Interlaminar Strength Research Articles

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.

An interlaminar fatigue damage model based on property degradation of carbon fiber-reinforced polymer composites

Interface delamination is a significant damage mechanism in fiber-reinforced polymer (FRP) laminated composites caused by interlaminar normal and shear stresses. This is due to the low interlaminar properties including interface stiffness and strength that continuously degrade to cause fatigue crack nucleation and growth. In this respect, an interlaminar fatigue damage model is proposed to accurately address the reliability aspect of the FRP composite materials and structures. The model considers a damage process that consists of two stages: (1) fatigue damage evolution due to interlaminar properties degradation to the onset of crack nucleation. A cyclic cohesive zone model with a bilinear softening behavior is introduced along with the fatigue life model to capture the mean stress effect of the interface. This is followed by (2) the crack nucleation process leading to the physical separation/debonding of the interface material point, as governed by the cycle-dependent fracture energy dissipation. The mechanical prediction of the interlaminar fatigue damage process is illustrated through finite element simulation of a mixed-mode flexural test of a carbon fiber-reinforced polymer (CFRP) composite beam subjected to fatigue loading. The results indicate that the interlaminar normal and shear crack opening displacement increases exponentially with the larger number of load cycles. In addition, the high-stress gradient is limited to the interface crack front zone with the normal and shear stress peaks at 69.2 and 16.5 % of the respective interlaminar strength. Moreover, controlled interlaminar crack growth is initially foreseen at 7.0 × 10-6 mm/cycle, followed by an abrupt crack “jump” at 287,000 load cycles.

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.

Influence of clamping methods on the ballistic performance of 3D shallow straight-joint woven fabrics

The interlaminar strength and designability of three-dimensional shallow straight-joint woven fabrics make them a promising material for personal protective armor. This study investigated the influence of clamping methods on the ballistic mechanism of three-dimensional shallow straight-joint woven fabrics. The study used a mesoscopic yarn-level full-size model developed based on the finite element method to analyze the energy absorption patterns, back face deformation, and damage morphology of the three-dimensional shallow straight-joint woven fabrics under different clamping methods. The results show that the clamping method has a significant influence on the ballistic performance of the fabric. With the weft-sides clamped, its ballistic limit velocity and failure morphology are similar to four-sides clamping, with more negligible energy absorption and failure area. Warp-sides clamped is second, while the corner held undergoes the maximum out-of-plane displacement and energy absorption, exhibiting an utterly different failure mechanism. This study shows that the clamping method of a fabric affects its ballistic performance through interaction with the fabric’s internal structure.

Enhancing carbon fiber reinforced aluminum laminates with cellulose paper interlayers: experimental characterization of tensile, flexural, and interlaminar fracture toughness

ABSTRACT The mechanical properties of fiber metal laminates (FML) are influenced by several various factors. Interface adhesion plays a particularly crucial role in interlaminar strength. Enhancing the interlaminar strength of carbon fiber reinforced aluminum laminate (CARALL) composites present a persistent challenge due to inherent weaknesses between metal and composite elements. Therefore, this study focuses on improving the interlaminar performance of CARALL composites by introducing cellulose paper interlayer at the metal/composite interface. The cellulose paper interlayer offers the advantage of being cost-effective and sustainable. Cellulose paper-interleaved CARALL composites were fabricated by vacuum bagging method and exhibited noteworthy improvements in mechanical properties. Comparative analysis with pristine samples revealed substantial enhancements, including a 15% increase in tensile strength, a remarkable 42% improvement in flexural strength, and a significant enhancement in mode-I fracture toughness by 65%. Furthermore, the cellulose paper interleaving played a crucial role in stabilizing fracture formation at the fiber-matrix interface, with mode II fracture toughness witnessing a 3% increase. Visual examination revealed the underlying toughening processes occurring in the interfacial area. This innovative approach of interleaving laminated composites with cellulose paper emerges as a sustainable and effective strategy, demonstrating the potential to fortify and toughen the interlaminar zones of CARALL composites.

