Showing papers in "Journal of Reinforced Plastics and Composites in 2023"

Influence of weave arrangements on mechanical characteristics of cotton and bamboo woven fabric reinforced composite laminates

Abstract: Due to the increasing demand for environmentally safe products, the use of composites manufactured from natural fibres has become one of the most widely researched subjects lately. In this present study, the influence of weave designs on the mechanical characteristics of bio-based cotton, bamboo and cotton/bamboo fabric reinforced textile epoxy composites is presented. The textile composites were prepared by compression moulding process with different fabric weight ratio of 70:30; 65:35; 60:40; 55:45 and 50:50. The composites were tested for interlaminar shear, flexural, compressive, tensile and impact strength as per ASTM standards. Compared to other weave design composite laminates, 45 wt.% loading of cotton and bamboo textile composite achieved the greatest results for all the tests in a simple weave pattern. The scanning electron microscopy results of the fractured samples exhibited superior interfacial bonding between the fabric and matrix, as evidenced by the presence of significantly reduced fibre bend, fibre pullout, tearing and crack. The prepared natural fibre composites (cotton/bamboo) can be used as an eco-friendly alternative to man-made materials that aid in pollution management. They are also less expensive, have higher mechanical characteristics and use less energy in the manufacturing process.

Effect of build orientation on tribological and flexural properties of FDM-printed composite PLA parts

Abstract: In the study, parts are fused deposition modeling–printed using polylactic acid filaments filled with copper, brass, bronze, tungsten, and carbon fiber. Wear and three-point bending tests are applied to FDM-printed specimens and it is yielded that on-edge oriented specimens had higher flexural strength while flat orientation contributed to wear resistance. Wear tracks are examined, and mostly abrasive wear is detected in the dry sliding condition while adhesive, fatigue and abrasive wear is observed in the wet sliding condition. Brass-filled polylactic acid specimens showed highest flexural strength and flexural modulus with 46.73 MPa and 1.53 GPa, respectively. Besides, the most wear resistant filler is determined as tungsten for dry and wet sliding conditions.

Characterization of recycled end-of-life rubber tire filled with black slag

Abstract: To date, discarded tires are reused in many applications, however, because of the enormous quantity decommissioned annually, it is essential to continue researching new recycling methods as well as applications to reduce waste and preserve new resources. In the present study, a simple recycling technology of end-of-life tire (ELT) powder is proposed, and the influence of steel slag as filler is assessed. Europe produces about 7 Mt of steel slag annually, and although most of it is reused as an artificial aggregate, about 15% is still landfilled. Also in the case of steel slag, the study of new applications is mandatory so that the combination of these two waste materials, in a 100% recycled composite fits across different industrial sectors facing the same environmental issue. It was found that the leaching of the slag incorporated in the rubber matrix is reduced and that the ELT powder recycled by this technology gives rise to a well-cohesive material. A good rubber-filler interaction was found by swelling test and differential scanning calorimetry (DSC) analysis. The slag reduces the friction coefficient and increases the thermal conductivity. The experimental results showed how some properties of recycled ELT can be improved by adding the steel slag.

Spray-coating of graphene nanoplatelets on sisal fibers and its influence on electromechanical behavior of biocomposite laminates

Abstract: Spray coating and vacuum-assisted resin infusion processes were implemented in this work to develop multifunctional biocomposite laminates. The biocomposites were fabricated using a bio-based epoxy resin reinforced with natural sisal fibers coated with graphene nanoplatelets (GNPs). A systematic characterization of material properties was performed to evaluate the mechanical, thermomechanical, electrical, and piezoresistive behavior of biocomposites with different GNPs contents (0, 1, 4, 6, and 8 wt.%). The mechanical tests revealed that adding GNPs to biocomposites has a slight positive effect on their flexural properties compared to the neat biocomposites (without GNPs). The electrical and thermomechanical tests showed that the electrical conductivity and glass transition temperature of biocomposites containing GNPs were enhanced significantly, achieving average values of 5.19 x 10-4 S/m and 63.29°C (26%), respectively. Regarding electromechanical tests, the biocomposites with 8 wt.% GNPs exhibited an excellent piezoresistive behavior under monotonic loading conditions, achieving a gage factor (strain sensitivity) of 3.56. Bending tests with cyclic loading were also performed, and cyclic reproducibility of the piezoresistive response of the biocomposites after 10 cycles was demonstrated, evidencing that the incorporation of GNPs onto sisal fibers by spray-coating produces an effective formation of conductive networks into biocomposites suitable for sensing applications.

