Showing papers in "Journal of Composite Materials in 2004"

Modeling the Unsaturated Flow in Liquid Composite Molding Processes: A Review and Some Thoughts

Abstract: The liquid composite molding (LCM) processes for manufacturing polymer composites involve injecting a thermoset resin into a fiber-packed mold cavity. Very often, the fiber preform behind the resin front is partially saturated during the mold-filling process in LCM, giving rise to an unsaturated flow in that region. This paper first discusses the role of dual-scale porous media in inducing unsaturated flow in certain directional (woven, stitched, or braided) mats due to the delayed impregnation of tows. The approach of using a sink term in the mass balance equation is compared with the traditional approach of using unsaturated permeability for modeling unsaturated flow in dual-scale directional mats. Later, the relation of unsaturated flow with the phenomenon of bubble formation and migration is discussed. The lack of connection between the unsaturated flow research on the one hand, and the bubble creation and migration research on the other, is highlighted next. A few suggestions are offered on related t...

Permeability-Porosity Relationship in RTM for Different Fiberglass and Natural Reinforcements:

Abstract: This study investigates the permeability-porosity relationship for glass and natural fiber mats. The estimation of the permeability-porosity relationship for glass mats is not new and has been investigated before by several authors, but in the present paper the same relationship was discovered for natural fibers. The experimental permeability values were fitted by the Carman-Kozeny equation with two fitting parameters: the Kozeny constant and the exponent of the porosity. The obtained permeability for the natural fiber mats shows higher values than glass fiber mat.

Interlaminar Stress Concentrations in Layered Structures: Part I - A Selective Literature Survey on the Free-Edge Effect since 1967

Abstract: Stress concentration phenomena in composite laminates are technically important situations. A well-known problem of this class is the free-edge effect in composite laminates or as a superordinated example the stress concentrations in the vicinity of free laminate corners (so-called free-corner effect). The present work is split into two parts. In the present contribution, after a short introduction to the given stress concentration problems in general we will survey relevant selected literature on the classical free-edge effect dating from 1967 until today. Beside accentuation on approximate closed-form analytic methods for the stress analysis in the free-edge effect situation, numerous references on numerical methods and investigations on the occurring stress singularities are also cited. In a subsequent paper we will present a simple closed-form method for the analysis of the stress fields in the vicinity of free laminate corners with arbitrary layup. The method is based on adequate stress shape assumpt...

The Integrity of Polymer Composites during and after Fire

Abstract: This paper reports on changes to the mechanical properties of woven glass laminates with polyester, vinyl ester and phenolic resins during fire exposure. Two sets of experiments were carried out. First, unstressed laminates were exposed to a constant one-sided heat flux (50kWm(-2)) for various times, and the residual post-fire strength at room temperature was reported. In a second series of experiments, laminates were tested under load. The times corresponding to a given loss of properties were 2-3 times shorter than in the previous case. It was found in both cases that modes of loading involving compressive stress were more adversely affected by fire exposure than those involving tension. A simple 'two-layer' model is proposed, in which the laminate is assumed to comprise (i) an unaffected layer with virgin properties and (ii) a heat-affected layer with zero properties. For residual properties after fire, the 'effective' thickness of undamaged laminate was calculated using this model and compared with measured values. A thermal model was employed to predict the temperature and the residual resin profile through the laminate versus time. Comparing the model predictions with the measured values of effective laminate thickness enabled simple criteria to be developed for determining the position of the 'boundary' between heat-affected and undamaged material. For post-fire integrity of unloaded laminates, this boundary corresponds to a Residual Resin Content (RRC) of 80%, a criterion that applies to all the resin types tested.

Dynamic Compressive Response and Failure Behavior of an Epoxy Syntactic Foam

Abstract: The high-strain-rate compressive behavior of an epoxy syntactic foam is examined in this study. A pulse-shaped split Hopkinson pressure bar (SHPB), modified for low-impedance material testing, was used to ensure that the samples deformed under dynamic equilibrium and at a nearly constant strain-rate. Dynamic stress equilibrium in the specimen was monitored for each experiment using piezoelectric force transducers mounted close to the specimen end-faces. Quasi-static experiments were also conducted to demonstrate rate effects of the foam, as well as to study its failure behavior. It was determined that the compressive strength of the foam increased with strain rate up to a transition strain rate of between 550 and 1030 s 1. For experiments conducted at strain rates above this transition range, strain-rate-induced damage caused the compressive strength of the foam to decrease. Based on the experimental results, a constitutive model with strain-rate and damage effects was developed, which described the test ...

