Introduction Fiber-reinforced polymeric composites(FRPC) have been extended from aerospace, automotive, and military applications into civil engineering structures[1,2]. There is an urgent need to assess the safety arid reliability of using polymeric composites in these applications. One particularly critical issue in regards to using polymeric composites in structural applications is their fatigue reliability in different environmental and loading conditions. Fatigue damages in polymeric composites for noncivil engineering applications have been extensively investigated. Studies of the effects of water and saltwater on fatigue behaviors of polymer composites have been reported. However, there is little quantitative research on the effects of civil engineering environments, namely, water, seawater, temperature, concrete pore solution, ultraviolet light, and loading on the fatigue of polymeric composites.
Optical coherence tomography (OCT) is a nondestructive and noncontact technique to image microstructure within scattering media. The application of OCT to highly scattering materials such as polymer composites is especially challenging. In this work, OCT is evaluated as a technique to image fiber tows and voids in two materials: an epoxy E-glass-reinforced composite and a vinyl-ester E-glass-reinforced composite. Features detected using OCT are compared with optical microscopy. Fiber architecture and voids of glass-reinforced polymer composites can be successfully imaged using OCT. The quality of the OCT image is strongly affected by the refractive index mismatch between the fibers and reinforcement. The largest sources of noise in the images arise from fiber lens effects, interference from within the sample, and a very large reflection at the surface.
Pultruded glass–fiber-reinforced vinyl ester matrix composite coupons were subjected to environmental aging in order to study their durability since such composites are of interest for infrastructure applications. Specimens were tested as-received and after aging in water or salt solutions at room temperature (25°C) or in water at 75°C for various times. The flexural properties (strength and modulus) were determined for bending perpendicular to the 0° orientations (0° being the pull direction) for all aging conditions. In addition, flexural properties in the 90° orientation and tensile properties in the 0° orientation were also measured for the as-received specimens and the specimens exposed to selected aging conditions. Both strengths and moduli were generally found to decrease with environmental aging. Comparing the size of the fracture mirrors on the broken ends of the fibers in aged and un-aged samples suggested that environmental aging decreased the in situ fiber strength. In addition, examination of the failure surfaces and comparisons between the strength of the 90° specimens suggested that degradation of the fiber/matrix interphase region also occurred during the aging process.
Composites using vapor-crown carbon fibers (VGCF), the most conductive of the carbon fiber types, are attractive for applications where low density, high strength, and at least moderate conductivity are required, such as electromagnetic interference shielding covers for spacecraft. The conductivity can be enhanced another order of magnitude by intercalation of the VGCF. If a high Z intercalate is used, the protection of components from ionizing radiation can be enhanced also. Thus, the intercalation of VGCF with IBr is reported. Since composite testing is required to verify properties, the intercalation reaction optimization, stability of the intercalation compound, scale-up of the intercalation reaction, composite fabrication, and resistivity of the resulting composites is also reported. The optimum conditions for low resistivity and uniformity for the scaled up reaction (20-30 g of product) were 114 C for at least 72 hr, yielding a fiber with a resistivity of 8.7+/-2 micro-Omega-cm. The thermal stability of these fibers was poor, with degradation occurring at temperatures as low as 40 C in air, though they were insensitive to water vapor. Composite resistivity was 20-30 micro-Omega-cm, as measured by contactless conductivity measurements, about a factor of five higher than would be expected from a simple rule of mixtures. The addition of 1.0 percent Br2, intercalated microfibers increased the resistivity of the composites by more than 20 percent.
Functionally graded composite materials often possess variations in constituent volume fractions across the material which lead to a variety of microstructures. Many micromechanical models have been used to relate the effective properties of these materials to volume fraction distributions. Herein, several of these models are examined and subjected to simple tests in order to determine their validity over this wide range of application. The self-consistent method surfaces as an attractive method that provides good estimates at minimal effort with no empirical fitting parameters. Using this method, it is shown that certain assumed property distributions presented in the literature must be used with care as they are not physically realizable for certain material combinations.
