bluuebunny/crossref_metadata_2025 huggingface dataset:
Failure Behavior of Plain Weave Fabric Laminates under On-Axis Umnaxial Tensile Loading: III—Effect of Fabric Geometry
Authors: Ganesh V. K. , Naik N. K.
Published: April 2011

The effect of fabric geometry on the failure behavior of two-dimensional orthogonal plain weave fabric laminates under on-axis uniaxial static tensile loading has been investigated using the two-dimensional woven fabric composite strength model. The weave geometrical parameters considered are: strand width, strand thickness and interstrand gap and the corresponding fiber volume fractions. The studies were carried out for three idealized laminate configurations. It was observed that there is a significant effect of weave geometrical parameters on the failure behavior.
Evaluating the Influence of Fiber Coatings on the Compression Strength of a Unidirectional Polymer Composite
Authors: Eskandari Saeed , Carman Greg P. , Case Scott
Published: April 2011

In this paper we present an analytical model to predict the compression strength of a composite containing coated, circular fibers. The theoretical development is based on the assumption that shear failure in the matrix governs the compression strength of a unidirectional composite material. The stress concentrations in the matrix are dependent upon the geometry of the fiber and the interphase or coating region around the fiber. Analytical results suggest that an optimum interphase might exist to maximize compression strain to failure. However, in general stiffer coatings appear to provide larger compression strength than do compliant coatings. To evaluate the analytical model, composites containing glass fibers coated with 5 different materials were manufactured. Tests on these samples support the trends predicted by the model and the strong dependence of compression strength predictions on coating properties.
Experimental and Finite Element Analysis of Pultruded Glass-Graphite/Epoxy Hybrids in Axial and Flexural Modes of Vibration
Authors: Nori Chandrasekhar V. , Mantena P. Raju , McCarty Tyrus A.
Published: April 2011

The effects of hybridization on the extensional and flexural dynamic properties of pultruded composite materials are reported in this paper. The composite materials considered were made up of unidirectional glass, graphite fibers in an epoxy matrix, and hybrids of glass-graphite/epoxy produced by the pultrusion manufacturing process. The dynamic storage modulus and loss factor of the rectangular and circular cross-section samples were first evaluated using the nondestructive impulse-frequency response vibration technique. The long and slender flat specimens were analyzed in both extensional and flexural modes of vibration in a free-free test configuration, whereas the thicker round specimens were analyzed only in the axial mode of vibration. The properties of the monofiber type glass/epoxy and graphite/epoxy composites were used as input to the finite element model for predicting and experimental validation of various pultruded (and hypothetical) glass-graphite/epoxy hybrid combinations. The modal strain energy method was used for computing the structural loss factors of the hybrid specimens based on the element stiffness matrices and estimated mode shapes in the fundamental mode of vibration.

Results of this investigation showed excellent agreement between experimental data and numerical predictions for the dynamic extensional properties of both flat and round specimens. The numerical results for dynamic flexural properties of flat specimens were found to have a similar trend as that obtained from the experimental technique. Small variations in flexural properties could be attributed to the irregular shapes (which were not taken into account in the finite element model) of layers in the hybrid combinations formed during the pultrusion manufacturing process. Results also demonstrated that while the extensional properties were independent of the fiber location and fiber packing geometry, the flexural properties were highly dependent upon these two factors. Previously reported data (which was obtained by testing these flat hybrid specimens in the flexural mode of vibration in a clamped-free boundary condition) were re-analyzed using the free-free configuration and modal strain energy method, yielding more reasonable results.
A Multicontinuum Theory for Thermal-Elastic Finite Element Analysis of Composite Materials
Authors: Garnich Mark R. , Hansen Andrew C.
Published: April 2011

Structural analysis of composite materials is severely limited by the lack of constituent information. This information is generally unavailable due to the continuum hypothesis associated with the geometric scale of the problem. Micromechanics has been the traditional approach to achieving insights into constituent behavior in a composite material. However, modelling micromechanical details in a structural application is frequently impractical.

In this paper, by example of linear elasticity, we suggest a viable bridge between structural analysis and micromechanics. The approach utilizes detailed finite element models of a representative volume element as a one-time preprocessor to characterize the structural stiffness of the material. This information is used to implement an analytical expression for constituent stress and strain fields (in the continuum sense) as a function of the structural fields in a conventional finite element analysis. The computational premium is minimal while the benefits and additional insights regarding constituent behavior are significant. Numerical results for constituent as well as structural fields are presented for thermal-elastic behavior of a composite plate with a hole. The results indicate dramatic differences in the constituent stress fields for a comparable structural stress caused by thermal and mechanical loads, respectively. A brief discussion of the potential extension to in-elastic analysis is also included.
Fiber Optic Sensor Development for Real-Time In-Situ Epoxy Cure Monitoring
Authors: Liu Y. M. , Ganesh C. , Steele J. P. H. , Jones J. E.
Published: April 2011

