bluuebunny/crossref_metadata_2025 huggingface dataset:
The PIR/PUR Ratio: A Novel Timer Conversion Tfst with High Correlation to the Factory Mutual Calorimeter for HCFC-141b Blown Polyisocyanurate Foams
Authors: Burns S. B. , Schmidt E. L.
Published: July 2017

In response to the environmental dangers posed by CFC-11 use, rigid polyisocyanurate foam insulation manufacturers are currently making the transition to HCFC-141b. This change has forced manufacturers to requalify their commercial roofing products on the Factory Mutual Calorimeter. Factory Mutual certification is demanded by the marketplace, but the cost of attainment is high. With HCFC-141b technology being perfected, and its eventual replacement looming on the horizon, it is evident a great deal of Calorimeter testing will be required in the near future.

There are a number of small scale tests in existence to measure fire resistance, but their effectiveness as predictors of Calorimeter performance has not been fully assessed. Trimer conversion is a crucial aspect of the fire resistance of polyisocyanurate foam, but a convenient and inexpensive measurement has not been documented. This paper introduces such a novel trimer conversion test, the PIR/PUR ratio.

A high degree of correlation exists between Calorimeter results and the PIR/PUR ratio. Strong correlations also exist between the Calorimeter and certain other small scale tests. However, relying on only one test to predict calorimeter performance is insufficient. Consideration must be given to all relevant tests. Forming a composite index from the individually relevant test results, using weighted factor techniques, provides the best indication of Calorimeter performance.

The information in this paper should enable polyisocyanurate insulation manufacturers to make better predictions of Calorimeter performance, avoiding costly failures.
Zero-ODP Blowing Agents for Polyurethane Foams
Authors: Creazzo J. A. , Hammel H. S. , Cicalo K. J. , Schindler P.
Published: July 2017

As we move toward final phaseout of chlorofluorocarbon blowing agents, new compounds and technologies are emerging to produce the polyurethane foams essential to so many products. In considering zero-ozone-depleting compounds that are currently available, the potential hydrofluorocarbon (HFC) candidates are gases at ambient conditions. Development programs are under way to understand feasibility and demonstrate use of one candidate, HFC-134a, to replace CFC-11.

This paper summarizes basic data developed to support commercial process conversion from CFC-11 to HFC-134a. Topics include a comparison of physical properties, results of vapor pressure and solubility studies with different polyols, effects of different surfactants on HFC-134a solubility, confirmation of product stability in foaming applications, B-side system viscosity effects, and materials-of-construction compatibility data.

With respect to processing, the paper summarizes experience developed in modifying equipment to handle higher B-side mixing and storage pressures and discusses effects of process settings on foam quality.

Finally, the paper summarizes results of commercializing 134a/polyurethane foam systems for both insulating and non-insulating applications.
The Effect of Tertiary Amine Catalyst Systems for Lower Density Rigid Foam with HCFC-141b
Authors: Harada S. , Isayama Y. , Kitagawa H. , Morii M. , Fukushima T.
Published: July 2017

The replacement of blowing agent from CFC-11 to HCFCs is proceeding in rigid polyurethane foam applications in order to meet the revised Montreal Protocol. The foams using HCFCs, however, have about 5 to 10% higher thermal conductivity than that using CFC, owing to the high thermal conductivity of HCFCs. Though improved HCFC blown foams have realized similar thermal conductivity to conventional CFC-11 blown foams, these foams have about 15 to 20% higher densities. This performance level can not be accepted as an insulation foam for refrigeration appliances.

Kao Corporation has developed new catalyst systems, which is Kaolizer KLP-200 series. The use of our new catalysts for HCFC-141b blown foams can realize low thermal conductivity, derived from formation of finer cell structure by acceleration of initial gelling reaction, and has performance to improve flowability in the foaming process at the same time. These characteristics give about 10 to 15% lower density foams than that using conventional catalysts in comparison with similar thermal conductivity. In addition, the formulation using Kaolizer KLP-200K can reduce the level of HCFC-141b compared with that using conventional catalysts to obtain the expected thermal conductivity of the foams, provided these foams have the same density.

This paper describes the effect of Kaolizer KLP-200K for HCFC-141b blown rigid foam. The system using Kaolizer KLP-200K results in lower density foam and reduced HCFC-141b compared to that using conventional catalysts. Moreover, Kaolizer KLP-200 is effective for cyclopentane blown rigid foam.
Thermal Conductivities of Composite Materials
Authors: Behrens Ernst
Published: July 2017

A field solution for long wavelengths of the equation of heat conduction is obtained for a composite material with orthorhombic symmetry. This yields exact expressions for the three principal thermal conductivities. They are valid for any number of ortho rhombic constituents and even for continuously varying thermal conductivities within the elementary cell. For a lamellar composite, the average thermal conductivities of the material are expressed directly as integrals over the space-dependent thermal conductivi ties. For a filamentary composite, such explicit expressions are obtained only in the Wigner-Seitz approximation and only for a stepwise variation of the thermal conductivities within the ele mentary cell. In the same manner, the average thermal conduc tivity of a composite with cubic symmetry, e.g. isotropic spherical inclusions in a cubic lattice, is calculated. Analogies with previous results on diffusion coefficients and elastic shear moduli are discussed.
Influence of End Constraint in the Testing of Anisotropic Bodies
Authors: Pagano N.J. , Halpin J.C.
Published: July 2017

