Interlaminar Toughening Research Articles

Effective interlaminar toughening of carbon fiber/epoxy composite laminates by extremely low loadings of monolayer graphene oxide

AbstractAn interlaminar ultrafine spraying method was proposed for monolayer graphene oxide (GO) modified CFRP nanocomposite laminates. The well‐dispersed monolayer GO nano‐solutions were prepared by multi‐level dispersion and then sprayed on carbon fiber/epoxy prepreg by the ultrafine atomizing technique. A series of GO/CFRP laminate specimens with different GO loadings were fabricated for our fracture toughness tests and SEM characterization. The test results indicated that the Mode I fracture toughness () was enhanced by 140% with quite a low fraction of monolayer GO nanosheets. The distinct toughening effect at such a low level of nano‐contents was attributed to the sufficient quantity of monolayer GO nanosheets and also a uniform distribution, which was found to be more important than the volume/weight fraction as the principal structural parameter. Thus, the proposed interlaminar toughening approach owns the virtues of good effectiveness and especially low cost owing to the largely reduced weight percentage of nanographene, showing a promising potential of industrial scale‐up.Highlights Proposed an interlaminar ultrafine spraying method for monolayer GO nanosheets. Found that the quantity of GO nanosheets is vital in polymer toughening. Realized significant toughening with extremely low loadings of monolayer GO. Provided a way with high toughening effect and low cost for industrial scale‐up.

Investigation on anisotropic toughness and cracking behavior of woven lay-up Fiber Metal Laminates with two-level customized interfaces

Interlaminar toughening for metal-polymer interfaces in woven lay-up Fiber Metal Laminates (w-FMLs) is a critical concern, demanding isotropic mechanics and higher energy dissipation. Customizing interfaces with interlocking structures like dimples or grooves has proven effective in meeting these requirements, while the resulting two-level features introduce greater complexity in toughening effects, and interfacial mechanics are influenced by factors such as fiber orientation and resin distribution. To understand the anisotropy effect and toughening mechanics induced by different two-level features, three types of two-level customized interfaces of w-FMLs are examined. Edge shear tests are utilized to evaluate interfacial mechanics and toughness, with a focus on tracing crack propagation behaviors. Through numerical simulation and characterization, the toughening effect and its mechanisms are unveiled, revealing that differences in toughness pinning effect and local plastic deformation of the customized interfaces are the primary determinants of interlaminar toughness. Additionally, a coupling effect between the woven core and interfaces is observed, showing that the superposition of the tougher orientation of the woven core on the strong toughness pinning orientation of the interfaces is a crucial factor in determining excellent overall interlaminar toughness.

Study of synergistic and competitive mechanisms on toughening CFRP composites with intrinsic and extrinsic multiscale interleaves

Delamination damage is a common failure mode of carbon fiber reinforced polymer (CFRP) laminates, which severely limits their application. This paper proposes a novel interlaminar toughening structure based on intrinsic and extrinsic multiscale toughening mechanisms using nanoscale multi-walled carbon nanotubes (MWCNTs) and microscale core-shell rubber (CSR). The resin sample M-0.5/C-8, containing 0.5 wt% MWCNTs and 8 wt% CSR, exhibited significant improvements in fracture toughness, with increases of 183.3 % in the mode-I critical stress intensity factor (KIC, Resin) and 412.0 % in the critical energy release rate of resin (GIC, Resin). However, its flexural strength and modulus decreased by 3.1 % and 3.3 %, respectively. Introducing MWCNTs and CSR into the interlayers of the CFRP, the interleaved toughened CFRP laminate exhibited a 123.3 % improvement in the mode-I critical energy release rate (GIC, Comp) and a 79.6 % increase in the mode-II critical energy release rate (GIIC, Comp) during the crack propagation stage, without sacrificing flexural performance. This remarkable enhancement in interlaminar fracture toughness results from the simultaneous triggering of nanoscale extrinsic toughening and microscale intrinsic toughening during crack growth. The flexural properties and fracture toughness of Epoxy/MWCNTs/CSR composites were also investigated to understand the properties transformation from resin matrix to CFRP. Morphological characterization and numerical analysis were used to analyze the synergies and constraints between two energy dissipation behaviors.

