Showing papers in "Journal of Composite Materials in 2003"

Numerical simulation of mixed-mode progressive delamination in composite materials

Abstract: A new decohesion element with the capability of dealing with crack propagation under mixed-mode loading is proposed and demonstrated. The element is used at the interface between solid finite elements to model the initiation and non-self-similar growth of delaminations in composite materials. A single relative displacement-based damage parameter is applied in a softening law to track the damage state of the interface and to prevent the restoration of the cohesive state during unloading. The softening law is applied in the three-parameter Benzeggagh-Kenane mode interaction criterion to predict mixed-mode delamination propagation. To demonstrate the accuracy of the predictions, steady-state delamination growth is simulated for quasi-static loading of various single mode and mixed-mode delamination test specimens and the results are compared with experimental data.

S2-Glass/Epoxy Polymer Nanocomposites: Manufacturing, Structures, Thermal and Mechanical Properties

Abstract: This paper is primarily focused in studying the effects of nanoclay particles such as montmorillonite on improving mechanical and thermal properties of fiber reinforced polymer matrix composite materials. Basic correlations between polymer morphology, strength, modulus, toughness, and thermal stability of thermoset nanocomposites were investigated as a function of layered silicate content. S2-glass/epoxy-clay nanocomposites were manufactured through an affordable vacuum assisted resin infusion method (VARIM). The nanocomposites are formed during polymerization when the adsorbing monomer separates the clay particles into nanometer scales. Transmission electron microscopy (TEM) and wide angle X-ray diffraction(WAXD) were used to characterize the morphology of the dispersed clay particles. The thermal properties such as onset of decomposition and glass transition temperatures were determined by Thermo Gravimetric Analysis (TGA) and Dynamic Modulus Analyzer (DMA). Mechanical properties such as interlaminar sh...

Exact Solution for Functionally Graded Anisotropic Elastic Composite Laminates

Abstract: Exact solutions are derived for three-dimensional, anisotropic, linearly elastic, and functionally graded rectangular composite laminates under simply supported edge conditions. The solutions are expressed in terms of an elegant formalism that resembles the Stroh formalism, and the composite laminates can be made of multilayered functionally graded materials with their properties varying exponentially in the thickness direction. The present solution extends Pagano's solution to the functionally graded material, and can serve as a benchmark to the modeling of functionally graded composite laminates based on various numerical methods. Typical results of the present solution are discussed for a single functionally graded plate and a bi-layer plate with a functionally graded layer overlying a homogeneous layer. For both plates, a simple load is applied on their top surfaces. It is shown that with a suitable functionally graded layer, the tensile stress on the top surface (or the compressive stress on the bott...

Mechanical Properties and Water Absorption Behavior of Composites Made from a Biodegradable Matrix and Alkaline-Treated Sisal Fibers

Abstract: Biocomposites were produced using a biodegradable material as matrix, and sisal fibers as reinforcement. The biodegradable material is a commercial product called MaterBi-Y, which is based on a cellulose derivatives and starch system. The characterization of these biocomposites was not done before and it is necessary in order to select a material instead of nonbiodegradable matrices. An alkaline treatment was performed in order to improve the mechanical properties of the fiber. The effect of the treatment on fiber properties and on impact, flexural, and tensile properties of composites were determined. Fiber content enhances the tensile properties of the biodegradable matrix. Water sorption studies were performed. The experimentally observed tensile properties (modulus and tensile strength) of short sisal fiber-reinforced cellulose derivatives/starch composites with different fiber loading are compared with the calculated values obtained from the existing theories of reinforcement.

Determination of residual stress in composite laminates using the incremental hole-drilling method

Abstract: The cooling conditions used in the forming process of composite materials play an important role in the creation ofresidual stress. In this study, a new method for measuring residual stress in composite laminates is presented. Three cooling conditions were used to produce different residual stress levels. Residual stresses in [02/902]s and [08] laminate have been measured by the incremental holedrilling method combined with 3-D finite element modelling. A software which quickly calculates all the coefficients for each increment was developed. The automatic procedure can be used to calculate the calibration coefficient for any type oflaminate (ply number, mechanic characteristics,...) and whatever the number of increments and their depths. The different results show that this method provides access to the in-depth distribution and through thickness ofresidual stress in the laminate with a good accuracy and practicality.

