bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Extrusion on Fiber Orientation and Breakage of Aluminar Short Fiber Composites
Authors: Kang C. G. , Kang S. S.
Published: April 2011

The short fiber reinforced aluminum composite material was fabricated by the stirring method, and extruded at high temperature at various extrusion ratios. The hot extrusion load of volume fraction 15% metal matrix composites and base alloy A17075 has been compared. The fiber length distribution, fiber breakage and fiber orientation are investigated to determine the fiber behaviour before and after hot extrusion.

The tensile strength of the hot extrusion billet was experimentally determined for different extrusion ratios, and compared with theoretically calculated strength.
Image Processing Based Method of Predicting Stiffness Characteristics of Short Fiber Reinforced Injection Molded Parts
Authors: Sirkis J. S. , Cheng A. , Dasgupta A. , Pandelidis I.
Published: April 2011

An automated method is presented for predicting the effective stiffness properties of injection molded parts reinforced with non-random distribution of short fibers. The methodology first employs image processing techniques to determine the probability distribution of fiber orientation (PDFO) and probability distribution of fiber length (PDFL), both of which considerably influence the anisotropy of the mechanical behavior of fibrous composites. Simple classical semi-empirical models are then utilized in conjunction with the PDFO and PDFL measurements to predict the elastic stiffness of the molded part. This automated method for predicting the stiffness of fibrous composites is applied to a glass (fiber)/resin (matrix) short fiber injection molded composite specimen.
Notched Strength Prediction of Holed Woven Glass/Epoxy Laminates with Tan's Models
Authors: Xiao Jiayu , Bathias Claude , Lai Dawei
Published: April 2011

The point strength model and the minimal strength model proposed by Tan were applied to predict the notched strength of three woven Glass/Epoxy laminates (two orthotropic and one quasi-isotropic) under monotonic uniaxial loading. In the two models, the stress field around the hole for a finite-width notched laminate was expressed by a numerical solution which was determined essentially by the characteristic roots of the notched laminate, and was anew determined in each rigidity degradation step because the rigidity and compliance matrices of the laminate (which give the coefficients in the characteristic equation) vary with the loading due to damages. The characteristic dimensions defined and adopted in the two models were also anew determined in each critical ply. The results show that the strength reduction factor (SRF) curves as a function of hole diameters determined with the characteristic dimensions of the ± 45 ° plies for the laminates studied can give much better notched-strength prediction results than with those of the 900 plies especially for the orthotropic laminates. The damage mechanisms in the woven laminates are very different from those in non-woven laminates.
Gate Location Optimization in Liquid Composite Molding Using Genetic Algorithms
Authors: Young Wen-Bin
Published: April 2011

Manufacture of polymer composites using the process of liquid composite molding involves injection of a reactive resin into a closed mold with preplaced fibrous reinforcements. There are many studies that address the computer simulations for mold filling. The simulation programs provide the simulated mold filling results for a known gate location. However, they do not discuss the method for determination of the gate locations, which is one of the important issues in the total mold design. In this paper, a gate location optimization method was developed based on minimization of the mold filling pressure, uneven filling pattern, and temperature difference during the mold filling process. A genetic algorithm was used to search the optimal gate location. This method, in general, has a better chance to locate the near global optimum than the gradient based search method, especially in the problems with multiple variables and a large search space.
Delamination and Buckling in 3D Composites
Authors: Cox B. N.
Published: April 2011

Elementary results are applied to the buckling of stitched laminates and woven composites with three-dimensional (3D) reinforcement that contain delamination cracks. The through-thickness fibers are assumed to provide continuous, linear restoring tractions opposing the deflection of the delaminated layer adjacent to the crack. With the boundary condition that the ends of the delaminated layer are clamped and with deflections permitted in one direction only, there exists a characteristic length a0 for buckling: if the length, 2a, of the delamination crack exceeds 2a0, then, when buckling occurs, it will consist of waves of period 2a, and will usually not span the whole delamination. A simple expression is derived for the minimum density of through thickness reinforcement required to suppress buckling of the delamination layer prior to failure by other mechanisms. It is shown that for typical densities of through-thickness reinforcement in current stitched laminates and 3D woven composites, the length 2a0 will rarely exceed the thickness of the delaminated layer by as much as an order of magnitude. Furthermore, the current generation of these materials are probably considerably overdesigned in the quantity of through-thickness reinforcement they contain for suppressing buckling.
Cumulative Damage of Fiber-Reinforced Elastomer Composites under Fatigue Loading
Authors: Lee B. L. , Liu D. S.
Published: April 2011