Inspired by nacre: α-ZrP three-dimensional network structures for interlaminar toughening of laminates

This work enhancing the mechanical properties of composite laminates by using a three-dimensional network structure that mimics the nacre-like structure. The optimal interlaminar structure was developed by utilizing α-ZrP with ultrasonic-assisted exfoliation for 6 min. The mode-I critical energy release rate (GIC) and mode-II critical energy release rate (GIIC) of the toughened composite are 104.44 % and 74.20 % higher than those of the unmodified sample. Observation of the region around the crack by SEM and OM reveals that the three-dimensional network structure across the fiber fabric undergoes energy dissipation behaviors, such as fracture, slip and pull-out when cracks penetrate thin walls. In addition, the effect of the thin-walled structure of the α-ZrP aerogel on interlaminar crack propagation is discussed, taking into account the inhomogeneous and multi-surface of the composite interfaces, and the toughening mechanism is investigated.

Inter-layer failure and toughening mechanisms of carbon/aramid hybrid fiber composites interleaved with micro/nano pulps under low-velocity impact load

Hybrid fiber-reinforced polymer (HFRP) composites are widely used in aerospace structures because of their excellent overall performance. However, it is still challenging to address the issue of high sensitivity to delamination between dissimilar material interlayers in HFRP composites. This study combines experimental and numerical simulation to analyze the low-velocity impact behavior and interlaminar damage of two types of HFRP. Based on a micromechanical model, an equivalent aramid pulp (eAP) toughening laminate model is developed. The impact behavior of the HFRP toughen by eAP only in dissimilar material interlayers is simulated based on the model. The effect of eAP areal density on the impact behavior and evolution of interlaminar damage is analyzed. Results show that the maximum force and impact stiffness of eAP-toughened HFRP increase initially with the increase in eAP areal density, and then decrease slowly. The area and extent of damage of dissimilar material interlayers in eAP toughened laminates is significantly reduced. Finally, the interlaminar toughening and failure mechanisms by eAP-toughening only in dissimilar material interlayers of the HFRP composites are systematically revealed from fiber bridging and damage transfer perspectives.

Towards developing advanced CFRP with simultaneously enhanced fracture toughness and in‐plane properties via interleaving CNT/PEI hybrid veils

AbstractCarbon nanotube (CNT)‐polyetherimide (PEI) hybrid nanofibrous veils were prepared through the electrospinning process to function as an interlayer in carbon fiber reinforced polymer composites (CFRP), and the effects of the addition of the hybrid fiber veils on the interlaminar fracture properties and flexural properties of CFRP were investigated. The results showed that interleaving CNT/PEI hybrid veils can effectively avoid long‐troubled trade‐off between mode‐I interlaminar fracture toughness and in‐plane properties when using thermoplastic toughening materials. Specifically, the GIc value and flexural strength of the laminates were increased by as much as 86.2% and 14.3%, respectively when CNT content was reached up to 0.7 wt% in CNT/PEI hybrid veil. This method adopts ultra‐low PEI and CNT additions, and the limited quantity of these additives demonstrates a notably high interlaminar toughening efficiency while concurrently enhancing the strength, rendering it a promising strategy for both toughening and strengthening.Highlights PEI is used as a carrier to transport CNT to the CFRP interlaminar layers. PEI‐CNT hybrid fiber veils improve GIC and flexural strength of CFRP. PEI‐CNT hybrid fiber veils impede crack expansion through various mechanisms. This method adopts ultra‐low PEI and CNT additions.

Interlaminar reinforced carbon fiber/epoxy composites by electrospun ultrafine hybrid fibers

The demand for high-strength composites in the aerospace industry is increasing; however, their low-impact resistance poses a danger to aircraft. Consequently, significant attention has been devoted to researching the interlaminar toughening of carbon fiber-epoxy composite laminates, focusing on electrospun fibers due to their high porosity and specific surface area. Our previous work explored the potential of poly(aryl ether nitrile) as an interlaminar toughening option. Nonetheless, these materials exhibited a decline in flexural properties. To address this concern, we developed poly(arylene ether nitrile)-poly(ε-caprolactone) ultrafine hybrid fiber membranes via electrospinning to enhance the interlaminar performance of composite laminates. The interlaminar fracture toughness demonstrated a remarkable improvement of 132.5 %. Additionally, the flexural strength in-creased by at least 12.3 %, while the flexural modulus experienced a minimum in-crease of 17.9 %. The interlaminar shear strength improved by approximately 27%–28 %, and the impact strength increased by 99.2 %. This study demonstrates the significant potential of electrospun hybrid fiber membranes in improving the interlaminar properties of carbon fiber-epoxy composite laminates, con-tributing to developing safer and more durable materials for the aerospace industry.

