bluuebunny/crossref_metadata_2025 huggingface dataset:
The Prevention of Kink Band Formation at Free Edges of Unidirectional Composite under Compressive Load
Authors: Zhang Guigen , Latour Robert A.
Published: April 2011

Compressive strength of fiber reinforced polymer (FRP) composites is related to many factors such as fiber and matrix constituent properties, interfacial bond properties, specimen geometry, free-edges, fiber waviness, voids, stress concentrations, among others. In this paper, an analytical study has been performed to investigate the effects of free-edges on the compressive strength of a uniaxial composite laminate; matrix nonlinearity and fiber waviness are also incorporated in the model. Results of this study indicate the compressive strength of fibers located along a free-edge is much less than that of internal fibers, implying that compressive failure should be initiated at free-edges, which is in agreement with experimentally observed behavior. Furthermore, also as expected, matrix nonlinearity and initial fiber curvature result in dramatic decreases in compressive strength of free-edge fibers. However, results indicate that. theoretically, a thin layer of matrix placed over free-edge fibers should stabilize the free-edge fibers against microbuckling to the extent that their microbuckling strength is even greater than the internal fibers. This finding may have significant implication for designing composite structures for the support of compressive loads.
Creep and Physical Aging in a Polymeric Composite: Comparison of Tension and Compression
Authors: Gates Thomas S. , Veazie David R. , Brinson L. C.
Published: April 2011

To help address the lack of engineering data on the time-dependent behavior of advanced polymeric composites, a combined experimental and analytical research program was initiated to investigate the similarities and differences of the effects of physical aging on creep compliance of IM7/K3B composite loaded in tension and compression. Recently developed novel experimental apparatus and methods were employed to test two matrix dominated loading modes, shear and transverse, for two load cases, tension and compression. The tests, run over a range of sub-glass transition temperatures, provided material constants, material master curves and aging related parameters. Assessment of these new results indicated that although trends in the data with respect to aging time and aging temperature are similar, differences exist due to load direction and mode.

An analytical model proposed previously by the authors for tension loading was used for predicting long-term tension and compression behavior using short-term data as input. These new studies indicated that the model worked equally as well for the tension or compression loaded cases. Comparison of the loading modes indicated that the predictive model provided more accurate long-term predictions for the shear mode as compared to the transverse mode. Parametric studies showed the usefulness of the predictive model as a tool for investigating long-term performance and compliance acceleration due to temperature.
Determination of the Mechanical Properties of Composite Materials by Tensile Tests. Part II: Strength Properties
Authors: Gommers B. , Verpoest I. , Van Houtte P.
Published: April 2011

A new data evaluation scheme is presented for the measurement of tensile strengths at different angles. The method is applicable for all composite materials that follow the Tsai-Wu failure criterion. It is general in the sense that it can be used for both tri-clinic and monoclinic or higher symmetry materials. With this method, tensile strength data at different angles can be made consistent. It is also shown that the shear strength can be determined by tensile tests if in addition a biaxial test and two or three compression tests (for respectively monoclinic or higher symmetry and triclinic materials) are performed. As an example, the method is applied to a glass/epoxy woven fabric composite and two types of knitted fabric composites. The best test angles to determine the strength parameters for these materials happen to be the same angles that were optimal for the determination of the elastic properties.
The Yeh-Stratton Criterion for Stress Concentrations in Fiber-Reinforced Composite Materials
Authors: Yeh Hsien-Yang , Chern Chyi
Published: April 2011

The purpose of this study is to investigate the Yeh-Stratton Failure Criterion with the stress concentrations in fiber-reinforced composites materials under tensile loadings. The Yeh-Stratton Failure Criterion developed from the initial yielding of materials is based on a macromechanics point of view. To investigate this criterion, the influence of the material anisotropic properties and the far field loading on the composite materials with a central hole or a normal crack were studied. Special emphasis was placed on definng of the crack tip stress field and their applications.

The study of the Yeh-Stratton criterion for damage zone stress field on fiber-reinforced composites under tensile loadings was compared with several other fracture criteria, including the Tasi-Wu Theory, the Hoffman Theory, the Fischer Criterion, and the Cowin Criterion. The theoretical predictions from these criteria are examined by experimental results.
Design Optimization of Fiber Reinforced Plastic Composite Shapes
Authors: Qiao Pizhong , Davalos Julio F. , Barbero Ever J.
Published: April 2011