Fracture mechanisms under fatigue loading were assessed in the case of nylon fiber-reinforced elastomer matrix composite which represents the actual carcass of bias aircraft tires. Under uniaxial tension, the angle-plied carcass composite specimens were subjected to a considerably large interply shear strain before failure. The composite specimens exhibited semi-infinite fatigue life when stress amplitude was below a threshold level, i.e., fatigue endurance limit. Under cyclic stresses exceeding the endurance limit, localized damage in the form of fiber-matrix debonding and matrix cracking was formed and developed into delamination eventually leading to gross failure of the composite. The process of damage accumulation was accompanied by heat generation and a continuous increase of cyclic strain (dynamic creep). Fatigue lifetime and the resistance to damage accumulation of aircraft tire carcass composite were strongly influenced by cyclic frequency. The use of higher cyclic frequency resulted in shorter fatigue lifetime at a given stress amplitude and lower fatigue endurance limit. The extent of dynamic creep at gross failure, which is defined as the increase of cyclic strain beyond initial elastic deformation, was roughly independent of stress amplitude under the frequency of 1 Hz, but decreased with higher stress amplitude when the frequency was raised to 10 Hz. Obviously a critical level of dynamic creep exists for gross failure of the composite and this level appears to be independent of the stress amplitude at low frequency. When the frequency is high enough, heat generation due to hysteretic loss is expected to degrade the materials. In this situation, the critical level of dynamic creep for gross failure may be reduced by the loss of matrix flexibility as well as fiber-matrix bonding strength. The study also examined the effect of load sequence on the damage accumulation process of composite based on dual stress level fatigue testing. Palmgren-Miner's linear damage accumulation theory tended to overestimate the cumulative damage of angle-plied carcass composites. As an alternative to PalmgrenMiner's rule, a linear cumulative damage model based on dynamic creep was utilized. The sum of dynamic creep to gross failure was found to be unity indicating that the level of cumulative damage is independent of the load sequence.
Fiber-Reinforced Plastic Grid for Reinforced Concrete Construction
Authors: Schmeckpeper Edwin R. , Goodspeed Charles H.
Published: April 2011

The objective of this paper was to evaluate the suitability of Fiber-Reinforced Plastic (FRP) grids for use as a structural reinforcement in slab-type concrete structures. The behavior of concrete beams reinforced with FRP grids was experimentally investigated. Two different types of FRP grids were tested; the first type used carbon fibers, the second type used a mixture of carbon and E-Glass fibers. The mechanical properties of these two FRP grids were determined. For each of the two types of reinforcement, five concrete beams were tested to failure. The flexural behavior, as characterized by the load-deflection response, was monitored throughout the tests. The results from the flexural tests on FRP reinforced concrete beams showed that the failure mode, measured deflections and ultimate loads were consistent with predictions.
Fabrication of Metal Matrix Composite by Infiltration Process-Part 1: Modeling of Hydrodynamic and Thermal Behavior
Authors: Assar Abd-Elwahed M. , Al-Nimr Moh'D A.
Published: April 2011

Modeling of hydrodynamic and thermal behavior during fabrication of metal matrix composites by infiltration process is presented. In terms of this model, the governing equations which describe the hydrodynamic and thermal behaviour of a liquid metal squeezed through a column of ceramic particles are solved. As a result, the time required for the liquid metal to penetrate completely throughout the column and the transient temperature distribution within the column are obtained. Also for a complete squeezing process, without freezing of the liquid metal at any location, the model is able to determine the initial melt superheat. The required melt superheat, for complete squeezing, is given as a function of the imposed pressure. It is found that increasing the imposed pressure will reduce the required superheat of the melt.
Indentation of Spherical Head Indentors on Transversely Isotropic Composite Plates
Authors: Suemasu Hiroshi , Kerth Steffen , Maier Martin
Published: April 2011

Impact phenomena can be resolved into a local indentation problem and a global forced vibration. In the present paper the local indentation problem is studied. The relation between the transverse force and the indentation is obtained by a superposition of the solution of the indentation of spherical head projectile in semi-infinite space and that of Boussinesq problem. An explicit expression of the relation is given. In the formula, the effect of the plate thickness on the indentation is introduced. The results agree well with the experimental results done by Tan and Sun and coincide with the finite element results. When the plate becomes thinner and/or the load is larger, the effect of the thickness on the indentation stiffness can not be neglected. The topic in the present paper has been rather extensively studied. However, the relative simplicity of this solution has some merit.
An Eddy Current Technique for the Measurement of Constituent Volume Fractions in a Three-Phase Metal-Matrix Composite
Authors: Pitchumani R. , Liaw P. K. , Yao S. C. , Hsu D. K. , Jeong H.
Published: April 2011

An eddy current technique is developed for the measurement of reinforcement and intermetallics concentrations in extruded particulate-reinforced metal-matrix composites. The technique employs a theoretical conductivity model which accounts for the constituent phases, together with a measurement of two anisotropic conductivities on the composite, for determining the reinforcement and intermetallics volume fractions.Towards implementation of this technique, an analytical model is described which relates the anisotropic conductivities of extruded particulate-reinforced composites to their microstructures. The volume fractions were calculated using eddy current measurements made on a wide range of silicon carbide particulate (SiC,) reinforced aluminum matrix composite extrusions. The calculated values are shown to be in good agreement with the actual metallographically measured volume fractions of the reinforcement and intermetallics. The technique may be adopted for an on-the-fly assessment of product quality in a processing environment.
Model for Determining the Vent Locations and the Fill Time of Resin Transfer Molds
Authors: Boccard Alexis , Lee Woo Il , Springer George S.
Published: April 2011

