bluuebunny/crossref_metadata_2025 huggingface dataset:
Performance Limits for Stiffness-Critical Graphitic Foam Structures. Part II: Comparisons of Foams, Foam-Core and Honeycomb-Core Sandwiches in Bending/Shear
Authors: Hall Richard B.
Published: April 2011

Part I of this study [1] described the concept of graphitic structural foams and indicated their substantial potential for saving mass in stiffness-critical structures. Upper bounds, considered achievable, on ligament moduli were assumed in a well-known, empirically-based relation [2] modeling the foam stiffness versus porosity. The resulting hypothetical graphitic foams were compared to commercially available, high-modulus solids and foams for simple structural applications, and were found to offer the potential for dramatic mass reductions. The present paper compares the potential stiffness-critical performances of neat graphitic foams in plate cylindrical bending to those of honeycombcore sandwich materials, as well as evaluating the foams as potential sandwich core materials. These evaluations require estimates of the foam shear modulus and shear deformation, which were not included in Part I, in order to model the foam deflection to the order standard for sandwich modeling. Three separate foam constitutive models ([2], [3] and elements of [2]-[4]) are considered in the plate bending comparisons, based on upper bound ligament stiffnesses in analogy to [1]; all three assumptions produce similar results, especially for foam porosities exceeding 90 percent. It is found that the influence of shear deformation [5] grows dramatically at porosities exceeding 97 percent, where higher-order theories should be appealed to for accuracy. As the construction of a sandwich of a given stiffness is not unique for given facing and core materials, only mass-optimum sandwiches were compared; each sandwich incorporated the same graphite-epoxy material for face sheets. The neat graphitic foams and foam core sandwiches were found to be competitive on mass and thickness bases with the honeycomb-core sandwich materials for porosities exceeding roughly 90 percent. The graphitic foams and foam-core sandwiches offer the potential for increased in-plane stiffnesses together with the bending stiffnesses associated with honeycomb sandwiches. Finally, the graphitic foams and foam-core sandwiches are briefly compared to the solid materials considered in Part I, including now the effects of shear deformation, and the foams are found to offer mass advantages similar to those predicted in Part I. In summary, Parts I and II indicate that graphitic foams offer exciting potential as additional materials for the design engineer's menu, and may be especially advantageous for stiffness-critical applications involving complex loadings/geometries where minimum mass drives the design.
A Plane-Strain Finite Element Model for Process-Induced Residual Stresses in a Graphite/PEEK Composite
Authors: Li M. C. , Wu J. J. , Loos A. C. , Morton J.
Published: April 2011

A plane-strain, linear elastic finite element model with temperature-dependent matrix properties was developed to analyze residual stresses in graphite/PEEK composites. The residual stress model takes into account the mismatch of the thermal expansion coefficients and the crystallization shrinkage of the matrix. Good agreement between the reported transverse normal stress data and the modeling result was observed in the analysis of a [040] T APC-2 laminate processed at a 35°C/s surface cooling rate. Furthermore, [010/906] T APC-2 laminates were manufactured at different cooling rates to verify the model. The induced residual thermal deformations were measured by a shadow moiré system. The model estimated the out-of-plane displacement of the non-symmetrical laminates accurately. The optimum processing cycles, which minimize the residual stresses and maximize the mechanical properties of composite materials, were found to be different for different lay-ups. Therefore, in order to minimize the residual stresses and to optimize the mechanical performance of the composite laminates, an appropriate processing cycle must be chosen for each specific lay-up.
Effects of Voids on the Mechanical Strength and Ultrasonic Attenuation of Laminated Composites
Authors: Jeong H.
Published: April 2011

The effects of voids on the mechanical strength and ultrasonic attenuation have been investigated on a number of laminated composite systems containing a wide range of void content. Graphite fiber reinforced specimens were fabricated both from unidirectional tape and woven fabric prepreg and their interlaminar shear strength and frequency dependent ultrasonic attenuation were measured together with a careful study of microstructure. The measured strength and attenuation slope were found to depend not only on the void content but on the void geometry associated with the prepreg structure of composites. A shear fracture criterion similar to the Mar-Lin equation for notched tensile fracture stresses was used successfully to correlate the fracture strength of porous laminates. The unidirectional laminates showed much higher void sensitivity than the fabric laminates, i.e., more reduction in shear strength and higher attenuation slope for a given void content. It was shown that the void content may be estimated from the measured attenuation slope and that a good correlation exists between the attenuation slope and the shear strength of porous composite laminates.
Propagation of Hole Machining Defects in Pin-Loaded Composite Laminates
Authors: Persson Erik , Eriksson Ingvar , Hammersberg Peter
Published: April 2011