A global approximation method to optimize material architecture and cross-sectional area of new fiber reinforced plastic (FRP) composite beams is presented. The sections considered are intended for applications in short-span bridges. The beams are subjected to transverse loading, and the optimization constraints include deflection limit, material failure, and elastic buckling. Assuming a laminated structure for the pultruded FRP shapes, experimentally-verified micro/macromechanics models are used to predict member structural behavior. The design variables include the cross-sectional geometric dimensions and the material architecture. The constraint functions are defined through a global approximation at a number of design points, and the approximate constraint equations are obtained through multiple linear regressions and are defined as power law functions of the design variables. The proposed method can concurrently optimize the dimensions and material architecture of a given shape, and as an illustration, a new winged-box (WB) shape is optimized. The present optimization approach combined with existing knowledge on FRP shapes can be used to develop various new shapes and to create a new family of efficient FRP geometries for the civil structural market.
Acoustic Emission from Depressurization to Detect/Evaluate Significance of Impact Damage to Graphite/Epoxy Pressure Vessels
Authors: Downs K. S. , Hamstad M. A.
Published: April 2011

Six aerospace-type, filament-wound graphite/epoxy pressure vessels were studied. Four of these cylindrical vessels had each received a single, controlled impact; four had received thermal exposures including moderate heat and/or cryogenic cold. Acoustic emission (AE) was monitored during a proof test sequence (after the impact/thermal exposure), including the final depressurization from proof pressure (i.e., the second unload cycle). A single set of wideband AE sensors was used to simultaneously record parameter-based AE and waveform-based AE on two independent AE instrumentation systems. Some slight to moderately large differences in AE activity for impacted vs. non-impacted vessels were noted for AE from Felicity ramps and holds at pressure. However, dramatic differences were apparent in quantities of AE from the final depressurization cycle: impacted vessels experienced unload AE activity which was a minimum of an order of magnitude greater in quantity than the unload AE activity for non-impacted vessels. This difference provided a distinctive means of identifying those vessels with impact damage. A newly defined "Shelby ratio" was introduced as a means of quantitatively assessing the unload AE; correlations were obtained between vessels' residual strength and the unload "Shelby ratios." Refinements of the parameter-based data analysis approach were made possible by waveform information. Additionally, examination of the waveform data of the unload cycle revealed a significant quantity of "friction" type waveforms having a repeated, lower frequency character; similar "friction" waveforms were able to be artificially generated by rubbing a pencil lead across a composite vessel surface.
Determination of the Mechanical Properties of Composite Materials by Tensile Tests. Part I: Elastic Properties
Authors: Gommers B. , Verpoest I. , Houtte P. Van
Published: April 2011

Elastic constants as measured by means of tensile tests at different angles, are interrelated by the transformation rules. Based on these rules, a new data evaluation scheme is proposed to make tensile test data consistent. It also offers a way to measure the shear modulus and the shear-extension coupling constants by simple tensile tests. The method is applicable for all types of anisotropic materials. For strong anisotropy, the end-constraint effect can be accounted for in an iterative way. As an example, results are shown for a glass/epoxy woven fabric composite and for knitted fabric composites with monoclinic and triclinic symmetry. Next, the optimal test design is looked for: which tensile tests have to be performed to get the best estimate of the elastic constants? This problem is addressed both theoretically and by means of simulations for the case of woven and knitted fabric composites.
Coupon Tests of Fibre Reinforced Plastics at Elevated Temperatures in Offshore Processing Environments
Authors: Hale J. M. , Gibson A. G.
Published: April 2011

A programme of tests has been conducted to characterise the strength reduction of three GRP composite materials as a function of temperature and testing environment.

The GRP materials and testing environments were selected to be of maximum relevance to the offshore industry. The materials were E-glass fibre reinforcement in matrices of three resins: two epoxies and phenolic. The testing environments were sea water and crude oil condensate. The temperature range tested was ambient to 150°C.

The tests were performed on coupon specimens laminated using fibres in the form of woven mat. The fibres were aligned so as to allow testing in both fibre dominated and resin dominated failure modes. A novel environmental tensile testing facility was developed for this work, and this is described briefly.

Results are presented for the strength of the three materials as a function of temperature in each failure mode when tested dry. These are then compared with equivalent results for the specimens tested in the liquid environments. This is the first reported measurement of GRP strength in liquid environments at these very high temperatures.

It is demonstrated from the results that strength reduction is exhibited predominantly in the matrix dominated failure mode, though a small but significant strength reduction is also seen in the fibre dominated mode. Causes for these strength reductions are identified as lowered glass transition temperature and degradation of the glass and glass/resin interfaces respectively. The strength reductions for each combination of material and environment are quantified.

Finally, appropriate limits to operating temperature are suggested for each material when used in an offshore environment.
A Method to Predict the Effect of Thermal Residual Stresses on the Free-Edge Delamination Behavior of Fibre Reinforced Composite Laminates
Authors: Unger W. J. , Hansen J. S.
Published: April 2011

A method is presented which accounts for the effect of process-induced thermal residual stresses on the free-edge delamination behavior of fibre reinforced laminates. A strain energy release rate fracture mechanics formulation is derived based upon the Irwin crack closure integral. The finite element method is then used to calculate Mode I and Mode II crack opening displacements and stiffnesses for free-edge delamination cracks originating at a specified ply interface from which strain energy release rates due to thermal and mechanical loading are then obtained. The effect of processing temperature on strain energy development is included in the calculation of thermal energy release rates by using experimentally measured material property data.