One of the most elementary concepts in elasticity theory is that of a uniform state of stress. Producing such a state of stress in the laboratory, however, is not a trivial task. A common experiment in composite mechanics—the tension test of off-angle composites-is discussed in this paper and the influence of end constraint on the uniform stress field is investigated. Analytical and experimental evidence is presented to show the serious effects caused by con ventional clamping devices.
Fabrication of Metal Matrix Composite Materials
Authors: Thornton H. Richard
Published: July 2017

The analysis of the limiting factors associated with ultimate strength of metal matrix composite materials provides a basis for applying boundary conditions to potential fabrication techniques. The boundary conditions are formulated through an understanding of fiber degradation, fiber-matrix interface and reactivity and processing effects (residual stress). The boron-aluminum system is considered for definition of boundary conditions. These condi tions are compared with test data generated from composites fabri cated by both liquid and solid state techniques. Considerations for future research in the area of metal matrix composite fabrication techniques are proposed.
Diffusion Bonding and Testing of Boron-Aluminum Composites
Authors: Joseph E. , Myers E.J. , Stuhrke W.F.
Published: July 2017

The tensile behavior of unidirectionally reinforced boron fila ment-aluminum composites was evaluated for a wide range of diffusion bonding parameters and a method of detailed analysis of the stress-strain behavior developed and described. The effect of these variations in processing conditions on the tensile behavior, including the fracture morphology of the resulting composites, was assessed. The value of the composite tensile strength at the opti mum combinations of time and temperature in the experimental situation was from 120 to 130 percent of the calculated rule of mixtures value. This demonstrated synergism is discussed and explained in terms of the developed internal multiaxial stress states in the composites.

The bond between the aluminum and the boron filament was investigated by electron microscope replica techniques. The replicas demonstrated that a strong bond between the filament and the metal matrix exists by showing failure in the metal rather than the bond area upon filament extraction.

The significance of the various diffusion bonding parameters in terms of the development of the multiaxially produced synergism is discussed in terms of the further development of these tech nologically important aluminum-boron composites.
Observations on Linear Anisotropic Viscoelasticity
Authors: Halpin J.C. , Pagano N.J.
Published: July 2017

The theory of anisotropic viscoelasticity is based upon the Onsager reciprocity relations. Experimental evidence presented here verifies this concept of matrix symmetry for time-dependent phenomena. A new experimental procedure for the characterization of orthotropic materials is presented.
Protective Coating by Surface Nitridation of Boron Filaments
Authors: Camahort J.L.
Published: July 2017

A protective coating is obtained by nitriding the boron filament surface. The existence of a boron nitride coating is shown by chemical analysis, optical microscopy, and electron microprobe analysis. The protective nature of the coating is illustrated metal lographically by using several composite fabrication techniques with pure aluminum and 2024 aluminum alloy as matrices. The interaction of aluminum with nitrided and as-received boron fila ments at 600, 700, and 800°C is studied using a simple experimental set-up. The boron nitride coating serves as an effective diffusion barrier in molten aluminum up to 10 min. at 800°C. Tensile tests on nitrided filaments show no degradation of mechanical properties due to the nitridation process. The results demonstrate the potential usefulness of the process to the development of improved boron- aluminum composites. The prevention of deleterious interactions between matrix and reinforcement should result in higher com posite strengths.
Momentless Design of Composite Structures with Variable Elastic Constants
Authors: Pao Y.C.
Published: July 2017

In recognizing the increasing availability of various composite materials, this paper promotes a new approach to the design of the plate and shell structures. It is based on the loading and shape require ments of the structure to determine a particular composite whose constituents and their geometric arrangements are such that a certain design criterion will be satisfied. For the preliminary study of such an approach, a momentless criterion is adopted in the present paper for the design of axisymmetrically loaded shells of revolution. To maintain the shell in a momentless state, the general equation which governs the shape, loadings and the material properties of the shell, and the displacements equations have been derived. Applications to the pressure vessels with ellipsoidal bulkheads show that composite materials with variable elastic constants will be needed for constructing a momentless vessel. Numerical examples are presented to illustrate the proposed method of approach and provide as guidelines for the development of new composites.
Factors Influencing the Properties of Whisker-Metal Composites
Authors: Schierding Royce G. , Deex Oliver Des.
Published: July 2017

Composites of aluminum reinforced with α-silicon carbide whiskers have been fabricated by a powder metallurgical technique. Although reinforcement was achieved, composite properties fall considerably below what is expected from the constituent properties. This is inter preted in terms of voids, fiber breakage, and imperfect fiber orientation and the conclusions are supported by a study of the fracture mode of samples tensile tested at temperatures to 500°C.
The End Problem for a Laminated Elastic Strip — II. Differential Expansion Stresses
Authors: Hess Milton S.
Published: July 2017

A plane elasticity solution is presented for the state of stress resulting from differential expansion in a composite made up of two rectangular strips bonded on an interface. The stress field that has long been known to exist in the middle of such a laminated strip (to the St. Venant approximation) includes neither normal nor shear traction on the bonded interface. These stress components are shown to be significant only within a distance from each end equal to the total thickness, and their distribution is determined for a wide range of elastic moduli and strip dimensions.