Investigations on toughening and self‐healing performance of plain woven composites with mendable polymer stitches

AbstractThis paper investigates toughening and self‐healing properties of plain woven composite structure with poly[ethylene‐co‐methacrylic acid] (EMAA) stitches. Firstly, the cantilever beam specimens with different stitched spacing were performed to determine inter‐laminar fracture toughness. Subsequently, the finite element models of EMAA stitched cantilever beam was established to predict the toughening effects based on nonlinear spring and cohesive zone model (CZM). The force‐displacement responds of experimental and simulation results show excellent correlation. Finally, the toughening and healing mechanism of the stitched plain woven structure were explored. Compared with the original specimen, the EMAA stitched structure shows an excellent toughening ability. After heating, the damaged structure was healed through pressure deliver mechanism to restore the interlaminar fracture toughness of the composite.Highlights Thermoplastic EMAA filaments with self‐healing properties were sewn into the woven composite. The EMAA stitched woven composite structure exhibits excellent interlaminar toughening and healing properties. Finite element model combined non‐linear spring and CZM was established to predict the toughening effects. The toughening and self‐healing mechanisms of EMAA stitched plain woven structure were revealed in detail.

Interlaminar performance of three nanomaterials film enhanced glass fiber reinforced polymer

The carbon nanotube (CNT) and graphene oxide are widely applied in Glass Fiber Reinforced Polymer (GFRP) to enhance interlaminar performance, even though they have an extremely expensive price. Herein, we provide a novel interlaminar toughening material: carbon black, which has a cheap price and good interlaminar properties. The relationship between different amounts of three nanomaterials and toughening efficiency is obtained through experiments (end notch bending (ENF) test) and the curves between different amounts and prices are revealed by the investigation. Moreover, a mathematical model of interlaminar performance, material consumption, and price is established, which can provide researchers with accurate, efficient, and inexpensive predictions in different application environments. On the other hand, a simple and efficient spraying method for making reinforcement layers is adopted in this research and we analyzed the microstructure strengthening mechanism of three nanomaterials by scanning electron microscopy (SEM).

Maximizing Interlaminar Fracture Toughness in Bidirectional GFRP through Controlled CNT Heterogeneous Toughening.

"Interleaving" is widely used for interlaminar toughening of fiber-reinforced composites, and the structure of interleaving is one of the important factors affecting the toughening efficiency of laminates. Several experiments have demonstrated that compared to continuous and dense structures, toughening layers with structural heterogeneity can trigger multiple toughening mechanisms and have better toughening effects. On this basis, this work further investigates the application of heterogeneous toughening phases in interlaminar toughening of bidirectional GFRP. CNT was selected to construct toughening phases, which was introduced into the interlaminar of composites through efficient spraying methods. By controlling the amount of CNT, various structures of CNT toughening layers were obtained. The fracture toughness of modified laminates was tested, and their toughening mechanism was analyzed based on fracture surface observation. The results indicate that the optimal CNT usage (0.5 gsm) can increase the initial and extended values of interlayer fracture toughness by 136.0% and 82.0%, respectively. The solvent acetone sprayed with CNT can dissolve and re-precipitate a portion of the sizing agent on the surface of the fibers, which improves the bonding of the fibers to the resin. More importantly, larger discrete particles are formed between the layers, guiding the cracks to deflect in the orientation of the toughened layer. This generates additional energy dissipation and ultimately presents an optimal toughening effect.

An integrated nanofiller spray and nanosecond pulse electrically-assisted method for synergistically interlaminar toughening and in-situ damage monitoring of CFRP composites

Nanomaterials have already been explored their potential for interlaminar toughening and crack-monitoring capabilities within the field of composite materials. However, efficient enhancement of interlaminar fracture toughness remains an unresolved challenge. Herein, a novel integrated filler spray and nanosecond pulsed electric field (nsPEF)-assisted curing technique is proposed for the fabrication of high-performance composite material system for the first time. The spraying technique proves to be an effective approach for depositing fillers, including carbon nanotubes (CNTs), graphene nanoplatelets (GNPs), and carbon nanofibers (CNFs), onto carbon fiber prepreg. Meanwhile, the nsPEF-assisted technique is utilized to enhance the wettability of resin to fiber, thus reducing void defects. The efficacy of this integrated technique is evidenced by the particularly low content of fillers (0.047 wt%). The experimental results show a substantial reduction of porosity in the composite by 49.36% (from 1.56% to 0.79%). Meanwhile, the mode I initiation and steady-state toughness of samples with CNTs and GNPs are remarkably improved by up to 47% and 82%, respectively, compared to the baseline. This represents the most significant improvement reported to date for non-functionalized CNTs and GNPs at such low concentrations. Moreover, the relative electrical resistance variation (ΔR/R0%) of CNT/GNP sensors used for in-situ damage sensing increased by ∼ 14%. This work pioneers the investigation of the synergistic effects of two promising techniques for preparing composite materials, offering advantages in terms of simplicity, multifunctionality, and the potential for industrial scalability.

