Interlaminar Toughening Research Articles

Enhanced interlaminar fracture toughness of CF/PEEK laminates by interleaving CNT-decorated PEEK films

The weak interlaminar properties severely hinder the high-property applications of carbon fiber (CF) reinforced thermoplastic composites. Here, interlaminar properties were significantly enhanced by introducing carbon nanotube (CNT) decorated polyether ether ketone (PEEK) films into the interlayer. PEEK films decorated with different CNT contents were fabricated by the spraying and thermoforming processes. Tensile test results show that the films with a CNT surface density of 45 mg/m2 exhibit superior tensile strength and Young's modulus due to the higher crystallinity and larger crystalline size. The introduction of this modified film into unidirectional CF reinforced PEEK laminates leads to an increase by 101% to 1.67 kJ/m2 in interlaminar fracture toughness, while the interlaminar shear strength is retained. Fractographic analysis reveals that the increase in interlayer resin content and crack path deflection are primary factors contributing to interlaminar toughening. Molecular dynamics simulation demonstrates that incorporating CNT in the interlayer resin significantly improved the interfacial bonding strength between fiber and resin, thus resulting in crack path deflection.

Short carbon nanotubes: From matrix toughening to interlaminar toughening of CFRP composites

Carbon nanotubes (CNTs) are widely used for strengthening and toughening of resin matrix. However, the improvement of resin matrix toughness can't be effectively transferred to carbon fiber reinforced polymer (CFRP) composites due to the “fiber filtration effect”. Here, short carbon nanotubes (SCNTs, average length <2 μm) were added to the epoxy resin and then used as the matrix of CFRP. The test results showed that 0.5 wt% SCNTs is the optimal mass fraction for matrix toughening, which enhanced the maximum strain, maximum stress and toughness of the epoxy resin by 43.3%, 10.1% and 65.9%, respectively, and the mode I interlaminar fracture toughness of resultant CFRPs was increased by as much as 86.7%. The superior interlaminar toughening efficiency of SCNTs can be ascribed to their smaller size, which enables them to penetrate into the tiny gaps of carbon fiber yarns, thus not only effectively avoiding the “fiber filtration effect” and making them evenly distributed in the matrix, but also strengthening the interfaces between epoxy resin and carbon fibers.

Interlaminar fracture toughness of carbon/epoxy composites laminates toughened by short carbon fiber

AbstractIn this paper, short carbon fiber (SCF) interlaminar toughened carbon fiber reinforced epoxy resin (CF/EP) composite laminates were prepared. The effect of SCF content on the mode II interlaminar fracture toughness (GIIC) and flexural properties of CF/EP composites was investigated. It was shown that the addition of SCF between the plies had an effect on the GIIC of the composites. When the SCF content was 10 g/m2, the GIIC increased from 1506.62 J/m2 to 1995.6 J/m2, which improved by about 32.46%. When the content of SCF was 5 g/m2, the flexural strength and flexural modulus increased by 36.92% and 98.48%, respectively, showing the best performance. The toughening mechanism of SCF was observed by SEM images as SCF debonding and pulling out from the resin and SCF fracture. The present study was also compared with our previous study on short aramid fiber interlaminar toughened CF/EP composites, and the test data and SEM images of the two experiments were compared to analyze the effect of different short fibers on the interlaminar toughening effect, which provides some reference for the selection of interlaminar toughening materials.

Hierarchical mode I interlaminar toughening of unidirectional CFRP laminates by the synergistic effects of CNT powders and veils

While carbon fiber reinforced polymer (CFRP) composites are widely used in structural components because of their excellent mechanical properties, delamination between reinforcing plies remains a well-known Achilles heel, weakening the composite structure and limiting their more widespread applications. This work tried to improve the interlaminar properties of unidirectional CFRP laminates by the synergistic effects of CNT powders and veils. The short CNT powders (average length <1 μm) were adopted for toughening the matrix, which effectively avoids the “self-filtration effect” commonly found in the VARTM process and allows the CNTs to penetrate between the tiny gap of the carbon fibers and be uniformly dispersed in the composites. In addition, an ultrathin CNT fiber veil (∼100 nm in thickness) was used as the interleaf material, offering the opportunity to reinforce the interlaminar bonding with minimal weight penalty and effectively avoid reductions in in-plane properties. The results showed that 0.5 wt% is the optimum loading level of CNT powders for matrix toughening. The Mode I interlaminar fracture toughness (propagation value, GIC,prop) of the unidirectional CFRP composites increased as much as 32 %, 62 % and 123 % when integrating CNT veil, CNT powder and CNT veil + powder, respectively. The figure of merit for interlaminar reinforcement, consisting of the change in interlaminar properties normalized by interleaf thickness and ply thickness, comes out as high as 2460, which far outweighs the state of the art.