Short aramid fiber reinforced thermotropic liquid crystalline polyester composites: Fabrication, thermal, and mechanical properties

Abstract: We conducted a study to analyze the impact of short aramid fibers (AFs) on the melt-rheological behavior, thermal transition, thermal stability, and mechanical durability of thermotropic liquid crystal polyesters (TLCPs). By using different AF loading contents ranging from 3–15 wt%, we produced TLCP matrix composites through masterbatch-based melt-compounding and injection-molding. The SEM images and FT-IR spectra demonstrate that the AFs are dispersed in the TLCP matrix with a microfibrillar structure through good interfacial adhesion caused by specific intermolecular interactions between the TLCP and AFs. As a result, the complex viscosity, shear storage/loss moduli, and thermal transition (melt-crystallization, glass transition, and melting) temperatures of the composites increase with increasing AF filler content. However, the melt-crystallization and melting enthalpies increase only at low AF loading contents of 3–5 wt%. At high AF contents of 7–15wt%, the enthalpies decrease owing to the partial aggregation of AF fillers. The thermogravimetric analysis proves that the thermal stability of TLCP/AF composites improves when the AF filler is introduced. The dynamic mechanical analysis using the stepped isothermal method shows that the addition of 5 wt% AF to the TLCP leads to an approximately 150% improvement in elastic moduli and long-term mechanical durability at elevated temperatures.

A study of the interactions of carbon based fillers in acrylonitrile butadiene rubber matrix for high deformation sensor applications

Abstract: A strain sensor was prepared by reinforcing acrylonitrile butadiene rubber (NBR)-5 parts per hundred of rubber (phr) carbon black (CBH) separately with small concentration (∼0.1phr) of reduced graphene oxide (GL), multi-walled, and carbon nanotube (NTL) via a combination of conventional solution and solid processing techniques. The interactions and the electronic properties among carbon based fillers NT, CB, G and their synergy effects (NBR-CBH-GL and NBR-CBH-NTL) were investigated by using density functional theory (DFT) modeling approach. The DFT predictions were in correspondence with the experimental results. The optimum design (NBR-CBH-GL) was found to show high curing, mechanical and improved electrical properties. On account of strain sensing performance, NBR-CBH-GL exhibited high gauge factor (GF) ∼105 at 0–40% strain, which was over 900% than NBR-CBH (GF ∼104 at 0–30% strain) and the highest reported so far. This was explained by the breaking of CB networks caused by tight NBR-G structures on straining, leading to high electrical resistance. The NBR-CBH-GL also demonstrated high stability and repeatability in the cyclic loading. In terms of applications, NBR-CBH-GL exhibited high capability for vibration detections and wearable sensing, especially for detection of human bodily motions like speeches, facial deformations, bending, and relaxation of the fingers.

Multi-scale modelling and life prediction of aged composite materials in salt water

Abstract: Tidal turbine infrastructure is currently in the large-scale prototype and short-term demonstration phase. However, the immediate requirement is to develop materials, processes and long-term life predictive facilities for tidal turbine plant that has decades of operational lifetime requirements. Computational modelling is a key tool to interpret the experimental data, understand the relevant mechanisms and provide a predictive capability for the performance of aged components for industries. The goal of this paper is a prediction of the long-term life of marine-based glass/epoxy and carbon/epoxy composite laminates aged in artificial seawater with 3.5% salinity based on Arrhenius degradation theory and tensile strength retention over 180 days ageing at room temperature and 60°C. Three different analytical models (linear and exponential) were implemented to calculate time shift factors and corresponding life in a real marine environment. Additionally, multi-scale modelling has been implemented via a representative volume element approach for square and hexagonal cells, and two-step homogenization of textile composites in accordance with nanoindentation testing for matrix/resin cells and fibre constraint cells after 90 days of immersion in saltwater. In general, the multi-scale modelling in ABAQUS and TexGen4SC was able to approximate (with about 10% difference) the mechanical properties of dry and aged composite laminates.