It is found that the distribution of normal stress at the interface is quite sensitive to the modulus and thickness ratios of the laminae. By making use of the form of this dependence, it may be possible to design bonded structures to withstand curing and/or temperature changes which would otherwise cause failure.
Boron Epoxy Laminate Test Methods
Authors: Lantz Robert B.
Published: July 2017

A discussion of necessary considerations for tensile and compressive test specimen design of laminated composite materials is presented. Particular attention is paid to comparing axially loaded specimens with traction loaded long beam flexure specimens. A large amount of test data, including strain gage measurements, has been compiled to pro vide a basis of comparisons of the tensile and compressive elastic properties and strengths of representative boron epoxy laminates by the two test methods.
On the Stability of a Mixture of Two Elastic Solids
Authors: Knops R.J. , Steel T.R.
Published: July 2017

We consider the stability of a mixture of two anisotropic linear elastic solids, which possesses internal friction due to the mechanical interaction between the solids. The interest in such a mixture lies in its possible application to binary alloys and to some composite materials. Further, since a pure metallic specimen is often difficult to obtain, many metals may actually consist of two or more metallic solids. It is shown that in general such a mixture will be asymptotically stable under mixed boundary conditions when certain definiteness conditions are imposed on the coefficients, even when the temperature remains constant. Instability of the mixture is also discussed and we then show that by imposing a boundedness condition on the solution stability can be recovered under very much less restrictive conditions on the coefficients.
Influence of Shear Moduli and Lattice Parameters on the Equilibrium of a Screw Dislocation
Authors: Pacheco E.S. , Mura T.
Published: July 2017

Using the Peierls model of a dislocation, the simultaneous effects of shear moduli and lattice parameters on the equilibrium of a screw dislocation in a material composed of soft and hard phases are examined. The dependence of the mobility of dislocations on the hardness of alloys due to these two forms of inhomogeneities is obtained. The existing literature on the subject of a screw dislocation in a bimetallic medium does not consider the effect of differences in lattice parameters and gives the result that a material with higher shear modulus will always repel a dislocation located in a material of lower shear modulus. In contrast to this, it is found that a material with higher shear modulus will either attract or repel a screw dislocation depending on the com bination of elastic constants and lattice parameters for the medium. The use of continuous distribution of dislocations removes the diffi culties encountered by a singular model when the dislocation approaches the interface.
Shear-Coupled Waves in a Thin, Helically Wrapped Cylindrical Shell
Authors: Reuter Robert C.
Published: July 2017

The propagation of long wavelength, axi-symmetric, elastic waves in a thin, semi-infinite, anisotropic, circular, cylindrical shell, resulting from a uniform, axial stress pulse on the free end, is investigated. Anisotropy of the shell is a consequence of its single layer, helical wrap construction. Shear-coupling is present in the plane of the middle surface of the shell and is accounted for in the analysis. Two waves are propagated which travel with different velocities, each consisting of non-zero components of stress and displacement in the axial and circumferential directions.
Thermodynamic Aspects of the Prevention of the Oxidation of Graphite Fibers in Metal-Matrix Composites
Authors: Huggins Robert A.
Published: July 2017

A general model for oxidation at the interface between graphite fibers and metallic matrices is presented and the important thermo dynamic parameters related to this phenomenon identified. The very significant influence of external oxides, carbide or other intermediate phases between the fiber and the matrix, and protective fiber coatings upon the thermodynamic factors which control the direction and magnitude of the driving force for mass transport are discussed.
Control Volume Analysis of Elastic Wave Front in Composite Materials
Authors: Pei Chi Chou  , Wang A.S.D.
Published: July 2017

A theory is proposed for the analysis of an elastic wave front in unidirectionally laminated as well as fiber reinforced composite mate rials. According to this theory, which is based on the concept of control volume, elastic wave front due to abrupt loadings propagates at a velocity which is bounded by the wave velocities of the fiber material and of the matrix material. The theory includes a nonlinear effect of area change; the wave front velocity is a function of the impact intensity. But for linearly elastic materials, this non-linear effect is small; if it is neglected, then wave front speed in the com posite becomes independent of the impact intensity.