Exploring the interlaminar toughening potential of carbon nanoparticles: Structural and size effects

This work explored the influence of the intrinsic structure and size of different carbon nanoparticles on the morphologies of their corresponding aggregated structures, as well as their interlaminar toughening effect in carbon fiber reinforced polymer composites. CNTs and Grs with different sizes were selected as building blocks of toughening layer, which were firstly well-dispersed into acetone and then spray-coated onto the surface of carbon fabrics. The results indicate that one-dimensional filamentous CNTs are prone to forming loose and porous layers, exhibiting better interlaminar toughening effects. On the contrary, the layers formed by two-dimensional sheet-like graphene are relatively dense at microscale, which weakens the interlaminar fracture toughness of the laminated plate. In addition, CNTs with short length have a larger BET surface area at nanoscale and form a porous layer at microscale, showing best interlaminar toughening efficiency.

Experimental and numerical investigation into interlaminar toughening effect of chopped fiber-interleaved flax fiber reinforced composites

Experimental and numerical investigation into interlaminar toughening effect of chopped fiber-interleaved flax fiber reinforced composites

Electrospun poly (arylene ether nitrile) fibers for interlaminar toughening of carbon fiber epoxy composites

The limited shear strength and interlaminar toughness of carbon fiber epoxy composite laminates restrict their further promotion. Traditionally, interlaminar toughening has been achieved using polymers such as polyamide 66, polyamide 69, and poly (ε-caprolactone), but they are limited by physical or chemical performance. As a potential option, poly (arylene ether nitrile) (PEN) could be considered for interlaminar toughening due to its epoxy compatibility, thermal resistance, and excellent mechanical properties. In this work, we prepared PEN ultra-fine fibrous membranes with different areal densities by electrospinning. The best overall performance was obtained by adding 8 g/m2 of PEN fibrous membranes between the layers. The results indicate that PEN fibrous membranes increased interlayer shear performance by 12%, mode I fracture toughness by 69%, and pendulum impact performance by 6%. This work highlights the potential of electrospun thermoplastic PEN fibers for improving the interlaminar toughening of composite laminates.

On the interlaminar fracture behavior of carbon/epoxy laminates interleaved with fiber‐hybrid non‐woven veils

AbstractThis study investigates the effect of fiber‐hybrid non‐woven veils on the interlaminar fracture of out‐of‐autoclave carbon/epoxy laminates. The Mode‐I and Mode‐II interlaminar toughening performance and R‐curves behavior of out‐of‐autoclave composite laminates with carbon and thermoplastic fiber‐hybrid veils are investigated. Pure carbon veil (i.e., high‐stiffness fibers), pure polyphenylene sulfide veil (i.e., low‐stiffness fibers), and three variants of fiber‐hybrid carbon/polyphenylene sulfide veil with different fiber contents (i.e., combination of low‐ and high‐ stiffness fibers) are tested and compared for Mode‐I and Mode‐II interlaminar toughening performance. All the pure and fiber‐hybrid non‐woven veils used in this study have a fixed areal density (~20 g/m2). Resin infusion combined with out‐of‐autoclave curing is utilized to manufacture the laminates. The results show that the hybridization of carbon fibers with thermoplastic fibers can address the observed shortcomings of the pure carbon veils (i.e., unstable crack growth) and of the pure thermoplastic veils (i.e., falling R‐curve) in toughening interlaminar regions. Considering both the Mode‐I and Mode‐II interlaminar fracture behavior, it is shown that fiber‐hybrid non‐woven veil toughening offers rising R‐curves, constrained crack paths (inside the veil), and enhanced interlaminar fracture energies, which are desirable for structural applications.Highlights Pure carbon/thermoplastic veils have shortcomings in interlaminar toughening. Hyrid veils constrain crack path and offer rising R‐curve. The weight ratio of hybrid veils affects crack paths and R‐curves.

Multi-scale synergistic toughening of glass fiber/epoxy laminates with carbon nanotube-modified carbon fiber felt

Hierarchical toughening is commonly found in natural materials such as teeth, bone and seashells, which have evolved to withstand mechanical stresses. The incorporation of this mechanism into the design of synthetic materials, such as composites, has drawn much attention in recent decades. In this work, a carbon fiber felt (CFF) made of short carbon fibers was spray-coated with carbon nanotubes (CNT) to create a hierarchical structure that can be directly interleaved into glass fiber reinforced polymer (GFRP) composites for interlaminar toughening purpose. The results showed that the highest enhancement of GIC,ini and GIC,prop values reach as much as 211 % and 174 %, respectively, which far outweighs the state of the art. The fracture surfaces as well as crack propagation behaviors were comparatively investigated to get deep insight into the toughening mechanisms. It was found that multiscale fiber bridgings and interlaminar crossings are the key to achieving high interlaminar fracture toughness of laminate composites.

