system is proposed that simultaneously has an adequate temperature resistance, a high increase in mode I (+51%) and mode II (+96%) delamination performance and can be commercially produced. Interleaving nanofibrous veils can potentially solve the issue of the limited delamination resistance encountered in composite laminates, but industrial upscaling has always been impeded by one or more critical factors. These constraining factors include a limited temperature stability of the nanofibers, a lack in simultaneous mode I and II delamination performance increase and the complexity of the electrospinning system because non-commercial polymers or specialty nanofibers (e.g. coaxial) are required. In this paper, a robust electrospinning system is proposed that is the first to overcome all major hurdles to make nanofiber toughening industrially viable. A new class of nanofibers based on biosourced polyamide 11 and its poly(ether-block-amide) co-polymers is used to deal with those shortcomings. The nanofibers have tuneable diameters down to 50 nm and cross-section morphologies ranging from circular to ribbon-shaped. The key to this work is the fundamental underpinning of the toughening effect using a broad range of interleaves with different mechanical and thermal properties, fiber diameters and fiber morphologies, all produced from the same bio-based base polymer. Generally, round and thin nanofibers performed better than larger and ribbon-like fibers. The relationship between the fiber morphology and the delamination performance is further underpinned using detailed analysis of the fracture surface. Ultimately, this results in a range of optimized nanofibrous veils capable of improving the delamination resistance considerably without suffering from the aforementioned drawbacks.