In this research activity, thermal conductivity of textile composites made from plain weaves and 3-D XYZ weaves is quantified. Plain weave composites are made from E-glass, Kevlar(R) and AS4 graphite fibers and epoxy resin. 3-D woven composites are manufactured from Toho graphite fibers and epoxy resin. The effect of fiber type and fiber volume fraction on the thermal conductivity of textile composites is investigated.The fabric Geometry Model is adapted to calculate the thermal conductivity of textile composites. Results using this tool are compared with experimental data and predictions using the Graphical Integrated Numerical Analysis. The analytical approach proved to be a good engineering tool to predict the thermal conductivity of textile composites.
This paper considers the transient dynamic response of a composite medium with arbitrary shaped voids. A boundary integral equation method is used to compute the displacement histories due to a loading pulse applied to the surface of a composite medium. The present methodology can also be used to study the response of a medium with near-surface cracks and elastic impurities. Numerical solutions are presented for graphite/epoxy, glass/epoxy and isotropic media. The influence of the degree of anisotropy of a medium, orientation and geometry of a void, time history of the loading pulse and the interaction between multiple voids are examined. It is found that the near-field displacements of a damaged medium differ substantially from an undamaged medium. The results reported here can be used for qualitative and quantitative nondestructive evaluations and also for computing the acoustic material signature of a composite medium with defects. The present results are also useful in the validation of inverse solution algorithms developed for material characterization.
Smart electronic packaging is necessary for solving the problem of protecting electronic devices, weight reduction and miniaturization, and many technical problems connected with the performance of these devices. To develop a guideline for these improvements, it is important initially to create some sound engineering estimates based on simple analytical considerations. In the present paper, such estimates are constructed as applied to the design project of the ground based radars (GBR) of the next generation. Some necessary technical requirements for the GBR structure are presented, and an estimate is made of the comparative performance of laminate composite materials in the GBR project, as compared to the aluminum alloy of the present GBR design. The menu method is suggested to solve the problem of the multi-criterion, optimal design of the panel thickness in the GBR design. Finally, the mechanism of sintering ceramics in order to control their density and strength, and hydridize them using superelastic SMA actuators against an impact, is considered.
Filament winding of composite structures generally causes fiber bands to weave or undulate throughout the structure in a non-periodic fashion, except for hoop-only winding. This causes the filament-wound fiber bands, made from multiple groups of filaments, called tows, to fail below the fiber strength of straight fibers. This fiber failure strength reduction is investigated by performing a finite element stress analysis and developing a strength-of-material type closedform solution for curved fibers. The stress analysis results from both methods are compared to actual test results conducted on single and double type weave patterns. The predicted failure stress from the analysis closely matches the experimental results. Application of this work to filament-wound composite pressure vessels is discussed.
The fundamental frequency of laminated plates with single-stepped lap joints was investigated analytically and experimentally. For the analysis, a two-dimensional finite element code was developed and the fundamental frequencies of the laminated plates were extracted from the calculation of the smallest eigenvalue. In the experiment, external excitation was applied to the plates using an impact hammer. The frequency response functions of the laminated plates were obtained by a fast Fourier transform function built in a dynamic signal analyser from which the fundamental frequencies were obtained. The fundamental frequency decreased as the thickness of the adhesive layer increased. When the stacking sequence of the composite laminates was [θ/-θ]2s, the fundamental frequencies decreased as θ increased. The analytical predictions were compared with experimental results. The fundamental frequencies of the laminated plates were closer to the results of plates without adhesive joints when the difference between the Young's modulus of the adhesive and the longitudinal modulus of the laminated plates was decreased.
Nondestructive characterization of a composite using ultrasonic technique has been conducted systematically on A12O3 short fiber reinforced AC8A aluminium metal-matrix composites. For this purpose, A12O3/AC8A with volume fraction of A12O3 short fiber varying up to 30% were fabricated by the squeeze casting technique. Ultrasonic properties of composites were measured by the ultrasonic immersion technique based on pulse-echo method. It was found that ultrasonic velocities were primarily correlated with the volume fraction of A12O3 short fiber. The elastic constants of composites including Young's modulus, shear modulus, and bulk modulus were determined on the basis of the longitudinal and shear wave velocities measured by an ultrasonic technique. The Young's modulus of the composites obtained by ultrasonic technique demonstrated a slightly more conservative value than those measured by the three-point bend test and showed relatively good agreement with the results predicted from the equal stress condition. It was also found that the attenuation and the backscattering behaviors of the composites were primarily related to the volume fraction of fiber and these relations could be used to predict the condition of fiber distribution in matrix nondestructively.