Properties of blends from pregelatinized starch with poly (vinyl alcohol) for hygienic-purposed disposable laundry bags

Abstract: This research was focused on the properties of poly (vinyl alcohol) (PVOH) blended with pregelatinized starch (PSt) as a suitable material to make laundry bags for infected clothes application. PVOH and PSt (0, 10, 20, 30, and 40 wt.%) with glycerol 20 phr were melt-blended by twin-screw extruders. The samples were processed into the film by single layer-blown film extrusion. From the results, it was found that PVOH, glycerol, and pregelatinized starch had intermolecular interactions with each other, forming hydrogen bonding interactions between PVOH/pregelatinized starch and glycerol. The glass transition temperature (Tg) of a blend was shifted to a higher temperature by increasing the pregelatinized starch content leading to a reduction in the percent crystallinity. The presence of pregelatinized starch slightly increased the melt flow rate (MFI)/melt volume rate (MVR), apparent viscosity, and viscosity average molecular weight (Mv) of the blends due to the chain entanglement but it decreased the water solubility time and the moisture content. A co-continuous phase with small coalescence was found when 30% pregelatinized starch was added, increasing the elongation at break to 171%. On the other hand, the pregelatinized starch content was 40%, the elongation at break reduced to 154%.

Flexural behavior of RC continuous beams having hybrid reinforcement of steel and GFRP bars

Abstract: Fiber reinforced polymers (FRP) have been considered to be the alternative material to steel in reinforced concrete structures (RC) with the advantages of corrosion resistance, non-conductivity, and high strength-to-weight ratio. FRP-RC beam is featured with higher deformability and larger crack width. However, there is no recognizable ductility in FRP-RC members. Due to the lack of the ductility of FRP-RC beam, it was suggested using hybrid reinforcement of steel and FRP to achieve a preferable strength and ductility. The flexural behavior of the concrete beams reinforced with hybrid reinforcement was investigated through series of six continuous beams. The positive moment GFRP reinforcement ratios of 0.75% and 1.25% were used and the steel reinforcement ratios were ranged to determine the optimum percentage to work efficiently with GFRP. The experimental results showed that increasing GFRP positive moment reinforcement ratio to be more than 1.4 [Formula: see text] increased the flexural capacity of the section but with less ductile behavior. However, increasing the steel positive moment reinforcement ratio leaded to the increase of both the flexural capacity and the ductile behavior in a condition that the steel positive moment reinforcement ratio was [Formula: see text]. It was shown that the flexural capacity predication of the hybrid section using the semi brittle elastic material method to present the de-bonding of moment that occurred at the failure stage had a good accuracy with the experimental results.

A study of post-processing for AFP-fabricated thermoplastic composites with void content and air permeability characterization

Abstract: Improving the quality of fiber-reinforced composite laminates using out-of-autoclave techniques has been a challenge for a long time. The recent trends toward the replacement of autoclave processes with out-of-autoclave processes for thermoplastic composites still lack a compressive recipe. This study first develops an understanding of the integrated effect of the automated fiber placement (AFP) lay-up process parameters (compaction force and lay-up velocity) and the type of post-processing: i) autoclave and ii) vacuum bag only (VBO) processing with two vacuum hold times, 2 and 24 h. The void content results highlight the potential of the VBO process to replace the autoclave with an improved void content of VBO with 24 vacuum hold times. Then, this result has been correlated with the morphology of voids after the AFP lay-up as isolated or connected via air permeability characterization. This study reveals that the high air permeability to effective porosity ratio of samples after AFP lay-up which is achievable with low compaction force facilitates the air removal mechanism during the VBO process with sufficient vacuum hold times.

