Interlaminar Fracture Toughness Research Articles

Improving the fracture toughness, electrical conductivity and electro‐thermal performance of hybrid glass/carbon fiber reinforced epoxy composites with surface‐sulfonated polypropylene fabrics

AbstractHybrid glass/carbon fiber (GCF) reinforced polymer composites are one of advanced composites with reduced cost, desired mechanical properties and special electrification performance. In the work, surface‐modified polypropylene (PP) non‐woven fabric was used as the interleaf for toughening these composites. The effects of the surface‐sulfonated PP fabric (PP/S) interleaves on various mechanical properties of the GCF reinforced epoxy (GCF/EP) composites were investigated firstly, including the mode II interlaminar fracture toughness and flexural strength. Comparing to the blank control sample, the GIIC of the hybrid composites interleaved with the sulfonated PP fabric was increased evidently, as well as its flexural strength. These demonstrate the great interleaving effect of PP/S fabric on the GCF/EP laminated composites. Moreover, the heterogeneous electrically conductive behavior and electro‐thermal effect of GCF/EP interleaved with and without PP/S were studied. Interestingly, comparing to the control sample, the conductivity of the interleaved GCF/EP composites increased in both two in‐plane direction. Due to the hybrid character of the plain woven GCF fabric, both the temperature increase rate and the equilibrium temperature of the GCF/EP composites were much larger than that of pure carbon fabric reinforced composites under the same electrification condition.Highlights Polypropylene fabric grafted with sulfonate group (PP/S) was fabricated. The tensile property of fabric almost unchanged after the sulfonation reaction. PP/S was more effective in toughening glass/carbon fiber/epoxy composite (GCF). PP/S increased the conductivity in in‐plane directions of the hybrid composites. GCF/EP displayed superior electro heating effect to CF/EP.

Introducing a nano-scale surface morphology parameter affecting fracture properties of CNT nanocomposites

Carbon Nanotube (CNT) particles and membranes improve interlaminar fracture toughness of nanocomposites. The novelty of this study is to introduce the effect of a new nanoscale parameter - CNT network (bundle) size to interphase thickness ratio - on modes I and II interlaminar fracture toughness of CNT Polymer Nanocomposites (PNCs). This investigation includes the stochastic distribution function of CNT bundle size, interphase thickness, and modes I and II fracture toughness of PNCs based on an extensive experimental study at the nano- and macro-scales. The Atomic Force Microscopy PeakForce Quantitative Nanomechanics Mapping technique was used to collect 500 datasets for a low-weight percentage of CNT PNC and 180 datasets for a high-weight percentage of CNT PNC. Each dataset included CNT bundle size and interphase thickness at the nanoscale. Double cantilever beam and end-notched flexure experiments were conducted to collect 600 modes I and II fracture toughness data. Two-, three-, and four-parameter Weibull models were used to simulate the nano- and macro-scale material properties of CNT PNCs. Results indicate that (i) a lower CNT bundle size to interphase thickness ratio improves fracture toughness, and (ii) a four-parameter Weibull model is the most efficient model for simulated data.

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

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

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.

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

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

Influence of backing layers on the interlaminar fracture toughness energy – Mode I – of quasi-unidirectional GFRP

Abstract Quasi-unidirectional glass fiber non-crimp fabrics consist of a unidirectional main layer in the 0° direction stabilized by a backing layer in the 90° direction with a significantly lower amount of fibers. It is known that the backing layers impair the fatigue performance of respective reinforced plastics due to their orientation perpendicular to the main load direction. They act as damage initiators for the failure of the load-carrying unidirectional fibers. In the present work, the positive influence of the backing layers on the interlaminar fracture toughness energy – Mode I – G IC is demonstrated by tests in accordance with DIN EN 6033. The presence of backing fibers in the delamination plane can improve the G IC values by up to 43 %. Furthermore, results from evaluation according to DIN EN 6033 and ASTM D5528 are shown to have a good agreement, if the correction factor for large displacements recommended in the ASTM standard is not applied. If it is applied, however, there is a clear gap of up to 19 % between the two standards because the DIN simply does not provide a correction for large deflections.

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.

Characterization of the damage mechanism in CFRP composites under mode I based on comprehensive analysis of AE signals

This study utilises several methods centred on acoustic emission (AE) waveform characteristics to conduct in-depth analysis of the damage mechanism of composite materials under mode I. Combined with the peak frequency statistics and continuous wavelet transform (CWT) of AE signals, three damage modes of laminated composites during loading are identified. The cumulative energy of three damage modes separated by variational modal decomposition (VMD) is used to characterize their damage evolution process. Based on the findings, delamination and fiber–matrix debonding are proven to be the dominant damage mechanisms for mode I. The comprehensive analysis results from AE's accumulated energy, peak frequency distribution, CWT, VMD, and sentry function can accurately detect the onset time of delamination in laminates. The equivalent crack length is calculated by the load value determined at the initial time of delamination, and the strain energy release rate calculated by the load value and crack length at this time is used as the interlaminar fracture toughness value of mode I. This research method will provide a new idea for the damage tolerance analysis of mode I delamination in laminated structures.

