Showing papers in "Applied Composite Materials in 2018"

Influence of the Hybrid Combination of Multiwalled Carbon Nanotubes and Graphene Oxide on Interlaminar Mechanical Properties of Carbon Fiber/Epoxy Laminates

Abstract: An effective strategy to improve the mode I and mode II interlaminar fracture toughness (G IC and G IIC ) of unidirectional carbon fiber/epoxy (CF/E) laminates using a hybrid combination of multiwalled carbon nanotubes (MWCNTs) and graphene oxide (GO) is reported. Double cantilever beam (DCB) and end notched flexure (ENF) tests were conducted to evaluate the G IC and G IIC of the CF/E laminates fabricated with sprayed MWCNTs, GO and MWCNTs/GO hybrid. Scanning electron microscopy was employed to observe the fracture surfaces of tested DCB and ENF specimens. Experimental results showed the positive effect on the G IC and G IIC by 17% and 14% improvements on CF/E laminates with 0.25 wt.% MWCNTs/GO hybrid content compared to the neat CF/E. Also, the interlaminar shear strength value was increased for MWCNTs/GO-CF/E laminates. A synergetic effect between MWCNTs and GO resulted in improved interlaminar mechanical properties of CF/E laminates made by prepregs.

Thermal and mechanical properties of 3D printed boron nitride – ABS composites

Abstract: The current work investigates the thermal conductivity and mechanical properties of Boron Nitride (BN)-Acrylonitrile Butadiene Styrene (ABS) composites prepared using both 3D printing and injection molding. The thermally conductive, yet electrically insulating composite material provides a unique combination of properties that make it desirable for heat dissipation and packaging applications in electronics. Materials were fabricated via melt mixing on a twin-screw compounder, then injection molded or extruded into filament for fused deposition modeling (FDM) 3D printing. Compositions of up to 35 wt.% BN in ABS were prepared, and the infill orientation of the 3D printed composites was varied to investigate the effect on properties. Injection molding produced a maximum in-plane conductivity of 1.45 W/m-K at 35 wt.% BN, whereas 3D printed samples of 35 wt.% BN showed a value of 0.93 W/m-K, over 5 times the conductivity of pure ABS. The resulting thermal conductivity is anisotropic; with the through-plane thermal conductivity lower by a factor of ~3 for injection molding and ~4 for 3D printing. Adding BN flakes caused a modest increase in the flexural modulus, but resulted in a large decrease in the flexural strength and impact toughness. It is shown that although injection molding produces parts with superior thermal and mechanical properties, BN shows much potential as a filler material for rapid prototyping of thermally conductive composites.

Surface Modification of Aramid Fibres with Graphene Oxide for Interface Improvement in Composites

Abstract: A novel method of biomimetic surface modification was used for aramid fibres aiming to enhance the interface properties between epoxy resin and the modified aramid fibre. Inspired by the composition of adhesive proteins in mussels, a thin layer of poly(dopamine) (PDA) was self-polymerized onto the surface of the aramid fibre. The graphene oxide (GO) was then grafted on the surface of PDA-coated aramid fibres. The microstructure and chemical characteristics of the pristine and modified fibres were characterised using Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), indicating successful grafting of GO on the PDA-coated aramid fibres. Single fibre tensile test and microbond test were carried out to evaluate the mechanical properties of the modified fibres. It was found that the fibre surface modification improved the interfacial shear strength by 210% and the fibre tensile strength was protected by GO-PDA coating.

Simulation of Mechanical Behavior and Damage of a Large Composite Wind Turbine Blade under Critical Loads

Abstract: Issues such as energy generation/transmission and greenhouse gas emissions are the two energy problems we face today. In this context, renewable energy sources are a necessary part of the solution essentially winds power, which is one of the most profitable sources of competition with new fossil energy facilities. This paper present the simulation of mechanical behavior and damage of a 48 m composite wind turbine blade under critical wind loads. The finite element analysis was performed by using ABAQUS code to predict the most critical damage behavior and to apprehend and obtain knowledge of the complex structural behavior of wind turbine blades. The approach developed based on the nonlinear FE analysis using mean values for the material properties and the failure criteria of Tsai-Hill to predict failure modes in large structures and to identify the sensitive zones.