Manufacturing composite laminates with controlled void content through process control

Abstract: To investigate and evaluate the effect of void features on the mechanical performance of composite materials, it is important to be able to manufacture samples with a range of controlled void content. A criterion of less than 2% of porosity is typically acceptable for industry. However, it is important to investigate the effect of void content above and below this range, as voids are typically unevenly distributed in composite parts, and so there are likely to be local void concentrations higher than 2% in some sections of the structures. In this paper, a novel manufacturing process that allows panels with a range of void contents to be manufactured in a controlled manner is introduced. This allowed an investigation of the effect of manufacturing parameters, such as time, pressure and temperature and material systems on the void content and morphology of the voids in specimens produced.

Interphase elastic modulus characterization in glass/epoxy composite using combined peridynamics and experimental method

Abstract: This paper introduces a novel combined technique to characterize the elastic modulus gradient for glass/epoxy interphase. The developed procedure is applicable to all fiber-reinforced polymer (FRP) composites. Interphase in FRP composites plays a vital role in the mechanical performance of the material. Scientists almost characterized this region using nanoscale test methods such as atomic force microscopy (AFM) and scratch tests. These local methods characterize the elastic properties at a specific region near the fiber and, usually, are utilized for all regions around the fiber. This paper proposes a combined peridynamic and single-fiber micro tensile test method to characterize the overall elastic modulus gradient at the interphase region. The advantages of peridynamic as a non-local method are used to model the multiscale regions of interphase, fiber, and matrix. Single-fiber micro tensile tests are performed by the authors, and variations of displacements and strains are measured by the digital image correlation method. The obtained displacements are used as target values in the developed peridynamic code, and by performing peridynamic analyses, the elastic modulus variation along the interphase is automatically extracted. Furthermore, the variations of stresses and strains are investigated at the specimens’ cross-section using different hypotheses for the interphase region.

Recycling effects on the bending, rheological, and structural properties of glass fiber-reinforced isotactic polypropylene composites

Abstract: In the present work, a combination of virgin polypropylene and E-glass fiber was subjected to ten (10) reprocessing cycles via extrusion and compression molding techniques to mimic recycling and its impacts on the bending properties of the composites. The samples were characterized using Fourier transform infrared (FTIR) spectroscopy, x-ray diffraction (XRD), scanning electron microscopy (SEM), and melt flow index (MFI). The results revealed a gradual depreciation in flexural properties after each reprocessing cycle. The XRD analysis indicated a substantial reduction of peak intensities, degrees of crystallinities, and average crystallite sizes, explaining the lowered flexural properties in addition to a possible reduction in glass fiber lengths (fiber attrition). Melt-processing behavior shows a progressive increase of MFI from 7 to 19.16 g/10 min, confirming the probable damage in molecular weight and loss of complex viscosity. Chemical and structural analysis showed no alteration in the polypropylene major functional groups. It is concluded that the reductions in molecular weight and composites’ properties occurred due to chain scission from recycling effects; hence, glass fiber-reinforced polypropylene composites can be recycled only three (3) times unless it is refreshed by the addition of virgin parts to compensate for the lost property.

Numerical investigation on the role of out-of-plane fiber direction on impact resistance of composite target

Abstract: In high-velocity projectile impacts, the contact duration is very short and the response of the target is governed by the local behavior of the material in the impact zone. On impact, a compressive stress wave is generated in the target, which travels in the thickness direction. This compressive stress wave plays a significant role in the failure of thick composite targets. In conventional laminates, the out-of-plane (transverse) compressive strength of composite resists the generated compressive stress wave which is not efficient since out-of-plane compressive strength is significantly lower than the fiber direction strength value. In the current numerical study, a non-conventional approach has been adopted by orienting the fibers in the impact direction to utilize the high compressive strength in the fiber direction to resist the projectile impact. In this regard, unidirectional composite rod segments are considered with fiber aligned in the projectile impact direction. The resulting effect on the impact energy absorption and the overall mechanical impact response is numerically studied and compared with conventional woven fabric laminates. Several configurations of these unidirectional fiber rod segments and the role of lateral confinement are also studied to understand their effect on the impact energy absorption. The numerical simulation results indicate an advantage for the proposed arrangement of composite (segments) rods with fiber in impact direction over the conventional woven fabric laminates.