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

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

Influence of Oxygen/Argon/Nitrogen multi-component plasma modification on interlayer toughening of UHMWPE fiber reinforced composites

The study reveals the influence of glow discharged three gas sources of Oxygen/Argon/Nitrogen plasma modification on interlayer toughening of ultra-high molecular weight polyethylene (UHMWPE) fiber reinforced composite. The failure modes under bending, tensile and interlayer fracture toughness testing were analyzed through acoustic emission (AE) technology. The results showed that Oxygen/Argon/Nitrogen multi-component plasma modified fiber introduced more hydrophilic groups such as hydrogen and hydroxyl groups, and significantly enhanced the binding of fiber and matrix. The GⅠC (Mode-I interlaminar fracture toughness) of multi-component plasma modified composites exhibited increased by 61.67 %, 40.88 % and 18.01 % compared to untreated, Oxygen and Oxygen/Argon plasma, respectively, while GⅡC (Mode-II interlaminar fracture toughness) increased by 38.57 %, 25.57 % and 23.09 %. AE cluster analysis exhibited that multi-component plasma modified composites formed an energy dissipation mechanism dominated by fiber breakage. Additionally, the oxygen, argon, and nitrogen plasma modification have a synergistic effect on interlayer toughening.

Multi-scale “core-sheath” structure of electrospun veils to enhance the interlaminar fracture toughness and in-plane properties of CF/poly(phthalazinone ether ketone) laminates

Carbon fiber reinforced polymer laminates' weak interlaminar properties limit application in high load-bearing structural components. Interleaving electrospun polymer veils or nanofillers between layers has been shown to improve laminate interlaminar fracture toughness (ILFT). In this work, we constructed a “core-sheath” reinforced veil based on a bionic principle inspired by the “tree-root system” by combining ZnO nanowires with electrospun nanofibres to achieve uniform nanocomponent distribution and directional modulation. The effect of “core” structures with different thermal properties and “sheath” structures with different dimensions on the mode I and mode II interlaminar fracture toughness were investigated. It was found that mode I and mode II ILFTs impose different requirements on the interlaminar reinforcement structure due to the different loading modes. The intact nanofiber morphology and longer ZnO nanowires are more advantageous for mode I ILFT. Mode II ILFT, however, is more dependent on the adhesion of the reinforcing veil to the matrix. Mode I and mode II ILFTs values increased by up to 75.6 % and 118 %, respectively, when enhanced with suitable reinforcement veils.

In-situ infrared annealing for laser-assisted automated fiber placement to enhance interlaminar properties without sacrificing laydown efficiency

Laser-assisted automated fiber placement (LAFP) in-situ consolidation (ISC) of thermoplastic composites suffers from poor interlaminar bonding. Repass treatment improves interlaminar bonding but sacrifices laydown efficiency. Here we propose an in-situ infrared annealing (IIA) strategy that combines infrared heat treatment with a second pressure roller immediately following the LAFP ISC process. This approach causes repeated interlaminar melting and slower cooling, reducing the porosity by 57.9% and increasing crystallinity by 89.2% compared to LAFP-manufactured carbon fiber-reinforced polyether-ether-ketone composites. Moreover, it completes the crystallization process with only a single placement rather than multiple placements by repass treatment. Interlaminar shear strength and mode I interlaminar fracture toughness are improved by 239.7% and 292.4%, respectively. These improvements are comparable to those achieved by repass treatment with a near-infrared laser. Overall, the IIA strategy offers a promising alternative to traditional post-consolidation processes while maintaining laydown efficiency.

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 investigation of the impact of functionalized MWCNT reinforcement on interlaminar strength of glass/epoxy composites: a comparative view

ABSTRACT In the present investigation, diverse functionalized states of multi-walled carbon nanotubes (MWCNT) were employed in fabricating glass/epoxy composites, with the MWCNT consistently incorporated at a 0.5 wt.% concentration. These functionalization states included pristine, hydroxyl, amine, and carboxylic conditions, systematically compared against neat glass/epoxy composites. The findings elucidated that all three functionalizations positively influenced the enhancement of interlaminar shear strength, mode-I interlaminar fracture toughness, and mode-II interlaminar fracture toughness properties. Notably, the amine-functionalized MWCNT exhibited superior performance relative to other formulations. The composite reinforced with amine MWCNT demonstrated significant improvements in ILSS, mode-I ILFT, and mode-II ILFT by 41.82%, 37.75%, and 40.93%, respectively. These enhancements are attributed to factors such as the presence of a more functional group, the quantity of individual MWCNT, and the introduction of additional damage mechanisms, including MWCNT pullout and bridging.