Analysis of a new fiber optic sensor based on the principle of Fresnel reflection is presented in order to develop a robust, flexible, readily embedded, high sensitivity and low-cost cure monitoring tool for intelligent control of composite manufacturing. This approach is distinct from previous work on on-line in-situ monitoring sensors in that the transducer is simply the fiber optic/epoxy interface. This leads to a more simple, less intrusive, and lower cost sensing system. The response of the sensor is a function of the mismatch in refractive index between the fiber optic end/resin interface. The refractive index of the resin is a nonlinear function of the temperature and the cure reaction of the resin. The sensing system detects the mismatch in the refractive index and generates a characteristic profile that gives the chemorheological information about the curing resin. Under isothermal cure conditions, the effect of the temperature on the refractive index mismatch can be eliminated. In this situation, the optical response of the sensor is only dependent on the cure kinetics and the state of the cross-linking in the material. A calibration method has been developed to interpret the optical response directly as the degree-of-cure of the resin. This work provides analysis of the fundamental sensor response and correlates it to the material state (degree-of-cure). These results provide a basis for extending the use of this sensor technology for on-line real-time cure monitoring and control.
On the Prediction of Mechanical Behavior of Particulate Composites Using an Improved Modulus Degradation Model
Authors: Wong F. C. , Kadi A. Ait
Published: April 2011

New micromechanical models for the prediction of particulate composite mechanical behavior have been developed. The models use an energy balance concept to account for nonlinear behavior due to particle debonding and incorporate a composite modulus prediction routine based on an improved Mori-Tanaka method. This method permits particle interaction effects to be taken into account and allows the stiffness matrix for voids or vacuoles to be explicitly stated in the model. To demonstrate the characteristics of the improved Mori-Tanaka method, comparisons with 2-phase and 3-phase modulus data were made. The micromechanical models developed for void and vacuole formation were evaluated against available mechanical behavior data. Comparisons showed that the model derived for vacuole formation predicted the mechanical behavior correctly for a range of composites which contained inclusion volume fractions ranging from 0.2 to 0.4. The inability of the models to predict the initial stress-strain behavior of a composite containing a volume fraction of 0.22 of well bonded particles suggests nonlinear matrix effects need to be included in the present model formulation.
Design and Optimization of a Thermoplastic Tow-Placement Process with In-Situ Consolidation
Authors: Pitchumani R. , Gillespie J. W. , Lamontia M. A.
Published: April 2011

Thermoplastic tow-placement with in-situ consolidation offers the potential for rapid fabrication of composite parts for a variety of applications. Physical models and model-based and experimental analysis of the process phenomena have been previously reported by the authors and other investigators in the literature. This paper presents a methodology for practical design and optimization using the available theoretical process models. Processing windows are developed based on considerations of material degradation through weight loss, final void content, and dimensional change of the tows, all of which determine laminate quality and thus, part performance. Optimum line speed and heat input variations with composite thickness are identified based on parametric studies to maximize interfacial bond strength and minimize fabrication time, subject to constraints on the above-mentioned quality-related parameters. The processing windows and optimum profiles are presented in terms of the controllable process variables, which allows for a ready implementation of the results in practice.
The Drapability of Aligned Discontinuous Fiber Composites
Authors: Tsuji Nobuyuki , Springer George S. , Hegedus István
Published: April 2011

The drapabilities of uncured unidirectional aligned discontinuous fiber composites were investigated, the drapability being defined as the ratio of the tip deflections, in one minute, of a discontinuous to a continuous fiber reinforced composite canti-levered beam. Equations were developed for calculating the deflections of such composite beams taking into account large deflections, and elastic and viscoelastic material behavior. On the basis of the model, a computer code was written which, for given fiber and matrix properties, discontinuous fiber geometry, fiber volume fraction, and fiber orientation, provides the stresses and strains in and the displacements of the composite beam, and the drapability. Tests were performed with beams made of uncured discontinuous and continuous fiber composites. Tip deflections were measured during the tests and were compared with tip deflections calculated by the computer code. Good agreements were found between the data and the results of the present model. Using the computer code, parametric studies were also conducted to assess the effects of fiber geometry, fiber volume fraction and fiber orientation on the drapability of uncured composites, and the effects of fiber geometry on the on-axis elastic moduli of discontinuous cured fiber composites. The results of these studies indicate that while discontinuous fibers increase the ability of uncured composites to drape, such fibers reduce the longitudinal modulus of cured composites.
Influence of Short Glass Fiber Addition on the Mechanical Properties of Sisal Reinforced Low Density Polyethylene Composites
Authors: Kalaprasad G. , Joseph Kuruvilla , Thomas Sabu
Published: April 2011