This paper investigates the low cycle fatigue and fracture behavior of cast-then-hot-extruded 6061 aluminum reinforced with alumina particles. The effect of heat treatment and volume percentages of the particle (0–20%) on the axial fatigue behavior and fracture toughness was investigated. Cyclic hardening of the under-aged and the peak-aged samples was observed while cyclic softening of the over-aged specimens was recorded. Although fatigue data follow the Coffin-Manson's model, the fatigue resistance of the composites was lower than that of the matrix alloy regardless of aging and percentage of the reinforcement. Plane strain condition was marginally achieved due to insufficient sample thickness. The measured fracture toughness for these composites however was low compared with that of the matrix alloy and decreased with increasing reinforcement content. Scanning electron microscopy reveals that a crack starts at a machine defect and propagates through or avoids a particle depending on the relative position of the crack front and the particle. Three failure modes were found for this advanced material: cleavage fracture of a particle, cohesive delamination between matrix and particle and void coalescence in the matrix.
A direct approach to the evaluation of the effective permeability, permittivity and transport properties of an arbitrary composite is presented in terms of gradient and flux-density concentrators. A set of requirements are presented, which are imposed on the effective properties and ultimately result in conditions of admissibility for the concentrators. In the scope of bi-constituent, poly-phase composites, two approximate choices for the concentrators are discussed in detail: the Hatta-Taya theory and the poly-inclusion theory. The Hatta-Taya formulation is shown, in general, to yield an effective property which is unsymmetric and which depends on the matrix properties at unitary volume fraction of the embedded material. The poly-inclusion theory is here applied for the first time to second-rank properties. Regardless of the constitution, morphology and texture of the inhomogeneities, the poly-inclusion approach is shown to satisfy all admissibility requirements with the exception of consistency here defined to indicate form identity of mutually inverse properties. A proof is presented which infers relationships between the symmetry group of the orientation distribution function and the symmetry group of the effective properties for special classes of composites. Bounds of order n are presented for macroscopically homogeneous and isotropic composites comprised of an arbitrary number of anisotropic constituents. The case of n = 2 corresponds to the Hashin-Shtrikman bounds which to date appear to have only been calculated for composites with isotropic constituents. Application of the effective properties to the analysis of functionally graded materials (FGMs) is addressed.
An analysis is presented of the dynamic response and stresses in a flexible carbon fibre composite strip that is struck by a rigid missile. The relative approach between the colliding missile and the strip was obtained in a stepwise fashion in the time domain by considering the compatibility of forces and displacements at the interface. The stresses in the region of contact were calculated using Hertz contact theory modified to represent material anisotropy. The plate significantly deflects near the contact patch during the period of contact. This deflection reduces the normal contact force acting between the missile and plate, and prolongs the period of contact.
The transient response of a fibre composite laminated plate due to a variety of impulsive line loads is examined. These loads act either on the outer surfaces or at an interface within the plate. The resulting straight crested waves travel through the plate in a direction at right angles to the load line. The analysis is based on taking a double transform of the displacement and stress components and solving the equations of motion subject to the appropriate boundary and interface conditions. An approximate numerical inversion technique is used to recover the solution to the problem. The results are in the form of graphs displaying the time histories of the normal surface displacement at epicentre and at distances varying from one to ten plate thicknesses.
Composite materials incorporating 3-D woven textile reinforcements have potential for applications in structural components. This paper examines the construction, manufacture and analysis of three-dimensional woven carbon fibre composites as a means of providing data for the validation of a CAD/CAM system for the design of engineering components. In order to achieve the high volume fractions and low void content required for aerospace applications, accurate positioning of yarns within the preform and the design of the moulding tool were crucial while the autoclave processing conditions required modification. The study demonstrates that 3-D woven structures can be successfully converted into complex composite components and provides valuable data for a future knowledge based design and manufacturing system.