To verify the model, (+352/-352/02/902)s graphite-reinforced PEEK tensile test specimens were manufactured and tested to failure. Measured delamination strains agree well with predictions when the effect of thermal residual stresses is included. Neglecting residual stresses over-estimates the tensile delamination strength by approximately 32%.
Design Optimization of Composite Plates: Part I—Design Criteria for Strength, Stiffness, and Manufacturing Complexity of Composite Laminates
Authors: Manne Philippe M. , Tsai Stephen W.
Published: April 2011

Design criteria are essential for describing the objectives of optimization. Strength, stiffness, and manufacturing are addressed.

The mechanical criteria, for strength and stiffness, provide with the magnitude by which the thickness of a given laminate must be scaled in order to just meet all requirements. For first-ply-failure strength, a quartic failure criterion was derived from the quadratic Tsai-Wu criterion, for unsymmetric and symmetric laminates subject to mixed loadsets. For stiffness, maximum displacement constraints can be selectively set on all components and modulus. Combined strength and stiffness control is also analyzed. While developed for single elements, these criteria are also suited for designing entire laminated plates.

Assessing manufacturing complexity without undertaking a detailed cost analysis is achieved by an index which interprets numerically qualitative issues such as simple and complex layups. The new manufacturing complexity index takes into account variations in thickness, the steepness of plydrops and the number of zones across a structure.
A Novel Technique for Penetrator Velocity Measurement and Damage Identification in Ballistic Penetration Experiments
Authors: Espinosa Horacio D. , Lu Hung-Cheng , Xu Yueping
Published: April 2011

A novel experimental configuration that can simultaneously record projectile velocity histories and target back surface out-of-plane motion in penetration experiments has been developed. The technique was used to investigate failure mechanisms during ballistic impact of an S-2 glass fiber woven composite with 60% fiber volume fraction. Microscopy studies performed on recovered samples clearly show interply delamination, fiber breakage, ply inelasticity, and fiber kinking as the major failure modes in these composites. Recorded penetrator velocity histories indicate the failure process is rate dependent.

Three well defined regions with different failure zones are observed in the laminate. In a region at the rear of the target plate, Region A, extensive delamination between plies is seen leading to bulge formation. Damage is observed in front of the penetrator with substantial fiber shearing. In a middle region, Region B, tensile fiber failure and large fiber deflection, to accommodate the lateral expansion generated by the steel penetrator, are observed. At the projectile entrance, Region C, fiber microfracture followed by fiber tensile failure is believed to be the failure mode in this region. Noticeable delamination is also produced in plies close to the front specimen surface. Two major fiber failure modes are observed in the micrographs, fiber kinking and cracking. Well defined kink bands are seen in Regions B and C on plies with fibers oriented perpendicular to the penetration direction. The formation of kink bands appears to be the result of compressive failure due to lateral motion of the plies away from the advancing steel penetrator.
Computational Simulation of Interlaminar Crack Extension in Angle-Ply Laminates due to Transverse Loading
Authors: Kamiya Shoji , Sekine Hideki , Yagishita Yoshimasa
Published: April 2011

We try to computationally predict the two-dimensional interlaminar crack extension behavior in angle-ply laminates with different fiber-orientations subjected to transverse loading. Unlike previous investigations, a prediction method free from any assumption on the shape of interlaminar cracks is aimed in this study. We focus our attention onto the local energy release rate along the interlaminar crack front. Under the concept of constant critical energy release rate, the interlaminar crack extension in three-layered angle-ply laminates with various fiber orientations is computationally simulated. Simulation results appear basically in good agreement with experimental results, and the variation of the shape of interlaminar cracks can be automatically predicted without any prior knowledge of their appearance.
Modified Mixed-Mode Bending Test Apparatus for Measuring Delamination Fracture Toughness of Laminated Composites
Authors: Shivakumar Kunigal N. , Crews John H. , Avvat Vishnu S.
Published: April 2011