Consideration has also been given to the determination of the shear force along the interface between the reinforcing fiber and the matrix. It is found that for certain combination of fiber and matrix, the shear stress attains an extremum value which may cause debond ing of the composite under moderate impact loading.
Time Dependent Static Strength and Reliability for Composites
Authors: Halpin J.C. , Kopf John R. , Goldberg William
Published: July 2017

An experimental investigation was conducted to study the fracture properties of laminated composites under uni-axial tension. It was found that the distribution of breaking strength of a composite ma terial under a constant stress rate or variable stress history can be described accurately by the Weibull distribution. The shape param eter of the distribution was shown to remain nearly constant under varying loading conditions, temperatures and strength degradations. A relation between the shape parameter and breaking strength- lifetime curve was established. Using these data, a reliability plan was developed, taking into consideration order statistics, sample size and time dependence. A proof test experiment was performed which showed that, for this particular material at least, proof testing is an effective method of removing weak members from a population. It is concluded that composite materials can be designed to yield structure with a higher degree of reliability per unit weight than its equivalent metallic structure.
Exact Natural Frequencies for Cross-Ply Laminates
Authors: Jones Alfred T.
Published: July 2017

Vibration of a simply supported cross-ply laminated plate infinite in length is analyzed. Existence of plane strain conditions allow an exact solution of the field equations. Curves of variation of exact natural frequencies versus wavelength for several cases of a two-ply laminated plate are calculated. The first two mode shapes are plotted for various wavelengths and compared to classical plate theory. Flex ural vibration appears to be well approximated by classical theory up to moderate wavelengths. Extensional vibration is approximated mar ginally only for very long wavelengths.
Characterization of Graphite Fiber Surfaces with Raman Spectroscopy
Authors: Tuinstra F. , Koenig J.L.
Published: July 2017

Raman spectroscopy is used to characterize the fiber surface of graphite and carbon fibers. A correlation exists between the Raman spectrum of the fiber surface and the shear strength of the com posite. Effects of surface treatments are reflected in the Raman spec trum as well as the shear strength of the composite. Many graphite fibers have a weak surface layer where the graphite planes are oriented parallel to the surface.
Longitudinal Shear Deformation of Composites— Effective Shear Modulus
Authors: Sendeckyj G.P.
Published: July 2017

An exact analytical solution is given for a case of longitudinal shear loading of an unidirectional fiber reinforced composite. The analysis takes into account random spacing between fibers, random variation of fiber radii, and the variation in shear modulus from fiber to fiber. An "exact" expression is derived for the effective shear modulus of the composite. For the case of a regular array of equal fibers, the expression simplifies considerably. Excellent agreement is found with numerical solutions. Extension of the results to viscoelastic composites is indicated.
Filament Fracture and Postimpact Strength of Boron-Aluminum Composites
Authors: Reed R.P. , Schuster D.M.
Published: July 2017

A preceding study of the fracture behavior of shock-loaded boron-aluminum composites indicated that these materials can have a spall resistance substantially greater than that of unreinforced alu minum. In the present investigation, filament fracture and postimpact tensile strength of a similar composite system were studied. Filament damage was produced by the thin-flyer, plate-impact technique using momentum traps to suppress spall. The specimens contained approxi mately 50 volume percent boron filaments in a plasma-sprayed 6061- aluminum matrix. Filaments were in two mutually perpendicular di rections and the specimens were impacted normal to the plane of reinforcement. Filament fracture was shown to be an important impact damage mechanism in brittle-filament reinforced composites. Filaments were fractured during the initial passage of the loading wave by interactions between adjacent or contacting filaments and by focusing of stresses in isolated fibers. The tensile strength of specimens was reduced by two-thirds after severe impact. However, the impact velocity necessary to produce a substantial decrease of postimpact tensile strength was much higher than that required to produce spall in unreinforced 6061 aluminum. The impact velocity required to cause degradation in postimpact strength was independent of the tape-layup fabrication process, i.e., brazed versus diffusion bonded.
The Control-Volume Approach to Hugoniot of Macroscopically Homogeneous Composites
Authors: Tsou F.K. , Pei Chi Chou
Published: July 2017

The control-volume approach is applied to a composite that is macroscopically homogeneous and consists of arbitrary number of constituents. This includes true mixtures, certain alloys and any kind of composite whose constituents have dimensions much smaller than the composite. The present work represents a generalization of the control-volume approach which has been proposed recently for uni directional fiber reinforced composites. From the present theory, the Hugoniot of the composite may be computed from the equations of state of its constituents. The calculated results of two sample com posites are presented and compare satisfactorily with test data.
Interlaminar Stresses in Composite Laminates Under Uniform Axial Extension
Authors: Pipes R. Byron , Pagano N.J.
Published: July 2017

The response of a finite-width composite laminate under uniform axial strain is treated through the application of classical elasticity theory. Finite-difference solution techniques are employed to obtain solutions for stresses and displacements throughout the region. Results for material properties typical of a high modulus graphite-epoxy composite material system are presented which explain the mechanism of shear transfer within a symmetric laminate. In addition, results of this work are compared to those given in a recent approximate formulation.
Modeling the Viscoplastic Behavior of Fiber-Reinforced Thermoplastic Matrix Composites at Elevated Temperatures
Authors: Ha Sung Kyu , Wang Quiling , Chang Fu-Kuo
Published: July 2017