Failure Modes of Foam Core Sandwich Beams under Static and Impact Loads

Abstract: In recent times, sandwich structures have been widely used in load bearing structures due to their high specific stiffness and high specific strength. Some sandwich structures, such as those used i...

Effect of Chemical Treatment on the Mechanical Properties of Starch-Based Blends Reinforced with Sisal Fibre

Abstract: Biocomposites were produced using polycaprolactone (PCL) and starch as the matrix, and washed and treated sisal fibres as reinforcement. The matrix consists of a biodegradable product commercially known as MaterBi-Z®, which is based on a PCL, starch and additive.Alkaline and acetylation treatments were performed on the fibre in order to enhance the adhesion degree and the compatibility between the fibre and the matrix. The effect of chemical treatment on morphology, physical and chemical properties and tensile properties of fibres and composites were determined. Tensile properties of the biodegradable composite were improved by the presence of the fibre. However, the untreated fibres behaved as better reinforcement than the acetylated and alkalitreated fibres. This was attributed to an impairment of the mechanical properties of the acetylated fibres and incompatibility of the alkali-treated fibres. The results observed are supported by SEM analysis of fibres and the composite materials.

Mechanical Property Characterization of a Polymeric Nanocomposite Reinforced by Graphitic Nanofibers with Reactive Linkers

Abstract: Mechanical property characterization including bending, tensile, and fracture properties for a new functionalized nanofiber/epoxy composite were conducted. Results show that there was only very little increase in mechanical properties of nanocomposites although we used GCNF-ODA reactive linkers to improve the interface. The interfacial stress level of nanocomposites should be much higher than that of traditional composites because of high property mismatch between the nanoscale reinforcement and the matrix. In order to design strong and stiff nanocomposite materials, one should use aligned nanofibers with a relatively large volume or weight fraction. Also, the length of the nanofiber should be long enough and its diameter not very small in order to facilitate the interfacial load transfer mechanism.

Nonlinear Tensile and Shear Behavior of Macro Fiber Composite Actuators

Abstract: The Macro Fiber Composite (MFC) actuator, developed at the NASA Langley Research Center, offers much higher flexibility and induced strain levels (~2000μσ, peak-to-peak) than its monolithic piezoceramic predecessors. The focus of this work is twofold; to measure the four independent linear elastic engineering constants of the orthotropic MFC actuator under short-circuit electrical boundary conditions using standard tensile testing procedures, and to use these experimental results to characterize the nonlinear tensile and shear stress–strain behavior and Poisson effects using various plastic deformation models. The results can then be readily incorporated into the piezoelectric constitutive equation and ultimately into structural actuation models that accurately consider nonlinear mechanical behavior.

Finite Element Micromechanics for Stiffness and Strength of Wavy Fiber Composites

Abstract: In this paper, a micromechanical model of a composite lamina material with fiber waviness is described. Results are presented and discussed with regard to stiffness and strength predictions for composite lamina. A micromechanical model of a unit cell from periodically distributed unidirectional waved cylindrical fibers embedded within matrix is proposed to withdraw the different material stiffness parameters. Finite element analysis of the periodic unit cell characterizing the structural stiffness of the composite material is carried out to determine the average stress and strain components. The composite stress-strain relations are then employed to determine the stiffness parameters. Numerical results for a typical composite constituted of polymer matrix and carbon fibers in the form of periodically hexagonal packing and initially sinusoidal waviness are presented for different amplitude to wavelength ratios and a range of fiber volume fractions. The results reveal the presence of local periodic-antisymm...

The Design, Matching and Manufacture of Auxetic Carbon Fibre Laminates

Abstract: Either an in-plane or out-of-plane negative Poisson’s ratio,, in continuous carbon fibre/epoxy resin composites can be achieved, providing the fibre volume fraction and anisotropy are high enough, by selecting suitable stacking sequences. This paper examines the use of specially designed software which allows the designer to match the mechanical properties of laminates with predicted negative to those with similar mechanical properties but positive. This has allowed auxetic and matched carbon fibre/epoxy resin laminates to be specifically designed. These laminates were then fabricated and tested, with good agreement found to theoretical predictions. This study, then, provides a route to evaluating the effect of a negative alone on properties such as fracture toughness and impact resistance.