A modified mixed-mode bending test apparatus was developed to measure delamination fracture toughness and fatigue delamination growth rates of laminated composites. The test apparatus is compact, lightweight, and has very few attachments. The use of needle bearings and a lightweight loading lever makes the apparatus particularly suitable for fatigue tests. The design parameters of the apparatus were selected to minimize the geometric nonlinear effects. The total weight of the aluminum lever and the saddle is about 1.02 lb (4.5 N). The lever is graduated to directly read the load position and calculate G I/ G II ratio. For this apparatus, simple linear equations for mode I and II strain energy release rates ( G) were derived and were found to be accurate. The test apparatus was used to measure mixed-mode fracture toughness of unidirectional IM7/5260 graphite/Bismaleimide composites. The delamination growth was stable for mode I loading and mixed-mode loading with G I/ G II = 4.55/1 and 1.16/1. Whereas, it was unstable for mode II and mixed-mode loading with G I/ G II = 1/3.4. The average mode I and mode II fracture toughnesses were 1.53 lb/in (268 N/m) and 13.53 lb/in (2,370 N/m), respectively. The average fracture toughness for mixed-mode loading with ( G I/ G II) = 4.55/1, 1.16/1, and 1/3.4 was 2.18 lb/in (382 N/m), 3.15 lb/in (552 N/m), and 7.44 lb/in (552 N/m), respectively. The linear mixed-mode delamination fracture criteria ( G Ic m/ G IIc m) found to give a lower bound solution. The delamination growth resistance and the mixed-mode ratio were practically constant.
Optimum Design of Thick-Walled Composite Rings for an Energy Storage System
Authors: Ha Sung K. , Jeong Hee-Moon , Cho Young-Soo
Published: April 2011

An optimum design has been performed to maximize the specific energy density (SED) of a composite flywheel rotor for an energy storage system. The flywheel rotor consists of multiple rings, and the interferences and ply angles vary in the radial direction. For the structural analysis the rotor is assumed to be an axisymmetric thick laminated shell with a plane strain state. Considering the ring-by-ring variation of fiber orientations, a symmetric local stiffness matrix was derived for each ring. Using the stiffness matrix, the continuity conditions of radial stresses and displacements between the rings with a consideration of the interferences can be easily incorporated. A symmetric global stiffness matrix is then obtained assembling the local stiffness matrices. Displacements are obtained by solving the global stiffness matrix, and the stresses in each ring are then calculated. Three-dimensional intra-laminar quadratic Tsai-Wu criterion for the strength analysis is used yielding the strength ratio for each ring. The optimization has been also performed maximizing the kinetic energy stored in the rotor. For that purpose the sensitivities of the strength ratios and displacements with respect to the design variables have been derived. As a result, the optimal design has attained 2.4 times of total energy compared to the case of 0 degree ply angle and no interference. The effects of interferences are found to be much more considerable than those of fiber orientations.
Development of a Stochastic Free Edge Delamination Model for Laminated Composite Materials Subjected to Constant Amplitude Fatigue Loading
Authors: Bucinell R. B.
Published: April 2011

A stochastic delamination growth model with parameters written in terms of fracture mechanics, material, and laminate structural parameters is developed for composite laminates subjected to constant amplitude cyclic loading. The model captures the discrete stochastic nature of the delamination growth process in laminated composite materials. Experimental data obtained using AS4/3501-6 coupons with a laminate stacking sequence of [±45/90/0], verifies that the model predicts the mean and variance associated with the delamination growth. Changing only one parameter in the model enables the model to predict delamination growth behavior at several load levels. Two parameters are identified that could possibly expand the model's predictive capability to include laminate geometries other than those used to correlate the model.
Manufacturing of Co-Cured Composite Aluminum Shafts with Compression during Co-Curing Operation to Reduce Residual Thermal Stresses
Authors: Cho Durk Hyun , Lee Dai Gil
Published: April 2011

Shafts made of carbon fiber composite have higher fundamental bending natural frequency when the stacking angle of fiber is close to zero from the axial direction compared to that of metals such as aluminum and steel, which is an important property for power transmission shafts. However, the small stacking angle not only reduces the torque transmission capability of composite shafts but also makes the joining of composite shafts to other materials difficult.

If a shaft is made of both carbon fiber composite and metal such as aluminum, the bending natural frequency as well as the torque transmission capability of the shaft can be increased: the carbon fiber composite increases the bending natural frequency and the aluminum increases the torque transmission capability.

In this paper, a hybrid shaft was manufactured by co-curing carbon fiber epoxy composite to an aluminum shaft to increase the bending natural frequency and damping without reducing the torque transmission capability of the shaft. In order to reduce the residual thermal stress at the interface of the two materials, the aluminum shaft was compressed by giving a compressive preload during co-curing operation.

From the fatigue test and finite element analysis of the co-cured hybrid shafts, it was found that the compression by a preload increased the torque transmission capability of the hybrid shafts.
Effect of Stress Concentration on Tensile Fracture Behavior of Carbon-Carbon Composites
Authors: Kogo Yasuo , Hatta Hiroshi , Kawada Hiroyuki , Machida Terufumi
Published: April 2011

The effect of stress concentrations on tensile fracture behavior of carbon-carbon (C/C) composites was investigated using circularly holed specimens and double-edge-notched (DEN) specimens.