On the static failure behaviour of co-cured composite joints with enhanced interlaminar fracture energies via multiscale toughening

This paper is focused on the multiscale toughened co-cured composite interfaces of carbon/epoxy laminates using vacuum infusion and out-of-autoclave curing—with an emphasis on the role of enhanced interlaminar fracture energies on the static failure of laminate joints. To toughen co-cured interlaminar regions, three toughening routes are used: (i) epoxy matrix toughening with 10 wt% core–shell rubber (CSR) nanoparticles, (ii) 10 g/m2 polyphenylene sulfide (PPS) non-woven micro-fibre veil toughening, and (iii) multiscale toughening by combining epoxy matrix toughening with 10 wt% CSR nanoparticles and 10 g/m2 PPS veils. Double cantilever beam and end-notch flexure tests are conducted with the toughened interfaces for mode-I and mode-II fracture energies. Moreover, three co-cured joint configurations (viz. double-notch single lap, stepped scarf and skin-stiffener joints with the three toughening routes) are tested to characterise the role of enhanced interlaminar fracture energies on the static failure of laminate assemblies. The results show that the three toughening routes have significantly altered the R-curve behaviour and interlaminar fracture energies and that the degree of enhancement in mode-I and mode-II fracture energies is sensitive to the type of toughening route. Based on the material systems tested, the mode-I fracture initiation energy is enhanced by ∼150% with multiscale toughening, ∼65% with particle toughening, and ∼60% with veil toughening; whereas, the mode-II fracture initiation energy is enhanced by ∼85% with veil toughening and ∼35% with multiscale toughening—but reduced by ∼20% with particle toughening. The joint tests show that the toughening routes affect the static failure of the three joint configurations and that the strength and damage evolution of the co-cured joints depend on the interlaminar fracture energies. The strength of the co-cured single lap joints is increased by ∼50% with multiscale toughening and ∼40% with veil toughening; whereas, the strength of the co-cured skin-stiffener joints is enhanced by ∼35% with multiscale toughening and ∼45% with veil toughening. The failure mechanisms observed on the fracture surfaces indicate that the strength/failure behaviour of the co-cured joints depends on the dominant interlaminar fracture mode and the corresponding fracture energies. Overall, it is shown that the multiscale interlaminar toughening routes that are used to address the issue of delamination can also be effectively explored to enhance the static failure of co-cured laminate assemblies by toughening the co-cured interfaces/bondlines.

Understanding Mode I interlaminar toughening of unidirectional CFRP laminates interleaved with aligned ultrathin CNT fiber veils: Thickness and orientation effects

Ultrathin CNT fiber veils (100–300 nm thick) were used as interleaves to improve the interlaminar fracture toughness of unidirectional carbon fiber reinforced polymer composites. These veils, drawn from vertically aligned CNT forests, were directly deposited at the mid-plane of CFRPs either longitudinally or transversely to the carbon fiber. The results revealed that triggering multiple toughening mechanisms is the key to achieve high toughness FRP composites. When the veils were oriented parallelly to the carbon fiber, the crack propagates in a “zig-zag” manner, not only interacting with more CNTs, but also triggering more carbon fiber bridgings, leading to a steeply “rising” R-curve. The superiority and effectiveness of this technique is exemplified by the extremely thin toughening layers (about 300 nm thick) needed to remarkably improve the propagation value of Mode I interlaminar fracture toughness of the CFRP by approximately 107%. The Mode I toughening factor (ηI), defined as the change in Mode I interlaminar fracture toughness per interleaf-to-ply thickness ratio, comes out as high as 713, which is far outweigh the state-of-the-art. However, transversely-placed CNT veils will seriously block the formation of carbon fiber bridgings, remarkably degrading the Mode I interlaminar toughness of the laminates.