Mechanics of Failure of Composite Laminates with an Embedded Fiber Optic Sensor

Abstract: Embedding fiber optic sensor (FOS) in fiber reinforced composites caused resin pockets and geometric disturbance of the reinforcing fibers around the optical fiber. The disturbance angle varied from 6.2 to 8.2 and the resin pocket area varied from 7 to 10 times the cross-sectional area of the sensor. These two act like defects and cause premature failure initiation. Failure initiation was hypothesized as due to transverse stress concentration. A static experimental study was conducted to evaluate the effect of an embedded optical fiber in unidirectional composite laminates. Inclination of FOS with respect to reinforcing fiber (loading direction) varied from 0 to 90 and the laminate thickness varied from 8 to 20 plies. The embedment had practically no loss of tensile modulus and about 10% loss of tensile strength while compression strength reduction was as large as 40%.

Process and performance evaluation of the Vacuum-Assisted Process

Abstract: Vacuum-Assisted Resin Transfer Molding (VARTM) is widely used for large-scale composite manufacturing of civil and defense applications. Here, the infusion process reduces part costs due to a decrease in labor, material, and equipment expenses compared to other composite manufacturing techniques. However, in order to replace conventional manufacturing methods for aerospace-quality parts such as autoclave processing, the VARTM process repeatability and part quality must be improved. The Vacuum-Assisted Process (VAP) (Filsinger, J., Lorenz, T., Stadler, F. and Utecht, S. (2001). Method and Device for Producing Fiber-reinforced Components Using an Injection Method, German Patent WO 01/68353 A1.) developed and patented by EADS Deutschland uses a gas-permeable membrane to allow for uniform vacuum distribution and continuing degassing of the infused resin.W.L. Gore & Assoc. GmbH has developed a suitable membrane in co-operation with EADS. The membrane is placed over the fabric layers, sealed to the tool, and co...

Characterization of impact properties and damage process of glass/epoxy composite laminates

Abstract: This paper investigates the responses of glass/epoxy composite laminates subjected to impact loading. It presents a new technique to characterize the impact properties and to correlate them with the damage process of the composite laminates. The technique, called the energy profiling technique, is based on instrumented impact tests and the least-squares method. The technique gives specific definitions of the penetration and perforation thresholds of the composite laminates. It also defines the range of the penetration process and quantifies the energy absorption efficiency of the composite laminates. The primary damage modes in the damage process of the composite laminates can also be correlated with the impact properties by using the energy profiling technique. The advantages of using this technique to analyze the experimental data obtained from impact tests are demonstrated by investigating the glass/epoxy composite laminates with various impactor sizes, laminate thicknesses, fiber orientations, and a s...

Measurements of Thermal Properties of Carbon/Epoxy and Glass/Epoxy using Modulated Temperature Differential Scanning Calorimetry

Abstract: Major potential of composite materials relies in the nonlinear behavior triggered by their inhomogeneous nature. Particularly in heat diffusion, composite materials present a high variation of thermal properties as a function of temperature. Therefore, the spectrum of a propagating thermal wave can contain higher harmonics of the excitation frequency. The amplitude of these harmonics depends on the range of temperatures developed inside the material. This study is focused on the mathematical formulation of the relationship between thermal properties and temperature. To this end, the heat capacity and the thermal conductivity of Carbon/Epoxy and Glass/Epoxy cross-ply laminates were determined in a temperature range of interest for the aircraft industry using an ASTM method based on Modulated Temperature Differential Scanning Calorimetry. The results are indispensable toward a nonlinear treatment of heat diffusion phenomena and the respective exploitation for nondestructive testing.

Elastic-Plastic Model of Adhesive-Bonded Single-Lap Composite Joints

Abstract: An analytical model for determining adhesive stress distributions within the adhesive-bonded single-lap composite joints was developed. ASTM D3165 ‘‘Strength Properties of Adhesives in Shear by Tension Loading of Single-Lap-Joint Laminated Assemblies’’ test specimen geometry was followed in the model derivation. In the model derivation, the composite adherends were assumed linear elastic while the adhesive was assumed elastic-perfectly plastic following von Mises yield criterion. Laminated Anisotropic Plate Theory was applied in the derivation of the governing equations of the bonded laminates. The adhesive was assumed to be very thin and the adhesive stresses are assumed constant through the bondline thickness. The entire coupled system of equations was determined through the kinematics relations and force equilibrium of the adhesive and the adherends. The overall system of governing equations was solved analytically with appropriate boundary conditions. Computer software Maple V was used as the solution...

Effects of Non-Negative Mean Stress on the Off-Axis Fatigue Behavior of Unidirectional Carbon/Epoxy Composites at Room Temperature

Abstract: Influences of non-negative mean stress on the off-axis fatigue behavior of unidirectional composites are elucidated on the basis of a non-fragmentary set of data on the T800H/2500 carbon/epoxy lami...