Experimental Study and Numerical Modelling of Low Velocity Impact on Laminated Composite Reinforced with Thin Film Made of Carbon Nanotubes

Abstract: In this work, polymer laminated composites based on Epon 862 Epoxy resin, T300 6 k carbon fibers and carbon nanotubes (CNTs) were tested with the aim to elucidate the effect of CNTs on impact properties including impact force and capacity to absorb impact energy. The polymer matrix was reinforced by a random distribution of CNTs with fraction ranging from 0.5 to 4.wt%. Composite panels were manufactured by using the infusion process. Taylor impact test was used to obtain the impact response of specimens. Projectile manufactured from a high strength and hardened steel with a diameter of 20 mm and 1.5 kg of mass was launched by a compressed gas gun within the velocity of 3 m/s. Impact force histories and absorbed energy of specimens were recorded. A numerical model was employed to simulate the impact performance. This model has been accomplished by forming a user established subroutine (VUMAT) and executing it in ABAQUS software. Finally, the effect of CNTs amount on dynamic properties of laminated composites was discussed.

Computational Homogenization of Mechanical Properties for Laminate Composites Reinforced with Thin Film Made of Carbon Nanotubes

Abstract: Elastic properties of laminate composites based Carbone Nanotubes (CNTs), used in military applications, were estimated using homogenization techniques and compared to the experimental data. The composite consists of three phases: T300 6k carbon fibers fabric with 5HS (satin) weave, baseline pure Epoxy matrix and CNTs added with 0.5%, 1%, 2% and 4%. Two step homogenization methods based RVE model were employed. The objective of this paper is to determine the elastic properties of structure starting from the knowledge of those of constituents (CNTs, Epoxy and carbon fibers fabric). It is assumed that the composites have a geometric periodicity and the homogenization model can be represented by a representative volume element (RVE). For multi-scale analysis, finite element modeling of unit cell based two step homogenization method is used. The first step gives the properties of thin film made of epoxy and CNTs and the second is used for homogenization of laminate composite. The fabric unit cell is chosen using a set of microscopic observation and then identified by its ability to enclose the characteristic periodic repeat in the fabric weave. The unit cell model of 5-Harness satin weave fabric textile composite is identified for numerical approach and their dimensions are chosen based on some microstructural measurements. Finally, a good comparison was obtained between the predicted elastic properties using numerical homogenization approach and the obtained experimental data with experimental tests.

A Comparison of Curing Process-Induced Residual Stresses and Cure Shrinkage in Micro-Scale Composite Structures with Different Constitutive Laws

Abstract: In this paper, three kinds of constitutive laws, elastic, “cure hardening instantaneously linear elastic (CHILE)” and viscoelastic law, are used to predict curing process-induced residual stress for the thermoset polymer composites. A multi-physics coupling finite element analysis (FEA) model implementing the proposed three approaches is established in COMSOL Multiphysics-Version 4.3b. The evolution of thermo-physical properties with temperature and degree of cure (DOC), which improved the accuracy of numerical simulations, and cure shrinkage are taken into account for the three models. Subsequently, these three proposed constitutive models are implemented respectively in a 3D micro-scale composite laminate structure. Compared the differences between these three numerical results, it indicates that big error in residual stress and cure shrinkage generates by elastic model, but the results calculated by the modified CHILE model are in excellent agreement with those estimated by the viscoelastic model.