Fire reaction and post-fire impact properties of flax fibre reinforced composites containing intumescent flame retardants

Abstract: The current research aims at investigating the fire reaction and post-fire impact characteristics of flax fibre reinforced composites based on different polymer matrices, such as epoxy resin and polypropylene (PP). The effects of stacking sequences of polymer sheets and flax fabrics within the composites are explored by a cone calorimeter. Heat release and smoke production results indicate that a top surface and number of flame-retardant polymer layers played significant roles in determining the fire reaction properties. Furthermore, post-fire impact properties of flax-epoxy and flax-PP composites depending on burning periods are analysed in this study at the first time. A fully instrumented drop-weight impact testing demonstrates that an increase in heat exposure time led a gradual decrease in impact properties of the composites due to fire-induced damages on fibres and polymers. However, pre-melting and re-consolidation of PP are beneficial to have higher impact energy and force of the flax-PP composite over those of the flax-epoxy counterpart. In addition, the char formation of composites due to intumescent flame retardant enhances the fire reaction properties of composites, whereas there is no significant influence on the post-fire impact characteristics due to highly brittle nature of carbonaceous char.

Shielding performances of short carbon fibers and tungsten particles reinforced benzoxazine resin matrix composites

Abstract: This present investigation reports, for the first time, the properties of the polybenzoxazine bio-based furfurylamine reinforced using the hybrid short carbon fibers and tungsten particles. The results revealed good compatibility between the matrix and the filers using scanning electron microscopy, due to the use of the silane treatment that led to enhanced interfacial bonding. In addition, the mechanical data disclosed that the bending and impact values were respectively increased to 210 MPa and 9 KJ/m2 for the composites containing 20%wt of the short carbon fibers and 20%wt tungsten particles. The hybrid composite shielding properties were evaluated using the cobalt-60 as an irradiation source, and the nuclear results depicted that the half-value length and the tenth-value length were increased as compared to those of the pure polymers. Based on the thermal stability studies, the reinforced hybrid composites also showed excellent thermal resistance.

An experimental study on the role of different void removal mechanisms in VBO processing of advanced thermoplastic composites

Abstract: The current study is focused on understanding the role of different void removal mechanisms in VBO processing of advanced thermoplastic composites. For this purpose, two commercially available Carbon/Poly-Ether-Ketone-Ketone (C/PEKK) tape materials were evaluated, distinct in morphologies, such as surface roughness and fiber-matrix distribution, and physical characteristics, mainly the presence of volatiles. The VBO consolidation results proved that the void reduction and removal mechanisms varied depending on the tape material. However, restricting the void reduction mechanism to dissolution and diffusion alone. Depending on the tape material, a significant difference in the consolidation dwell time was observed to achieve <1% void content parts. Thus, indicating that despite the tapes having the same polymer matrix, they differ in their diffusion behavior. The difference in the times required for consolidation may be due to the following. Firstly, the diffusion coefficients may be different for the two tapes. Although the matrix material in both tapes is PEKK, the exact formulation is unknown. Secondly, the volume of gases, which comprises entrapped air and the volatiles that evaporate during the process, that need to be removed maybe be different.

FE micromodel of a cord-rubber composite tube sample subjected to uniaxial tension

Abstract: The aim of this study is to examine displacements, strains and stresses of a railway composite cord-rubber air brake tube undergoing uniaxial tension by microscale modelling. The micromodel is based on the macromodels (by matching the boundary conditions of the micromodels with displacements of the macromodels) created previously by authors of this article. The reinforcing yarns are described by an orthotropic, elastic material model, whereas the matrix has been described by a 2 parameter Mooney-Rivlin model, which all have been validated before by a uniaxial tensile test and a three-point bending test. Force-displacement curves of the micromodel and experimental results show a considerably good agreement. Failure of the reinforcement layers does not occur in case of the uniaxial tension of the composite tube sample, there was no failure event observed during the experiment of the tube sample. Load transfer mechanism has been demonstrated and characterized numerically by strain and stress results inside and between reinforcement layers (implying shear plays a dominant role in the load-transfer mechanism). The microscale nature of the FE model ensures that the strain and stress results are representative for the reinforcement layers.