Simple models were developed for determining the vent locations and the fill time of resin transfer molds containing thin flat preforms with isotropic permeabilities. The in-plane shape of the preform is arbitrary and the preform may contain impermeable inserts. The vent locations required to avoid trapping air bubbles are determined by geometric considerations. The time required to fill the mold is calculated by treating the resin flow inside the mold as partly radial and partly channel-like flow. Tests were performed with different shape molds with and without inserts. The vent locations required to avoid air bubbles and the time required to fill the mold were determined. The experimentally established vent locations were in good agreement with the vent locations given by the model. The measured and calculated fill times also agreed well.
Thermo-Oxidative Aging of Composite Materials
Authors: Tsotsis T. K.
Published: April 2011

A series of tests was conducted on two carbon/epoxy systems to assess key factors in the characterization of the thermo-oxidative stability of composite materials. Mechanical properties (open hole compression strength, fracture toughness, and unidirectional compression strength) were determined from specimens aged in air at 177°C (350'F) up to 5,000 hours. Compression properties declined with increasing aging time and did so in a shorter period of time than given in previously reported tensile tests from other workers. Compression strengths were seen to decline by up to 30% with only a 2% weight loss. Thus, percentage weight loss cannot and should not be used as a criterion for material acceptance without corresponding mechanical test data. Moreover, glass transition temperature is shown to be a poor indicator of thermo-oxidative stability. Thermogravimetric analysis gave an activation energy for oxidation roughly 50% that of thermal degradation alone. Glass transition temperatures were observed to rise significantly for specimens aged at temperatures above 121° C (250'F) in air. After 1,000 hours at different temperatures, differential scanning calorimetry as well as glass transition measurements suggest that higher conditioning temperatures lead to more highly cross-linked microstructures which may not be representative of actual aging at use temperatures. Thus, it is recommended that accelerated aging methods should rely more on increased-oxygen pressure and less on higher temperature to obtain meaningful aging mechanical properties.
The Effective Thermal Conductivities of Composites with 2-D Arrays of Circular and Square Cylinders
Authors: Lu Shih-Yuan
Published: April 2011

A boundary collocation scheme suitable for calculation of the effective thermal conductivity of 2-dimensional periodic arrays is developed. Investigated 2-dimensional arrays include the square arrays of infinitely long circular and square cylinders and hexagonal arrays of infinitely long circular cylinders. The effective thermal conductivity is studied subject to variation in two system parameters: the inclusion volume fraction, f, and the conductivity ratio of the inclusion to the matrix, k2/ k1 (= σ). The parameter domain investigated is complete, ranging from 0 to the close packing in inclusion volume fractions and from 0 to ∞ in conductivity ratios.

The effectiveness of different configurations in improving (if k2/ k1 > 1) or reducing (if k2/ k1 < 1) the effective thermal conductivity of the composite is studied with the three arrays. With enhancing (insulating) inclusions, the square array of infinitely long square cylinders is found to be the most efficient configuration in improving (reducing) the effective thermal conductivity of the composite among the three arrays for f below a first crossing volume fraction, f c1, around 0.56 For f greater than fc1, the square array of infinitely long circular cylinders becomes the most effective configuration. Contour plots of the relative effective thermal conductivity, k eff/ k1, in the f - log (σ) space are obtained, which can be used to infer values of σ from measurement of k eff/ k1 at some given f.
Two-Dimensional Modeling of Composite Pinned-Joint Failure
Authors: Lessard Larry B. , Shokrieh Mahmood M.
Published: April 2011

The modeling of damage in a laminated composite pinned-joint presents many difficulties because of the inherent complexity of the failure process. The joint area is a region with stress concentrations thus a complicated stress state exists. In order to model progressive damage from initial to final failure, finite element methods are used rather than closed form stress analyses. Two approaches for the finite element technique can be used: a simple two-dimensional linear model or one that has been enhanced with non-linear assumptions. Adding non-linear material behaviour and large deformation theory are two improvements that can be made to a linear finite element model. Failure analysis must be a logical combination of suitable failure criteria and appropriate material property degradation rules. It is understood that a three-dimensional analysis may now be a better alternative, but the goal here is to fly understand the limits of two-dimensional modeling for solving the composite pinned-joint problem.
In-Situ Observations in SEM of Degradation of Graphite/Epoxy Composite Materials due to Seawater Immersion
Authors: Grant T. S. , Bradley W. L.
Published: April 2011