As for the circularly holed specimens, the tensile fracture stress was much higher than that estimated from the maximum stress criterion, which suggest that major stress relaxation mechanisms should exist. On the other hand, the linear elastic fracture mechanics can be applied to the DEN specimen, which means the damaged zone should be small enough compared with the notch length. In order to discuss the magnitude of the stress relaxation, damaged regions of the two kinds of testing geometry were estimated using the point stress criterion. The estimation led to remarkable difference in the size of the damaged regions, which will explain the difference in the magnitude of the stress relaxation.

Through the observations of fractured specimen, it was deduced that not only the shear deformation but delamination along fiber bundles and opening of transverse crack would relax the stress concentrations. The other mechanism was also proposed based on the testing results, that is strength increase in the damaged region.
Simplified Analysis of Residual Stresses in In-Situ Cured Hoop-Wound Rings
Authors: Gabrys C. W. , Bakis C. E.
Published: April 2011

A simple theoretical/empirical hybrid method for the prediction of residual stresses in in-situ cured, thick, hoop-wound, thermosetting composite rings is presented. The method avoids the need for complicated characterizations of resin and fiber properties that affect residual stress development during the manufacturing process. New results from a series of experiments with in-situ cured hoop-wound glass and carbon reinforced epoxy composites show that residual stresses due to non-thermal sources such as winding tension and resin shrinkage during cure are essentially constant for fixed manufacturing process parameters and resin system, regardless of ring geometry and fiber type. Also, the combined effect of winding tension and resin shrinkage becomes progressively less important in comparison to thermal effects as the ID/OD ratio of a ring decreases. The analysis method is demonstrated by using the data acquired with a single ring to predict the residual stress in another ring with different fiber, ID, and OD. Equations for the thermal expansion of hoop-wound composite rings were corrected from the published version and verified for a range of ring geometries, two different types of fibers, and two resin systems. The analysis tools outlined in this paper aid the design and fabrication of multiple-concentric-ring structures subjected to varying temperatures, such as composite flywheels for energy storage.
Computation of Homogenized Coefficients for a Viscoelastic Composite Reinforced with Spherical Inclusions
Authors: Labed Nadia , Turbe Nicole
Published: April 2011

The viscoelastic composites are being more widely used in the industry. While remaining light materials with good mechanical stiffness characteristics, the viscoelastic composite materials provide interesting absorption properties and can reduce the vibratory disturbances.

The paper is concerned with the behavior of a viscoelastic composite incorporating isotropic viscoelastic balls embedded in isotropic resin subjected to harmonic vibrations. The wavelength considered is large compared to one period of the medium, and therefore, the homogenization of the medium is substantiated allow to replace the highly heterogeneous material by a homogeneous material whose behavior depends on the mechanical properties and geometry of the various components. The homogenized coefficients are obtained by resolving elementary problems in which only complex quantities are present. These coefficients are numerically computed using a finite element method in 3-dimensional space of complex numbers.

Two examples are proposed allowing to trace the change in the various coefficients which are homogenized as a function of the pulse. These coefficients are only angular dependent frequencies and are approximated using a law of n parameters. This enables an important reduction in computational time.
Design of Particle Reinforced Heat Conducting Composites with Interfacial Thermal Barriers
Authors: Lipton Robert
Published: April 2011

Two phase particle reinforced heat conducting composites are considered. We treat the case when there is an interfacial thermal barrier between phases. We provide novel rules of thumb for selecting the particle size distribution and minimum particle size necessary for constructing composites with effective properties greater than that of the matrix. The rules are based on new energy dissipation inequalities obtained in the work of Lipton (1996, Journal of Applied Physics, 80:5583-5586).
Compressive Behavior of Plain Weave Lamina
Authors: Kollegal M. G. , Sridharan S.
Published: April 2011

An investigation into the micromechanics of a plain weave lamina subjected to compressive loading is presented. Three dimensional finite elements, embodying a realistic description of the spatial variation of the material properties and yarn geometry, are used to model a repetitive portion of the plain weave fabric. A detailed examination of the internal stresses and strains is carried out. Possible mechanisms causing intra-yarn and inter-yarn failure are described. It is shown that geometric nonlinear effects alone are not significant in the compression response of plain weaves, but in conjunction with material nonlinearity precipitate failure. The reduction in strength due to the inclusion of material nonlinearity of the matrix surrounding the yarns is found to be substantial.
Effective Elastic Constants of Particulate Composites with Inhomogeneous Interphases
Authors: Wang Wei , Jasiuk Iwona
Published: April 2011