An investigation was performed to study the mechanical response of fiber-reinforced thermoplastic matrix composites at elevated temperatures. The primary concern of the study was the effect of temperature on the viscoplastic behavior of the composites. During the investigation, an analytical model was developed for analyzing the response of both unidirectional and multidirectional composites at elevated temperatures. The model considers that the fibers have a linearly elastic behavior but the matrix responds viscoplastically with temperature. Micromechanics theories were adopted and developed to integrate fibers and matrix properties for predicting the response of unidirectional composites. Numerical algorithms were developed based on the lamination theory for evaluating the response of the laminated composites. Based on the model, a computer code was also developed for calculating the mechanical response of thermoplastic matrix composites subjected to both thermal and mechanical loading. In order to verify the model, experiments were also conducted for multidirectional APC-2 carbon/PEEK fiber composites and fairly good agreements were found between the predictions and the data.
Effect of Fiber Treatment on the Mechanical Properties of Hybrid Fiber Reinforced Polystyrene Composites: IV. Use of Glass Fiber and Sawdust as Hybrid Fiber
Authors: Maldas D. , Kokta B. V.
Published: July 2017

Hybrid combinations of sawdust and surface-treated glass fiber were used as reinforcing filler for polystyrene composites. Under study was the effect of various parameters [e.g., wood species (e.g., hardwood aspen and softwood spruce), pretreatment of sawdust (e.g., coating with polymer + isocyanate or with a silane coupling agent and grafting), filler loading, composition of sawdust: glass fiber, as well as grades of polystyrene] on the mechanical properties of the composites. The extent of improvement in mechanical properties depends on each of the variables. Treated sawdust/surface-treated glass-filled composites offered the best balance of mechanical properties compared to those of non-treated composites.
Static Strength of Adhesively Bonded ARALL-1 Joints
Authors: Long R. S.
Published: July 2017

ARALL® Laminates are a family of structural composite materials which consist of thin aluminum alloy sheets alternating with aramid fiber/epoxy prepreg layers developed for fatigue-critical applications requiring light gage sheet. To obtain the benefits associated with ARALL Laminates, the joint regions must be designed to carry the required loads without incurring a substantial weight penalty. This paper considers the static strength of adhesively bonded ARALL-1 single and double lap joints. Single and double lap joints with various overlap lengths were fabricated using two standard ARALL thicknesses: 3/2 ARALL-1 (three layers of aluminum with 2 prepreg layers) and 5/4 ARALL-1 (five layers of aluminum with four prepreg layers). These joints were tested in uniaxial tension. A shear lag analysis based on failure of the adhesive was used to predict the failure load as a function of the joint configuration and overlap length. A finite element analysis was used to determine the interlaminar shear and tensile stresses within the laminate. The results of these two analysis techniques were compared to the experimental results.

The results show that the maximum adhesive and adherend stresses calculated using the shear lag analysis agree with those calculated using finite element techniques. While the shear lag analysis gave accurate predicted failure loads for 312 ARALL-1 single and double lap joints, the predicted failure loads for 5/4 ARALL-1 single lap joints were generally higher than the experimental failure loads. For the 5/4 ARALL-1 specimens, the experimental failure loads were dependent on the observed failure mode. The premature failure can be attributed to interlaminar tensile and shear stresses in the prepreg layers closest to the bondline. The interlaminar failure observed in these joints can be predicted using the interlaminar shear and tensile properties and an appropriate interaction equation.
Processing and Characterization of Laminated SiC Whisker Reinforced Al2O3
Authors: Kragness E. D. , Amateau M. F. , Messing G. L.
Published: July 2017

Laminated silicon carbide (SiC) whisker/alumina (Al2O3) composites were fabricated by tape casting and hot pressing. Whisker loadings of 10 and 20 vol % were produced with a predominance of fiber orientation in the direction of casting. Elastic, thermal and mechanical properties of the composites were measured in the longitudinal and transverse directions. The elastic moduli for 10 and 20 whisker vol% Al2O3 composites were approximately the same as unreinforced alumina. The coefficient of thermal expansion was reduced by the addition of the whiskers. The fracture toughness of the composites increased with whisker content from 3.8 MPa m1/2 for pure alumina to 8.0 MPa m1/2 for 20 vol% whiskers in the longitudinal direction.
Global-Local Methods for Thermoelastic Stress Analysis of Thick Fiber-Wound Cylinders
Authors: Luo J. , Sun C. T.
Published: July 2017

Efficient global-local methods are developed for stress analysis in thick laminated cylinders consisting of a repeating sublaminate. The effective moduli and thermal expansion coefficients are obtained from the typical sublaminate and used to obtain global solutions. A refining procedure is introduced in which the global solution is used as boundary conditions for the sublaminate which is then solved by using the exact thermoelasticity formulation. Solutions obtained using these global-local methods are compared with the exact solutions for a number of composite cylinders.
Nonlinear Analysis of Stiffened Laminated Panels with Various Boundary Conditions
Authors: Sheinman Izhak , Frostig Yeoshua , Segal Alex
Published: July 2017