This paper presents the evaluation of enhancement in the mechanical properties of short sisal fiber reinforced polyethylene composites by the incorporation of short glass fiber as an intimate mix with sisal. Intimately mixed short glass-sisal hybrid fiber reinforced polyethylene composites (GSRP) were prepared by solution mixing technique. The effects of fiber orientation and alkali treatment on sisal fiber in GSRP were studied. Addition of relatively small volume fraction of glass (0.03) to the sisal reinforced polyethylene matrix (SRP) enhances the tensile strength of longitudinally oriented composites by about 80%. Addition of the same volume fraction of glass to the alkali treated sisal incorporated SRP enhances the tensile strength by more than 90%. The flexural strength of the longitudinally oriented composites was also studied. The incorporation of glass fiber ( V f = 0.03) to SRP enhances the flexural strength by more than 60%. The effect of hybridization on water absorption tendency of the sisal fiber was studied by immersing SRP and GSRP in boiling water for 3 hours. It was observed that water uptake of GSRP was 2 to 4 times less than that of SRP composites. Halpin-Tsai equation for composites was tried for calculating the tensile modulus of longitudinally oriented GSRP.
The Effects of Fiber Surface Modification and Thermal Aging on Composite Toughness and its Measurement
Authors: Bowles Kenneth J. , Madhukar Madhu , Papadopoulos Demetrios S. , Inghram Linda , McCorkle Linda
Published: April 2011

A detailed experimental study was conducted to establish the structure-property relationships between elevated temperature aging and (1) fiber-matrix bonding, (2) Mode II interlaminar fracture toughness, and (3) failure modes of carbon fiber/PMR-15 composites. The fiber-matrix adhesion was varied by using carbon fibers with different surface treatments. Short beam shear tests were used to quantify the interfacial shear strength afforded by the use of the different fiber surface treatments.

The results of the short beam shear tests definitely showed that, for aging times up to 1000 hr, the aging process caused no observable changes in the bulk of the three composite materials that would degrade the shear properties of the material. Comparisons between the interlaminar shear strength (ILSS) measured by the short beam shear tests and the G,,, test results, as measured by the ENF test, indicated that the differences in the surface treatments significantly affected the fracture properties while the effect of the aging process was probably limited to changes at the starter crack tip. The fracture properties changed due to a shift in the fracture from an interfacial failure to a failure within the matrix when the fiber was changed from AU-4 to AS-4 or AS-4G.

There appears to be an effect of the fiber/matrix bonding on the thermo-oxidative stability of the composites that were tested. The low bonding afforded by the AU-4 fiber resulted in weight losses about twice those experienced by the AS-4 reinforced composites, the ones with the best TOS.

The results are in agreement with those of previous work completed by the authors.
Analytical Prediction of Flexural Behavior of Concrete Beams Reinforced by FRP Bars
Authors: Almusallam Tarek H.
Published: April 2011

An analytical model to predict the flexural strength and deformations in concrete beams reinforced with fiber reinforced plastic (FRP) bars is presented. It is based on the equilibrium of forces and compatibility of deformation. The model uses a more rational representation of the behavior of all composite constituents. It also accounts for the concrete tensile stresses, the variation of stresses, strains and curvature over the depth of the cross section and the length of the beam. The model predicts the moment-curvature, load-deflection, and load-strain of any of the composite constituents. The model predictions were checked against the experimental results of nine test beams reinforced with steel or FRP bars. The behavior of the concrete tested beams were characterized by their moment-curvature, load-strain in the reinforcement, and load-deflection response up to failure. The results indicate that the analytical model predicts the behavior of GFRP reinforced concrete beams with good accuracy. Furthermore, the accuracy of the model was checked by comparing its capacity prediction with that of a parallel model available in the literature based on measured data and reported by other researchers. Results of the comparison showed that the suggested model provided better prediction.
Residual Strength of an ARALL Laminate Containing a Crack
Authors: Jin Zhi-He , Mai Yiu-Wing
Published: April 2011

The residual tensile strength of a cracked ARALL (ARamid-fibre-reinforced-polymer/ALuminium Laminates) sandwich with an aramid-fibre/epoxy ply in the middle and two aluminium alloy plies on the outside is studied in this paper. It is assumed that both a Dugdale strip yielding zone in the aluminium layers and a strip damage zone in the fibre composite layer are developed concurrently at the crack tip. Two important parameters have been identified and these are the maximum separation displacement δ0 of the fibre composite damage zone and the metal deformation parameter J c/σ ys where J c is the critical J-integral and σ ys is the yielding strength of the metal. The requirement that the strength of ARALL is comparable to that of their metal counterpart can be met if δ0 is close to J c/σ ys
Progressive Damage and Residual Strength of a Carbon Fibre Reinforced Metal Laminate
Authors: Lawcock Glyn , Ye Lin , Mai Yiu-Wing
Published: April 2011