Experimental characterization and numerical modelling of bending behavior of carbon fiber unidirectional thermoset prepregs

Abstract: To enhance the prediction accuracy of forming simulations of carbon fiber unidirectional (UD) thermoset prepregs, particularly the wrinkle predictions, bending behavior of UD prepregs in the forming process needs to be accurately characterized and modelled. In this paper, an improved bending test system was proposed to characterize the bending behavior of UD prepregs at different forming temperatures and loading rates. It is found that both the temperature and the loading rate have significant impact on the bending stiffness of UD prepregs. An obvious nonlinear bending behavior can be noted in the fiber direction. The loading rate sensitivity of bending stiffness in the fiber direction decreases with temperature increasing. To simulate the bending deformation, a superimposed membrane-shell element model was employed to achieve the decoupling of tensile-bending deformation. Membrane element simulated the in-plane tensile and in-plane shear deformations of UD prepregs, while shell element primarily accounted for the bending deformation. Fiber orientation dependent nonlinear bending stiffness was accounted in the model. Deformation simulations were conducted for vertical cantilever bending, horizontal cantilever bending, and compression wrinkling of UD prepregs. Good agreement is noted between simulation and experiment, demonstrating efficiency of the model and validity of measured bending stiffness.

Laser powder bed fusion of glass-filled polyamide-composite with enhanced physical properties

Abstract: Glass-filled polyamide (GF/PA) is an important engineering material for automotive industry because of its excellent physical, thermal and mechanical properties. But it is challenging to produce high quality functional parts through additive manufacturing technology like selective laser sintering (SLS). It is due to fact that SLS made parts exhibit high dependence on the sintering conditions. Thus, it becomes essential to adjust these sintering conditions accurately to improve physical properties like density and hardness to enhance the widespread use of this technology. Therefore, this study presents a statistical analysis of the SLS process parameters led by a design of experiment to extract information about their influence on hardness and density of the PA 3200 GF composite. GF/PA with a refresh rate of 60:40 was used for the part fabrication and the effect of five key sintering parameters has been considered for analysis. Experiments revealed that poor material integrity and weak interaction between the particles at lower energy density were the main reason for the lower hardness and density of fabricated parts.

Statistical tensile and flexural fatigue lives of unidirectional CF/PP laminates

Abstract: The accelerated testing methodology (ATM) for the statistical life prediction of carbon fiber reinforced plastics (CFRP) under creep and fatigue loads based on the viscoelasticity of matrix resin has been proposed by authors. This method has been successfully applied to the life predictions of unidirectional CFRP with thermosetting epoxy resin as the matrix under tensile and flexural loads. In this paper, this method was applied to the fatigue life prediction of unidirectional CFRP (CF/PP laminates) with thermoplastic polypropylene (PP) as the matrix under tensile and flexural loads, and the effect of matrix viscoelasticity and load cycles on the fatigue behavior of CF/PP laminates were discussed under tensile and flexural loads. First, the viscoelasticity of matrix PP was measured. Second, the static and fatigue strengths of CF/PP laminates were statistically measured at various temperatures and frequencies under tensile and flexural loads. Third, all of material parameters in the fatigue strength formulation in ATM were determined using measured data, and the statistical long-term fatigue strengths of CF/PP laminates were predicted under tensile and flexural loads. As results, comparison with tension and bending test results clarified that the long-term tensile fatigue strength of CF/PP laminates decreases at an accelerated rate as the number of cycles increases, and that the flexural fatigue strength is affected by temperature and frequency rather than by the number of cycles to failure.