Hygrothermal progressive damage in open-hole compression of composite laminates with aligned carbon nanotube interlaminar reinforcement studied by X-ray micro-computed tomography

Hierarchical nanoengineered composite laminates were created with through-thickness-aligned carbon nanotube (A-CNT) reinforcement of unidirectional carbon (micro) fiber/epoxy laminae in their failure-prone interlaminar regions, forming a multiscale “nanostitched” architecture. Motivated by previous mechanical enhancements for similar nanostitched laminates in ambient environments, hygrothermal effects on the 3D strengthening and toughening mechanisms of nanostitched laminates are investigated here for the first time, focusing on standard open-hole compression (OHC) testing of quasi-isotropic laminates. We report that the hygrothermal conditions of −55 °C/DRY and 100 °C/DRY correspond to ultimate strength increases of 2.6% and 5.9%, respectively, for nanostitched laminates over the baseline; and, room temperature/50% relative humidity (RT/50% RH) and 100 °C/wet conditions exhibit no change in ultimate strength, suggesting correlation of positive nanostitch effects with increased polymer brittleness. Subsequently, damage progression in step-wise interrupted OHC loading for selected hygrothermal conditions (RT/50% RH and 100 °C/DRY) was performed in coordination with high-resolution ex situ X-ray micro-computed tomography for various load steps up to 98% of failure load. No clear differences due to nanostitch in the damage progression are found up to 98% of ultimate strength, suggesting that the reinforcement effect is associated with interlaminar strength at ultimate load, and not pre-ultimate damage. This study contributes first insights from high-resolution experimental mapping of nanostitched composite progressive damage subjected to hygrothermal conditions that may guide and inform mechanical enhancement approaches and improved progressive damage models. Future work is suggested to expand the environmental conditions considered, as well as visualize progressive damage nearer to failure to identify nanostitch effects on complex multimodal failure.

The role of printing parameters on the short beam strength of 3D-printed continuous carbon fibre reinforced epoxy-PETG composites

It is well known that printing parameters strongly affect the mechanical performance of 3D printed parts. This work explores the role of i) extruder temperature, ii) print speed, and iii) layer height on the interlaminar strength of 3D-printed continuous fibre-reinforced composites. The carbon fibre-reinforced thermoset filament is printed concomitantly with polyethylene terephthalate glycol (PETG) thermoplastic filament in a single nozzle, characterising a continuous fibre co-extrusion (CFC) process. There is a significant variation in the short beam strength for composites printed with different parameters. The load–displacement curves have a similar pattern, with clear load peaks followed by a plastic zone. Optical micrographs and computed tomography (CT) scans reveal that the microstructure is dependent on the printing parameters. Image analysis elucidates the various mechanisms of void formation. Following the application of a three-way ANOVA and statistical tests to quantify the effects and interactions among variables, the analysis concludes that the extruder temperature has the highest influence, followed by print speed and layer height. When considering all possible interactions between the factors, the interaction between print speed and layer height is the most impactful.

Impact of strain rate on mechanical properties of polylatic acid fabricated by fusion deposition modeling

AbstractThe primary objective of this research is to investigate the impact of strain rate on the mechanical properties of polylactic acid (PLA) fabricated through fused deposition modeling. PLA material was fabricated in a grid pattern with 75% infill density, a 0.16 mm layer height, and a ± 45° printing direction. This study specifically investigated tensile strength, flexural strength, and interlaminar strength with respect to varying crosshead speeds (1, 2, 3, 4, and 5 mm/min). The highest tensile strength noted was 40.13 MPa at a cross—head speed of 5 mm/min. Maximum elongation of 7.1% was noted on cross head velocity of 1 mm/min, and the maximum interlaminar strength reached 76.04 MPa at 4 mm/min. At a crosshead speed of 4 mm/min, the tensile strength measured was 38.36 MPa, which is a value close to the maximum result achieved in the tensile test. Interlaminar strength was notably higher at a crosshead speed of 4 mm/min, indicating a strong integrity of the printed layers. The values for elongation percentage and flexural strength were also moderate at a crosshead speed of 4 mm/min, which was 4.9% and 55 MPa, respectively. Therefore, crosshead speed of 4 mm/min appears to be an optimum factor for testing 3D printed PLA parts. This research sheds light on understanding the critical influence of strain rate in the mechanical performance of 3D printed PLA.