An experimental study on the residual strength behaviour of a Carbon Fibre Reinforced Metal Laminate (FRML) was conducted. Mechanical tests were carried out to determine the laminate tensile property profiles and residual strength of notched coupon laminates. The effect of notch geometry was studied for both centre cracks and centre circular holes with notch sizes ranging from 10 to 40 mm. The reduction in residual strength for the saw-cut specimens was greater than that for the circular hole specimens. C-scan and optical microscopy associated with a chemical etching technique were used to investigate progressive damage and failure mechanisms in these laminates. Damage modes such as delamination, crack growth and fibre breakage were observed. Stable crack growth was seen in both the aluminium and composite layers for the saw-cut specimens, but no visible crack growth in the aluminium layers in the circular hole specimens was found although cracking in the composite layer occurred. It is concluded that delamination ahead of the notch tip is attributed to cracking of the composite layer. The average stress criterion developed for fibre reinforced composites was applied to correlate residual strength data and good agreement was obtained.
A Quadratic Yield Function for Fiber-Reinforced Composites
Authors: Chen J. K. , Allahdadi F. A. , Sun C. T.
Published: April 2011

A simple, 3-D yield function that is quadratic in stresses was proposed to describe the plastic behavior of fiber composites. It relaxes the two usually used assumptions that hydrostatic stress does not influence plastic deformation and that the total plastic dilatation is incompressible. It is also general in nature to allow for composites with various fiber volume fractions and different fiber arrays. The applicability of this quadratic yield function to fiber composites was examined, and the accuracy of the elasto-plasticity model was verified by using the macro stress-strain data generated by a 3-D nonlinear micromechanics model. Because this anisotropic plasticity model is simple and is in the general form of those widely used in existing numerical plasticity codes, it can easily be incorporated into the existing codes with little effort.
Interactive Simulation of Resin Transfer Molding
Authors: Ismail Youssef M. , Springer George S.
Published: April 2011

A computer code was developed for simulating the resin transfer molding process. The unique feature of the code is that it is interactive such that, during the simulation process, the user can (a) change the locations of the inlet ports and exhaust vents, and (b) add, remove, open and close ports and vents, and (c) change the inlet pressure of flow rate at the inlet ports. Additionally, the user can "rewind" the simulator to any previous time in the mold filling process, make any of the above changes and then continue the simulation. The computer code has two main components, the Simulator and the Graphic User Interface (GUI). The Simulator is a finite element code which calculates the resin flow, and the temperature, degree of cure and viscosity of the resin inside the fiber preform. The GUI serves as the interface between the user and the Simulator, and is used to provide the commands to the Simulator and to display the results. Sample results are presented which illustrate the use of the computer code (designated as SUPERTM) in selecting the port and vent locations, their opening and closing sequences such that the fiber preform is filled completely in the shortest time.
Progressive Crush of Resin Transfer Molded Square Tube Stiffened Beam Elements
Authors: Karbhari Vistasp M.
Published: April 2011

Processes such as Resin Transfer Molding (RTM) present the composites designer with the opportunity to combine tailored architectures with near-net-shape preform elements to fabricate components that not only reduce the number of joining operations through parts integration, but also enable the integration of a number of functions. The combination of structural and energy absorption functions requires the design of components that go beyond simplistic elements such as tubes. In this paper the progressive crush response of multi-celled square tube stiffened beam type elements fabricated using the RTM process is investigated. It is shown that the use of glass-carbon reinforced hybrids can result in very efficient structures capable of achieving high levels of initial peak load, mean crush load and specific energy absorbance with the same architecture. Comparisons are made between a variety of preform lay-up and reinforcement strategies on the basis of load, energy and damage characteristics, and the role of the stiffening flange between the tubes based on the reinforcement therein.
Residual Stresses in Composites with Coated Particles
Authors: Pan Ming-Jen , Green David J. , Hellmann John R.
Published: April 2011

There is a growing interest in understanding the residual stress state in particulate composites that contain a thin interfacial phase. In this work, new analytical solutions were obtained for the residual stresses in composites with (1) a coated particle in an infinite matrix and (2) finite concentration of coated particles. The explicit expressions provided an easy evaluation of the influence of constituent properties on residual stresses in such systems. The interactions amongst the constituents are complicated but are demonstrated graphically in an example.
The Effect of Surface Flaws on SiC Fiber Strength in an SiC/Ti-Alloy Composite
Authors: Gambone M. L. , Wawner F. E.
Published: April 2011

The strength distribution of SiC fibers has been measured after chemical extraction from a titanium-alloy matrix composite both in the as-consolidated state and after heat-treatment at 950°C for 430 hours. The strength and fracture mode of fibers extracted from the as-consolidated composite was comparable to that of virgin fiber. However, fibers from the heat-treated composite demonstrated lower tensile strength and fractured from surface flaws created by chemical reaction between the fiber and matrix. A fracture mechanics analysis of the relationship between the size of these surface flaws and fiber tensile strength enabled the calculation of the fiber fracture toughness (2.2 MN V4). The validity of this toughness value for internally-initiated tensile failures was verified by comparison of observed and predicted flaw sizes.
Prediction of Filling Time and Vent Locations for Resin Transfer Molds
Authors: Chen Y. F. , Stelson K. A. , Vollert V. R.
Published: April 2011