The effect of immersion in seawater on the mechanical properties of three graphite/epoxy composite materials has been studied. The transverse tensile strength was found to be reduced by 17% in one of the systems with essentially no change in the other two systems studied. Direct observation of fracture in the SEM as well as microindentation measurements of the interfacial shear strength have been used to explain these observations. The 17% decrease in transverse tensile strength was associated with degradation of the interfacial strength, resulting in a change in the fracture mechanism from primarily matrix cracking to interfacial failure. Little difference was found in the behavior of composites immersed in distilled water and in seawater at ambient pressure or seawater at 20.7 MPa pressure.
Multiple Cracking in Angle-Ply Composite Laminates
Authors: Wang J. , Karihaloo B. L.
Published: April 2011

The stress field in a cracked [(±θ°) n2/(90°) n2] s angle-ply fibre-reinforced composite laminate is solved by using Fourier transforms and dual integral equation formulation. The (90°) n1 sublaminate is characterized by periodically distributed multiple transverse intralaminar cracks. The stress intensity factor at each crack tip and the crack-induced interfacial stresses are calculated. Both are found to be significantly influenced by the closeness of the crack tip to the bimaterial interface, the crack spacing and ply angle θ of the constraining sublaminates. The variation of the SIF explains the physical mechanisms behind constrained cracking in composite laminates, while the perturbation of the interfacial stress field caused by the transverse cracks reveals that as the cracks approach the bimaterial interface, there is a considerable increase in interfacial stresses. The crack driving force and the crack-induced interfacial stresses decrease considerably when the multiple cracks are densely distributed. From the design point of view, the results also suggest that by a proper choice of ply angles in angle-ply laminates the crack growth in the individual lamina can be greatly retarded.
Response of Multi-Element Foam-Filled Preform RTM Structures, II: Low-Velocity Impact and Post-Impact Crush Response
Authors: Karbhari Vistasp M.
Published: April 2011

Resin transfer molding has opened the potential for structures incorporating multi-element preform architectures and cores, tailored for combinations of requirements that would not previously have been possible in one structure. The response of foam-filled RTM structural elements to low-velocity impact and post-impact progressive crush is investigated. Six different architectures based on a stiffened plate-type structural element were tested, and comparisons of behavior prior to and after impact are made based on overall response and damage mechanisms. Differences in preform architecture are shown to result in differences in post-impact crush response through the triggering of damage modes that are architecturally driven. Failure mechanisms are seen to be driven by the use of a woven layer in the outer preform element, or through damage imparted to the central rib as a function of the damage events. Results based on the comparison of crush and post-impact crush response indicate that preform selection should be made based on architectural optimization linked to overall repeatability and uniformity of performance under a range of loading regimes for maximum efficiency in cases where structural stability and crush are important metrics of performance.
Model Laws for Curing of Thermosetting Composites
Authors: Hojjati Mehdi , Hoa Suong V.
Published: April 2011

Model laws for curing of thermosetting composites are established. The transient heat conduction coupled with the kinetic equation and the initial and boundary conditions are non-dimensionalized. Dimensionless parameters are extracted from the non-dimensional governing equations and boundary conditions. Model laws are constructed based on these dimensionless parameters. It is shown how one can use the model laws to design and fabricate a model composite structure for a prototype one. The model can be employed for the experimental purposes. Since both model and prototype have the same behavior at the homologous points and times, the model experimental results can be interpreted for the prototype one. The use of different systems of materials in the design process of a model is investigated. It is shown that due to the different kinetic equations for the different resin systems, a feasible way to design a model is to use the same resin, but the different fibers. For this case, it can be shown that for the same initial temperature and cure cycle for both model and prototype, the temperature and degree of cure at the homologous points are the same. Also to transform the model experimental results to the prototype, it is not necessary to know the resin kinetic equation. Therefore, one can use a less expensive model to determine the optimum cure cycle by performing experiments, and then the results can be used directly for the more expensive prototype.
Weibull Analysis of the Strength of Carbon Fibers Using Linear and Power Law Models for the Length Effect
Authors: Padgett W. J. , Durham S. D. , Mason A. M.
Published: April 2011

The single-filament test measurements for strength of carbon fibers obtained by Bader and Priest [5] are analyzed using a broader class of Weibull models with the scale parameter assumed to be a function of fiber gauge length. Two specific models are proposed for the fiber gauge length effect on tensile strength: The power law model and a linear model. The methods of maximum likelihood estimation and Weibull plots provide tools for the parameter estimation and assessment of the fitted models. In particular, the models provide excellent prediction of the fiber strength distributions at longer gauge lengths not tested, but illustrate that extrapolation to very short gauge lengths yield larger predicted fiber strengths than those actually observed experimentally, and extrapolation to very long gauge lengths yield smaller predicted fiber strength than observed experimentally. Analysis of the Bader-Priest data using the proposed models supports the observed "end effects" (or "clamp effects) for short gauge lengths studied initially by Phoenix and Sexsmith [3] and reported also by Stoner, Edie and Durham [4].
Properties of Composite Laminates Reinforced with E-Glass Multiaxial Non-Crimp Fabrics
Authors: Wang Youjiang , Li Jian , Do Pete B.
Published: April 2011