Evolution of Failure Mechanisms in 2D and 3D Woven Composite Systems Under Quasi-Static Perforation:

Abstract: The effects of reinforcement geometry, in 2D and 3D woven fabric-reinforced composites, on the progression of damage and perforation failure at quasi-static loading rates (10-80 mm/s) are investigated The broad classes of glass-fiber-reinforced systems that were examined include 2D plain-woven laminates, 3D orthogonally woven monolithic systems, and 3D orthogonally woven laminates The experimental results indicate that the 3D laminates consistently had greater damage tolerance than the 2D laminates and the 3D monolithic composites The enhanced damage tolerance of the 3D systems is due to unique energy absorption mechanisms, which involve the crimped portion of z-tows

Quasi-Static and Dynamic Compressive Behaviors of a S-2 Glass/SC15 Composite

Abstract: A pulse-shaped split Hopkinson pressure bar (SHPB) was employed to determine the dynamic compressive mechanical responses and failure behaviors of a S-2 glass/SC15 composite along two perpendicular directions under valid dynamic testing conditions. The loading pulses in SHPB experiments were precisely controlled to ensure that the composite specimen deforms at a nearly constant strain rate under dynamically equilibrated stress during dynamic compression. Quasi-static experiments were conducted with an MTS and an Instron to study material rate sensitivity over a wider range. The compressive stress-strain behaviors along both directions were found to be strain-rate sensitive, but with different strain-rate sensitivities. A compressive constitutive model with strain-rate and damage effects was modified to accurately describe both quasi-static and dynamic compressive stress-strain behaviors of the composite material along the two perpendicular directions.

Progressive Failure Behaviors of 2D Woven Composites

Abstract: This paper focuses on understanding the progressive failure behavior of woven composites. Five weaves, i.e. plain, 4-, 5-, 8-harness satin and twill, are considered. Rather than developing a new progressive failure analysis approach, the focus is placed on comparing the damage behaviors of the various weaves predicted by the selected failure criterion and property degradation model. The loading conditions include uniaxial tension and compression. The discussions focus on (1) the effect of the woven architecture on the predicted progressive failure behaviors (2) the similarities and difference in the damage initiation and evolution mechanisms between the plain and satin weaves and (3) the sensitivity of the predicted progressive failure behavior to assumptions about geometric nonlinearity and the property degradation model. The results have shown that the weave architecture (i.e. weave pattern) has significant effects on the predicted progressive failure behaviors even if the composites have the same overa...

General Techniques for Exploiting Periodicity and Symmetries in Micromechanics Analysis of Textile Composites

Abstract: General formulas for obtaining boundary conditions for micromechanics analysis of textile composites are developed based on the concept of equivalent subcell, which is the smallest region that has

Relationship between Dynamic Mechanical Properties and Water Absorption of Unsaturated Polyester and Vinyl Ester Glass Fiber Composites

Abstract: The effect of hydrothermal degradation on polyester and vinyl ester mat glass fiber composites on viscoelastic properties is investigated by dynamic mechanical thermal analysis and flexural modulus. The adhesion between fiber and matrix is studied by short-beam shear test and by SEM microphotographs. Storage modulus in the glass state (Tg - 50 K) and rubber state (Tg + 50 K) is also analyzed for the matrix and composites samples, before and after water immersion. Loss tangent (δ) is analyzed taken into account the values obtained for the materials in moisture sorption experiments.

Modeling of Heat Transfer and Void Dynamics for the Thermoplastic Composite Tow-Placement Process

Abstract: Thermoplastic composite tow-placement technology is a nonautoclave consolidation process that offers the potential to significantly reduce fabrication costs by using in situ consolidation as a meth...

Modified Short Beam Shear Test for Measurement of Interlaminar Shear Strength of Composites

Abstract: A modified short beam shear test was proposed to measure interlaminar shear strength (ILSS) of fiber reinforced carbon-carbon composites (CCC). The direct loading in the short beam shear (SBS) test is replaced by an indirect loading through a rubber pad and an aluminum seat. This loading reduced flexure stresses to less than one fourth of the SBS test and eliminated transverse shear stress concentration under the load point. Both beam and contact finite element analyses were conducted to validate the proposed method. The finite element analysis showed that the ILSS calculated from beam equation overestimates the strength by 5% for shear flexible composites. This reduction was found to be the same for both polymeric and carbon matrix composites. The test data confirmed the interlaminar shear failure and very low data scatter in measured strength. The measured ILSS of T300 CCC was 2.68 ksi and it is one of the highest values reported in literature.