Improved interlaminar fracture toughness of carbon fiber/epoxy composites by a combination of extrinsic and intrinsic multiscale toughening mechanisms

Carbon fiber reinforced polymer (CFRP) composites with hierarchical structure interleave composed of nanoscale core-shell rubber (CSR) and microscale short carbon fiber (SCF) were fabricated. The hierarchical structure interleave exhibits superior interlaminar toughening efficiency than either CSR or SCF alone. Triggering intrinsic and extrinsic toughening mechanisms at different scales by CSR and SCF, respectively, the mode-I critical energy release rate (GIC) and mode-II critical energy release rate (GIIC) of multi-phase interleaved CFRP composite are 127% and 154% higher than those of unmodified sample. While the tensile properties and glass transition temperature (Tg) are not compromised. The fracture toughness of epoxy/CSR system was also investigated to better understand the effect of matrix toughness on interlayer toughness of CFRP. Numerical analysis and fracture surface observations revealed the synergistic toughening mechanisms in SCF/CSR interleaved CFRP composites. The synergy between nanoscale intrinsic toughening mechanism and microscale extrinsic toughening mechanism, evolving with crack propagation, contributes to a greater energy absorption. The usefulness of the present study for preparation of high-performance CFRP is discussed.

Translaminar enveloping ply for CFRP interlaminar toughening

Developing low-cost and high-performance interlaminar toughening strategies for the advanced CFRP products has great engineering significances. Combining features on practical automated fiber placement and well-recognized 3D reinforcement technologies in the field of CFRP composites, herein, we alternately introduced a reinforcement ply in the manner of translaminar enveloping to generate interlocking and binding action into conventional lay-ups. Remarkably, more than 1700% for mode I (DCB) and 500% for mode Ⅱ (ELS) propagation fracture toughness improvements were experimentally investigated. The evolution of interlaminar toughening behaviors was specifically revealed. Multiple fracture energy absorption stages, successively including delamination tolerance, partial interface pull-out and through-thickness ruptures, reflect long-acting mode I toughening capacity. Under the multi-interface synergistic shear-delamination mechanism, the mode Ⅱ crack growth gets far more effectively suppressed. Meanwhile, the in-plane tensile and compressive strength could be maintained acceptably. We also gave insights into the technical superiorities of this original ply-level reinforcing work.

A CNT-Toughened Strategy for In-Situ Repair of Aircraft Composite Structures.

This study aimed to develop an in-situ field-repair approach, especially for aircraft composite structures, to enhance the interlaminar toughness of plain-woven composites (PWCs) by adding multi-walled carbon nanotubes (MWCNTs). MWCNTs were dispersed at each interface between prepreg layers by means of solvent spraying, with a density of 1.58 g/m2. Then, the layers were stacked with the predefined sequence and cured at 120 °C and 1 bar pressure, using a heat-repairing instrument. A standard double cantilever beam (DCB) test was used to investigate the interlaminar toughening effect that was due to the MWCNTs. For comparison, original samples were also prepared. The results indicated that the introduction of MWCNTs can favorably enhance the interlaminar toughness of PWCs in a field-repair approach and the Mode I fracture energy release rate, GIC, increased by 102.92%. Based on the finite element method (FEM) of continuum damage mechanics, the original samples and the MWCNTs toughening specimen under DCB Mode I fracture were modeled and analyzed. The simulation and the experiment were in good agreement. Finally, when the toughening mechanism of MWCNTs was explored with a scanning electron microscope (SEM), we found that a large amount of fiber-matrix (F-M) interface debonding and matrix cracking in mountain shape were the major modes of fracture, accompanied by a small amount of fiber breakage and matrix peeling for the MWCNTs-toughening specimens.

Simultaneously improve the mode II interlaminar fracture toughness, flexural properties, and impact strength of CFRP composites with short aramid fiber interlaminar toughening

AbstractThis work mainly focuses on the mode II interlaminar fracture behavior of carbon fiber composite that was interlaminar toughened by short aramid fiber modified epoxy resin. The short fibers were first dispersed in the epoxy matrix and then in the laminate interlayer area during the lamination process. The ENF test evaluated the Mode II interlaminar fracture toughness of laminates. The results show that mode II interlaminar fracture toughness increases with the addition of short fibers. When the fiber content is 10 g/m2, the toughening effect is the best at 2437.72 J/m2, which is about 61.8% higher than the untoughened sample. The toughening mechanism of CFRP laminates by adding short fibers was observed by mode II fracture surface SEM images. In addition, flexural properties and Charpy impact properties of short fiber modified resin interlayer toughened laminates were also studied. The results showed that with this method, flexural properties and impact strength were improved to a certain extent, proving that this method is an effective interlaminar toughening method.