Multiaxial knitted non-crimp fabrics (NCFs) are a relatively new form of reinforcement for composites. A study focusing on the mechanical properties of such composites has been carried out and the results are reported in this paper. E-glass/epoxy composites were manufactured by the Resin Transfer Molding (RIM) process using different NCFs. The composites were tested in uniaxial tension, three point bending, compression and short beam shear. Results on elastic modulus and strength were obtained and compared among the different testing conditions. Failure of the NCF composites was examined visually after the final failure of the specimens. The test results on tension and flexural moduli were compared with Classical Lamination Theory (CLT) predictions, and good agreements were observed. The test results and inspections of failed specimens revealed that the NCF composite laminates behaved like their tape laminate counterparts. Consequently, low cost high perfomance composite laminates may be achieved through the use of NCF fabrics as reinforcing materials.
Finite Element Free Vibration Analysis of Laminated Composite Stiffened Shell
Authors: Goswami S. , Mukhopadhyay M.
Published: April 2011

Free vibration analysis of composite stiffened shell panels has been carried out for the first time. As there is no published literature available on the problem, the finite element technique has been applied using two elements, namely the nine-noded Lagrangian element and the heterosis element, for obtaining new results. An improved version of the stiffener modelling is presented here. In this modelling, stiffeners can be placed anywhere inside the element and it need not pass through the nodal lines. Moreover, both elements have the capacity of accommodating curved boundaries and incorporating transverse shear deformation. Results for stiffened composite cylindrical shells and doubly curved shells with different boundary conditions and various laminae orientations have been presented for both concentric and eccentric stiffeners. Parametric studies considering different variables have also been carried out.
Fatigue Behaviour of Polymer Composite Sandwich Beams
Authors: Shenoi R. A. , Clark S. D. , Allen H. G.
Published: April 2011

The static and flexural fatigue characteristics of foam cored polymer composite sandwich beams are investigated. The skins of the beams are made from hybrid glass-aramid fibres set in epoxy resin and the core materials are linear and cross-linked polymer foams. The applied load in a ten-point configuration approximates a uniformly distributed load throughout the span of the beam which is simply supported at the ends. The testing frequencies are 0.33-0.91 Hz. Failure modes relate to both core shear and skin failure.
Effect of Interfacial Strength on Erosion Behavior of FRPs
Authors: Miyazaki N. , Hamao T.
Published: April 2011

Experiments were carried out to study the effect of interfacial strength between a matrix material and fibers on the solid particle erosion behavior of FRPs. The FRPs used were epoxy resins unidirectionally reinforced by carbon fibers. Although more than ten papers have been published concerning the erosion behaviors of FRPs, it has not been clarified how the interfacial strength between a matrix material and fibers affects the erosion behavior of FRPs. In the present erosion experiments, FRPs with treated fibers or untreated fibers were used as test materials to examine the effect of interfacial strength between a matrix material and fibers on the erosion rate. According to the tensile tests, the FRPs with treated fibers show higher interfacial strength between a matrix material and fibers than the FRPs with untreated fibers. The results of the erosion tests show that the FRPs with treated fibers have higher erosion resistance than the FRPs with untreated fibers due to higher interfacial strength between a matrix material and fibers.
Through-the-Thickness Tension Strength of 3-D Braided Composites
Authors: Avva Vishnu S. , Allen Harold G. , Shivakumar Kunigal N.
Published: April 2011

Low cost of manufacturing textile composites make them a candidate for manufacturing affordable aerospace vehicles. However the material test methodologies and database are needed before they can be confidently applied. The study thus far has been on characterization of mechanical properties of two-dimensional (2-D) textile composites. This paper presents through-the-thickness strength test methodology, analysis, and data for 3-D, triaxially braided composites. The braid preform [012 k/±β12 k] 45% axial was manufactured using a four-step circular braiding machine and BASF G30-500, 12k graphite fiber tows. The preform was consolidated with Tactix 123 matrix. L-beam specimens were tested in tension and through-the-thickness strength was calculated from the measured failure load and the curved beam equation. Strength of braided composites was compared with an equal thickness and ply stacking AS4/3501-6 graphite/epoxy laminated specimen. The average strength of braided, equivalent laminate, and unidirectional composites was 24.2, 28.8, and 40.9 MN, respectively. Through-the-thickness strength of both braided and laminated composites was found to be a matrix dependent property. Failure modes in braided composites were matrix cracks, tow cracks, and tow separation; whereas failures in laminates were delaminations and ply-cracks. Reduction of size of resin pockets in braided composites may improve the through-the-thickness strength.
On the Law of the Transverse Matrix Crack Growth in Brittle Polymeric Matrix Composite Laminates under Static Loadings
Authors: Xu Luo-Yu
Published: April 2011