We consider a composite material reinforced with spherical inclusions, which are arranged in a matrix in a statistically isotropic way and they are bonded to the matrix via an interphase. We represent the interphase as an inhomogeneous region (a functionally graded material) and consider an idealized model in which the interphase is linear elastic and isotropic and has the Young's modulus and Poisson's ratio varying in radial direction. We predict effective elastic constants of such a composite and study the influence of the spatial inhomogeneity of interphase on these constants. In the analysis we use the composite spheres assemblage method [1] to evaluate the effective bulk modulus and the generalized self-consistent method [21 to obtain the effective shear modulus. We show that the radial variation in the Young's modulus of interphase has an influence on the effective elastic moduli as compared with the corresponding uniform interphase case when particles are stiffer than the matrix and the effect is negligible when particles are softer than the matrix for the composite systems considered. We also show that the variation in Poisson's ratio has a much smaller influence on the effective bulk modulus than the inhomogeneity in the Young's modulus.
The Interlaminar Mode I Fracture of IM7/LaRC-RP46 Composites at High Temperatures
Authors: Wang W. X. , Takao Y. , Yuan F. G. , Potter B. D. , Pater R. H.
Published: April 2011

In this paper, the interlaminar Mode I fracture behavior of a IM7/LaRC-RP46 composite laminate at elevated temperatures was investigated by DCB (double cantilever beam) static fracture testing. Straight-sided DCB specimens were made from unidirectional laminates and tested at three high temperature levels of 232°C (450°F), 288°C (550°F) and 343°C (650°F) at five crosshead rates from 0.51 mm/min. (0.00033 in./sec) to 6.1 mm/min. (0.004 in./sec). Unlike the linear load-displacement response observed at room temperature, the load-displacement response at high temperatures behaves nonlinearly before reaching the maximum load. Two crack growth regions of slow crack extension and relatively fast crack propagation were observed and the slow crack extension caused the nonlinear load-displacement response. The initial fracture toughness, G IC decreased slightly, while the propagation value increased by nearly 30% to 100% with increased temperature. The materials exhibited the highest fracture toughness at 288°C among the three different tested temperatures. The complex combination effects of fiber bridging and the change of matrix properties and fiber/matrix interfacial adhesion at elevated temperatures are found to be the major reason resulting in the variation of G IC with temperature. After the temperature increased beyond T g (glass transition temperature), which is approximately 310°C, the propagation fracture toughness decreased drastically and aging effects of the high temperature on the matrix are observed. No significant effects of crosshead rate on the G IC were observed in the range of the present rates. However, the crosshead rate is found to significantly influenced the crack growth rate.
Effect of the Coating on the Stresses and Strains in an Embedded Fiber Optic Sensor
Authors: Van Steenkiste Régis J. , Kollár László P.
Published: April 2011

Coated fiber optic sensors operating on interferometric principles have recently been considered for measuring strains and temperature inside composite materials. Owing to the complex interactions between the sensor and the material surrounding it, the relationship between the sensor output and the strains and temperature inside the material cannot be determined by simple tests. In general, the relationships providing the bridge between the sensor output and the engineering values of strain and temperature must be established via analytical models. Closed form expressions are derived which relate the strains in the composite far from the sensor to the strains inside the optical sensor. Strains were calculated inside a sensor with and without a coating in order to assess the effect of the coating on the sensor strains.
Delamination Resistance of Post-Stretched Fibre Metal Laminates
Authors: Vlot A. , Van Ingen J. W.
Published: April 2011

Fibre Metal Laminates (FML) consist of bonded alternating thin aluminium and fibre/epoxy layers. Post-stretching of FML after curing is accompanied by plastic deformation which causes a reversal of the internal stresses due to curing which is favourable for their fatigue properties. The influence of this post-stretching on the delamination resistance was studied. The GI, of FML with aramid fibre (ARALL) was increased by post-stretching. No influence was found on the GI, of glass fibre based FML (GLARE). The GI,, of FML was hardly influenced. The mode I and II delamination resistance of GLARE is significantly higher than that of ARALL. There appeared to be no relationship between these G,-values and the delamination of realistic structural details: the delamination of doublers as applied in aircraft wing panels. Post-stretching increased the delamination resistance of doubler specimens and despite its higher G,-values GLARE doubler specimens showed the same delamination resistance as ARALL. Although the delamination resistance of post-stretched doubler specimens was increased, the delamination progression after exceeding the delamination stress was for post-stretched material much faster than for as cured material, due to the larger amount of strain energy stored in the laminates during the post-stretching operation. Delamination fatigue tests indicated two competing mechanisms on the delamination growth rate. Therefore the delamination fatigue behavior is largely influenced by post-stretching, but the effect can be either positive or negative.
Intralaminar Failure Stresses Due to Centrifugal Forces in Polar-Anisotropic Circular Plates
Authors: Tutuncu Naki
Published: April 2011

Failure stresses due to centrifugal forces in circular plates of general polar anisotropy are determined. The plate was assumed to be symmetrically laminated, each lamina being oriented at an arbitrary angle with respect to the radial coordinate. Since the plate considered had unconstrained and stress-free outer boundary, it was possible to obtain closed-form solutions for radial and circumferential displacements. The intralaminar failure stresses are then calculated and, using the Tsai-Wu tensorial failure theory, the safety factors for each lamina interface has been determined. The material used for numerical results was chosen arbitrarily for purely illustrating purposes. The results have shown that it was preferable, solely for the increased strength purposes, to have fibers placed in radial direction for the particular material and the lay-up chosen.
A Non-Quadratic Plasticity Model Suitable for Metal Matrix Composites
Authors: Chen J. K. , Beraun J. E.
Published: April 2011