Significantly enhancing fracture toughness of epoxy composite with promising γ-FeOOH@Fe2O3 hybrid nanoparticles by magnetic field assistance

The toughening of epoxy resin (EP) and the interlaminar toughening of carbon fiber reinforced composite (CF/EP) laminates have been widely concerned. In this work, the needle-like γ-FeOOH nanoparticles were prepared by liquid phase deposition-air oxidation method, and then were calcined under different conditions to obtain γ-FeOOH and γ-Fe2O3 hybrid nanoparticles (γ[email protected]2O3). Effect of calcination condition of γ[email protected]2O3 and magnetic field assistance on fracture toughness (KIC) of EP was systematically investigated. Then the selected γ[email protected]2O3 with the best toughening effect were used to improve the mode I interlaminar fracture toughness (GIC) of CF/EP laminate. The resulting γ[email protected]2O3 have a length of around 1 ​μm, a diameter of around 100 ​nm and the Ms of 8.99–45.96 emu/g. After calcinated at 250 ​°C for 1 ​h, the γ[email protected]2O3 containing 24 ​wt% FeOOH and 76 ​wt% Fe2O3 achieved the best toughening effect. Under a magnetic field of 0.09 ​T, the KIC of the γ[email protected]2O3/EP composite (2.45 ​MPa ​m1/2) is 81.7% and 66.7% higher than that of neat epoxy and the composite without magnetic field induction, respectively. Furthermore, the GIC of the γ[email protected]2O3/CF/EP composite (0.914 ​kJ/m2) is also significantly increased by 88.8% and 51.8% compared to that of CF/EP and the corresponding composite without magnetic field induction, respectively.

The electrical conductivity and interlaminar fracture toughness of composite laminate interleaved with silver-plated nylon mesh

A composite laminate with high conductivity and high mode I fracture toughness was prepared by using a silver-plated nylon mesh as the interleaf. Through in-depth comparison with the silver-plated non-woven veil interleaved composites, the mechanisms of the conductivity improvement and interlaminar toughening were discussed. The conductivities along the fiber direction, in transverse to fiber direction and through thickness direction reached 739 S/cm, 734 S/cm, and 5.4 S/cm, respectively, which increased by 196%, 344,500%, and 4326%, respectively, and are much higher than these of the other modified systems. At the same time, the mode I interlaminar fracture toughness increased by 112%. Mechanism study shows that the conductivities along the fiber direction and in transverse to fiber direction are greatly affected by the interleaf conductivity, while the conductivity through thickness direction is little affected. The toughening mechanism of the plain or silver-plated nylon mesh is much different from that of the non-woven veils. Both of the composites interleaved with the plain or silver-plated nylon mesh have higher mode I interlaminar fracture toughness, but less improved mode II toughness. The effect of silver-plating on the mode I and mode II fracture toughness was studied.

Study on 3D multi-performance composite films of Fe3O4 decorated CNTs/graphene oxide

In this work, three-dimensional (3D) Fe3O4 decorated CNTs/graphene oxide (Fe3O4-CNTs/GO) composite film was used to enhance glass fiber reinforced polymer (GFRP) resulting in three excellent performances of interlaminar toughening, resistance to temperature changes, and electromagnetic interference (EMI) shielding were all achieved. The end notch bending (ENF) test results showed that Fe3O4-CNTs/GO film enhanced GFRP was 60.51% and 79.64% higher than the baseline (GFRP without reinforcement film) after RT or temperature cycle treatment, while the GFRP of pure CNTs film was 21.79% and 26.34% higher than the baseline, respectively. The change in interlaminar toughness of Fe3O4-CNTs/GO samples by temperature cycle was also lower than that of the other two samples (Fe3O4-CNTs/GO film: 4.15%, pure CNTs film:11.15%, baseline: 14.35%). The average EMI shielding effectiveness of Fe3O4-CNTs/GO films with a thickness of only 0.052 mm was 25 dB, about 500% higher than the 5 dB of the control group (ordinary CNTs/Fe3O4/GO films, thickness 0.047 mm). Scanning electron microscopy (SEM) further confirmed the existence of the 3D frame structure and interlamellar strengthening interface of Fe3O4-CNTs/GO films.