Fabrication of GNPs sprayed carbon fiber and its effects on the interfacial and mechanical behavior of reinforced epoxy multiscale laminated composite

AbstractThe current research compares the effects of modifying carbon fiber [via spraying graphene nanoplates (GNPs)] and/or GNPs reinforcement on the mechanical and interfacial behavior of carbon fiber epoxy composites. Using a spraying method, the epoxy matrix was reinforced, and carbon fiber was modified with 0.2 and 1 wt% of GNPs, respectively. Tensile, interlaminar, and dynamic mechanical analyses (DMA) tests were conducted on the composite to assess the role of GNPs in enhancing mechanical, interfacial, and thermomechanical properties. The obtained results indicate that the spraying technique effectively enhanced the ultimate tensile strength (UTS) and interlaminar fracture toughness (ILFT) of the composite. The UTS improved by 31%, while the ILFT significantly improved by 38%. Additionally, assessing the modulus gradient through the interface shows improved interfacial interaction between modified carbon fiber and reinforced epoxy. Scanning electron microscopy examination of the fractured surfaces indicated that the existence of nanofillers with high surface area contributed to significant improvements in adhesion between the modified fiber and reinforced epoxy matrix, leading to enhanced interfacial, mechanical, and thermomechanical properties of CFRPs for structural applications.Highlights Incorporation of surface modified carbon fibers and GNPs reinforcement in the epoxy matrix to enhance the interfacial interaction. Evaluation of the effects of GNP nanofiller on interfacial and mechanical performance of the composite. Fracture surfaces in ILSS, tensile, and mode I test were analyzed using FESEM. Utilization of nanoscale modulus mapping for the fiber‐matrix interface assessment.

Investigation of Mode I and II Interlaminar Fracture Toughness of Nylon/Glass Fiber Laminates with Various Layup Orientations

Investigation of Mode I and II Interlaminar Fracture Toughness of Nylon/Glass Fiber Laminates with Various Layup Orientations

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

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

Multifunctional composite material based on piezoelectric nanofibers and Cu-CFRP electrodes for sensing applications

To monitor possible failures of a composite, several Structural Health Monitoring (SHM) systems have been developed. However, these methods typically involve embedding commercial sensors within the laminate, potentially compromising the material's strength.In this study, a self-sensing composite laminate was fabricated by interleaving poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) piezoelectric nanofibers between Glass Fiber Reinforced Plastic (GFRP) prepreg plies. Instead of conventional metallic sheets, hybrid Copper-Carbon Fiber Reinforced Plastic (Cu-CFRP) was used as electrodes to collect piezoelectric signals. This innovative approach offers two main advantages: enhanced interlaminar fracture toughness due to nanometric piezoelectric fibers and an intrinsic connection between copper wires and carbon, eliminating the need for additional electrical cables within the laminate. The effect of stacking sequence parameters on the self-sensing laminate's electromechanical response was investigated using a Design of Experiment (DoE) based on the Box-Benken method. Additionally, a lumped electric circuit model was employed to gain analytical insights into the piezoelectric behavior of the laminates.

The effect of resin properties on toughness translation in interleaved composites

Interleaving is a well-recognized method for enhancing the interlaminar fracture toughness of composite materials. However, theoretical toughness is often not realized since cracks tend to propagate through regions of lower toughness. While most studies have focused on the effect of interleaf properties, e.g., thickness and interleaf resin properties, on composite interlaminar fracture toughness, the effect of matrix resin properties on interleaved composite toughness has been overlooked. Recently, we hypothesized that there is a relationship between toughness translation and the ratio of matrix to interleaf resin toughness. In this work, we use additive manufacturing to test this hypothesis with a range of resins in interleaved composite. Toughness is quantified via mode I delamination resistance. Our results confirm our hypothesis that the ratio of matrix to interleaf resin fracture toughness, i.e., the ratio of [Formula: see text], is directly correlated to the degree of toughness translation. More specifically, a ratio of approximately 0.5 is required for effective interleaving; below 0.5, the toughness translation is significantly below the theoretical value. This approach has important implications for the design and fabrication of tough composites via choice of resins.

Influence of Interfacial Interaction and Composition on Fracture Toughness and Impact Properties of Carbon Fiber-Reinforced Polyethersulfone.

In this study, the interlaminar fracture toughness and impact strength of polyethersulfone reinforced with continuous carbon fibers were studied. Interlaminar fracture toughness tests were performed using the double cantilever beam method. It was shown that surface modification using the thermal oxidation method of the carbon fibers can strongly increase the interlaminar fracture toughness of the obtained composites. Thus, the maximum value reached 1.72 kJ/m2, which was 40% higher than the fracture toughness of the composites reinforced with initial carbon fibers. Moreover, fractographic analysis using a scanning electron microscope allowed us to highlight the main reasons for the dependence of fracture toughness on fiber content and surface modification conditions of the carbon fibers. It was shown that the main factor that allowed for an increase in fracture toughness was the enhanced interfacial interaction between the fibers and polymer matrix. Additionally, it was found that expectedly, there was a good correlation between interlaminar fracture toughness and interlaminar shear strength results. However, a negative influence of surface modification on the impact properties of composites was found. Such behavior occurred because of higher structural stability and lower exposure to delamination in multiple layers of the composites reinforced with the modified carbon fibers. It was found that impact energy reached ~150 kJ/m2 for the polyethersulfone-based composites reinforced with initial fibers, while the composites reinforced with modified carbon fibers showed impact energy values of only ~80 kJ/m2. Nevertheless, surface modification of carbon fibers using the thermal oxidation method can be an effective method for improving the performance properties of polyethersulfone-based composite materials.