Curing of Thick Thermoset Composite Laminates: Multiphysics Modeling and Experiments

Abstract: Fiber reinforced polymer composites are used in high-performance aerospace applications as they are resistant to fatigue, corrosion free and possess high specific strength. The mechanical properties of these composite components depend on the degree of cure and residual stresses developed during the curing process. While these parameters are difficult to determine experimentally in large and complex parts, they can be simulated using numerical models in a cost-effective manner. These simulations can be used to develop cure cycles and change processing parameters to obtain high-quality parts. In the current work, a numerical model was built in Comsol MultiPhysics to simulate the cure behavior of a carbon/epoxy prepreg system (IM7/Cycom 5320–1). A thermal spike was observed in thick laminates when the recommended cure cycle was used. The cure cycle was modified to reduce the thermal spike and maintain the degree of cure at the laminate center. A parametric study was performed to evaluate the effect of air flow in the oven, post cure cycles and cure temperatures on the thermal spike and the resultant degree of cure in the laminate.

Principles and Applications of Microwave Testing for Woven and Non-Woven Carbon Fibre-Reinforced Polymer Composites: a Topical Review

Abstract: Carbon fibre-reinforced polymer (CFRP) composites have been increasingly used by aerospace and other industries for their high specific stiffness and strength properties. When in service, non-destructive testing (NDT) methods are required to monitor and evaluate the structural integrity. Microwave-based detection techniques offer the advantages of non-contact, no need for a coupling medium or sensors bonded to the object surface and relatively easy setup. This paper is intended to provide a comprehensive overview of the currently available microwave techniques appropriate for carbon fibre/polymer composites. The electromagnetic properties of carbon fibre composites associated with microwave testing are discussed first. Then, the microwave methods are categorised into self-sensing methods, near-field induction methods, near-field resonance methods, far-field sensing methods and the methods with combination of other NDT (e.g., microwave-based thermography). Principles and applications of each kind are demonstrated in detail. Discussions of the advantages and limitations in addition to research trends of microwave testing methods are presented.

Evaluation of Basalt Fibre Composites for Marine Applications

Abstract: Basalt fibres offer potential for use in marine structures, but few data exist to evaluate the influence of seawater immersion on their mechanical behaviour. This paper provides the results from a study in which basalt fibre reinforced epoxy composites were aged in natural seawater at different temperatures. Tests were performed under quasi-static and cyclic loading, first in the as-received state then after saturation in natural seawater. Results are compared to those for an E-glass reinforced composite with the same epoxy matrix. They indicate similar mechanical performance for both materials after seawater saturation.

Numerical Evaluation of Dynamic Response for Flexible Composite Structures under Slamming Impact for Naval Applications

Abstract: The deformable composite structures subjected to water-entry impact can be caused a phenomenon called hydroelastic effect, which can modified the fluid flow and estimated hydrodynamic loads comparing with rigid body This is considered very important for ship design engineers to predict the global and the local hydrodynamic loads This paper presents a numerical model to simulate the slamming water impact of flexible composite panels using an explicit finite element method In order to better describe the hydroelastic influence and mechanical properties, composite materials panels with different stiffness and under different impact velocities with deadrise angle of 100 have been studied In the other hand, the inertia effect was observed in the early stage of the impact that relative to the loading rate Simulation results have been indicated that the lower stiffness panel has a higher hydroelastic effect and becomes more important when decreasing of the deadrise angle and increasing the impact velocity Finally, the simulation results were compared with the experimental data and the analytical approaches of the rigid body to describe the behavior of the hydroelastic influence

Improvement of Progressive Damage Model to Predicting Crashworthy Composite Corrugated Plate

Abstract: To predict the crashworthy composite corrugated plate, different single and stacked shell models are evaluated and compared, and a stacked shell progressive damage model combined with continuum damage mechanics is proposed and investigated. To simulate and predict the failure behavior, both of the intra- and inter- laminar failure behavior are considered. The tiebreak contact method, 1D spot weld element and cohesive element are adopted in stacked shell model, and a surface-based cohesive behavior is used to capture delamination in the proposed model. The impact load and failure behavior of purposed and conventional progressive damage models are demonstrated. Results show that the single shell could simulate the impact load curve without the delamination simulation ability. The general stacked shell model could simulate the interlaminar failure behavior. The improved stacked shell model with continuum damage mechanics and cohesive element not only agree well with the impact load, but also capture the fiber, matrix debonding, and interlaminar failure of composite structure.