A nonlinear analysis of stiffened laminated composite panels under various boundary conditions is presented. The nonlinear differential equations, expressed in terms of the out-of-plane displacement and the Airy stress function, are solved by separating the variables into eigenfunctions in conjunction with a finite-difference scheme. The vibration and buckling eigenfunctions of an isotropic beam are examined as the displacement functions, and a modified Galerkin procedure are used to minimize the error involved in the field equations, the continuity requirements and the boundary conditions. Results are presented for the isotropic and anisotropic cases, with different boundary conditions.
Buckling of Unidirectional Composites Containing Noncircular Fibers: Part II—Nonelliptical Sections
Authors: Tan Seng C.
Published: July 2017

A simple micro-macro approach is applied to study the buckling of unidirectional composites containing a single row or multiple rows of rectangular, triangular and C-shell fibers. The fiber volume fraction and the distribution of fiber slanted angle are taken into account in this model. The concept of ratio of critical buckling stresses is utilized to study this class of problem. The result reveals that the critical buckling stresses of unidirectional composites containing noncircular fibers are affected significantly by the fiber volume fraction. The critical buckling stress of a unidirectional composite can be improved using noncircular fibers under certain conditions. The relations of critical buckling stress versus fiber slanted angle and the critical buckling stress versus laminate thickness are generally different for different fiber shapes.
Reduction of Impact Damage in KFRP through Replacement of Surface Plies with Glass Fabric Plies
Authors: Kumar Prashant , Rai Badri
Published: July 2017

The surface ply on each side of an angle ply KFRP laminate was replaced by a balanced ply of glass fabric to study the impact induced damage. The laminates were impacted by a stainless steel projectile accelerated through an air gun. The replacement of surface plies reduced the projected impact damage for incident impact energies greater than 6.5 J per mm thickness of the laminate. Delamination damages on individual interfaces, studied through the technique of the thin slice method, were observed to be reduced considerably on all the interfaces. The presence of the glass fabric ply suppressed the spalling at the rear interface drastically.
Acoustic Emission in 2-D Carbon-Carbon Coupons in Tension
Authors: Gorman Michael R.
Published: July 2017

Carbon-carbon coupons were loaded and unloaded in uniaxial tension to progressively higher loads. Hysteresis in the material was noted especially after the stress-strain curves became nonlinear at about 2000 μ. Each subsequent reloading showed an increase in the linear range similar to work hardening in metals except that the modulus was lower on each cycle. The onset of acoustic emission was monitored on each cycle and the computed felicity ratio correlated with the change in stiffness.
Impact Properties of Three-Dimensional Braided Graphite/Epoxy Composites
Authors: Gong J. C. , Sankar B. V.
Published: July 2017

An experimental study of the response and damage of three-dimensional braided graphite/epoxy composite due to sub-perforation velocity impact was carried out in this research. Simply supported square plates were impacted with an instrumented impact pendulum and also a projectile fired by a gas gun. Hemispherical nose impactors of two different diameters, 12.7 mm and 25.4 mm, were used in the pendulum tests. In addition, static flexure tests were performed. Impact damage was assessed using X-radiography, and compression after impact tests. Damages involved in impacted braided panel are matrix cracking in resin pockets, separation of fiber tows, and fiber tow breakage, mostly in fiber bundle crimp areas. A quasi-isotropic laminate was impact tested with pendulum for comparison of impact tolerance between the two composite systems.
Effect of Fiber Orthotropy on Effective Composite Properties
Authors: Knott Tamara Wright , Herakovich Carl T.
Published: July 2017

Classical form analytical expressions are presented for four of the five effective elastic constants of a transversely isotropic composite material containing ortho-tropic fibers. Circumferentially and radially orthotropic fibers are considered. The properties of composites with orthotropic fibers are compared to those with isotropic and transversely isotropic fibers. It is shown that the morphology of the fiber can have a significant effect on composite properties. It is also shown that fiber morphology has a significant effect on the stress distributions in a composite cylinder.
The Radial Distribution of Axial Stress around a Single Fiber in Epoxy Resin Undergoing Water Uptake
Authors: Xu Z. R. , Ashbee K. H. G.
Published: July 2017

Model composites consisting of a single axial fiber in a cylindrical block of epoxy resin, to one end of which is bonded a flexible glass cover slip and to the other end a rigid substrate, have been self-stressed by immersion in water in order to cause the resin to swell. Changes in the positions of Fizeau fringes created between the free surface of the cover slip and an optical flat have been used to follow flexing of the cover slip, and the displacement fields so measured have been used to estimate (1) the resin swelling and, in particular, the constraint imposed on the axial component of this swelling by the inter-facial bonding between fiber and resin, and (2) the distribution of normal stress in the cover slip and hence in the underlying resin and fiber.
Influence of Specimen Tabs on the Compressive Strength of a Unidirectional Composite Material
Authors: Adams Donald F. , Odom Edwin M.
Published: July 2017

An experimental study was performed to determine the effect of type of tabbing material and tab taper on the measured axial compressive strength of a unidirectional carbon/epoxy material. Two types of tabbing materials (hot-rolled, low carbon steel and glass-fabric/epoxy) and two tab taper geometries (untapered and 30° taper) were utilized. Additionally, the influence of partial tab debonding on the measured compressive strength was determined. A standard IITRI compression test fixture was used. Tapered steel tabs provided consistently higher compressive strengths than tapered glass-fabric/epoxy tabs, or untapered steel or glass-fabric/epoxy tabs. Deliberately introducing regions of tab debonding in untapered glass-fabric/epoxy tabbed specimens resulted in only a small decrease in compressive strength.
Longitudinal Splitting in Epoxy and K-Polymer Composites: Shear Lag Analysis Including the Effect of Fiber Bridging
Authors: Nairn John A. , Liu Siulie , Chen Hsichieh , Wedgewood Alan R.
Published: July 2017