Ballistic Penetration of Conically Cylindrical Steel Projectile into Plain-woven Fabric Target – A Finite Element Simulation:

Abstract: This paper gives a finite element model to simulate the entire process of multilayered fabric target perforated by conically cylindrical steel projectile on the basis of the description of the actual structure of fabric considering crimps of warp and weft yarns. Commercially available finite element code Ls-Dyna is incorporated with the Weibull constitutive equations of filament yarns at high strain rate to simulate the ballistic impact response. The projectile and yarns in the fabric are meshed with a eight-node hexahedron element (Solid 164 in Ls-Dyna). From the comparisons of the residual velocities of the projectile, and deformation and damages of the fabric target after ballistic perforation between the experimental results and theoretical ones, it is proven that at the yarn-structural hierarchy, the finite element method with explicit algorithm and code Ls-Dyna could precisely simulate the ballistic impact between the steel projectile and the fabric target. It is also found from calculations that on...

Compressive Failure of Impacted NCF Composite Sandwich Panels - Characterisation of the Failure Process:

Abstract: In the present study, non-crimp fabric (NCF) composite face sheet sandwich panels have been tested in compression after impact (CAI). Damage in the face sheets was characterised by fractography. Co ...

Modeling of friction and shear in thermostamping of composites - Part I

Abstract: The effects of processing parameters on the friction coefficient between commingled glass-polypropylene plain-weave fabric composites (Twintex) and the steel tool during thermostamping processes are under investigation. This work focuses on the effect of fiber orientation, fabric velocity, normal force, and resin viscosity (through variations in tool and fabric temperatures) under conditions similar to those in the thermostamping processes. In comparison to the experiments conducted to date, velocity, normal force, and tool temperature have the greatest effect on the friction coefficient. The effect of tool temperature on the friction coefficient dominated the effect of initial fabric temperature on the friction coefficient. Based on the effect of these parameters, a phenomenological model has been incorporated into ABAQUS/Standard as a user-supplied friction subroutine. This model was first used in a finite element model of the friction test. A good agreement was found between the experimentally measured...

A Statistical Approach to Permeability of Clustered Fibre Reinforcements

Abstract: The focus is set on mesoscale modelling of permeability of real fabrics used in composite manufacturing. Of particular interest is the effect of expected perturbations from perfect geometries, such as fibre bundle crimp, on the permeability. To start with, variational methods are used to calculate the permeability of individual gaps between fibre bundles. Based on this study a network of unit cells is formed enabling studies of two-and three-dimensional flow through the structure. From such an analysis the overall permeability of an arbitrary distribution of unit cell permeabilities can be calculated. Here random and controlled distributions are simulated. The former is an approximate representation of a continuous strand mat and the latter may describe Non-Crimp Fabrics. The result is that for random distributions, the permeability decreases with the maximum variation in unit cell while for a controlled permeability distribution the overall permeability can as well increase as decrease depending on the t...

Effect of Platelet Dispersion on the Load Transfer Efficiency in Nanoclay Composites

Abstract: Load transfer efficiency of nanoclay platelets in a polymer matrix wasinvestigated. Both tactoid nanocomposites and uniformly dispersed nanocompositeswere considered. The unit cell suitable for mod...

Experimental Investigation of the Effect of Fiber-mat Architecture on the Unsaturated Flow in Liquid Composite Molding:

Abstract: In liquid composite molding technologies such as RTM, this study of the inlet-pressure history for the constant flow-rate 1-D flow experiment reveals that the measured pressure profile, which droop downwards as in earlier studies on the unsaturated flow, is at a variance with the linear pressure profile predicted by the physics used for state-of-the-art LCM mold filling simulations. The droop along with the error in the inlet-pressure predictions increase with an increase in the fibermat compression. The effect of fiber-mat architecture on the droop in the inletpressure profiles is studied and significant droops are observed for various stitched mats as compared to the woven mats. This study repudiates the long-held view that mere presence of cylindrical or elliptical tows in the woven or stitched fiber-mats automatically leads to the unsaturated flow characteristic of dual-scale porous media; rather the presence of continuous uninterrupted macrochannels along the flow direction for preferential channel-f...

Through-Thickness Mechanical Characterization of Woven Fabric Composites

Abstract: The objective of this study was to investigate the interlaminar orthrough-thickness deformation and failure of woven fabric composites subjected toquasi-static loading and develop testing methods a...

In Situ Fracture Toughness Testing of Core Materials in Sandwich Panels

Abstract: The in situ fracture toughness of six core materials was measured using a new test method. The materials tested were seven composite sandwich panels fabricated using vacuum-assisted resin transfer ...