Micro-tomography based Geometry Modeling of Three-Dimensional Braided Composites

Abstract: A tracking and recognizing algorithm is proposed to automatically generate irregular cross-sections and central path of braid yarn within the 3D braided composites by using sets of high resolution tomography images. Only the initial cross-sections of braid yarns in a tomography image after treatment are required to be calibrated manually as searching cross-section template. The virtual geometry of 3D braided composites including some detailed geometry information, such as the braid yarn squeezing deformation, braid yarn distortion and braid yarn path deviation etc., can be reconstructed. The reconstructed geometry model can reflect the change of braid configurations during solidification process. The geometry configurations and mechanical properties of the braided composites are analyzed by using the reconstructed geometry model.

Investigation into the Fiber Orientation Effect on the Formability of GLARE Materials in the Stamp Forming Process

Abstract: Glass-reinforced aluminum laminate (GLARE) is a new class of fiber metal laminates (FMLs) which has the advantages such as high tensile strength, outstanding fatigue, impact resistance, and excellent corrosion properties. GLARE has been extensively applied in advanced aerospace and automobile industries. However, the deformation behavior of the glass fiber during forming must be studied to the benefits of the good-quality part we form. In this research, we focus on the effect of fiber layer orientation on the GLARE laminate formability in stamp forming process. Experimental and numerical analysis of stamping a hemisphere part in different fiber orientation is investigated. The results indicate that unidirectional and multi-directional fiber in the middle layer make a significant effect on the thinning and also surface forming quality of the three layer sheet. Furthermore, the stress-strain distribution of the aluminum alloy and the unique anisotropic property of the fiber layer exhibit that fiber layer orientation can also affect the forming depths as well as the fracture modes of the laminate. According to the obtained results, it is revealed that multi-directional fiber layers are a good alternative compared to the unidirectional fibers especially when a better formability is the purpose.

Nonlinear Multi-Scale Modelling, Simulation and Validation of 3D Knitted Textiles

Abstract: Three-dimensionally (3D) knitted technical textiles are spreading into industrial applications, since their geometric, structural and functional performance can be tailored and optimized on fibre-, yarn- and fabric levels by customizing yarn materials, knit patterns and geometric shapes. The ability to simulate their complex mechanical behaviour is thus an essential ingredient in the development of a digital workflow for optimal design and manufacture of 3D knitted textiles. Here, we present a multi-scale modelling and simulation framework for the prediction of the nonlinear orthotropic mechanical behaviour of single jersey knitted textiles and its experimental validation. On the meso-scale, representative volume elements (RVEs) of the fabric are modelled as single, interlocked yarn loops and their mechanical deformation behaviour is homogenized using periodic boundary conditions. Yarns are modelled as nonlinear 3D beam elements and numerically discretized using an isogeometric collocation method, where a frictional contact formulation is used to model inter-yarn interactions. On the macro-scale, fabrics are modelled as membrane elements with nonlinear orthotropic material behaviour, which is parameterized by a response surface constitutive model obtained from the meso-scale homogenization. The input parameters of the yarn-level simulation, i.e., mechanical properties of yarns and geometric dimensions of yarn loops in the fabrics, are determined experimentally and subsequent meso- and macro-scale simulation results are evaluated against reference results and mechanical tests of knitted fabric samples. Good agreement between computational predictions and experimental results is achieved for samples with varying stitch values, thus validating our novel computational approach combining efficient meso-scale simulation using 3D beam modelling of yarns with numerical homogenization and nonlinear orthotropic response surface constitutive modelling on the macro-scale.