Mechanical Properties and Analysis of Two-body Abrasive Wear Behaviour of Graphene Modified Carbon/Epoxy Composites Using Taguchi’s Technique

The present work emphasizes the effect of graphene nanoplatelets (G) filler loading on mechanical and abrasive wear behavior of carbon fibre reinforced epoxy (C/E) composites. Graphene nanoplatelets were mixed with epoxy framework using a temperature-controlled magnetic stirrer and then ultrasonically treated. The parameters considered for the abrasive wear study are the applied load in N (5, 10 and 15), abrading distance in m (75, 150, and 225) and weight percentage of reinforcement (0, 1, and 1.5). The incorporation of 1 wt. % G into C/E composites increases hardness by 14 % and interlaminar laminar strength by 19 % when compared to C/E composites. According to the Taguchi design of tests, a filler loading of 1 wt. % G, an abrading distance of 225 m, and an applied load of 15 N are ideal. Analysis of variance (ANOVA) was done to establish the dominant parameter, and the filler loading with abrading distance was shown to be significant. With 36.4 %, the filler loading had the biggest influence on the composite specific wear rate. The combination of filler loading with 1 wt. %, load of 15 N, and abrading distance of 225 m yields the lowest specific wear rate. The involved wear mechanisms during the abrasive wear process have also been explained with scanning electron micrographs.

Interlaminar Adhesion Study of Fused Filament Fabrication (FFF)-Printed Soft and Rigid Bilayer Structure with Composites

Multi-material Additive manufacturing (AM) has opened new opportunities for the creation of multifunctional structures that enables value-added structural product designs. Among the multi-material AM techniques, multi-nozzle fused filament fabrication which is a type of material extrusion technique is found to be the more popular choice for multi-material polymer fabrication. One major challenge of multi-material additive manufacturing of polymers is the poor mechanical strength at the interface of the dissimilar materials such as polylactic acid (PLA) and thermoplastic polyurethane (TPU) due to the lack of chemical affinity. Therefore, understanding the mechanical strength at the interface of these dissimilar materials becomes an important topic as it allows product designers to do necessary tweak to the design to compensate for the weaker link in the structural design. In this work, we investigated the tensile strength and the shear strength of different combinations of PLA and TPU and their respective nanocomposites, as well as the fatigue analysis of the bi-layer structures made of these dissimilar materials in a 3-point bending test configuration. Generally, when functional fillers or particles are added to the polymer in composites, they tend to adversely affect the interlaminar adhesion property and fatigue life of the soft-rigid bilayer structure. It was found, that the interlaminar tensile strength and the interlaminar shear strength can reduce as high as 44% and 78%, respectively, compared to the baseline samples with no fillers.

Prediction and adjustment of elastic properties in three‐dimensional orthogonal woven composites through dual‐scale modeling

AbstractThree‐dimensional (3D) orthogonal woven composites can overcome the drawback of weak interlaminar strengths in traditional laminated composites and exhibit excellent resistance to out‐of‐plane impact, however, the significant difference in properties in the in‐plane orthogonal direction will limit the potential range of application thereof. Predicting elastic properties and exploring methods of adjustment are therefore of paramount importance. Using computed tomography (CT) scanning to obtain detailed parameter models, a corresponding dual‐scale model of microscopic representative volume element (RVE) and mesoscopic RVE was established, and the accuracy of the model was experimentally validated, which can predict the elastic properties of materials. In addition, the influences of the yarn fiber volume fraction and weaving distance on elastic properties were evaluated using the elastic modulus visualization method, and a formula integrating these two factors is provided for adjustment of the in‐plane orthogonal Young's modulus.Highlights Experimentally validated microscale and mesoscale RVE models were established. The influences of factors on elastic properties were visualized. The relationship between Young's modulus and influencing factors was derived. A reliable strategy for adjusting the in‐plane Young's modulus was proposed.