The characteristics of the matrix crack growth were studied experimentally and analytically. A probabilistic-analytical method, the "Characteristic Curve Method (CCM)" is developed to correlate the initial defects and matrix crack densities. The concepts of the "equivalent applied loading" and the "equivalent crack density" are introduced to reveal the physical essence of the matrix cracking and eliminate the different matrix crack growth features in different laminates. Several characteristic curves for matrix crack growth of different materials are established and some similar material constants which control the matrix crack growth are found. The prediction of the matrix crack initiation and the saturation crack density are summarized, combined with the CCM approach, the whole matrix crack growth processes in many CFRP and GFRP laminates subjected to static loadings can be predicted well.
Multiple Delaminations and their Severity in Circular Axisymmetric Plates Subjected to Transverse Loading
Authors: Suemasu Hiroshi , Majima Osamu
Published: April 2011

Instability problem of multiple penny-shape interlaminar delaminations in circular axisymmetric plates subjected to a transverse concentrated loading is theoretically studied. The authors consider a case that all the delaminations located at the same intervals have a same size and propagate simultaneously. Then, they consider two cases that one of the delaminations is larger or smaller than the rest. The energy release rates are obtained in closed forms and the results are compared with those obtained numerically via a finite element analysis. The present solutions agree excellently with the finite element results. When the load is constant, the energy release rate is constant regardless of the size of the delaminations and increases linearly with the number of the delaminations. When one of the delamination is larger or smaller than the others, the energy release rate of the shorter delamination or delaminations become much larger than that of the longer delamination(s), that is, only the smaller delamination(s) tends to propagate until it grows up to the size of the larger delamination(s). Multiple delaminations, if they exist, have a tendency to grow self-similar way.
Thermally-Induced Shapes of Unsymmetric Laminates
Authors: Peeters L. J. B. , Powell P. C. , Warnet L.
Published: April 2011

Models to describe the shapes of unsymmetric laminates produced with a flat mould have been already developed by several researchers. These models are reviewed and on some points theory is extended.

The experimental work done so far to confirm the developed theoretical models is rather limited. This especially applies for angle-ply laminates, on which no experimental work has been reported yet. This paper presents new experimental work that has been conducted both on cross-ply and angle-ply laminates over a wide range of laminate dimensions. The experiments show that the influence of the side length on the curvature is very well predicted both for cross-ply and angle-ply laminates, also the bifurcation point behaviour is clearly observed. Possible causes of differences between theory and experiment are outlined. The theoretical predictions are especially sensitive to the transverse thermal expansion coefficient of the individual layers.
Can the Compressive Response of Fiber-Reinforced Composites be Modelled by Means of Layered Arrays?
Authors: Weitsman Y. Jack , Chung I.
Published: April 2011

The validity of some basic premises employed in modelling of the compressive response of fiber-reinforced composites is examined in the present article.

It is shown that the deformation fields associated with the compression of layered media yield inadmissible results when applied to hexagonal and square arrays of circular fibers. Consequently, predictions of compressive strength, which derive from the prevailing representations of layered geometry may not apply to realistic circumstances.
Mechanisms and Quantification of Spalling Failure in Laminated Composites under Shock Loading
Authors: Gupta V. , Pronin A. , Anand K.
Published: April 2011

The free surface spalling phenomenon in 2D carbon/carbon (C/C) and carbon-polyimide (C/P) laminates was studied by subjecting them to laser-produced nano-second rise-time compressive stress pulses. The compression pulse travels through the laminate normal to the plies and, upon reflection into a tension pulse from the sample's front surface, leads to fiber/matrix debondings either in the bundles within a ply or at the interply interface. The stress pulse amplitude responsible for initiating such a damage was determined by recording the free surface velocity by an optical interferometer, and on the continuum scale, this is referred to as the effective interply strength. Since by employing a special specimen geometry we were able to determine the onset of such delaminations, its characterization in terms of an effective interply strength should be acceptable. Additional information on the attenuation and the dispersion characteristics associated with the propagation of such stress pulses was also obtained for both types of composite micro-structures. Using the interply strength as the local failure criterion, the information on the dispersion characteristics allows for a complete design of a composite component under impact conditions.
Fracture Behavior of Unidirectional Commingled-Yarn-Based Carbon Fiber/Polyamide 6 Composite under Three-Point Bending
Authors: Choi N. S. , Yamaguchi H. , Takahashi K.
Published: April 2011