Process Modelling of Curing Process-Induced Internal Stress and Deformation of Composite Laminate Structure with Elastic and Viscoelastic Models

Abstract: In this paper, two kinds of transient models, the viscoelastic model and the linear elastic model, are established to analyze the curing deformation of the thermosetting resin composites, and are calculated by COMSOL Multiphysics software. The two models consider the complicated coupling between physical and chemical changes during curing process of the composites and the time-variant characteristic of material performance parameters. Subsequently, the two proposed models are implemented respectively in a three-dimensional composite laminate structure, and a simple and convenient method of local coordinate system is used to calculate the development of residual stresses, curing shrinkage and curing deformation for the composite laminate. Researches show that the temperature, degree of curing (DOC) and residual stresses during curing process are consistent with the study in literature, so the curing shrinkage and curing deformation obtained on these basis have a certain referential value. Compared the differences between the two numerical results, it indicates that the residual stress and deformation calculated by the viscoelastic model are more close to the reference value than the linear elastic model.

Damage evaluation of laminated composite material using a new acoustic emission Lamb-based and finite element techniques

Abstract: In this paper, a very promising procedure is proposed to evaluate delamination using Acoustic Emission (AE) technique in composite laminates. First, a new procedure was developed to decompose the fundamental Lamb wave modes in small size specimens. The damage mechanisms in End Notched Flexure (ENF) in woven and unidirectional specimens were then discriminated using Fuzzy Clustering Method (FCM). Afterwards, the crack-arrest phenomenon was examined in each specimen. After that, experimental and Cohesive Zone Modeling (CZM) techniques were conducted to characterize the delamination using ENF specimens. The results showed how, it is possible to successfully decrease the effect of propagating media such as attenuation of AE signals using the new proposed methodology. As a final point, the results of this study could lead to efficiently distinguishing different damages in laminated composite using AE Lamb-based technique.

New Design Concept for an Excavator Arms by Using Composite Material

Abstract: The purpose of the present paper is to lightweight design an excavator arms, by using a different materials and in particular composite material. Specifically, the research is based on the study of a commercial excavator, by determining its geometry and analyzing the load conditions to which it is exposed. These are determined in relation to either the load diagram of the machine or the possible utilities of the excavator, such as the rotation of the machine. The materials used and implemented in the different analytical and numerical elaborations are classic construction steel S 355 (UNI EN 10025–3), high-resistance steel S 890 (UNI EN 10025–6), aluminum Al 6063 T6 (UNI EN 573–3) and the composite material made by carbon fiber and epoxy resin. The adopted constraints for the design of new arms with different materials, non-conventional for these applications, are numerous. The new solutions must present a safety factor either with respect to the yield tensile strength or to the critical load of buckling greater than or equal to the one determined for the excavator in its original geometrical conformation. Another criterion, which has heavily conditioned the geometry of the arms, was given by the fact that the developed solutions must present a very similar value of the maximum displacement in the different load conditions analyzed. A new geometry for arms made by composite material was developed. It was an elliptical conic section, instead of the classic rectangular section, in order to use the filament winding technological process. As for the adoption of the composite material, we focused on the study and the design of this material as long as the interaction with the extremities (made of aluminum) which are interfaced either with the link between the arms or with the elements of the hydraulic plant which serves for the arms movement. From the results developed, it emerges that the solution developed by adopting composite materials is the one that permits the maximum weight reduction for all arms, about 68.1%, which can be seen as an increment of the maximum mass transportable about 45.5% i.e. passing from 5000 kg to 7277 kg.

Experimental Investigations of Compressed Sandwich Composite/Honeycomb Cylindrical Shells

Abstract: This article explains in some details the behaviour of thick, deep cylindrical sandwich panels subjected to compressive loads. In general, experimental results indicated that two different forms of failure have been observed – the first corresponds to the overall buckling and the second to the facesheet wrinkling. The obtained experimentally damages of shells are verified and validated with the use of the FE analysis, 2-D and 3-D both in the linear and non-linear approach. The unidirectional strain gauges were applied to detect the initiation of the overall buckling mode.