The propagation of hole machining defects in pin-loaded carbon/epoxy laminates subjected to uniaxial tension fatigue loading was investigated using two non-destructive examination (NDE) techniques. The KTH-method, a new method which gives defect-free1 holes, was used to machine holes in specimens. For comparison, holes were also machined using two traditional methods (DIXI and Dagger drills) causing varying extent of damage. X-ray radiography and computerised tomography were used to give two- and three-dimensional images, respectively, of defect propagation after 50,000-250,000-500,000-cycles, and after failure. Permanent hole elongation was monitored during cyclic loading. The results showed that the propagation of defects occurred in a controlled and stable phase until a sharp fatigue limit was reached (about 400,000 cycles for the DIXI specimen and about 500,000 cycles for both the KTH and Dagger specimens). During the stable phase, the global structural response of the specimens was affected very little by the damage propagation. At the fatigue limit, extensive damage development and defect propagation occurred, resulting in similar large permanent hole elongations in all specimens.
Biaxial Testing of Unidirectional Carbon-Epoxy Composite Using Biaxial Iosipescu Test Fixture
Authors: Balakrishnan M. V. , Bansal B. , Kumosa M.
Published: April 2011

In this study, the biaxial Iosipescu test fixture has been employed to determine the failure properties of a unidirectional carbon-epoxy composite subjected to combined shear and transverse tensile stresses. The range of possible biaxial stress states has been extended by varying the fiber orientations in the Iosipescu specimens. Results obtained from the shear tests conducted using the biaxial Iosipescu test fixture have been compared with independent Iosipescu tests conducted at the University of Idaho, and with the 100 off-axis tension test. It is found that the Iosipescu shear testing of specimens with a fiber angle of 450 can provide reliable estimates for the transverse tensile strength of the material. Using this approach, the biaxial failure properties have been derived in the complete range of shear-transverse tension stress space.
A Discussion on the Damage Mechanism and the Apparent Fracture Strength of Notched Fiber-Reinforced Composite Laminates
Authors: Kamiya Shoji , Sekine Hideki
Published: April 2011

Apparent fracture strength of fiber-reinforced composite laminates depends on the notch tip radius even if it is evaluated in terms of the local parameters such as the stress at notch tips or stress intensity factors. Although numbers of phenomenological explanations have been made, this phenomenon has not yet been physically clear enough even today. In order to elucidate its key mechanism, our interest is here focused on the interlaminar crack extension from notch tips in cross-ply laminates subjected to mode-I loading. We find a stochastically expected upper bound of interlaminar crack extension due to the probabilistic breakage process of fibers in load-bearing laminae inside the region covered with interlaminar cracks. This upper bound, i.e., the critical length of interlaminar crack extension, is inherent to the laminate and corresponds to its notched strength. The well-known variation in apparent fracture strength of notched composite laminates with respect to the notch tip radius is clearly explained as the scale effect of this constant critical length in the different displacement distributions ahead of notch tips with different radii.
Edge Stresses in a Composite Strip Subjected to Axial Temperature Gradients, Part I: Development of the Theoretical Solution
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 is utilized to obtain the plane elasticity solution for the stresses and displacements in a multilayer laminated anisotropic strip subjected to a temperature gradient that is arbitrarily symmetric in the longitudinal direction. The method of analysis utilized departs from previous works in that an eigenfunction solution is developed assuming a length coordinate expansion of the stresses and displacements, with an exponential variation in the thickness coordinate direction. This avoids the shortcoming of the nonorthogonal Fadle-Papkovitsch eigenfunctions in that an orthogonal series repesentation suitable for modeling of nonuniform as well as uniform temperature distributions is obtained. The resulting eigenfunctions satisfy the necessary equations of equilibrium, conditions of displacement compatibility, requirements on interlaminar stress and displacement continuity, traction free surface conditions, and shear traction free edge conditions. As interfacial conditions and strain compatibility are satisfied exactly, the approach overcomes the inter-facial approximations and average sense compatibility invoked in previous complementary virtual work approaches. The solution does not exactly satisfy the free edge normal traction requirement since only resultant force is enforced to zero; however, convergence for enforced zero transverse slope at the strip ends can be established, as the eigenfunctionsare orthogonal. Thus the solution is exact for these edge conditions.

In Part II of this paper, numerical results are presented for several examples and compared to those obtained from our own MSC/NASTRAN finite element analyses. The results indicate the shear and peel stress concentrations and axial stress distributions are in excellent agreement with the finite element analyses for the zero edge slope boundary condition. Also, good correlation was determined with finite element analysis results for the free edge boundary condition over the range of problems considered. This correlation with the finite element numerical results verifies the solution and indicates application of the solution as an approximation to free edge engineering problems is reasonable for abroad range of practical cases.
Torque Capacity of Co-Cured Tubular Lap Joints
Authors: Choi Jin Ho , Lee Dai Gil
Published: April 2011