Flexure-response and fracture behavior of unidirectional commingled-yarn-based carbon fiber (CF)/polyamide (PA) 6 composite were investigated using three-point edgewise and flatwise bend tests. Difference of the experimental flexure data from that expected was attributed to poorly bonded fiber/matrix interfaces and deviated fiber bundle orientations. For the fracture test, four kinds of notch direction were adopted: edgewise notches parallel (L) and transverse (T) to the major direction of fiber bundles, and flatwise notches parallel (ZT) and perpendicular (ZT) to this direction. Strength and absorbed fracture energy for L and ZL specimens exhibited high sensitivity to notching. ZL specimens exhibited the lowest values of the critical stress intensity factor, K c which were slightly superior to those of unfilled PA6 matrix. T specimens showed the largest values of the strain energy release rate Gc which were mostly occupied by the plastic fracture energy: the large plastic energy coincided with generation of the crushing-mode failure on the compression side. Enlargement of the compression area was analyzed by means of the rigidity reduction resulting from the fracture progression.
Study on the Characteristic Curve of Stiffness Degradation Caused by Transverse Matrix Cracking in Multidirectional Composite Laminates
Authors: Xu Luo-Yu
Published: April 2011

By introducing two concepts of the "equivalent residual lamina stiffness" and the "equivalent crack density" to eliminate the different stiffness degradation characteristics caused by transverse matrix cracks, the degradation of the three lamina stiffness components can be described by a same characteristic curve which is independent of the laminate layup. But the residual stress has some effect on the stiffness degradation, especially to the inplane Poisson's ratio of the laminate. Results show that for the same matrix crack density, the residual Poisson's ratio is always less than the residual shear modulus and Young's modulus. Due to the existence of the matrix crack saturation state, the degradation of the lamina stiffness has the low boundary value; this value seems to be independent of the laminate configuration and materials.
Object-Oriented Approach to Optimize Composite Laminated Plate Stiffness with Discrete Ply Angles
Authors: Todoroki Akira , Sasada Naonobu , Miki Mitsunori
Published: April 2011

A system for designing the optimum stacking sequence combination of laminated composite plates using an object-oriented approach was presented here. In this design system, a stacking sequence combination of a laminated composite plate is optimized such that the actual strain under an applied load is reduced with the lowest ply number being subject to some empirical combinatorial restrictions on the stacking sequence. Two optimization approaches are provided here. In the first approach, the ply that most drastically reduces the actual strain of a composite plate is stacked one by one on the laminate. This approach is called the sequential decision approach in this study. In the second approach, the optimal solution to the stacking sequence problem was efficiently obtained by the branch-and-bound procedure. By using these approaches, the optimized stacking sequences were successfully obtained.
Orthotropic Creep and Complex Moduli of a Viscoelastic Composite Reinforced with Aligned Elliptic Fibers
Authors: Li J. , Weng G. J.
Published: April 2011

A general micromechanical viscoelastic theory is developed to study the effective creep behavior and nine complex parameters of an orthotropic composite consisting of a viscoelastic matrix and aligned elliptic cylinders. With such a microgeometry it is first shown that two Maxwell or two Voigt constituents generally do not make a Maxwell or a Voigt composite, but under the conditions that the ratios of the shear modulus to the shear viscosity are equal for both constituents and that both Poisson's ratios remain unchanged in the course of deformation, a Maxwell and a Voigt composite can be constructed. The orthotropic creep compliances then are examined as the cross-sectional aspect ratio α (the thickness-to-width ratio) of the elastic elliptical cylinders changes from circular one (α = 1) to lamellar one (α→ 0) under six loading directions. It is found that under the three tensile loadings the composite with the lamellar structures gives rise to the strongest creep resistance. As the aspect ratio increases the tensile creep resistance also weakens, with the traditional circular fibers providing the poorest reinforcement. But the creep behaviors under the three shear loadings are sensitive to the loading direction as well. The real and imaginary parts of the nine complex moduli under a harmonic loading at low frequency are also examined as a function of the aspect ratio, the volume fraction of elastic cylinders and the loading frequency. The results show that the real parts of all these nine effective complex moduli will reach their elastic counterparts as the frequency increases. The imaginary parts, however, are dependent upon the cross-sectional shape and the loading frequency, but all of them reduce to zero when the volume concentration reaches 1 or the loading frequency increases to infinity.
Impact of Composite Structures—The Momentum Balance Method
Authors: Christoforou A. P. , Yigit A. S.
Published: April 2011

The momentum balance method which is based on the impulse-momentum principle is used to study the transverse impact of a simply supported composite beam. In this method the effects of local contact behavior is accounted for through the use of an appropriate coefficient of restitution obtained from an elastic-plastic contact law. This greatly simplifies the problem by decoupling the equations of motion, and for the problem studied, facilitates an analytical solution. The momentum balance solution is compared to the numerical solution of the same problem using the nonlinear elastic-plastic contact law. The proposed method gives very good results for the post impact behavior of both the impacter and the beam. In certain cases, it yields satisfactory results for the dynamic behavior even during contact. The momentum balance method, with an appropriate coefficient of restitution obtained from the contact law, is physically the same as using the contact law itself, with the difference that the local deformation is neglected.
Modelling Lamina Longitudinal Compression Strength of Carbon Fibre Composite Laminates
Authors: De Morais A. Balacó
Published: April 2011