Meso-Scale Finite Element Simulations of 3D Braided Textile Composites: Effects of Force Loading Modes

Abstract: Meso-scale finite element method (FEM) is considered as the most effective and economical numerical method to investigate the mechanical behavior of braided textile composites. Applying the periodic boundary conditions on the unit-cell model is a critical step for yielding accurate mechanical response. However, the force loading mode has not been employed in the available meso-scale finite element analysis (FEA) works. In the present work, a meso-scale FEA is conducted to predict the mechanical properties and simulate the progressive damage of 3D braided composites under external loadings. For the same unit-cell model with displacement and force loading modes, the stress distribution, predicted stiffness and strength properties and damage evolution process subjected to typical loading conditions are then analyzed and compared. The obtained numerical results show that the predicted elastic properties are exactly the same, and the strength and damage evolution process are very close under these two loading modes, which validates the feasibility and effectiveness of the force loading mode. This comparison study provides a suitable reference for selecting the loading modes in the unit-cell based mechanical behavior analysis of other textile composites.

Development & Characterization of Green Composites Using Novel 3D Woven Preforms

Abstract: Green composites are the emerging materials made using natural fibers and environmentally degradable matrix such as green epoxy. Natural fiber composites are the motivation of researchers for low to medium impact applications as well as structural applications like automobiles. In this research work, 3D orthogonal layer to layer (LL) and through the thickness (TT) woven structures with different interlocking patterns, used as preforms in composites are presented. The mechanical properties of preform as well as associated composites are studied on equivalent fiber volume fraction. Jute yarn was woven into four layered 3D woven structures. The use of bridgeable and sustainable fiber, with its prospective use with the biodegradable matrix, is the objective of this work. The focus of this study is to improve mechanical performance by changing weave pattern, so that the resulting composite is robust in design.

The Need to Use Generalized Continuum Mechanics to Model 3D Textile Composite Forming

Abstract: 3D textile composite reinforcements can generally be modelled as continuum media. It is shown that the classical continuum mechanics of Cauchy is insufficient to depict the mechanical behavior of textile materials. A Cauchy macroscopic model is not capable of exhibiting very low transverse shear stiffness, given the possibility of sliding between the fibers and simultaneously taking into account the individual stiffness of each fibre. A first solution is presented which consists in adding a bending stiffness to the tridimensional finite elements. Another solution is to supplement the potential of the hyperelastic model by second gradient terms. Another approach consists in implementing a shell approach specific to the fibrous medium. The developed Ahmad elements are based on the quasi-inextensibility of the fibers and the bending stiffness of each fiber.

Hybrid Composite Using Natural Filler and Multi-Walled Carbon Nanotubes (MWCNTs)

Abstract: This paper presents an experimental study on the development of hybrid composites comprising of multi-walled carbon nanotubes (MWCNTs) and natural filler (oil palm shell (OPS) powder) within unsaturated polyester (UP) matrix. The results revealed that the dispersion of pristine MWCNTs in the polymer matrix was strongly enhanced through use of the solvent mixing method assisted by ultrasonication. Four different solvents were investigated, namely, ethanol, methanol, styrene and acetone. The best compatibility with minimum side effects on the curing of the polyester resin was exhibited by the styrene solvent and this produced the maximum tensile and flexural properties of the resulting nanocomposites. A relatively small amount of pristine MWCNTs well dispersed within the natural filler polyester composite was found to be capable of improving mechanical properties of hybrid composite. However, increasing the MWCNT amount resulted in increased void content within the matrix due to an associated rapid increase in viscosity of the mixture during processing. Due to this phenomenon, the maximum tensile and flexural strengths of the hybrid composites were achieved at MWCNT contents of 0.2 to 0.4 phr and then declined for higher MWCNT amounts. The flexural modulus also experienced its peak at 0.4 phr MWCNT content whereas the tensile modulus exhibited a general decrease with increasing MWCNT content. Thermal stability analysis using TGA under an oxidative atmosphere showed that adding MWCNTs shifted the endset degradation temperature of the hybrid composite to a higher temperature.