The co-cured joint is an efficient joining technique because both curing and joining for composite structures are achieved simultaneously. In this paper, the torque capacities of the co-cured tubular lap joint with and without knurling of the pyramid type were experimentally measured. From the experiment, it was found that the excess resin of composite prepreg should not be extracted much because the excess resin played a role as an adhesive in the co-cured tubular lap joint whose steel adherends were not knurled. Also, it was found that the torque capacity of the co-cured joint was increased as the knurling size of the pyramid type on the surface of the steel adherend was increased.
Modeling Compression Failure after Low Velocity Impact on Laminated Composites Using Interface Elements
Authors: De Moura M. F. S. F. , Gonçalves J. P. M. , Marques A. T. , De Castro P. M. S. T.
Published: April 2011

Low velocity impact damage can significantly reduce the residual strength of laminated composites. This kind of damage (mostly delaminations) is very dangerous for the structures because it is not apparent to the naked eye and, in some cases, it can reduce the compressive residual strength up to 60%. In this work, a numerical model for predicting the compression failure of laminated composites containing delamination caused by low velocity impact was developed. An interface finite element, previously developed by the authors, was used. This element is compatible with twenty-seven node isoparametric hexahedral elements and enables modeling the behavior of the damaged interface, taking into account a three-dimensional stress state, the interpenetration constraint and the propagation of delamination. In order to verify the numerical model, some experimental work was done. The experimental work, performed on carbon-epoxy (04, 904)5 and (904, 04), laminates, included low velocity impact tests using a drop weight testing machine, followed by X-Ray damage characterization and compression tests using a fixture system similar to IITRI system. The numerical and experimental results were compared and good agreement was obtained.
Microstructural Differences between Woven Mat and Acrylic Binder TMC and their Relationship to Fatigue Damage Modes
Authors: Harmon D. , Jerina K. L. , Sastry S. M. L.
Published: April 2011

Extensive microstructural evaluations were performed on two titanium matrix composite (TMC) materials to examine any chemical and/or physical differences which explain fatigue crack growth patterns observed in these materials. These examinations included optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and Auger analysis. The results of these studies indicate that fatigue damage modes, in addition to being affected by obvious parameters like load and temperature are also affected by the process in which the TMC is consolidated. Fiber surface damage is visible on one of the two TMC systems which may have influenced the observed fatigue damage mode. The fiber surface damage can be traced back to the pressure applied to the laminate during consolidation. This paper provides a detailed explanation for the occurrence of the different fatigue damage modes and, therefore, provides an explanation for the significant difference in fatigue life of the two materials.
Aspects of the Failure and Postfailure of Fiber Composites in Compression
Authors: Kyriakides S. , Ruff A. E.
Published: April 2011

In this paper the analysis of the compressive failure of an AS4/PEEK composite started in [10] is continued. The compressive strength of this material is typically 60% of its tensile strength. It has been long suspected that this is related to fiber misalignment. Two types of fiber imperfections were found in the material analyzed: imperfections introduced during the manufacturing of the prepreg, and fiber waviness introduced during the lamination and final curing of the composite. Failure due to uniaxial compression takes the form of kink bands of distinct widths and inclinations. Kink bands were preserved by conducting the experiments on circular cross section cylindrical rods which were radially confined. Post-failure examination of the kink bands showed them to be at angles ranging from 11°-16° to the x2 direction of the material. Furthermore, all kink planes were found to have normals in the two orthogonal planes containing the imperfections.

The problem was idealized as a two-dimensional solid with alternating layers of matrix and fibers. The models account for material and geometric nonlinearities and the two types of fiber imperfections seen in this composite. The response is characterized by a load maximum which represents the strength of the material. The strength is strongly influenced by the imperfection wavelength, amplitude and spatial distribution. The load maximum is followed by fiber bending confined to narrow inclined zones. The fibers eventually break into distinct kink bands because of excessive bending. The inclinations and widths of the highly deformed zones just before fiber breaking is expected to start were found to be similar to those of the kink bands measured in the experiments.
Estimation of Sink Mark Depth in Compression Molded SMC Parts with Substructures
Authors: Jeong Jin-Ho , Im Yong-Taek
Published: April 2011