A previously reported algorithm for predicting the last point to fill in a porous mold is presented. This algorithm is modified for the prediction of the filling time and vent location in anisotropic RTM molds of various geometries with time dependent boundary conditions. The proposed method is computationally efficient and requires, at most, the solution of two linear systems of equations. For a number of test cases, the performance of the method is investigated on comparing predicted vent locations with those obtained using a complete filling simulation. It is demonstrated that the proposed method can predict fill times and vent locations with the same accuracy as a direct filling simulation but at a much reduced computational cost. Finally, it is shown how an alternative interpretation of the results from the vent location algorithm can be used to predict the location of all the potential dry spots (vent locations) in an RTM mold.
Energy Absorption Characteristics of Hybrid Braided Composite Tubes
Authors: Karbhari Vistasp M. , Falzon Paul J. , Herzberg Israel
Published: April 2011

Increasing demands for greater automotive fuel economy coupled with increased crashworthiness are forcing a move towards the use of tailored materials and structures. Among available composite structures, textile composite assemblies such as achieved through braiding present immense potential for the tailoring of structural response. This study is focused on the determination of the effect of braid related parameters on the progressive crush of composite tubes. Parameters studied include the type of fiber, braid yield, type of 2-D braid pattern and number of layers used. The use of hybrids is investigated and it is shown that the appropriate use of such constructions can lead to very efficient structures in progressive crush.
Strength of Composite Laminates Containing Holes and Subjected to Complex Loading Conditions
Authors: Ireman Tomas , Eriksson Ingvar
Published: April 2011

Stress and failure analyses of composite laminates containing holes are presented. Both the finite element method (FEM) and an analytical method, based on complex stress functions, are used to determine the stress distributions around the holes. The Point Stress Criterion (PSC) and the Damage Zone Criterion (DZC) are used to predict the tensile strength of test specimens subjected to complex loading conditions. A comprehensive test program was carried out to establish criteria parameters and to generate data to validate the stress and failure analyses. For this purpose a special test fixture was designed. Good agreement between predicted and measured results was found.
Applied Problems of Mechanics of Composite Structures
Authors: Tarnopol'Skii Yu. M. , Kulakov V. L.
Published: April 2011

The mechanics of composites as an independent branch of the mechanics of deformable media is in the stage of growth, development and formation. Its branches have been expanded to various extents; many problems are far from completion, yet the advances in engineering mechanics of composites are indisputable. These advances are connected with the solution of applied problems in the most advanced industries. The first stage of the development of the mechanics of composite structures, mainly intuitive, has been completed. It is appropriate to give preliminary resume and consider further directions of investigations. In this regard it is of great interest to consider experience of the scientists of former USSR having made considerable contribution to the mechanics of composite structures. The present review is based on the examples belonging to the scope of interest of the authors and, naturally, does not claim to exhaust the theme.
Delamination Failure of a Single Yarn Glass Fiber Composite
Authors: Ebeling T. , Hiltner A. , Baer E. , Fraser I. M. , Orton M. L.
Published: April 2011

When the constraint of the fill yarns was removed from a woven fiber composite, delamination followed an intrayarn pathway. Despite extensive fiber-matrix debonding during crack growth, resin toughness translated directly to composite toughness. This suggests that damage from intrayarn cracking contributes significantly to toughness of woven fiber composites even though the crack is expected to follow an interply pathway. Effects of fiber undulation were isolated from the effects of yarn interlacing by studying specimens a single yarn in width. In the double cantilever beam (DCB) test, intrayarn crack growth caused the crack to follow the undulating pathway of the fibers. The orientation of the crack changed as the crack moved along the undulating fibers; thus, the extent of mode II loading relative to mode I loading increased as the fibers deviated from the plane normal to the loading direction. Because composite toughness increases with the degree of mode II loading, the variation in toughness induced a characteristic stop-start crack growth mechanism. The toughness of the matrix was varied by changing the soft segment content of the urethane-methacrylate resin. Soft segment content affected the crack initiation toughness more than crack arrest toughness. Consequently, longer crack jumps were observed with the tougher resins.
The Influence of Loading Rate on the Mode II Interlaminar Fracture Toughness of Composite Materials
Authors: Cantwell W. J.
Published: April 2011