Based on the assumption that uniform dilatation has negligible effect on the yield point, a high-order yield function of non-equal stress exponents in conjunction with a non-associated flow rule is proposed to formulate 3-D plastic constitutive equations for fibrous metal matrix composites (MMC). The involved nine plastic parameters, six for the yield criterion and three for the plastic potential, can be easily calibrated with the macro stress-strain data of simple tension, pure shear, and bi-axial tension tests. It is demonstrated numerically that this theory describes anisotropic plasticity of boron-aluminum (B/Al) composites very well. It is also shown that the uniform dilatation assumption is adequate for other fiber composites. By properly selecting the plastic parameters, the present model can reduce to the nine-parameter quadratic yield function developed for graphite fiber/organic matrix composites [1], Hill's quadratic yield model [2], and von Mises and Tresca isotropic yield criteria.
A Model for Moisture Absorption in Cross-Ply Composite Laminates with Matrix Cracks
Authors: Lundgren Jan-Erik , Gudmundson Peter
Published: April 2011

A homogenized analytic model for average moisture uptake in composite laminates containing periodically distributed matrix-cracks is presented. It is shown that the model, compared to FE-calculations, well decribes moisture absorption in a cross-ply composite laminate containing periodically distributed transverse matrix cracks in the 90° plies. The homogenized model is based on a two-phase model consisting of the crack channels and the composite material respectively. An analytic solution to the two-phase model for moisture uptake in composite laminates is presented. The model can be used to judge whether micro cracks are of importance or not concerning moisture absorption.
Nondestructive Evaluation of Interlaminar Bonding for in Situ Consolidation of Polymer Composites
Authors: Mantell S. C. , Linstrom E. J.
Published: April 2011

A sensor for detecting poor interlaminar bonding between a topmost ply and composite substrate was developed and validated. The sensor was designed for the specific application of real time process control for in situ consolidation processes, such as thermoplastic tow placement. The sensor design consists of two piezoelectric transducers oriented in a pitch catch arrangement. The transmitting transducer is angled so as to produce surface waves in the composite panel. These waves interact with defects at the top ply/substrate interface. The received signal was analyzed for wave speed, attenuation and energy dissipation (power spectral density). Experimental validation of the sensor is presented. Several composite panels were made under various manufacturing conditions so as to create poor interlaminar bonding at the topmost ply. Interlaminar bonding was then evaluated by destructive and nondestructive measures. There was good agreement between the destructive and nondestructive measures of top ply bond strength.
Modeling of Plain Weave Fabric Composite Geometry
Authors: Kuhn J. L. , Charalambides P. G.
Published: April 2011

Woven unit-cell geometry functions are presented for a balanced plain weave fabric composite. Geometry models for polymer and ceramic matrix woven systems are developed. For polymer matrix woven systems, the space between bundles is fully occupied by the inter-bundle matrix phase which is modeled as a porous elastically isotropic medium. On the other hand, the woven unit-cell geometry model for ceramic matrix systems allows for the presence of both disperse porosity as well as large scale voids within the interbundle matrix region as needed to simulate the complex microstructure resulting from the Chemical Vapor Infiltration (CVI) processing technique. The geometry models take into account the existence of space between tows, the undulation of the tows, and the actual tow cross-sectional shape. Both the warp and transverse tows are modeled using piecewise continuous functions throughout the unit-cell domain. Equal accuracy in both the warp and fill directions, as required for biaxial and in-plane shear loading studies, is built into the geometry surface functions. The effects of the unit-cell geometry on the matrix and tow volume fractions for both the PMC and CMC systems are explored. Three-dimensional finite element meshes developed using these new mathematical geometry models are presented for both the porous matrix and matrix layer models.
The Effects of Delamination on the Fatigue Behavior of Composite Structures
Authors: Barboni Renato , Carbonaro Rolando , Gaudenzi Paolo
Published: April 2011

The aim of the present study is to investigate the role played by delamination on the fatigue behavior of centrally holed CFRP laminates. To reach this objective, both theoretical and experimental studies were performed. As to the theoretical analysis and the relevant numerical predictions of the 3D state of stress, the study proposes the use of a finite element procedure based on a multi-layer higher-order laminate theory developed by the authors in conjunction with the application of an interlaminar failure criterion. The approach assumes that significant information on the fatigue life of holed laminate can be obtained by performing a static prediction only. Two types of rectangular laminates with stacking sequences [+45)4/(-45)4] s, and [0/90] 3 manufactured with T300/934 are examined. The static and the fatigue failure of the above mentioned laminates were then investigated in a series of experiments in order to verify the initiation of delamination both in the static case and during the fatigue loading process, ultrasonic nondestructive inspections were also made. The obtained results show a significant correspondence between the experimental static and fatigue behavior of the laminates and the theoretical predictions. They show also that an efficient numerical tool of the 3D state of stress prediction is used.
Crystallinity of On-Line Consolidated Thermoplastic Composites
Authors: Pistor Christoph M. , Güçeri Selçuk I.
Published: April 2011