Sink mark, a surface defect along the centerline over a substructure is observed when compression molding of SMC (Sheet Molding Compounds) applies in fabrication of a thin plate with substructures. The magnitude of sink mark formed depends on process parameters used in manufacturing process. The surface quality of the external panels in automotive industry is so critical that this kind of defect should be eliminated during manufacturing stages. Thus, the effect of process parameters on sink mark depth formed in the compression molded thin plate with substructures was experimentally investigated in the present study. In order to estimate the effect of the process parameters such as molding temperature, mold closing speed, depth of the rib, corner radius of the rib, and final molded part thickness of flat portion on the magnitude of sink mark depth under the given experimental conditions, the non-dimensionalized molding parameters were determined based on physical relevances among various parameters. Then, the non-dimensionalized depth of sink mark was determined through dimensional analysis based on the experimental data. It was further tested by comparing the estimated values of sink mark depth with the experimental data. According to this comparison, reasonably accurate prediction was obtained. Compared to the previous study in the literature, the sink mark depths were quantitatively determined according to the various processing parameters and the non-dimensional equation to predict the sink mark level was derived in the present investigation. In addition, the orientation and distribution of fiberglass which directly affect the formation and magnitude of the depth of sink mark were investigated by taking the photographs of the cross-sectional area of the compression molded specimen by SEM (Scanning Electron Microscope). The non-dimensionalized equation for prediction of sink mark depth proposed in the present investigation will be useful in estimating the magnitude of sink mark depth in compression molded SMC plates with substructures and the experimental observations were useful in understanding the formation of sink mark during compression molding.
Static and Dynamic Torque Characteristics of Composite Cocured Single Lap Joint
Authors: Lee Seung Woo , Lee Dai Gil , Jeong Kwang Seop
Published: April 2011

In this paper, the static and dynamic torque capacities of the cocured joints were observed with respect to several bonding parameters such as surface roughness, bonding length and stacking angle and compared with those of conventional adhesively bonded joints. In order to analyze the stresses of the cocured joint, finite element analysis was performed incorporating the nonlinear shear behavior of epoxy resin. The dependence of the static torque capacities on the bonding parameters was investigated numerically.
Moisture Ingression in Honeycomb Core Sandwich Panels: Directional Aspects
Authors: Cise D. M. , Lakes R. S.
Published: April 2011

Moisture ingression was studied in several composite sandwich panels. No observable ingression was found in a panel with HRP core of density 0.13 g/cm3 (8.0 lb/ft3). Significant moisture ingression occurred in a panel with Korex 3.0-lb core with density 0.048 g/cm3, (3.0 lb/ft3). It was faster in the Y-axis (core ribbon) direction, per unit distance than in other directions
Design of Composite Laminates for Strength, Weight, and Manufacturability
Authors: Fine Andrew S. , Springer George S.
Published: April 2011

A procedure is presented for determining the layups of unidirectional fiber reinforced composite plates (with or without cutouts) subjected to multiple in-plane and out-of-plane loadsets, such that the plate satisfies the Tsai-Wu Quadratic Strength Failure criterion, is of low weight, and has a layup which is easy to manufacture. Additionally, the desired cure cycle is generated for thermosetting matrix composite plates such that during cure the maximum allowed temperature is not exceeded, after cure the plate is fully cured and fully compacted, and the cure is achieved in the shortest time.

The design of the layup for strength and weight (utilizing a finite element method for the stress analysis) is based on the Hybrid Algorithm for Laminate Optimization, modified to result in plates in which preassigned regions are of uniform thickness. The design of the layup for ease of manufacture is based on rules. The cure cycle is determined by simulating the cure process of the thermosetting matrix with a deterministic model "controlled" interactively by an expert system.

A computer code SMARTOPT was developed which combines all the above features of the design process and, for specified geometry, material properties, and applied loadsets, provides the desired layup and cure cycle. Results for sample problems were generated which demonstrate the usefulness of the computer code for designing composite plates. The results also show that with proper design, plates made of composite materials can be made lighter than plates made of metals.
Studies on Mechanical Properties of Wood-Polymer Composites
Authors: Razi P. S. , Raman A. , Portier R.
Published: April 2011

Composite plates of wood chips and high density polyethylene (HDPE) matrix were prepared and characterized. Mechanical properties of these composites were evaluated by tensile, static 3-point bending and nail withdrawal tests according to ASTM-D1037-89 (for wood-base fiber and particle panel materials). The tensile strength and break strain of the composites were obtained from the tensile test; and the modulus of rupture (MOR), stiffness and peak load (P) were obtained from the static 3-point bending test. Nail pull-out tests were carried out to obtain information on the adhesion of nail in the composites. It was found that these wood-polymer composites exhibit attractive mechanical properties for application as particle board. The effect of wood chip size and volume percentage on the mechanical properties of these composites was also studied. The resultant fracture surfaces of stretched and bent samples were analyzed using an environmental scanning electron microscope (ESEM)
The Effect of Cooling Rate on Free-Edge Stress Development in Semi-Crystalline Thermoplastic Laminates
Authors: Domb Moshe M. , Hansen Jorn S.
Published: April 2011

A numerical model is developed for prediction of the process-induced thermal residual stresses in thermoplastic composite laminates. The model addresses the development of the three-dimensional residual stress state in fracture-critical free-edge regions as well as through-thickness stress variations. The current approach and analysis provide a unique capability for the investigation of the influence of thermal processing and structural parameters on the resulting buildup of residual stresses during manufacturing. Therefore, it can assist in the design and analysis of thermoplastic composites in terms of tailoring of mechanical and strength characteristics.