Cured Shape of Cross-Ply Composite Thin Shells

Abstract: For a composite laminated plate, it has been found that classical laminate theory (CLT) can not always predict the final cured shape correctly and geometric nonlinearity must be considered. For com...

Interlocked Composite Grids Design and Manufacturing

Abstract: Composite grid structures made from pultruded unidirectional glass or carbon ribs promise to provide unmatched performance/cost combination of any composite panels. A new manufacturing method for a...

Fatigue and Reliability Analysis of Unidirectional GFRP Composites under Rotating Bending Loads

Abstract: Rotating bending fatigue tests have been conducted on unidirectional glass fiber reinforced polyester (GFRP) composites Standard test specimens were manufactured in form of circular rods with vari

Rapid inspection of composite skin-honeycomb core structures with ultrasonic guided waves

Abstract: Guided wave inspection of composite skin-honeycomb core structures is an efficient and sensitive alternative to other common inspection methods. This paper shows that sweeping experimentally through the dispersion curves is an effective way to experimentally locate guided wave modes sensitive to skin-core delamination. Composite skin-Nomex honeycomb core specimens were developed with simulated delaminated areas. The delaminated areas were detected with guided waves and confirmed with conventional ultrasonic testing methods. Calculated phase velocity dispersion curves are given to define the practical phase velocity and frequency ranges. Example wave structures in this range are given to illustrate the change in sensitivity as frequency is swept for a given mode.

A comparison of oven-cured and induction-cured adhesively bonded composite joints

Abstract: The advantages of using adhesives for joining composite structures are now well accepted. Adhesive joints may offer, over bolted joints, advantages such as a lower assembly weight, a superior stress transfer and an improved fatigue resistance. However, in some applications the above advantages may be offset by the processingconditions required to cure the adhesive. Indeed, in the conventional oven curing process the thermal energy must diffuse through the composite layers to heat the joint interfaces, resultingin longand expensive processingtime as well as wasted energy. A novel method of achieving adhesive bonds is addressed in the present study. The method of electromagnetic heating is well suited for rapid and efficient localized heatingof adhesive bond lines, provided suitable susceptors are used at the interfaces. This paper presents the results of a study on the use of induction heating for bondingcomposite adherends. Single-lap shear tests and double-cantilever beam fracture experiments were perfor...

Failure Load Prediction of Mechanically Fastened Composite Joints

Abstract: With the wide application of fiber-reinforced composite material in aero-structures and mechanical parts, the design of composite joints has become a very important research area because they are often the weakest regions in composite structures. In this paper, a failure area index method is proposed to predict failure loads of mechanically fastened composite joints under plane stress condition with the same stacking sequence. The suggested failure area index method is shown to produce very favorable comparisons with measured failure loads of mechanically fastened composite joints with the difference well within 10% for all 17 cases investigated.

On-Line Characterization of Bulk Permeability and Race-Tracking During the Filling Stage in Resin Transfer Molding Process

Abstract: Resin Transfer Molding (RTM) is widely used to manufacture polymer composite materials. In this process, the fiber preform is placed in a closed mold and thermoset resin is injected to saturate the preform. After the resin cures the mold is opened and the net shape composite part is obtained. With RTM, one is capable of making complex and high quality composite parts with short cycle times. However, by introducing more complexity into the part, one also introduces higher probability of disturbances, such as race-tracking of resin during impregnation along preform edges. This can lead to incomplete saturation of fiber preform forming flaws such as dry spots in the composite part. The strength and existence of race-tracking is a function of the fabric type, perform manufacturing method, and their placement in the mold. It can vary from one part to the next in the same production run and usually it is not repeatable. The characterization of race-tracking and preform permeability is a key input for simulation...

Processing of Iron Oxide-epoxy Vinyl Ester Nanocomposites:

Abstract: An ultrasonic method is used to disperse iron oxide particles of two different sizes in an epoxy vinyl ester resin. The dispersion quality is assessed from sedimentation, density measurements, and scanning electron micrographs. By varying parameters such as power and duration of sonication, optimum dispersion conditions are suggested. Dispersion is most effective in the early stage of sonication and becomes more difficult to improve later. Once separated, however, the particles do not come into direct contact with one another because of the steric hindrance. The nano-size (29 nm) particles delay curing whereas the larger (200 nm) particles do not. The results indicate a strong interaction between nanoparticles and the resin.