Processing of high-quality thermoplastic composite parts by laser-assisted tape consolidation involves in-situ melting and solidification of the thermoplastics matrix material. In contrast to autoclave processing, fusion of the matrix material occurs locally, preventing both the development of residual stresses and fiber buckling in the inner layer. High temperature thermoplastics such as Polyetheretherketone (PEEK) or Polyetherketoneketone (PEKK), are semicrystalline. The level of crystallinity depends strongly on the local thermal history as well as other processing parameters. In the present study the effects of these parameters on the thermal properties, such as the melting and cold crystallization point, are determined utilizing Differential Scanning Calorimetry (DSC). These thermal properties can give information about possible degradation of the polymer induced during the manufacturing process. The material selected for this study (PEEK/Carbon composite) exhibits a decrease of melt and cold crystallization temperature for increasing laser powers where the latter is more pronounced. The crystallinity, calculated from regular DSC data, showed wide variations. Therefore, modulated DSC measurements were also performed, which are considered to be more accurate for crystallinity measurements due to their ability to record endo- and exothermal events during cold crystallization and melting respectively. The results of these measurements show a significant trend of increasing crystallinity with laser power. Furthermore, a significant decrease of the glass transition temperature of the material was also observed. This was believed to be caused by a molecular weight reduction due to heat fragmentation during processing. The reduction of molecular weight is expected to be responsible for the increase in crystallinity with laser power, since the glass transition temperatures were lower at higher laser power settings.
The Effect of Interfacial Reactions on Residual Stress Fields within Composites
Authors: Debray K. , Martin E. , Quenisset J. M.
Published: April 2011

The study has been devoted to methodological aspects related to the design of fiber/matrix interfacial zone in refractory composites. Titanium matrix composites reinforced by SiC filaments were chosen to illustrate the various facets of the study. The high temperatures of processing and use lead to thermally induced residual stresses which play such an important and often unfavorable role in the composite behavior that designing new materials inevitably requires the evaluation of these stresses. Making use of a classical method for computing these thermal stresses showed the interposition of a compliant carbon interphase between fiber and matrix is inefficient to release significant hoop stresses. Besides, submitting fiber/matrix interfaces to high temperature generally gives rise to chemical interactions whose effects are often unfavorable. However, the formation of new interphases is sometimes accompanied by volume reductions leading to chemically induced residual stresses. A method has been proposed to compute these stresses. Considering various types of interactions which can be involved in titanium matrix composites (TiB2/Ti, C/Ti, C/Si) has shown that chemical interactions are able to generate significant compressive hoop stresses. Their control through high temperature treatments could be used to moderate thermally induced tensile stresses whose excessive levels in hoop direction initiate radial microcracking.
Stress and Plastic Strain Fields during Unconstrained and Constrained Fabrication Cool Down of Fiber-Reinforced IMCs
Authors: Baxter Sarah C. , Pindera Marek-Jerzy
Published: April 2011

Two mathematical modes are presented for the thermal cool-down phase of composite fabrication and applied to a specific material system (A12O3/Ni3Al). The first is a baseline model of idealized processing conditions. The second extends the process model by including a tooling constraint generally associated with fabrication techniques such as hot isostatic pressing, vacuum hot pressing or die casting. In previous work, little attention has been devoted to the investigation of the constraining effects of the tooling phases, used to fabricate unidirectional metal matrix composites, on the residual stress evolution.

Various pressure loading histories can be applied to each model. The stresses and plastic strains that develop during the cool-down are calculated at the micromechanical level by the generalized method of cells, which is embedded into both global models. It is shown that residual stresses, at the end of the process, differ very little due to either the boundary conditions of the models or pressure loading histories. The stresses that evolve during the cool-down, however, are significantly different for the two models. In the unconstrained model the driving mechanism for the developing stresses and plastic strains due to thermal cool-down is the mismatch between the coefficients of thermal expansion of the fiber and matrix materials. In the constrained model this effect becomes secondary to the mismatch between the thermal expansion coefficients of the composite and the constraining can. In the constrained model, in-plane tensile stresses develop which are large enough to produce damage in the form of transverse fiber cracks, matrix cracking or filament breaks within the fiber tow. Since the damage producing stresses predicted by this model can be traced to the difference between the effective coefficient of thermal expansion of the processing can and the composite, steps can be taken to prevent damage by tailoring the properties of the processing can to the composite.