Comparison of 2D and 3D measurement methods for evaluating laser structured aluminum surfaces using fractal dimension

The interlaminar strength of mechanically interlocked polymer-metal hybrids is strongly determined by the surface geometry at the interface. To improve the interlaminar strength, laser beam machining is among the most applied technologies for manufacturing a predefined and complex surface structure. Previously, the fractal dimension of interfaces in polymer-metal-hybrids has been introduced as scale-independent measure. It has been correlated quantitatively with the interlaminar strength using values derived from cross-sectional images. Therefore, the quality of the prediction directly depends on the accuracy of the measured surface data. However, a holistic acquisition of surface structure for the determination of the fractal dimension including 3D methods has not been investigated so far. In this study, laser machining was used to generate five different pin-structures on samples of the aluminum alloy EN AW-6082. Surface characterization was realized by light microscopy (2D), laser scanning microscopy (3D) and X-ray microscopy (3D). Subsequently, the fractal dimension was determined using a box-counting algorithm as well as slit-island method. The results show that all methods regarded are appropriate to determine the fractal dimension, but differ in the specific results. The functional characteristics of each method are described in detail and potentials for applications are outlined.

Surface microstructuring by Ultrasonic Vibration Assisted Deformational Machining (UVADM) – Simulation and experimental results

Polymer-metal hybrids are a lightweight solution for many industrial applications, especially in the automotive or aircraft sector. However, due to their poor chemical bonding the manufacturing of such compounds requires the use of adhesive layers or the conditioning of the metallic surface to achieve form closure and high joint strength. Ultrasonic Vibration Assisted Deformational Machining (UVADM) is a novel process that generates functional microstructures with high production rates, aiming to form defined burr structures with undercuts, that have a significant impact on the interlaminar strength of polymer-metal hybrids (PMH). A 3D FEM simulation using Abaqus/Explicit software was applied to design and analyze process conditions including tool geometry. Material deformation was described by the Johnson-Cook model. Simulation results were validated experimentally using two different MCD tools. For the UVADM experiments, specimens of the aluminum alloy EN AW-6082 were used. In the tests, a constant ultrasonic frequency (20.42 kHz), machining speed (100 m/min and 200 m/min), and peak-to-peak amplitude (7 µm) were used. Geometrical surface properties were analyzed by optical methods and SEM. The study demonstrates a conformity between simulated and experimental results, highlighting the potential of UVADM for an increased productivity in the surface conditioning for polymer-metal hybrids.

Machine-learning based process monitoring for automated composites manufacturing

AbstractAutomated fibre placement (AFP) is an advanced robotic manufacturing technique which can overcome the challenges of traditional composite manufacturing. The interlaminar strength of AFP-manufactured composites depends on the in-situ thermal history during manufacturing. The thermal history is controlled by the choice of processing conditions and improper interfacial temperatures may result in insufficient bonding. Being able to better predict such maintenance issues in real-time is an important focus of smart manufacturing and Industry 4.0 to improve manufacturing operations. The data analysis of real-time temperature measurements during AFP composites manufacturing requires the temperature profiles from Finite Element Analysis (FEA) based simulations of the AFP process to better predict the quality of layup. However, the FEA simulations of the AFP process are computationally expensive. This study focuses on developing a digital tool enabling real-time process monitoring and predictive maintenance of the AFP process. The digital tool constitutes a machine learning-based surrogate model based on results from Finite Element Analysis (FEA) simulations of the AFP process to predict the in-situ thermal profile during AFP manufacturing. Multivariate Linear Regression, Multivariate Polynomial Regression, Support Vector Machine, Random Forest and Artificial Neural Network (ANN)-based models are compared to conclude that ANN based surrogate model performs best by predicting the important parameters of thermal profiles with a mean absolute percentage error of 1.56% on additional test data and reducing the time by four orders of magnitude as compared to FEA simulations. The predicted thermal profile can be compared with the real-time in-situ temperatures during manufacturing to predict the quality of the layup. A GUI application is developed to provide predicted thermal profiles data for analysis in conjunction with real-time temperatures during manufacturing enabling monitoring and predictive maintenance of the AFP process and paving way for the development of a digital twin of the AFP composites manufacturing process.