The applied surface cooling rate from the melt has a strong effect on the free-edge stress levels obtained at room temperature, and in particular the interlaminar stresses. Results are shown for the case of typical cross-ply and quasi-isotropic APC-2 (graphite/polyether-ether-ketone) laminates.
Design Optimization of Composite Plates: Part II—Structural Optimization by Plydrop Tapering
Authors: Manne Philippe M. , Tsai Stephen W.
Published: April 2011

A preliminary design tool was developed to optimize structures such as beams and plates, with optional cutouts, subject to multiple loadsets and any boundary conditions. SUNSPOT provides with simple, tapered and manufacturable layups.

Sublaminates, made of multiple plies at various orientations, combining one or more materials, are repeated or dropped in different zones of the structure, yielding the plydrops taper while ensuring physical continuity of the fibers in all composite layers.

The orientation and thickness of each ply group in this reference sublaminate as well as its number of repetitions in the zones across the structure, are optimized using the Nelder-Mead algorithm in conjunction with parametric SAMCEF Finite Element Analyses, all integrated within SUNSPOT.

The optimization objective is minimum weight for first ply failure strength. Maximum displacement constraints and economical manufacturing can be enforced additionally. Therefore, extended Tsai-Wu strength failure criteria were derived; new stiffness control criteria and a manufacturing complexity index were developed [1,2].
Micro-Stress Analysis of Woven Fabric Composites by Multilevel Decomposition
Authors: Vandeurzen Ph. , Ivens J. , Verpoest I.
Published: April 2011

A new micromechanics model is presented to predict the local stresses within woven fabric composites under three-dimensional mechanical loading. The model accounts for fabric geometry, yam interactions and matrix distribution. First, by constructing a multilevel decomposition scheme, the composite unit-cell is split up into matrix and yam cells. Then, by a multistep homogenization procedure, a link is established between the external loading and the internal stresses. The principle idea lies in the interpretation that the "stress concentration factors" can be computed at each step by applying the complementary variational principle. We achieve a straightforward analytical stress model for woven fabric composites which has not yet been reported. In addition to modeling the stress fields, the three-dimensional stiffness matrix is computed in this paper using the "stress concentration factors." The proposed modeling technique yields numerical stiffness results comparable to those obtained by three-dimensional finite element modeling.
Damage Growth Investigation in a Random Fiber Composite Beam by Moiré Interferometry
Authors: Zhang Y. , Sikarskie D. L. , Miskioglu I. , Post D.
Published: April 2011

In this paper, high sensitivity laser moire interferometry was used for observation and analysis of in-situ damage initiation and growth in random fiber composite beams. The technique was found to be very effective. The general approach was to load the beams to successively increasing load levels, and then unload, observing the residual field. Damage showed up as anomalies in the fringe patterns. In the four-point-bend tests, it was found that observable microscopic failure initiated in the tension region first at about 40% of the ultimate failure load, while failure initiation in the compression region started at about 50% of the ultimate failure load. To determine the effect of this early and asymmetric damage growth in the tension and compression regions, the neutral axis shift of the beam was determined as a function of increasing load. This was done by reloading the beams (after observing the residual field) to a small load well below any damage threshold. It was found that very little shift occurs indicating the stiffness of the beam was largely unaffected by the microstructural damage over a major range of loading including close to failure. This unexpected behavior is most likely caused by progressive failure of the chopped fibers in orientations at or near 90° to the length of the beam.
Probabilistic Micromechanics/Macromechanics for Ceramic Matrix Composites
Authors: Murthy Pappu L. N. , Mital Subodh K. , Shah Ashwin R.
Published: April 2011

Ceramic matrix composites (CMCs) are known to display a considerable amount of scatter in their properties due to variations involved in fiber/matrix properties, interphase properties, interphase bonding, amount of matrix voids, and many geometric or fabrication process related parameters such as ply thickness and ply orientation. This paper summarizes the preliminary studies related to the incorporation of formal probabilistic descriptions of the material behavior and fabrication related parameters into micromechanics and macromechanics for CMCs. This process involves a synergistic coupling of two existing methodologies: namely ceramic matrix composite micro- and macromechanics analysis, and a fast probability integration (FPI) technique to obtain probabilistic composite behavior/response. Preliminary results in the form of cumulative probability distributions and information on the response probability sensitivities to primitive variables for a unidirectional SiC/RBSN ceramic matrix composite are presented. The cumulative distribution functions are computed for composite moduli, thermal expansion coefficients, thermal conductivities and longitudinal tensile strength at room temperature. Variations in the constituent properties that directly affect the above mentioned composite properties are accounted for via assumed probabilistic distributions. Collectively the results show that the present technique provides valuable information on the composite properties and sensitivity factors which are useful to the design/test engineers. Furthermore, the present methodology is computationally more efficient than a standard Monte-Carlo simulation technique and the agreement between the two is excellent as shown via select examples.
Microstructural Analysis of Composite Tubes Made from Braided Preform and Resin Transfer Molding
Authors: Tsai J. Stephen , Li S. J. , Lee L. James
Published: April 2011