The shear-lag model has previously been used to derive a fracture mechanics analysis of longitudinal splitting in finite-width double edge notched unidirectional composites ( J. Comp. Mat., 22:561, 589 [1988]). In this paper we extend that analysis to account for the effect of fibers bridging across the longitudinal split. The new analysis has been used to measure the longitudinal splitting fracture toughness, Glc, of composites of Hercules AS4/3501–6 and Avimid* K-polymer/Magnamite® IM-6 graphite. The effect of resin toughness was further investigated using laminates from a series of experimental resins based on the K-polymer chemistry. The 3501–6 epoxy laminates have a Glc of 196 J/m1 and the Avimid® Klaminates have a Glc of 414 J/m2. The Glc for the K-polymer based laminates was independent of laminate thickness but depended on the neat matrix toughness and on the processing conditions. The inclusion of fiber bridging in the fracture analysis has a significant effect on the reported fracture toughnesses. We claim that these new results give the correct fracture toughness and that previous results that ignored fiber bridging over estimated the laminate toughness.
Flexural Testing of General Multi-Layered Composites
Authors: Mottram J. T.
Published: July 2017

Three-point bend tests were performed on two 32-layer balanced symmetric laminates. The advanced composite material is HI-TEX 46–8B carbon fibre in epoxy resin E7K8T, with a 59.5 % fibre volume fraction. A graphical technique based on classical beam theory was used to measure specimen longitudinal flexural and out-of-plane shear moduli. Likely sources of error were examined. A special case of the classical lamination plate theory was found to give excellent predictions of the flexural modulus. Comparison with the experimental data demonstrated that omission of the in-plane coupling stiffness had a more profound effect than omission of the shear coupling stiffnesses. To provide the engineer with a design tool a high-order analytical technique was developed. Its purpose is to predict shear deformation in multi-layered symmetrical strips, subjected to three-point bending. Exact elasticity solutions for specially orthotropic examples were used to establish its potential performance. Measured moduli for the two laminations established the shear contribution to the central displacement. Comparison with the computational model was found to be excellent for thin strips. As the span was reduced the numerical predictions gave higher displacement from shear. For the two laminations tested, shear effects can be neglected for span-to-thickness > 40. The work presented confirms the safe use in the composite community of spans 30 to 40 times the specimen thickness.
Effects of Nonuniform Crystallinity on Stress Distributions in Cross-Ply Graphite/PEEK (APC-2) Laminates
Authors: Lee K. , Weitsman Y.
Published: July 2017

This paper presents analytical/computational results for the inplane stresses in a moderately thick, cross-ply, graphite/PEEK (APC-2) laminate accounting for effects of nonuniform PEEK crystallinity across the plate's thickness. These effects are incorporated as crystallinity-induced variations in the longitudinal modulus EL, including its nonlinear dependence on strain.
Evaluation of Interfacial Frictional Properties for Fiber-Reinforced Ceramic Composites
Authors: Yeh J. R.
Published: July 2017

An analytical model is developed to determine the frictional properties of the fiber-matrix interface in ceramic matrix composites using the indentation test. This model includes the effect of the interfacial friction stress arising from the transverse expansion of the fiber subjected to the compressive load. Two intrinsic interfacial properties, an interfacial friction limit (Tmass) and an interfacial coefficient of friction (μ), are determined for a SiC fiber-reinforced lithium aluminosilicate glass-ceramic matrix composite. It is found that during the indentation test, the interfacial friction coefficient is more pronounced at lower values of applied load. As the load increases, the behavior is dominated by the interfacial friction limit.
Repeated Impact Failure of Continuous Fiber Reinforced Thermoplastic and Thermoset Composites
Authors: Jang B. P. , Huang C. T. , Hsieh C. Y. , Kowbel W. , Jang B. Z.
Published: July 2017

An effort was made to study the damage mechanisms in both thermoplastic and thermoset composites in response to repeated low-velocity impacts. An instrumented falling-dart impact tester was used that provided the load-deflection and energy-deflection curves plus other numerical data on material properties. Both poly(phenylene sulfide) (PPS) and epoxy composites prepared from different preform styles and stacking sequences were tested. For any composite with given dimensions a threshold incident impact energy (Ec,) could be identified above which delamination would occur at the first impact. As a consequence, both the stiffness and the strength of this composite were reduced and these properties could be measured using the same impact tester as a function of impact cycles. If instead a subcritical incident energy (Ein < Ec) was imposed on the material, no appreciable property loss would be observed until a critical number of impact cycles (Nc) was reached. In this subcritical stage, matrix cracking preceded delamination as impacts were repeated. Both the number and size of delamination cracks were then found to increase as the number of repeated impacts was increased. An elastic strain energy approach was developed to predict the threshold incident energy values of various composites and the prediction was found to be in good agreement with the experimental data. Distinct failure mechanisms were found to exist between epoxy and PPS composites.
A Constitutive Relation for the Viscous Flow of an Oriented Fiber Assembly
Authors: Pipes R. Byron , Hearle J. W. S. , Beaussart A. J. , Sastry A. M. , Okine R. K.
Published: July 2017