Improving delamination resistance of 3D printed continuous fiber-reinforced thermoset composites by multi-scale synergistic toughening of mono-component polyetherketone-cardo

Continuous fiber-reinforced thermoset composites (CFRTCs) 3D printing offers a promising solution to fabricate lightweight, high-strength sophisticated composite structures. However, the delamination resistance of 3D printed CFRTCs is decreased by the weak fiber-matrix and interlayer adhesion caused by the process principle. To increase the interlayer toughness of 3D printed CFRTCs, this study developed a printing matrix toughened by various polyetherketone-cardo (PEK-C) forms by modulating its dissolution state. The results showed that the interlaminar toughening effects of the particle dispersion and dissolved dual form of PEK-C were superior to the insoluble particles or the dissolved PEK-C. As a result, the mode I and mode II interlaminar fracture toughness increased by 112.38 % and 189.01 %, respectively. And the synergistic effect of dual-form PEK-C was determined. Fractographic investigation revealed that the dissolved PEK-C experienced the reaction-induced phase separation initiating a nanoscale thermoplastic phase and developing a multi-scale PEK-C toughening system with the microscale PEK-C particles. Moreover, morphological observation of the particle and PEK-C phases demonstrate multi-scale synergistic toughening mechanisms of mono-component PEK-C. This study presents an innovative technique for interlayer toughening applicable to the CFRTCs 3D printing, illustrates the toughening principle, and shows its promise as a general strategy.

Optimizing interlaminar toughening of carbon-based filler/polymer nanocomposites by machine learning

Currently, most designs for interlayer toughening of carbon-based filler/polymer nanocomposites are highly dependent on experimental iterative trial and error, and there is no rational design framework. This work uses machine learning to build a fast and accurate predictive model and assess the extent to which key features affect performance, giving researchers ideas for designing new materials and greatly improving efficiency. A training database is built by first collecting the features of the domain that affect the interlaminar performance. A stacking model fusion of the three machine learning models was then performed to construct a highly accurate fast prediction model. Besides, the importance of key features is evaluated during model training using the Random Forest Algorithm (RFA). Finally, by predicting the performance of materials and analyzing the importance of characteristics to guide material preparation, the development cycle is shortened and costs are reduced.

Free-Standing CNT Film for Interlaminar Toughening: Insight into Infiltration and Thickness Effects.

Carbon fiber reinforced polymer composites have the advantages of being lightweight, having high strength and designability, and having been extensively used. However, the interlaminar toughness and delamination resistance of these composites are relatively poor due to their laminated structure and intrinsic brittleness of resin matrix. In this paper, commercialized free-standing carbon nanotube (CNT) films, drawn from CNT forests, were used to toughen the interlaminar interfaces of the composites. The effects of resin infiltration state and thickness of CNT films on the interlaminar toughening effect were systematically investigated. The results show that the pre-infiltration treatment of CNT films with acetone diluted epoxy resin solution can effectively improve the degree of resin infiltration. Compared with the samples containing untreated CNT film, the Mode I and Mode II interlaminar fracture toughness of the treated samples were significantly improved. The GIC reached a maximum of 1412.42 J/m2 at a CNT film thickness of 5 µm, which was about 61.38% higher than that of the baseline. At a CNT film thickness of 15 µm, the GIIC reached a maximum value of 983.73 J/m2, approximately 67.58% higher than that of the baseline. The corresponding toughening mechanisms were also systematically analyzed.

Study on Mechanical Properties of Carbon Nanotube Reinforced Composites.

In this study, carbon fiber composite laminates were modified by carbon nanotube films. In-plane and out-of-plane compression tests were carried out in a wide strain rate range (10-3-103/s). Results display that the out-of-plane compressive properties are improved by CNT interlaminar toughening because CNT can hinder the propagation of interlayer cracks; however, the dynamic in-plane compression performance is decreased due to the lack of resin in CNT film that leads to delamination inside of CNT film in advance. To optimize the material preparation process, two methods were used to prepare the mode I fracture test: (a) curing the prepreg by autoclave process; and (b) curing of resin preform by vacuum resin-transmitted molding (VARTM). Results showed that CNT prolonged the crack propagation path and improved the interlaminar fracture properties when the preform was infiltrated with resin and cured by VARTM. In addition, it was found that the interlaminar thickness was almost linear with the number of CNT layers.