This paper investigates the manufacturing method and mechanical properties of composite tubes made from braided fiber preforms. The tubes molded by Vacuum-Assisted Resin Transfer Molding (VARTM) process with different resins, fiber reinforcements and braiding architecture were tested for weeping and burst strength. Both biaxial and triaxial braided fiber preforms were used. The failure modes of tubes made from these two braiding architectures were different. For biaxially reinforced tubes, the initial cracks are aligned along the direction of the fiber tows, while for triaxially reinforced tubes, the initial cracks along the longitudinal direction appear first. Tubes with and without flanges were also fabricated. The strength tests indicated that the tube with flanges is much stronger than the one without flanges. Two theoretical models based upon mechanics of material were employed to predict the mechanical properties. The pressure vs. deformation curves from the weep/burst tests are consistent with the theoretical predictions in the elastic region.
The Durability of E-Glass/Vinyl Ester Composite Materials Subjected to Environmental Conditioning and Sustained Loading
Authors: Buck Stephanie E. , Lischer David W. , Nemat-Nasser Sia
Published: April 2011

A series of experiments has been performed to observe the effects of the combination of sustained load, elevated temperature, and moisture on the durability of an E-glass/vinyl ester composite material. This durability is evaluated in terms of the ultimate tensile strength of the material. The results have confirmed that moisture at elevated temperatures reduces the ultimate tensile strength of the material, as reported in previous studies. In these experiments, a sustained load has been applied to samples during the conditioning process and has been found to further reduce the composite durability. Conditioning also decreases the strain to failure, with a greater decrease overall for samples which were loaded during conditioning.
Repair of Composite Laminates-II: Models
Authors: Ahn Sung-Hoon , Springer George S.
Published: April 2011

Models were developed for calculating the failure loads of composite laminates repaired by scarf and uniform lap techniques. The models take into account anisotropy of each ply in the laminate and in the repair ply and non-elastic behavior of the adhesive or resin interlayer between the laminate and the repair patch. On the basis of the models two computer codes were written for generating numerical values of the failure loads. The first code is for calculating the failure loads of laminates repaired by the uniform lap technique, the second for calculating the failure loads of laminates repaired by the scarf technique. Failure loads calculated by the models, and the corresponding computer codes, were compared with data, and good agreements were found between the results of the models and the tests.
An Experimental Study of Low Velocity Impact Damage in Woven Fiber Composites
Authors: Siow Y. P. , Shim V. P. W.
Published: April 2011

A study is made on the low velocity impact response and post-impact mechanical capacity of woven fiber [0/90,-45/45,0/90]s carbon epoxy composite plates. This complements the work done by numerous other researchers who have also examined low velocity impact of composites, but have focused on unidirectional, cross-ply or quasi-isotropic laminates. In the present study, post-impact static uniaxial tension, compression, as well as tension-compression fatigue tests are performed. The damage mechanisms for woven laminates are found to be predominantly delamination and fiber breakage, with the area of impact-induced delamination increasing linearly with impact energy for the range of energies examined. Damage extent and type are also dependent on the curvature of the impactor tip and deformation generated by a sharp impactor is more localized. The existence of a threshold energy level below which no delamination discernible by C-scan occurs is noted. In contrast with previous findings, it is observed that the peak in the impactor deceleration-time response is not associated with the onset of fiber failure, which can occur earlier. The amount of energy absorbed when this peak occurs therefore does not indicate the energy required to initiate fiber breakage. It is observed that residual tensile strength is a function of delamination area and impactor tip radius. For static compression and tension-compression fatigue, the residual load-bearing capacity is only dependent on delamination area. Under compression, delamination promotes the micro-buckling of fibers, whereas in tension, failure is predominantly via fiber breakage. For a common impact energy and impactor, a damaged specimen is weaker in compression than in tension.
Manufacturing and Mechanical Properties of Glass Bubbles/Epoxy Particulate Composite
Authors: Lin Wei-Hwang , Jen Ming-Hwa R.
Published: April 2011

The objective of this work is to manufacture particulate composites. to measure their compressive mechanical properties and to model the behavior by finite element analysis. Syntactic foams, glass bubbles in epoxy resin, have low density and hence are of particular interest for subsea applications. The research reported here is undertaken in the context of a submersible manipulator. In the laboratory two types of specimens are manufactured and tested, including (a) coupons of the same particle size with varied particle volume fractions and (b) coupons of the same particle volume fraction with different particle sizes. From the compressive test, we receive the fracture strength and observe the fracture mechanism. Also the buoyance is obtained. In order to correlate the experimental data, the three-dimensional (3-D) finite element method of axis symmetry and cubic center symmetry is used numerically. The predicted values and locations of the maximum tensile stresses of specimens agree well with the failure mechanism. It is found that small particles are stronger than matrix, whilst larger particles are weaker than matrix. A decrease in particle volume fraction increases the strength at fixed particle size. An increase in particle size increases the strength at fixed particle volume fraction.
A Strength of Materials Formulation for Thin Walled Composite Beams with Torsion
Authors: Massa Julio C. , Barbero Ever J.
Published: April 2011