A constitutive relation for an equivalent, homogeneous fluid is developed for the anisotropic viscous flow of an oriented assembly of discontinuous fibers suspended in a viscous fluid. The anisotropic viscous compliance matrix can be expressed in terms of three constants by assuming the equivalent fluid to be incompressible and the micro-structure to have axial symmetry (transversely isotropic). By means of a micromechanics analysis, the three terms of the constitutive relation are expressed in terms of the viscosity of the matrix fluid, the fiber aspect ratio, and the fiber volume fraction. A comparison of the viscosity terms reveals that the elongational viscosity in the fiber direction varies as the square of the fiber aspect ratio and a complex function of the fiber volume fraction. Further, the ratio of the axial elongational viscosity to the transverse elongational viscosity and both axial and transverse shear viscosities was shown to be 104-106 for fiber aspect ratio of 102-103, except at extreme values of the fiber volume fraction.
Damage Tolerance of Woven Graphite/Epoxy Buffer Strip Panels
Authors: Kennedy John M.
Published: July 2017

Graphite/epoxy panels with S-glass buffer strips were tested in tension and shear to measure their residual strengths with crack-like damage. The buffer strips were regularly spaced narrow strips of continuous S-glass. Panels were made with a uniweave graphite cloth where the S-glass buffer material was woven directly into the cloth. Panels were made with different width and thickness buffer strips. The panels were loaded to failure while remote strain, strain at the end of the slit, and crack opening displacement were monitored. The notched region and nearby buffer strips were radiographed periodically to reveal crack growth and damage. Except for panels with short slits, the buffer strips arrested the propagating crack. The strength (or failing strain) of the panels was significantly higher than the strength of all-graphite panels with the same length slit. Panels with wide, thick buffer strips were stronger than panels with thin, narrow buffer strips. A shear-lag model predicted the failing strength of tension panels with wide buffer strips accurately, but overestimated the strength of the shear panels and the tension panels with narrow buffer strips.
Prediction of the Compressive Strength of Thick-Section Advanced Composite Laminates
Authors: Chaudhuri Reaz A.
Published: July 2017

An analytical study, with the twin objectives of (1) development of a simple, yet comprehensive approach for prediction of the compressive strength of thick-section advanced composite (graphitc/cpoxy) cross-ply laminates and (2) explanation of unacceptably low strength of the (90N°/0°)k cylindrical specimens tested at David Taylor Research Center (DTRC) under hydrostatic compression, has been undertaken. A simple closed-form expression for the kink mode compressive strength has been derived, under the simplifying assumptions of small deformation, material linearity, local flatness and piecewise linear distribution of the surface-parallel displacements through the thickness, using the minimum potential energy approach. Initial fiber misalignment (maximum tangent angle), ultimate fiber shear strain and the two transverse shear moduli are found to be the key parameters, which limit the compressive strength of the aforementioned thick-section composite laminates. An analysis pertaining to the elastic plane strain in extensional deformation of the compressed composite reveals that kink band formation is primarily due to rotational instability of the fibers, and regardless of the event or sequence of events that may act as precursor(s), kink band formation, once triggered, will, in general, be the dominant (lowest energy) failure mode, especially in the presence of such defect as fiber waviness or misalignment. Critical kink band angle has been found to be bounded well away from zero (a fact consistent with experimental observations), and has been shown to be dependent on the ultimate shear strain of the fiber, maximum initial fiber misalignment tangent angle, θ0 and transverse moduli, Er, Gir and Grr, of the composite laminate. Numerical results for graphite/epoxy and glass/epoxy laminates demonstrate the extreme sensitivity of the former to the initial fiber misalignment defects, responsible for the lowering of the compressive strength of the thick cylinders tested at DTRC and the observed scatter in the test data. These results clearly indicate the sensitivity of kink band angle, β, for graphite/epoxy composites, to the maximum initial fiber misalignment tangent angle, θ0, in the lower range of values (2–3° or less).
Characterization of Elastic-Viscoplastic Properties of an AS4/PEEK Thermoplastic Composite
Authors: Yoon K. J. , Sun C. T.
Published: July 2017

The clastic-viscoplastic properties of an AS4/PEEK (APC-2) thermoplastic composite were characterized at 24°C (75°F) and 121°C (250°F) by using a one-parameter viscoplasticity model. To determine the strain rate effects, uniaxial tension tests were performed on unidirectional off-axis coupon specimens with different monotonie strain rates. A modified Bodner and Partom's model was also used to describe the viscoplasticity of the thermoplastic composite. The experimental results showed that visco-plastic behavior can be characterized quite well using the one-parameter overstress viscoplasticity model.