Interlaminar damage model for polymer matrix composites

Abstract: A constitutive model for fiber-reinforced composite materials with damage and unrecoverable deformation, which for the first time accounts for interlaminar damage, is presented. The formulation is based on Continuous Damage Mechanics coupled with Classical Plasticity Theory in a consistent thermodynamic framework using internal state variables. In-plane damage and novel formulation of interlaminar damage are included in order to describe the main failure modes of laminates structures. A novel implementation of the constitutive model into a finite element formulation incorporating geometric nonlinearity is presented. The model uses a small number of adjustable parameters, which are identified from available experimental data. Comparisons with experimental data for composite laminates under torsion loading are shown to validate the model for interlaminar damage. Coupled material and geometrical nonlinear analysis with simultaneous in-plane and interlaminar damage is demonstrated. The effect of warping on in...

Micro-hardness and flexural properties of randomly-oriented carbon nanotube composites

Abstract: The carbon nanotubes possess many unique mechanical and electrical properties, and have been appreciated as new advanced materials for nanocomposite structures, particularly for the development of nanocomposite films. Nanotubes may also be used as nano-reinforcements for matrix system for fibre-reinforced plastic structures in order to improve out-of-plane properties, thus increasing the delamination resistance. However, those properties are highly relied on the structural integrity and homogeneity of the nanotube composites. Unfortunately, only a little works have paid much attention on these issues recently. It has been obviously proved that the atomic architecture on the nanotube's surface may be affected after the nanotubes were chemically reacted with polymer matrix. The weak bonding force among the different layers (bonded by a weak Van Der Waals attractive force) of multiwalled nanotubes may also cause a discontinuous stress transfer from the outer-shell to the inner of the composites. This paper reports the micro-hardness and flexural properties of nanotube composites with different amounts of nanotubes content. Experimental measurements and microscopic observations of the nanotube-epoxy composites before and after the tests are discussed in detail. The results show that the hardness of the nanotube composites varied with different nanotube weight fractions. The flexural strength decreased by 10% for a nanotube composite beam with 2 wt. % of nanotubes. The SEM images also revealed that all nanotubes were completely pulled out after the flexural strength test due to a weak-bonding strength between the nanotube and matrix.

Microstrip Antenna for SAR Application with Composite Sandwich Construction: Surface-antenna-structure Demonstration

Abstract: A 5.3 GHz microstrip antenna for use in synthetic aperture radar (SAR) systems was developed with a composite sandwich construction, using composite laminates, Nomex honeycomb and aluminum alloy. This is the surface-antenna-structure (SAS) for application to load-bearing structural surfaces. The design concept originated from a composite sandwich structure and a multi-layer microstrip antenna. Design, fabrication and validation of structural/electrical performances were all demonstrated. To verify the structural rigidity, flexural behavior was observed under the three-point bending test and was compared with two kinds of composite sandwich beams consisting of CFRP–GFRP skins and Nomex honeycomb core. Electrical measurements of the fabricated antenna array were in good agreement with design requirements, and a comparative study found that SAS has good mechanical characteristics. The SAS concept can be extended to give a useful guide to manufacturers of structural body panels as well as antenna designers, p...

Impact Damage of Partially Foam-filled Co-injected Honeycomb Core Sandwich Composites

Abstract: The present study considers filling of honeycomb type cores with foam to produce sandwich constructions. The potential benefits of this approach are enhancement of damage resistance, and ability to process honeycomb type sandwich structures through cost-effective vacuum assisted resin transfer molding (VARTM). As weight penalty is incurred in complete filling of honeycomb cells with foam, an alternative approach to reduce weight is partial filling of the cells, without losing the advantage of VARTM processing of the core. Two cores are considered, a polyurethane foam for full filling of honeycomb cells, and syntactic foam for partial filling, in conjunction with carbon–epoxy facesheets. Their impact response was investigated under low and high velocity impact (LVI and HVI respectively). For both cores, the foam filling was found to provide confinement to the cells. The resistance to penetration, energy absorbed and damage modes in LVI and HVI were a function of core stiffness, extent of filling and number...