The effect of varying loading rate on the mode II interlaminar fracture toughness of a range of composite materials has been investigated. A screw-driven universal testing machine and a fully instrumented drop-weight carriage have been used to obtain crosshead speeds in the range 0.01 mm/min to 3 m/s. The results have shown that the value of GIIc for many composites is rate-sensitive, tending to increase with increasing loading rate. Damage development in the crack tip region has been investigated by examining the polished edges of a number of test specimens in a scanning electron microscope. It has also been shown that offsetting the center plies by a few degrees in order to reduce fiber bridging results in a greater degree of rate-sensitivity in GIIc. It is believed that the presence of fiber-bridging in standard unidirectional fracture-mechanics samples conceals much of the intrinsic viscoelastic response of the polymeric matrix. Failure mechanisms at the crack tip have been investigated using the Double Edge Notch Flexure specimen. A scanning electron microscope study of this region has shown that the size and nature of this zone is dependent upon the crosshead displacement rate.
Bond Behavior of Deformed GFRP Rebars
Authors: Ehsani M. R. , Saadatmanesh H. , Tao S.
Published: April 2011

The bond behavior of straight Glass-Fiber-Reinforced-Plastic (GFRP) reinforcing bars to concrete was experimentally investigated. Results of 48 beam and 18 pull-out specimens constructed with No. 3, 6, and 9 GFRP rebars are presented. The static tensile load was applied to the rebars in a gradual increment of load level until splitting failure of concrete, rebar pull out failure, or rebar fracture occurred. The slip between the rebars and concrete was measured at the loaded and free ends at each load level. Variables in the study included concrete compressive strength, embedment length, clear concrete cover, rebar diameter, and concrete cast depth. On the basis of the experimental results, it was concluded that the ultimate bond stress increases with higher concrete compressive strength and clear concrete cover, but decreases with larger concrete cast depth. The ultimate bond stress and the loaded end slip of the pull-out specimens were found to be greater than that of the beam specimens. Beam test data could be more realistic and accurate for use in determining the development length.
Effect of Heat Treatment and Interfacial Characteristics on the Work Hardening Behaviour of a SiC Reinforced Aluminum Metal-Matrix Composite
Authors: Manoharan M. , Gupta M.
Published: April 2011

An understanding of the work hardening behavior of particulate reinforced metal-matrix composites is crucial in optimizing the parameters for deformation processing of these materials. The microstructure and interfacial properties of these composites can be altered by the choice of the heat treatment used. These interfacial properties in turn determine the mechanical behavior of these materials. In this study, a SiC reinforced aluminum metal matrix composite was subject to two different heat treatments to modify the matrix/reinforcement interfacial characteristics. A model, developed earlier, was then used to relate the work hardening behavior to microstructural parameters. It is shown that the model is applicable for both the heat treatments studied.
Statistical Fatigue Life Prediction of Cross-Ply Composite Laminates
Authors: Diao Xiaoxue , Ye Lin , Mai Yiu-Wing
Published: April 2011

Within the framework of statistical models of residual strength and fatigue life established in previous work, a new model for determining the stress redistribution function is proposed to predict fatigue life of cross-ply laminates in terms of the principal fatigue mechanisms, i.e., progressive matrix cracking in 90° plies. The simple shear-lag analysis is applied to describe the cycle-number-dependent stress redistribution process and the stiffness reduction of laminates resulting from the progressive matrix cracking in transverse plies. From the strength degradation behaviour and the static strength distribution of 0° plies as well as the fatigue behaviour of 90° plies, the fatigue life of cross-ply laminates with various lay-ups can be calculated from the model. Predictions of fatigue life from the model are compared with experimental data for [0/902] s, [02/902] s and [0/904] s, graphite/epoxy cross-ply laminates, and good agreement indicates success of the approach.
A Comparative Study of Square Cell and Circular Cell Micro-Models
Authors: Sridharan Srinivasan , Jadhav Virendra R.
Published: April 2011

Accuracy and computational simplicity are both sine qua non for micro-mechanical models which would be candidates for incorporation in structural analysis treating combined nonlinearities. To date there are a number of micro-models and most of these employ either a square cell or a circular cell as the representative volume element. The present paper discusses the accuracy of typical micro-models belonging to either category. A simplified square cell model (SSCM) derived from elementary mechanics is presented and this model is shown to give results almost identical to those of Aboudi's method of cells (AMC). A finite element-based 3-phase cylindrical fiber model (FECM) is developed with the primary object of determining the stress variation at the micro-level and thus the initiation of local failure. This model is apparently the most accurate in the set of models considered in the paper.