The determination of the lamina longitudinal compression strength remains an unsolved problem. Non-valid failure modes are commonly observed in compression tests of unidirectional specimens. Micromechanical modelling has also proven to be a difficult task. It is believed that failure is initiated by fibre microbuckling, and that non-linear matrix behaviour and fibre misalignments play a major role. The main obstacles are thus the non-linear nature of the phenomenon and the scarcity of material data. Here 2D and 3D Finite Element models are presented. The Finite Element Method can easily deal with non-linear problems. The models are computationally inexpensive and can be easily implemented on a commercial code. A sensitivity analysis was performed using realistic input data. The model predictions are well above the unreliable experimental values so far available. Further, the most important variables were found to be the fibre misalignment angle and the matrix yield stress. The fibre spatial arrangement also affects the predicted results.

Reliable strength data are essential to validate the models here presented. Nevertheless this study shows that accurate predictions should be expected to prove difficult, mainly because of the complex matrix stress-strain behaviour and the statistical nature of fibre misalignments and spatial distribution. In addition, the important role of the matrix in model results raises the question of the existence of a unique lamina longitudinal compression strength (LLCS). Lay-up dependent transverse and shear stresses acting on the load aligned laminae should accordingly yield lay-up dependent LLCSs, which is confirmed by experimental data.
Optimum In Situ Strength Design of Composite Laminates. Part II: Optimum Design
Authors: Wang J. , Karihaloo B. L.
Published: April 2011

The stress norm developed in Part I is incorporated into the formalism of an optimization problem in order to enhance the load bearing capacity of multidirectional laminates. The optimization problem is solved for symmetric laminates subjected to any combination of in-plane loads. In this problem, the stress norm is the design objective to be minimized whereas the ply angles and thicknesses are the design variables. The solution of the formulated min{max) optimization problem is obtained by the bound-formulation method and mathematical programming. The gradients of objective functions and constraints needed in this method are calculated by a mixed analytical/numerical procedure. The results show that the optimization technique based on the in situ laminae strength parameters is a promising tool in the design of the composite laminates. The load bearing capacity of an optimally designed laminate can be increased several fold compared with that of a conventionally designed laminate.
Bearing Failure of Bolted Composite Joints. Part II: Model and Verification
Authors: Hung Chang-Li , Chang Fu-Kuo
Published: April 2011

An investigation was performed to study the bearing failure and response of a mechanically fastened laminated composite joint. The experimental work of the study was presented in Part I, and this paper summarizes the analytical work.

An accumulative damage model based on the damage mechanisms observed from the experimental study was developed to simulate the bearing failure in the laminated composite joints. The model consists of two parts: damage prediction and constitutive modeling. Damage accumulation criteria by modifying Hashin criteria were proposed to predict the mode and extent of bearing damage as a function of the stress state, and the relationships between material properties and bearing damage were established based on failure mechanisms.

A nonlinear finite element analysis based on the model was developed. Data obtained from this study and existing literature were used to validate the model. Overall, the predictions agreed with the data very well.
Dry Spot Formation and Changes in Liquid Composite Molding: II—Modeling and Simulation
Authors: Han K. , Lee L. J. , Nakamura S. , Shafi A. , White D.
Published: April 2011

Three types of dry spot changes, which have been described in Part I, are modeled in this study. Darcy's law and ideal gas law are used to model the Type I and Type II dry spot changes, while a simplified form of the two phase Darcy's law is used to model the Type III dry spot change. A computer code based on the control volume finite element method is developed to simulate the dry spot formation and changes. In order to save computation time, a coarse mesh rebalancing method is implemented in the code. Compared to the experimental results, this flow model predicts the Type I and Type II dry spot changes and the packing and bleeding process very well. It shows the right trend of Type III dry spot changes when compared to some preliminary experimental results.
The Energy Absorption of Composite Plates under Off-Axis Loads
Authors: Fleming David C. , Vizzini Anthony J.
Published: April 2011

A fixture for measuring the energy absorption of composite structures under off-axis loads is developed. The testing fixture stabilizes the specimens by providing simply-supported boundary conditions on the sides of the specimen to prevent global buckling. Using the fixture, graphite/epoxy plates are crushed edgewise against an adjustable inclined plane surface and the energy absorbency is measured. Consistent test data are obtained that are similar to those obtained using different specimen geometries, indicating the acceptability of the test method. Results suggest that the off-axis crushing response may be described by considering the occurrence of a failure mode separately from the quantitative performance of the mode. Under off-axis loads, the energy absorbency of a specimen is dependent upon the failure mode experienced, but not directly on the orientation of the applied load. However, load orientation affected the preferred failure mode.