Tensile response of additively manufactured carbon/nylon joints using optical fibers and digital image correlation

ABSTRACT Additive manufacturing (AM) has gained popularity due to reduced production times of low-cost and geometrically complex structures. Due to technological constraints such as print volume, time, and thermal stability of large AM parts, adhesive joints are commonly used in assembling smaller components in large-scale polymeric structures. This work investigates the feasibility of using embedded optical fibers to monitor strain distributions within AM composite (carbon fiber/nylon) single-lap shear (SLS) joints. Internal strain distributions within the adhesive and surface strain fields of the SLS joints with AM adherends under tensile loads were obtained using optical fibers and digital image correlation, respectively. Results indicate negligible influence of optical fibers on the failure strength and axial stiffness of 3D-printed SLS joints under quasi-static loading. Higher strains were observed at the bond edges within the adhesive (using optical fiber) and the surface (using DIC) of the joints due to the rotation of the bond under loading. All specimens failed in the adherends beyond the bonded region due to their low interlaminar strength.

Investigating the effect of recycled irregular metallic particles embedment on the interlaminar strength of polyamide-based composites

ABSTRACT In this research, for the first time, the effect of embedding irregular and jagged steel particles of recycled milling dendritic chips on the improvement of compressive and interlayer strength of thermoplastic polyamide6 (PA6)-based, 200 g/m2 plain weave E-glass-fabric-reinforced composite laminates was investigated. The goal here is to examine whether the sharp spikes on the particles can penetrate adjoining laminate layers to provide superior interface toughness compared to spherical toughening particles. In-plane compression tests, according to ASTM D695-15 standard, show that through thickness expansion leads to delamination and subsequent failure. Consequently, theoretical considerations suggest that enhancements in interface toughness should lead to an increase in compression strength. For this purpose, steel particles with three size ranges, 300–600 µm, 150–300 µm, and 75–150 µm, were prepared and embedded into the composite laminates using film-stacking and hot-pressing process, in four different volume fractions. The particle distribution, impregnation, and bonding with the PA6 matrix were analyzed using optical and scanning electron microscopy imaging. Overall, results demonstrate a very good mechanical bonding between the particles and the thermoplastic matrix. Some interlocks between the particles and composite laminates were observed which has led to an improvement in the compressive strength of the composite laminate.

The influence of thickness and recycled milled glass fiber fillers on the delamination resistance of polymer composite angle brackets

AbstractComposite structures with L, C, and T shape geometries are commonly used in the design of aerospace structural components. These complex structures are prone to delamination failure at the curved region. This study investigates the effect of thickness and recycled milled glass fiber fillers to improve the strength properties and delamination resistance of glass/epoxy composite angle brackets. Different thicknesses of 3.5 mm (16 layers), 5 mm (24 layers), and 6.5 mm (32 layers) specimens were subjected to four‐point bending test with acoustic emission (AE) monitoring for evaluating damage resistance. Mechanical results reveal that the specimen with a higher thickness of 6.5 mm has better damage tolerance, followed by 3.5 and 5 mm angle brackets. Moreover, the location of AE signals and % of AE hits noticed the initiation and propagation of delamination in 5 mm thickness specimen at the earlier stage of loading. However, the curved beam strength and interlaminar tensile strength (ILTS) of 5 mm thickness specimen were improved by 31.02% and 19.2% respectively, through the incorporation of 2 wt% of fillers. Further, the interfacial adhesion between the fillers and epoxy matrix was studied using Fourier‐transform infrared analysis to confirm the improvement of delamination resistance in 5 mm thickness specimen. Finally, the mechanical and AE results were well correlated with field emission scanning electron microscope images to characterize the damage mechanisms in composite angle brackets.Highlights Effect of thickness on the delamination resistance of composites was studied. Interlaminar properties of composite angle brackets were assessed. Acoustic emission parameters were used to study the damage mechanisms. Recycled milled glass fillers were employed to improve delamination resistance. Mechanical and acoustic emission results were correlated with field emission scanning electron microscope images.