Thermal Phenomena in Fiber-reinforced Thermoplastic Tape Winding Process: Computational Simulations and Experimental Validations

Abstract: This paper presents experimentally validated three-dimensional transient simulations of the thermal phenomena of the tape winding process, as well as a method to determine separately the heat transfer between the hot gas originating from a torch and the composite material. The computational model predicts the temperature of the incoming tape and the substrate during the winding process. Each numerical simulation is based on an explicit time integration scheme and covers the duration of the process. The simulation within each time step employs a steady-state model. This method takes into account the cyclic nonuniform heating of the material and the effect of the growing mass. The comparison of the simulation results with the experimental data shows good agreements. The experiments were performed with preconsolidated glass fiber-reinforced polypropylene tapes. The measurements were performed with infrared pyrometry. This technique can handle moving points during the entire process, and is nonintrusive.

Delamination Failure Investigation for Out-of-plane Loading in Laminates:

Abstract: In contrast to failure approaches at the lamina level or the micromechanics level, the present work concerns failure characterization at the laminate level. Specifically, attention is given to the ultimate failure characterization for quasi-isotropic laminates. This is in further contrast to the commonly used approaches for initial damage or progressive damage. It is shown that the analytical failure forms decompose into two modes, one for out-of-plane, delamination-type failure and one for in-plane, fiber-controlled-type failure. The work here is mainly given over to the delamination mode of failure. Experimental results are presented for laminates in this mode of failure. These results are then integrated with the analytical forms to give a simple criterion for delamination failure.

The Effects of Through-thickness Compression on the Interlaminar Shear Response of Laminated Fiber Composites

Abstract: The effects of through-thickness compression on the interlaminar shear response of laminated fiber composites were studied. The combined stresses were generated using a hollow cylindrical specimen that was subjected to axial compression and torsion. For both glass- and carbon-fiber composites, through-thickness compression resulted in a significant enhancement in the interlaminar shear stress and strain at failure. Under moderate compression levels, the failure mode changed from elastic to plastic. An attempt was made to predict the observed increase in shear strength for carbon fiber epoxy laminates using three-dimensional lamina failure criteria. Although all the failure theories correctly predicted the trend of increasing shear strength with compression, none were able to predict the full extent of the observed strength increase. These results indicate that improved models are needed for determining failure under a combined state of interlaminar stress. The experimental results demonstrate that there a...

Deformation Behavior of NiTi/Polymer Shape Memory Alloy Composites – Experimental Verifications

Abstract: Composites containing NiTi shape memory alloy (SMA) long-fiber, short-fibers or Ti long-fiber in a Polycarbonate (PC) matrix have been fabricated by the injection molding technique. Also, prestrained SMA long-fiber/Epoxy matrix composites have been fabricated. The fracture behavior and thermo-mechanical deformation behavior are examined; (1) Fracture behavior – uniaxial tensile tests up to fracture for SMA long-fiber and short-fiber composite (SMAC). (2) Thermomechanical deformation behavior – tensile loading–unloading tests for Pseudoelastic (PE) long-fiber/PC matrix composites. Several thermo-mechanical loading tests for Shape Memory Effect (SME) long-fiber/PC matrix and SME long-fiber/Epoxy matrix composites were used.The obtained results are as follows: (1) The stress–strain relation up to the final fracture of the Shape Memory Alloy Composites (SMACs) showed the repeated up-and-down of the stress which corresponds to the necking of the specimen, fiber fracture, and matrix fracture. The strain for the...

Characterization of Microcrack Development in BMI-Carbon Fiber Composite under Stress and Thermal Cycling

Abstract: The objective of this research was to determine the effect of thermal cycling on the development of microcracks in bismaleimide (BMI)-carbon fiber composites (5250-4 RTM/IM7 4-harness satin weave fabric). By clamping composite specimens on the radial sides of half cylinders having two different radii (78.74 and 37.96 mm), two different strain conditions with respect to the neutral axis (0.406 to 0.406% and -0.843 to 0.843%) were applied to the composites. Three different thermal cycling experiments: (1) -196 to 250°C, (2) 23°C to (i) 150°C, (ii) 200°C, (iii) 250°C, and (3) -196 to 23°C were performed as a function of stress, number of thermal cycles, heating or cooling rate, and humidity conditions. An in situ monitoring microscope was used to observe the microcrack development during the experiment. The results suggest that there is a higher probability of microcracking with increasing number of thermocycles, higher prestrain, and humidity. The principle findings are that the full cycles from 196 up to 2...