It is confirmed that the closed form expressions of Hashin and Rosen in conjunction with the expressions to determine the transverse shear modulus given by Christensen and Lo offer a viable approach to the determination of elastic constants. The AMC and SSCM too provide sufficiently accurate prediction of plastic constants. However for the determination of initial yielding under combined loading, a micro-model which gives the variation of stresses across the model is required. The problem of the prediction of first yield of a metal matrix composite is considered and the results of the FECM are compared with those of the simpler square cell models and the hexagonal array model of Dvorak. The square cell models fail in the prediction of the first yield in the context of stress states which are nearly hydrostatic. The present FECM model is seen to be very effective because of the uncoupling of harmonics in the solution process and promises to be an effective tool for the inelastic analysis of the composites.
On the Feasibility of Using Thermal Gradients for Active Control of Interlaminar Stresses in Laminated Composites
Authors: Kim T. , Steadman D. , Hanagud S. V. , Atluri S. N.
Published: April 2011

In this paper, a method is proposed to actively control interlaminar stresses near the free edges of laminated composites by through-thickness thermal gradients. Theoretical solutions are given for optimal steady-state through-thickness temperature distributions under uniaxial loading that are required to elmininate or reduce the interlaminar stresses below a prescribed level. The optimal solutions are obtained by minimizing appropriate performance indices that are functions of the far-field properties, with respect to the through-thickness temperature differences. In the second part, an experimental investigation is conducted on a glass/epoxy cross-ply laminate with embedded piezoelectric sensors and a thermal heater. Through the experiment, the feasiblity of the thermal control of interlaminar stresses is demonstrated.
Edge Stresses in a Composite Strip Subjected to Axial Temperature Gradients Part II: Evaluation of Numerical Results
Authors: Swett Dwight W. , Shiflett Geoffrey R.
Published: April 2011

In Part I of this paper, a combination of Airy stress functions and direct displacement functions was utilized to obtain the plane elasticity solution for the stresses and displacements in a multilayer laminated orthotropic strip subjected to a temperature gradient that is arbitrarily symmetric in the longitudinal direction. The solution is exact for the specific boundary conditions associated with satisfaction of zero slope of transverse displacement at the strip ends and shear traction free edges at the strip ends.

In Part II of this paper, numerical results are presented for several examples and compared to detailed finite element analyses for approximate zero edge slope and free edge boundary conditions. The results indicate the shear and peel stress concentrations and axial stress distributions for the boundary condition are in excellent agreement with the finite element analysis results for the zero edge slope case and correlate with the free edge boundary condition over a broad range of temperature gradient cases. This correlation with the free edge numerical analysis indicates the theoretical approach is feasible as a conceptual design tool for a reasonable range of free edge engineering problems.
A Technique for Characterizing Process-Induced Fiber Waviness in Unidirectional Composite Laminates-Using Optical Microscopy
Authors: Joyce Peter J. , Kugler Danielle , Moon Tess J.
Published: April 2011

A technique has been developed, using optical microscopy, for the characterization of localized fiber waviness in composite laminates. Since most of the process-induced waviness which develops in unidirectional thermoplastic laminates is clearly discretized into little packets or fiber-wrinkled regions, the spatial distribution of fiber waviness can be estimated by surface inspection of the laminates. The waviness in these fiber-wrinkled regions is approximately sinusoidal so the waviness parameters chosen were amplitude and wavelength. The waviness in each of the fiber-wrinkled regions is approximately coherent; thus, one measurement of the local fiber geometry is enough to characterize the wave packet. Another issue this technique attempts to address is the thru-thickness waviness. To investigate the presence of waviness through the thickness a two pronged approach is presented: 1) a general survey in which an entire laminate was carefully sectioned into small pieces and inspected in cross-section for the existence of fiber waviness below the part surface and 2) a rigorous three-dimensional serial reconstruction of a "typical" fiber-wrinkled region to illustrate the nature of the fiber waviness in these zones. The laminates were surveyed using a statistical sampling routine and the fiber-wrinkled regions were carefully measured using microscopy and image analysis both on the surface and through the thickness of the plates. Results from a series of plates are included to demonstrate the application of this technique.
The Effect of Test Conditions on the Fatigue Properties of a Phenolic Composite
Authors: Richardson M. O. W. , Branco C. M. , Ferreira J. A. M.
Published: April 2011

The paper reports experimental work carried out on a composite with a phenolic matrix and reinforced with glass fiber. The fatigue results were obtained on both unidirectional fiber composites and crossed fiber composites. For unidirectional fiber composites the two glass fiber treatments were investigated. All the fatigue tests were carried out on a mechanical fatigue machine equipped with frequency control and a furnace. The unidirectional composites were tested in the frequency range from 1.5 to 25 Hz and in the temperature range from 20 to 200°C. The crossed composites were tested in the frequency range from 2 to 20 Hz at 20°C. The paper discusses and draws conclusions about the fatigue damage in terms of variation of Young's modulus, the influence of the temperature and frequency on the fatigue strength and surface temperature variation during testing.
A Fatigue Crack Growth Model for Random Fiber Composites
Authors: Atodaria D. R. , Putatunda S. K. , Mallick P. K.
Published: April 2011