A simple methodology for the analysis of thin walled composite beams subjected to bending, torque, shear, and axial forces is developed. Members with open or closed cross section are considered. The cross section is modeled as a collection of flat. arc-circular, and concentrated area segments. Each laminated segment is modeled with the constitutive equations of classical lamination theory accounting for a linear distribution of normal and shear strains through the thickness of the walls, thus allowing for greater accuracy than classical thin walled theory when the walls are moderately thick. The geometric properties used in classical beam theory such as area, first moment of area, center of gravity, etc., are no longer used because of the variability of the materials properties in the cross section. Instead, mechanical properties such as axial stiffness, mechanical first moment of area. mechanical center of gravity, etc., are defined to incorporate both the geometry and the material properties. Warping, restriction to warping, and secondary stresses are considered. Failure predictions are made with customary failure criteria. Comparison with experimental results are presented.
Property-Based Optimal Design of Composite Materials and their Internal Architectures
Authors: Sadagopan D. , Pitchumani R.
Published: April 2011

Composite materials are increasingly finding use in diverse applications with a wide range of property and performance requirements. The ability to optimally tailor composite materials for these applications is of much practical importance, and forms the focus of this investigation. Composite materials tailoring refers to the concurrent manipulation of the materials composition and internal architecture of a composite material to achieve the desired properties. Since the wide variety of material combinations, reinforcement geometries and architectures to choose from poses a bewildering task of selection, a systematic approach to optimal tailoring of composite materials is a challenging design problem which is addressed in this article. This study presents a generalized optimal tailoring framework using the combinatorial optimization technique of simulated annealing in conjunction with a property model base consisting of analytical relationships between the composite properties and the microstructures. Optimal tailoring charts are developed, which provide for selecting optimal combinations of matrix and reinforcement materials, and reinforcement morphology, architecture, and volume fraction so as to meet the specified property and performance requirements. The study considers matrix materials that span the classes of polymers, metals and ceramics, while reinforcement geometries of unidirectional fibers, particulates and two-dimensional woven fabrics are considered.
A Model for Unsaturated Flow in Woven Fiber Preforms during Mold Filling in Resin Transfer Molding
Authors: Pillai K. M. , Advani S. G.
Published: April 2011

In this paper, the unsaturated flow encountered in the woven or stitched fiber mats used in RTM is studied for the case of constant rate injection into a 1-D mold. Such fiber mats are characterized as a dual scale porous medium and a two-layer model, based on the difference in the length scales of the intra-tow and inter-tow spaces, is proposed. The mass balance is derived from first principles for an idealized representation of such type of porous media and incorporation of a sink term in the macroscopic equation of continuity is established. First a simple sink function, the constant sink, is studied and the downwardly drooping profile of the injection pressure, reported in previous experiments, is shown to be a natural consequence of the absorption of resin by the tows. Next an On/Off type constant sink function is proposed; critical parameters of unsaturated flows such as pore volume ratio and sink strength are introduced. Then the two-layer model with rectangular cross section is extended to two other types of cylindrical cross sections encountered in woven mats. A numerical scheme to solve the governing equations of sink models is discussed and results of simple analytical models and the two-layer models are presented. These models classify the flow through such woven mats as either a developing or a developed flow depending upon whether all or some tows behind the resin front are unsaturated. Parametric studies to identify important parameters affecting the length of the partially saturated regions near the front are presented. The simple On/Off type sink model also predicts the slopes of the injection pressure generated by the two-layer models for developed flow with good accuracy. The inlet pressure curve of the two layer models reproduce the characteristic drooping trend. Finally, the effect of different cross sections of the two-layer model upon the inlet pressure history is studied.
Peel Test for Characterization of Polymer Composite/Concrete Interface
Authors: Xie Ming , Karbhari Vistasp M.
Published: April 2011

A specially designed peel test was used to characterize the interfacial bond strength between polymer composite and concrete. For the carbon fiber/epoxy composite and concrete system investigated in the present study, the crack was found to propagate mainly through the adhesive at the interface. The cohesive debonding inside the concrete substrate was found to be the least possible failure mode. A finite element analysis was also performed to calculate the strain energy release rate at the peel front in a peel test specimen and the numerical results agreed well with those predicted by a closed-form solution. The effect of different crack propagation paths along the composite/concrete interface on the fracture energy was predicted using the finite element analysis. The experimental observations in the present study were explained well by the numerical modeling.
Effect of Reinforcing Tabs on the Mode I Delamination Toughness of Stitched CFRPs
Authors: Jain Lalit K. , Dransfield Kimberley A. , Mai Yiu-Wing
Published: April 2011