The effect of adhesively bonded external hybrid patches on the residual strength of repaired glass/epoxy‐curved laminates

AbstractComposite materials are increasingly used in aircraft structural components with complex structures like L, C, or T shapes, which can cause delamination failure in the thickness direction under complex loading. This research focuses on the experimental investigation of the effect of different adhesive reinforcement phases with an adhesively bonded external patch to examine the residual strength and delamination resistance of repaired glass/epoxy curved samples. The damaged region in curved samples was repaired using adhesive reinforcement phases like neat epoxy, chopped glass fiber, and particulate glass fiber, along with adhesively bonded external patches like hybrid patches of glass/Kevlar (HP G/K), glass/carbon (HP G/C), and intraply patch carbon/Kevlar (IP C/K). The findings show that chopped glass fiber HP G/C patch‐repaired curved samples had a curved beam strength (CBS) and interlaminar tensile strength (ILTS) recovery of 222.87% and 149.54%, respectively, as compared to damage area removed samples. In addition, it exhibited higher peak force, residual strength, and less elongation when compared to HP G/K and IP C/K patch‐repaired curved samples. According to the findings of this research, composite angle brackets in aircraft structural components can be repaired utilizing an adhesively bonded HP G/C patch filled with chopped glass fiber at the dressed site.Highlights The effect of patch hybridization on repaired curved laminates was studied. Four‐point bending test was conducted on indented and repaired samples. Residual strength and delamination behavior were studied. Chopped glass fiber HP G/C patch repaired samples had higher CBS and ILTS. Delamination onset and propagation were assessed using Photographic images.

Hybridization Effect on Interlaminar Bond Strength, Flexural Properties, and Hardness of Carbon-Flax Fiber Thermoplastic Bio-Composites.

Hybridizing carbon-fiber-reinforced polymers with natural fibers could be a solution to prevent delamination and improve the out-of-plane properties of laminated composites. Delamination is one of the initial damage modes in composite laminates, attributed to relatively poor interlaminar mechanical properties, e.g., low interlaminar strength and fracture toughness. This study examined the interlaminar bond strength, flexural properties, and hardness of carbon/flax/polyamide hybrid bio-composites using peel adhesion, three-point bending, and macro-hardness tests, respectively. In this regard, interlayer hybrid laminates were produced with a sandwich fiber hybrid mode, using woven carbon fiber plies (C) as the outer layers and woven flax fiber plies (F) as the inner ones (CFFC) in combination with a bio-based thermoplastic polyamide 11 matrix. In addition, non-hybrid carbon and flax fiber composites with the same matrix were produced as reference laminates to investigate the hybridization effects. The results revealed the advantages of hybridization in terms of flexural properties, including a 212% higher modulus and a 265% higher strength compared to pure flax composites and a 34% higher failure strain compared to pure carbon composites. Additionally, the hybrid composites exhibited a positive hybridization effect in terms of peeling strength, demonstrating a 27% improvement compared to the pure carbon composites. These results provide valuable insights into the mechanical performance of woven carbon-flax hybrid bio-composites, suggesting potential applications in the automotive and construction industries.

Enhancing the fracture toughness of laminated composites through loop warp yarns

The delamination tendency is a key factor affecting the practical use of laminated composites. How to improve the interlaminar strength and interlaminar fracture toughness of laminated composite is a focus of research on composite materials. A three-dimensional (3D) weaving technique is employed to weave loop warp yarns (loops) with 3D fiber bundles instead of warp yarns on the basis of plain fabrics. The contribution of loops to the interlayer performance is revealed by double cantilever beam (DCB) tests, end-notch flexure (ENF) tests, flexural tests, and double notch shear (DNS) tests. Test results show that the mode I and mode II interlaminar fracture toughness values of the single-sided-loop two-dimensional (2D) woven laminated composite (SWLC) are 77% and 58.2% higher than the interlaminar fracture toughness values of the general 2D woven laminated composite (GWLC). As companied with the ones of GWLC, flexural strength and modulus of SWLC increase by 73% and 69.1%, and the interlaminar shear strength of SWLC increases by 45.7%. The loop yarns act as a link between the interlaminar matrix and the reinforcing fiber fabric and thus improve the poor interlaminar performance of GWLC. The reported information may provide a reference for the application of the SWLC in practice.