Viscoelastic Characterization of Transversely Isotropic Composite Laminae

Abstract: Understanding and designing for damping in composite laminates has become a topic of great interest; unfortunately, only limited viscoelastic property data is presently available. Direct experimental measurement of the three-dimensional viscoelastic properties is not simple to implement and, thus, an approach leading to the complete 3-D viscoelastic characterization using a reduced number of measured parameters is desirable. To address the difficulties related to direct measurement of properties, this work proposes a reduced number of material coefficients, which allow the specification of the viscoelastic constitutive relationships for a transversely isotropic material, based on only five independent dynamic stiffness parameters and three independent damping loss factors. Further, using this model, a method is developed, based on energy equations, which allows the viscoelastic properties to be evaluated from experimental data, collected from three bend-beam oscillatory tests and two measured Poisson’s ra...

Fracture Toughness of Carbon Foam

Abstract: Mode I fracture toughness of open cell carbon foam was measured using single edge notched four-point bend specimens. A micromechanical model was developed assuming a rectangular prism as the unit-cell. A small region surrounding the crack tip was modeled using finite elements. Displacement boundary conditions were applied to the boundary of the region based on linear elastic fracture mechanics for orthotropic materials. From the finite element results the Mode I stress intensity factor that will cause failure of a crack tip element was determined and it was taken as the predicted fracture toughness of the foam. A simpler model in which the foam consisted of struts of square cross section was also considered. The micromechanical simulations were used to study the variation of fracture toughness as a function of solidity of the foam. The good agreement between the finite element and experimental results for fracture toughness indicates that micromechanics can be an effective tool to study crack propagation in cellular solids.

Numerical Simulation of Pressure Variation and Resin Flow in Injection Pultrusion

Abstract: Injection pultrusion is a continuous process for manufacturing composite materials To produce good quality parts it is essential that complete wet out of the reinforcement fibers is achieved in th

On Modeling Bonds in Fused, Porous Networks: 3D Simulations of Fibrous–Particulate Joints

Abstract: The deformation and failure of fused, porous networks are key concerns in microscale power sources and open trusses for structural applications. We have found that the behavior of the connection points in these open networks is the most critical determinant of materials response. For many manufacturable material geometries, both stiffness and strength are largely controlled by these interparticle or interelement bonds, rather than by the response of the longer aspect ratio sections. Classic work in sintered materials is of limited applicability in this context, since formation of multiphase porous networks often involves volume-conserving interconnects, or fused sections created by additional material, rather than via (heated) compaction of particles or elements. Here we expand on previous work in 2D network behavior (reviewed in [1]) by focusing on the geometry and response of 3D interconnects between cylindrical elements. We present computational results for deformation of 3D interconnects in a large cl...

Fracture Behavior of Woven Composites Containing Various Cracks Geometry

Abstract: Recently, polymeric composite materials are widely used in automotive applications for reducing structural weight, saving fuel, and improving performance. Accidentalimpact damage is an important issue in the use of these composite materials as body components. The damages or cracks in the present work were represented by notched specimens. The test specimens were fabricated from polyester resin reinforced by woven E-glass fiber. A series of tests were conducted to measure the notched strength of composite laminates with various types of through-the-thickness cracks such as central normal notch (CNN), center inclined notch (CIN), single edge notch (SEN) and double edge notch (DEN). A Modified Point Stress Criterion (MPSC) was proposed for predicting the tensile strength of composite laminates with various aspect ratio (a/w), that is, notch length to specimen width ratio.The results show that the load–elongation diagrams of notched and unnotched glass fiber reinforced polyester (GFRP) specimens have a nonli...

Cryogenic Microcracking of Carbon Fiber/Epoxy Composites: Influences of Fiber-Matrix Adhesion

Abstract: The impact of fiber-matrix adhesion on the transverse microcracking of fiber reinforced polymeric materials thermally cycled at cryogenic temperatures was investigated using symmetric cross-ply carbon fiber/epoxy laminates containing fibers with different surface treatments. Past research explored the role of fiber-matrix adhesion in determining the room temperature properties of composite materials, but this work is original in that it examined how fiber-matrix adhesion affected the behavior of composite materials at cryogenic temperatures. Three fiber surfaces were used: Unsized but exposed to an oxidative surface treatment, epoxy sized, and surfactant sized. Modifications of the fiber surfaces changed the adhesion of the matrix to the fibers as determined by interlaminar shear strength and dynamic mechanical analysis. The extent of microcracking in the laminates exhibited a dependence on fiber-matrix adhesion, with high levels of adhesion corresponding to decreased microcracking.