Simple stitch-assisted carbon fiber-aluminum laminates via out-of-autoclave process

Abstract: Fiber–metal laminates (FMLs) are hybrid materials that provide the advantages of both metal and fiber-reinforced polymer composite (FRP) such as improved damage tolerance and impact strength, particularly in aerospace and automotive applications. However, the production of FMLs remains expensive because the manufacturing process is performed in an autoclave. In addition, delamination failure due to low interfacial adhesive strength between the metal and fiber layer is a problem that must be solved to improve the mechanical properties of FMLs. In this study, simple stitch-assisted carbon fiber-aluminum laminates were developed using vacuum-assisted resin transfer molding (VARTM). The FMLs produced through vacuum-assisted resin transfer molding were stitched with aramid fibers through holes on Al sheets for the resin flow associated with the VARTM. The mechanical properties of the FMLs were investigated by impact and compression after impact tests as well as double cantilever beam and flexural tests. The stitching effectively improved the interfacial strength and toughness owing to the anchoring between the aluminum and carbon fiber layers, thus preventing delamination crack propagation. As a result, the stitched FML with significantly improved interfacial adhesive and toughness was developed by the VARTM process, which will accelerate the application of FML to various lightweight structures.

In search of the quasi-isotropic laminate with optimal delamination resistance under off-axis loads

Abstract: Delamination is one of the most feared failure modes in laminated composites and yet there is a lack of well established procedures to find the ply configuration with the highest delamination resistance for a given geometry of the component and a set of elastic and strength constraints. This paper presents a methodology for determining the quasi-isotropic ply sequence more resistant to delamination under off-axis uniaxial tension. Delamination is considered to be triggered by interlaminar stresses at free-edges or by matrix cracks. Two different ply thicknesses (standard and thin) and two sets of ply orientations (multiples of π/4 or π/8) are considered. The results show that the use of thin plies enlarges the design domain by up to 70% and generates a practically isotropic safe space. The methodology presented is also suitable for optimizing the delamination resistance of other load cases with stress singularities.

Energy absorption capability of carbon/epoxy composite laminates: A novel numerical approach

Abstract: While composite structures absorb energy well during crash events, crush-induced failure mechanisms and their effects on energy absorption characteristics are still unclear. Due to the complexity of the process, Finite Element (FE) models applied to estimate energy absorption capability of composite structures require a considerable amount of computational time. This study presents a novel numerical approach for the crushing process of AS4/8852 flat coupon plates in which computational cost is decreased by reducing the number of interfaces between plies and modifying the relevant properties. Experimental and numerical studies are conducted for cross-ply specimens [0/90]ns in order to understand and discuss the failure mechanisms occurring during the crushing process in detail and the influence of plate thickness on Specific Energy Absorption (SEA). Moreover, to demonstrate the validity of novel FE model for different stacking sequences, same approach is applied to the [0/45/0/−45]s plate geometry. By this method, run time is decreased by more than 50%.

Reinforcing effect of functionalized graphene nanoplatelets in palm kernel polyol based polyurethane composite

Abstract: Shape memory polyurethane-based palm kernel oil (SMPU based PKOp) has been shown to possess good shape memory properties. However, SMPU based on vegetable oil bear poor mechanical properties, thus enhancing mechanical properties using reinforcement, such as graphene nanoplatelets (GNP) is favorable in term of performance and cost. Nevertheless, the high viscosity of the polymer and high surface energy of GNP are obstacles in achieving a good composite. Therefore, in this research, a series of acid functionalization process was conducted to enhance the distribution of GNP in SMPU matrix, as well as the effects of functionalized GNP on the mechanical and shape memory properties of PKOp based SMPU were investigated. Functionalized GNP-based SMPU composite (PU-F) reached maximum tensile strength at 1wt% of functionalized GNP, which is 150% higher compared to that of neat SMPU while tensile strain at break increased 1590% than that of neat SMPU at 0.25wt% functionalized GNP. However, the shape fixity was almost similar to that of pristine SMPU and all the composite samples exhibited 100% shape recovery.

Dynamic stress concentrations around a single fiber break in unidirectional composites: a 3D finite element analysis

Abstract: When a fiber break occurs in longitudinal tension of a unidirectional composite, dynamic stress concentrations arise, which can be different from the ones found considering only static loading. The current paper analyzes the dynamic stress concentration factors (SCF) around a fiber break in unidirectional carbon fiber/epoxy composites. 3D finite element models with random and hexagonal fiber distributions were analyzed to investigate the evolution of stress concentrations as a function of time and position. The results indicate that dynamic effects result in much higher SCFs with a larger effective area around the broken fiber. The increase of SCFs in the closest fibers was determined to be larger for lower fiber volume fractions due to the presence of dynamic effects. Similar to the static case, a lower volume fraction causes higher maximum dynamic SCF in random packings. Results also support the high prevalence of coplanar cluster breaks observed in the experiments.