A Unit-Cell Model for Predicting the Elastic Constants of 3D Four Directional Cylindrical Braided Composite Shafts

Abstract: In this work, the elastic constants of 3D four directional cylindrical braided composite shafts were predicted using analytical and numerical methods. First, the motion rule of yarn carrier of 3D four directional cylindrical braided composite shafts was analyzed, and the horizontal projection of yarn motion trajectory was obtained. Then, the geometry models of unit-cells with different braiding angles and fiber volume contents were built up, and the meso-scale models of 3D cylindrical braided composite shafts were obtained. Finally, the effects of braiding angles and fiber volume contents on the elastic constants of 3D braided composite shafts were analyzed theoretically and numerically. These results play a crucial role in investigating the mechanical properties of 3D 4-directional braided composites shafts.

Simulation and Experimental Validation of Spacer Fabrics Based on their Structure and Yarn's Properties

Abstract: Warp-knitted spacer fabrics are considered, which are plates or shells composed of two knitted plane layers connected by vertical beams. Our aim is to compute the effective stiffness and permeability of such spacer fabrics on the basis of their structure and properties of yarns and the monofil. In order to reduce the computational effort and simplify the computational model, homogenization and dimension reduction techniques are applied. They replace the fabric by an equivalent two-dimensional plate or shell with effective elastic properties. To compute the effective permeability, the fluid simulation is done on the fully resolved micro-structure. The paper demonstrates the algorithm on application examples. We compute the elastic properties of a spacer fabric and its effective permeability for different outer-plane compression stages. Numerical examples were performed by applying the multi-scale simulation tools, developed at Fraunhofer ITWM and by comparing with the corresponding experimental results, based on measurements performed at the TU Dresden. The developed algorithms and simulation tools enable a full virtualisation of the material design adapted to exposure scenarios in various technical application cases, i.e. infiltration processes with polymers in the field of fiber reinforced composites, which enables new discoveries for the designing and manufacturing process of 3D warp-knitted spacer fabrics.

Detection and Evaluation of Pre-Preg Gaps and Overlaps in Glare Laminates

Abstract: Gaps and overlaps between pre-preg plies represent common flaws in composite materials that can be introduced easily in an automated fibre placement manufacturing process and are potentially detrimental for the mechanical performances of the final laminates. Whereas gaps and overlaps have been addressed for full composite material, the topic has not been extended to a hybrid composite material such as Glare, a member of the family of Fibre Metal Laminates (FMLs). In this paper/research, the manufacturing, the detection, and the optical evaluation of intraply gaps and overlaps in Glare laminates are investigated. As part of an initial assessment study on the effect of gaps and overlaps on Glare, only the most critical lay-up has been considered. The experimental investigation started with the manufacturing of specimens having gaps and overlaps with different widths, followed by a non-destructive ultrasonic-inspection. An optical evaluation of the gaps and overlaps was performed by means of microscope image analysis of the cross sections of the specimens. The results from the non-destructive evaluations show the effectiveness of the ultrasonic detection of gaps and overlaps both in position, shape, width, and severity. The optical inspections confirm the accuracy of the non-destructive evaluation also adding useful insights about the geometrical features due to the presence of gaps and overlaps in the final Glare laminates. All the results justify the need for a further investigation on the effect of gaps and overlaps on the mechanical properties.