Fatigue crack growth behavior of a randomly oriented short carbon fiber rein-forced polyether ether ketone (PEEK) composite is studied at four different load ratios. Based on the experimental data, a new fatigue crack growth model is proposed which accounts for crack growth throughout the entire fatigue cycle and not just at the maximum load. The proposed model involves a weighted averaging procedure and is found to fit the fatigue crack growth rates at all load ratios in a single power law equation. Finally, it is observed that the fatigue threshold intensity of this material decreases with increasing load ratio.
Localized Load-Sharing Rules and Markov-Weibull Fibers: A Comparison of Microcomposite Failure Data with Monte Carlo Simulations
Authors: Durham S. D. , Lynch J. D. , Padgett W. J. , Horan T. J. , Owen W. J. , Surles J.
Published: April 2011

Monte Carlo simulations are compared with strength data from carbon fibers and composites made from similar fibers, in order to investigate several issues concerning the failure of fibrous composite materials under tensile load. Failure of,"bundles" consisting of up to 1000 fibers under several different localized load-sharing rules and two different models for fiber strength is simulated, and the chain-of-bundles model is used to simulate tows of varying gauge lengths. The relationships of composite strength, Weibull shape inflation and ineffective length are investigated for the new models. The issues of volume effects and critical crack growth are also discussed.
Material and Structural Characterization of the Dynamic Behavior of Shape Memory Alloy Reinforced Composites
Authors: De Santis S. , Trochu F. , Ostiguy G. , Sol H. , Berthelot J. M.
Published: April 2011

The purpose of this article is to characterize elastic and dissipative properties of Shape Memory Alloy (SMA) fiber reinforced composite materials. These properties are then used to evaluate the dynamic characteristics of a structure (modal parameters) using finite elements. A laminate level procedure is used to evaluate the complex bending stiffnesses of CuZnAl fiber reinforced composites. The modal strain energy finite element method is implemented to calculate the modal parameters of composite structures with different geometries and boundary conditions. Calculated results are compared to those obtained by experimental modal analysis. The study outlines the relatively good agreement of natural frequencies and the difficulties encountered in evaluating damping in these types of composites at both the experimental (modal analysis) as well as the numerical (finite element) levels.
Prediction of Momentary Transverse Creep Behavior of Thermoplastic Polymer Matrix Composites Using Micromechanical Models
Authors: Wen Y. F. , Gibson R. F. , Sullivan J. L.
Published: April 2011

The objective of this paper is to report on the application of two different micromechanical models for the characterization of the viscoelastic behavior of E-glass fiber reinforced polyetherimide (PEI) thermoplastic composites in the linear viscoelastic region. In particular, predictions are made for the momentary creep behavior of E-glass/PEI composites of various fiber volume fractions. Both micromechanics models, the Tsai-Hahn semi-empirical model and the finite element model, involved the use of elastic solutions and the elastic-viscoelastic correspondence principle. The models were validated by applying them to composites having three different fiber volume fractions. These predictions are obtained from micromechanical models using the experimental creep compliance of the neat resin (matrix), the known fiber volume fraction, and the assumption that the fibers are elastic. Finally, comparisons are made between measured and predicted momentary creep master curves for the composites.
A Consolidation Model for Polymer Powder Impregnated Tapes
Authors: Padaki S. , Drzal L. T.
Published: April 2011

Powder impregnation technology is a relatively new fabrication method for polymer powder composites. A number of processes using this processing route have been developed. However, the transformation of these preimpregnated materials into fully densified composite structures has been performed empirically. A fundamental understanding of the process would therefore prove a good resource to the composite industry. The overall objective of this study was to develop a mathematical model that would describe the consolidation process.

Experimental observations were used to categorize the process into three simple unit operations: heating, compression and cooling. Each of these stages was modeled independently. A one dimensional heat flow equation with appropriate boundary conditions was solved from the heating and cooling models. A three dimensional squeeze flow model with corresponding boundary conditions was used to describe the flow of the polymer in the lay-up. An existing fiber deformation model was used in order to predict the additional pressure required for consolidation. Data from experiments designed to test the heating and cooling model fit well with the simulations. Deviations from the model at short times were observed. However, the total heating and cooling times that were predicted were accurate. Experimental verification of the flow model showed that the time and pressure required for flow were extremely small compared to the fabrication cycle.
Thermoelastic Behavior of PEEK Thermoplastic Composite during Cooling from Forming Temperatures
Authors: Wang C. , Sun C. T.
Published: April 2011

In the forming of advanced thermoplastic composite structures, the stresses generated during cooling have a profound effect on the distortion and strength of the formed parts. In this paper, the thermoplastic behavior of a PEEK matrix thermoplastic composite during cooling was investigated. A procedure for determining temperature-dependent thermal expansion coefficients using off-axis composite specimens was introduced. A simple thermoplastic constitutive model was used to calculate thermal stresses during cooling. Based on this model, an explanation of a strong scissoring effect on the thermal expansion behavior of symmetric [±45] ns laminates and a calculation of warping in a non-symmetric laminate were presented