bluuebunny/crossref_metadata_2025_split huggingface dataset:
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.
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.
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.
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 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.
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.
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.
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.
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.
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.
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.