The Effects of Triggering Mechanisms on the Energy Absorption Capability of Circular Jute/Epoxy Composite Tubes under Quasi-Static Axial Loading

Abstract: The usage of composite materials have been improving over the years due to its superior mechanical properties such as high tensile strength, high energy absorption capability, and corrosion resistance. In this present study, the energy absorption capability of circular jute/epoxy composite tubes were tested and evaluated. To induce the progressive crushing of the composite tubes, four different types of triggering mechanisms were used which were the non-trigger, single chamfered trigger, double chamfered trigger and tulip trigger. Quasi-static axial loading test was carried out to understand the deformation patterns and the load-displacement characteristics for each composite tube. Besides that, the influence of energy absorption, crush force efficiency, peak load, mean load and load-displacement history were examined and discussed. The primary results displayed a significant influence on the energy absorption capability provided that stable progressive crushing occurred mostly in the triggered tubes compared to the non-triggered tubes. Overall, the tulip trigger configuration attributed the highest energy absorption.

Buckling and Post-Buckling Behaviors of a Variable Stiffness Composite Laminated Wing Box Structure

Abstract: The buckling and post-buckling behaviors of variable stiffness composite laminates (VSCL) with curvilinear fibers were investigated and compared with constant stiffness composite laminates (CSCL) with straight fibers. A VSCL box structure was evaluated under a pure bending moment. The results of the comparative test showed that the critical buckling load of the VSCL box was approximately 3% higher than that of the CSCL box. However, the post-buckling load-bearing capacity was similar due to the layup angle and the immature status of the material processing technology. The properties of the VSCL and CSCL boxes under a pure bending moment were simulated using the Hashin criterion and cohesive interface elements. The simulation results are consistent with the experimental results in stiffness, critical buckling load and failure modes but not in post-buckling load capacity. The results of the experiment, the simulation and laminated plate theory show that VSCL greatly improves the critical buckling load but has little influence on the post-buckling load-bearing capacity.

Design and modeling of a combined embedded enhanced honeycomb with tunable mechanical properties

Abstract: Honeycomb structures are increasingly being used in many important fields. A novel combined embedded enhanced honeycomb (CEEH) in developed in this paper based on the two existing embedded enhanced honeycombs, the single rib embedded enhanced honeycomb (SREEH) and the rhombic grid embedded enhanced honeycomb (RGEEH). Analytical model related to the in-plane Young’s modulus and Poisson’s ratio is built and validated by using two different finite element (FE) models (3D beam model and 3D solid model). The in-plane elastic behavior of the honeycomb is also investigated against the geometrical parameters by using the numerically validated analytical solutions. The results show that the new CEEH can achieve a wide range value of Poisson’s ratio and Young’s modulus by tailoring geometric parameters. The results also show that the new CEEH exhibits higher x- directional specific stiffness than SREEH while higher y- directional specific stiffness than RGEEH. Moreover, the new CEEH can weaken even eliminate the difference between the two principal directions which can be hardly achieved by the SREEH and RGEEH. Given these advantages, this new design may be promising in some applications. This work provides a new insight into the designs of embedded enhanced honeycombs.

Numerical Simulation of Thermal Response and Ablation Behavior of a Hybrid Carbon/Carbon Composite

Abstract: The thermal response and ablation behavior of a hybrid carbon/carbon (C/C) composite are studied herein by using a numerical model. This model is based on the energy- and mass-conservation principles as well as on the calculation of the thermophysical properties of materials. The thermal response and ablation behavior are simulated from the perspective of the matrix and fiber components of a hybrid C/C composite. The thermophysical properties during ablation are calculated, and a moving boundary is implemented to consider the recession of the ablation surface. The temperature distribution, thermophysical properties, char layer thickness, linear ablation rate, mass flow rate of the pyrolysis gases, and mass loss of the hybrid C/C composite are quantitatively predicted. This numerical study describing the thermal response and ablation behavior provides a fundamental understanding of the ablative mechanism of a hybrid C/C composite, serving as a reference and basis for further designs and optimizations of thermoprotective materials.