Finite element modelling of the elastic and dynamic behaviour of nonwoven flax/polypropylene composite

Abstract: 2D and 3D finite element models of nonwoven flax/polypropylene composite are developed in this work to estimate its elastic and dynamic properties. These models are based on an in-plane random generation of the nonwoven flax fibres embedded in 2D and 3D networks of polypropylene (PP) matrix with porosities. First, the elastic properties of the flax/PP nonwoven composite are numerically predicted and a comparison with the experimental results from tensile tests is conducted. Second, the dynamic properties resulted from free vibration tests, at different fibre weight ratios and porosity contents, are numerically determined. Finally, the flax fibre damping is predicted using the strain energy method and the finite element results. This allows proposing a first estimation of the damping coefficient of this natural fibre.

Estimation of cohesive zone model parameters for glass/Elium composites with different processing temperature

Abstract: Important estimates were made of the cohesive zone mode (CZM) characteristics used to predict the fracture of Elium composites processed at various temperatures in this study. First, tests for the end notched flexure (ENF) and double cantilever beam (DCB) on composites processed at different temperatures (24°C, 50°C, and 80°C) were performed. Based on the experimental results, the numerical models of damage evolution were developed. And six pairs of cohesive parameters (cohesive strength and cohesive stiffness) to the specified fracture energy release rates were obtained. Then, a Tsai approach was developed to predict the cohesive strength of the Elium composite with various process temperatures based on the obtained parameters. In order to predict the delamination behavior of the Elium composite, the obtained cohesive parameters were then used to a numerical model of short beam shear (SBS). The peak force of the simulations was discovered to be consistent with the outcomes of the tests. The findings served as a reference for damage assessments of Elium composites processed at various temperatures.

Impact performance of jute 3D woven auxetic reinforced thermoplastic (PC/PVB) composites

Abstract: Inducing auxetic nature is one of the emerging techniques to enhance impact tolerance in 3D woven composites due to their unique bidimensional-energy dissipation capability. The conventional 3D orthogonal structure has inherent auxetic nature; however, such auxeticity is completely restricted due to brittle thermoset resins-based composites. To overcome this problem, three types of 3D woven auxetic structures were developed and used as reinforcement with thermoplastic resins, that is, polycarbonate (PC) and polyvinyl butyral (PVB). The results revealed that the warp interlock structure showed the highest auxeticity, while the bidirectional interlock structure showed the least auxeticity. Charpy and low-velocity impact tests were performed to evaluate the effect of auxeticity on the impact properties of corresponding composites. Warp interlock with PC showed 49% and 47% higher Charpy impact strength and force, respectively, than bidirectional interlock with PC resin. Similarly, the low-velocity impact test results of warp interlock showed 32% and 32.5% higher impact force and absorbed energy, respectively, in warp direction than bidirectional interlock with PC resin.

Experimental and numerical investigation of core splicing configurations in an advanced composite sandwich structural system

Abstract: In this study, an investigation is presented on the effects of the honeycomb core splice gaps on the performance of advanced composite sandwich structures consisting of aluminum honeycomb core and carbon-fiber/epoxy facesheets. The experimental and numerical investigation explores the effects of core thickness, facesheet thickness, splice gap width, and the presence of core splice adhesive on the structural response of the composite sandwich structure. The specimens fabricated with the splices exposed to primarily bending and shear to understand further how the core splice interacts with the overall structural response under such loading conditions. In addition, 3D finite element models investigated the failure modes in the specimens by examining the strain fields for core-spliced sections of the sandwich structure under the experimentally observed loading conditions. Various failure modes are observed in the experiments ranging from core shear failure, facesheet debonding, or fracture depending on the location of the core splice, core, and the facesheet properties. The results illustrate the importance of proper detailing and analysis of core splices in composite structures made from sandwich designs.