Low‐cost carbon black‐loaded functional films for interlaminar toughening and in‐situ delamination monitoring of carbon fiber/epoxy composites

AbstractFunctional polyethersulfone (PES)/carbon black (CB) films were fabricated and interleaved into carbon fiber/epoxy composite laminates to simultaneously improve the mode I interlaminar fracture toughness (GIC) and delamination monitoring sensitivity. It was found that when the CB content in the functional film was 7.5 wt%, the initial and propagation GIC of the laminate achieved the greatest improvements of 99.16% and 82.83%, respectively. The toughening mechanism was mainly attributed to the formation of phase separation structure, the plastic deformation of PES and the pull‐out of epoxy and CBs. Meanwhile, a delamination increment‐resistance change relationship equation was proposed to predict the delamination increment of the laminate during mode I delamination. Moreover, the effect of CB content in the film on the delamination monitoring sensitivity was investigated. The pure PES film showed the largest sensitivity coefficient k of 4.55, which was much higher than that of Control (0.14). When the CB content was in the range of 2.5–7.5 wt%, both GIC and k remained at relatively high levels, indicating that PES/CB films have promising applications in advanced multifunctional composites due to their low cost, simple manufacturing process, and large‐scale industrial manufacturability.

Enhanced interlaminar fracture toughness of woven carbon fabric/epoxy composites by interleaving fluffy CNT veils

While fiber reinforced polymer (FRP) composites are widely utilized in structural components due to their favorable mechanical properties, delamination between reinforcing plies remains a major problem, weakening the composite structure and limiting more widespread applications of FRPs. Carbon nanotubes (CNTs) carry the promise of enhancing this poor out-of-plane mechanical performance, although their integration has been challenging. In this work, macroscopic CNT veils with controlled nano-meso structure were drawn from the gas-phase using a semi-industrial process and then integrated into woven carbon fiber/epoxy matrix composites utilizing a facile and scalable approach. Interlaminar fracture toughness (ILFT) of the resulting composites was determined in Mode-I (opening mode) test. Additionally, crack propagation and interlaminar toughening mechanisms were systematically analyzed by means of optical microscope, SEM, and Raman analysis. The results showed that mode I ILFT was improved as much as 60% when interleaving as-received fluffy CNT veils and also revealed that interlaminar crossing between CNT veil/CF interfaces is of paramount importance in toughening mechanism.

WITHDRAWN: Significantly enhancing fracture toughness of epoxy composite with promising γ-FeOOH@Fe2O3 hybrid nanoparticles by magnetic field assistance

Abstract The toughening of epoxy resin (EP) and the interlaminar toughening of carbon fiber reinforced composite (CF/EP) laminates have been widely concerned. In this work, the needle-like γ-FeOOH nanoparticles were prepared by liquid phase deposition-air oxidation method, and then were calcined under different conditions to obtain γ-FeOOH and γ-Fe2O3 hybrid nanoparticles (γ-FeOOH@Fe2O3). Effect of calcination condition of γ-FeOOH@Fe2O3 and magnetic field assistance on fracture toughness (KIC) of EP was systematically investigated. Then the selected γ-FeOOH@Fe2O3 with the best toughening effect were used to improve the mode I interlaminar fracture toughness (GIC) of CF/EP laminate. The resulting γ-FeOOH@Fe2O3 have a length of around 1 ​μm, a diameter of around 100 ​nm and the Ms of 8.99–45.96 emu/g. After calcinated at 250 ​°C for 1 ​h, the γ-FeOOH@Fe2O3 containing 24 ​wt% FeOOH and 76 ​wt% Fe2O3 achieved the best toughening effect. Under a magnetic field of 0.09 ​T, the KIC of the γ-FeOOH@Fe2O3/EP composite (2.45 ​MPa ​m1/2) is 81.7% and 66.7% higher than that of neat epoxy and the composite without magnetic field induction, respectively. Furthermore, the GIC of the γ-FeOOH@Fe2O3/CF/EP composite (0.914 ​kJ/m2) is also significantly increased by 88.8% and 51.8% compared to that of CF/EP and the corresponding composite without magnetic field induction, respectively.