Stitching advanced carbon fibre composites in the through-thickness direction enhances their interlaminar fracture toughness. To study this, composite specimens were fabricated from dry carbon fabric and resin using resin transfer molding (RTM) technique, and also from unidirectional prepreg tape. The stitching was performed with 2-ply (40 tex) Kevlar thread. It was observed during the mode I testing of RTM specimens using a double-cantilever-beam (DCB) test that the cantilever arms failed in bending, instead of crack propagating in a self-similar manner. Therefore, DCB specimens were reinforced with aluminium tabs along the length of the specimens. To understand the effect of tabs on the mode I toughness, prepreg specimens were tested with and without the tabs. The results indicated that the tabs altered the failure mechanism of the stitching thread and the improvement in the mode I delamination toughness was underestimated. Specimens with varying stitch density were studied. The steady-state toughness, GIRS, of unstitched and stitched specimens with and without tabs were compared. The addition of stitching was found to impart a 10-fold improvement in toughness. Experimental results were also compared with theoretical predictions from a micromechanics-based model developed previously, and good agreement was found.
Tensile Strength of Fiber-Reinforced Composites: II. Application to Polymer Matrix Composites
Authors: Curtin W. A. , Takeda N.
Published: April 2011

An analytic model for predicting the tensile strength of uniaxial fiber-reinforced composites is applied to several different graphite-fiber composites and shows good agreement with measured strengths. The model includes the effects of fiber statistical strength distribution, local load transfer from broken to unbroken fibers, interfacial shear strength, fiber pullout, and composite size. The model is applied in detail to AS-4 fiber/Epon828 matrix and T300 fiber/Epicote matrix composites, for which the requisite constituent and composite data are available in the literature. Data for the fiber strength at the critical length is obtained from single fiber composite experiments using the analysis of Curtin. For the AS-4/Epon828 material, the predicted tensile strength is 35% higher than the measured value if physical fiber volume fraction is used and 18% higher if a fiber volume fraction consistent with the measured composite Young's modulus is used. For the T300/Epicote system, the predicted strength is 35% higher if thermal stress effects in the s.f.c. test are neglected but only 20% higher if estimated thermal stresses are included. Strength predictions for an AS-4 fiber/thermoplastic matrix system and for two other T300 fiber-based composites are made using the same fiber strength data and comparable agreement (20%) is obtained. The theory predicts very modest changes in composite strength with increasing interfacial shear strengths, as observed experimentally. Overall, the model is in reasonable agreement with experiment when the constituent data is accurately determined, which establishes the model as the basic model for predicting uniaxial tensile strength in polymer matrix composites.
Thickness Effects on Pinned Joints for Composites
Authors: Liu Dahsin , Raju Basavaraju B. , You Junling
Published: April 2011

Mechanical joints are commonly used for joining composite laminates, especially thick section composites. In most studies of mechanical joints, the interactions between pin diameter (D) and composite dimensions such as width (W) and distance from hole center to composite end (e), resulting in W/D and e/D ratios, are the primary parameters for joint evaluation since composite laminates are usually very thin. However, as composite thickness increases, the contact interaction between pin and composite alters. Subsequently, the composite joint stiffness and joint strength are affected. Since the contact interaction is strongly affected by pin diameter (D) and composite thickness (H), this study investigated mechanical joints with combinations of various composite thicknesses and pin diameters. Composite material made of woven glass fabric and phenolic matrix was investigated. Sixteen joint configurations based on four composite thicknesses and four pin diameters were examined. Both experiments and finite element analysis were conducted in this study. It was found that the ratio of H2/ D2 could be used to distinguish the two primary damage modes, i.e. pin bending and bearing failure. Results also showed that thick composites with small pins and thin composites with large pins had lower efficiencies for joint stiffness and joint strength than those having similar dimensions between pin diameter and composite thickness.
Use of the Free Vibration Spectrum to Detect Delamination in Thick Composites
Authors: Penn L. S. , Jump J. R. , Greenfield M. J. , Blandford G. E.
Published: April 2011

A free vibration method was evaluated for damage detection in thick composite plates, whereby vibration was initiated by a single external mechanical pulse and was sensed by piezoelectric patches bonded to the surface of the composite plate. The investigation was conducted on unidirectional fiber glass/epoxy plates approximately 0.387 in (==10 mm) thick, containing controlled delaminations. The free vibration method described in this paper was found relatively insensitive for identifying small to medium delaminations in thick plates. Consequently, several frequency computations using a finite element model were performed to determine the influence of delamination size and location for a single delamination.