Effect of Ø 0.11 mm Z-pinning on the properties of composite laminates via an ultrasound guided insertion process

This paper develops an ultrasound guided Z-pinning process, which can insert finer Z-pins of Ø 0.11 mm (in comparison with traditional Ø 0.28 mm - 0.50 mm) with minimal in-plane damage and substantially improve the interlaminar performance of Z-pinned composite laminates. The experimental results show that for unidirectional fibre/epoxy laminates reinforced by 0.1 vol% of Ø 0.11 mm carbon Z-pins, the in-plane strength reduction is only 1.31% in tension and 3.46% in compression, respectively, while the Mode-I fracture toughness can be improved by 11 times in comparison with the unpinned specimens. The results can be attributed to the small damage to the microstructures of the pinned area, where long and narrow resin-rich zone with small in-plane fibre waviness and neglectable out of plane fibre crimp are represented. The interlaminar toughening mechanism of fine Z-pinned laminates is elucidated by the bridging law with the establishment the DCB testing model based on the traditional Z-pin reinforcement mechanism.

Influence of the glass transition of interlaminar particles on shear behaviour during cure of interlayer toughened thermoset composites

This work investigates the effect of the glass transition of interlaminar toughening particles on the in-plane (G12) and out-of-plane (G13) lamina shear modulus during cure for an interlayer toughened prepreg system, Toray T800SC/3900-2B. The dependence of lamina G12 and G13 on temperature and degree of cure (DoC) of the resin was determined using Dynamic Mechanical Analysis (DMA). Lamina G12 is mainly governed by resin properties, whereas lamina G13 is additionally affected by particle glass transition in two different ways: first, the particle glass transition changes interlaminar microstructural characteristics such as interlayer thickness, packing morphology of particles and interlaminar particle volume fraction depending on the cure path in the pre-gelation stage, which subsequently influences lamina G13 in the post-gelation stage; in addition, the particle glass transition reduces particle shear modulus and interlayer shear modulus in the post-gelation stage, which consequently reduces lamina G13.

Interlaminar mechanical properties of nano- and short-aramid fiber reinforced glass fiber-aluminum laminates: a comparative study

Delamination damages limit the application potential of Fiber metal laminates, hence improving the interlaminar mechanical properties has always been a research focus and challenge in this field. The toughening effect of two fillers, i.e., nano-aramid fibers (ANFs) and short-aramid fibers (ASFs), which are used at the interface of glass fiber-aluminum laminates, have been investigated. Chemical pretreatments of aluminum alloy surface were conducted to ensure the better adherence between the fiber composites and metal sheets. Results revealed that Mode-I and Mode-II fracture toughness of the laminates could be simultaneously improved when using the two fillers at the interface of glass fiber-aluminum laminates. Attributed to the better dispersion of ANFs in epoxy matrix, the toughening performance of ANFs is better than that of ASFs in this case. The mechanism of interlaminar toughening was revealed with electron microscopic observation of fracture morphology. Meanwhile, the finite element analysis based on bilinear cohesive zone model was adopted to predict the increased interlaminar tensile and shear strength.

Interlaminar toughening in carbon fiber/epoxy composites interleaved with CNT-decorated polycaprolactone nanofibers

A major problem with carbon fiber/epoxy (CF/EP) composites is their weakness to resist interlaminar delamination originating from the brittle epoxy matrix. Interleaving has been long studied as a solution. In this work, carbon nanotubes (CNTs) were uniformly decorated on electrospun polycaprolactone (PCL) nanofibers by ultra-sonication to form flexible interleaves for CF/EP laminates. Upon epoxy curing, PCL nanofibers were phase separated from epoxy, and CNTs could be uniformly distributed within the matrix. Results obtained showed that modes I and II interlaminar fracture energies of CF/EP laminates interleaved with CNT-decorated PCL nanofibers were enhanced by 68% and 44%, respectively. Synchronous improvement of 7.5% on the interlaminar shear strength of CF/EP laminates were also achieved. Moreover, the glass transition temperature of the interleaved laminates was maintained as that of epoxy because of the introduction of the CNTs.