Novel jet vapor deposition (JVD) processes offer considerable promise for the inexpensive synthesis of functionally graded (composite) materials (FGMs). Here, we explore microstructure-mechanical property relationships for a model Al/Cu metal-metal system and an Al/Al203 metal-metal oxide multilayered nanocomposite system fabricated by the JVD process. The 10μm thick AlCu multilayers were deposited on silicon wafers at a substrate temperature of ∼140°C. The A1 and Cu layers were of approximately equal thickness and were systematically varied from ∼20 to ∼1000 nm. The 20μm thick rmAlAl2O3 multilayers were deposited on glass slides at ∼250°C. The oxide layer thickness was held constant in the ∼2–6 nm range, whilst the Al layer thickness was systematically varied from ∼3 to ∼50 nm. The structure of the Al/Cu multilayers was polycrystalline and had a strong [111] texture, whereas the Al/Al2O3 multilayers consisted of amorphous aluminum oxide layers and polycrystalline metal layers with randomly oriented grains. The yield strength of the Al/Cu multilayers exhibited an inverse dependence upon layer thickness when the layer spacing exceeded ∼50 nm. When the AlCu layer spacing was thinner than ∼50 nm, the strength was better predicted by a Koehler image force model. A similar phenomenon was also found in the Al/Al2O3 multilayers. In this case the critical metal layer thickness for the transition from an Orowan to a Koehler type behavior was approximately 25 nm. This is consistent with theoretical predictions which indicate that the critical layer thickness of the low modulus consistuent decreases as the difference in shear moduli between the two constituent layers increases.
Two silicate matrix composites, Pyrex/Nicalon and BaO-MgO-Al2O3-SiO2 (BMAS)/ Tyranno, have been used to study composite stability with respect to time at temperature, and under applied stress. Samples aged in an oxidizing atmosphere have been tested in flexure at room temperature, and also by fibre “push-down” to investigate the interfacial properties. Tensile tests have been carried out from room temperature up to 1200°C on the BMAS material, and it was found that a steady degradation in strength occurred from 500 to 1100°C, with a small but significant increase up to 1200°C. Creep experiments have been performed on both the Pyrex and BMAS materials, it was found that Pyrex has a creeping matrix and elastic fibres below the matrix softening point, whereas the BMAS composite showed creep in both components, though at long times the creep rate was shown to be fibre controlled. A simple model for the development of strain with time is reported and used to obtain values for the creep rate of both the matrix and fibres. Activation energies were calculated for the creep processes in both matrix and fibres. The values obtained were: Pyrex, 256 kJ mol−1, BMAS matrix, 300 kJ mol−1 and the Tyranno fibres, 495 kJ mol−1.
The response of long, thin-walled, cross-ply composite tubes subjected to pure bending was studied analytically. The formulation includes three parts: pre-buckling response, material failure and bifurcation buckling. The pre-buckling response is analyzed using nonlinear kinematics to accommodate the ovalization of the cross-section. The formulation is based on the principle of virtual work and is used to generate a numerical solution procedure. The Tsai-Wu failure criterion is used to detect material failure in the pre-buckling response. The maximum stress criterion was also considered for comparison. Finally, the buckling analysis considers the possibility of bifurcation into modes containing periodic displacements along the axis of the tube. The tubes are assumed to be geometrically perfect and free of residual stress. Three materials-AS3501 graphite-epoxy, Kevlar 49-epoxy, and E-glass-epoxy-and three diameter-to-thickness ratios-50, 100 and 400-are considered. The moment-curvature response of the tubes is non-linear due to the ovalization of the cross-section (Brazier effect) which induces a limit moment instability. Either material failure or bifurcation buckling always occurs prior to the limit moment in the cases considered. Little difference was observed between the failure loads predicted by the Tsai-Wu and the maximum stress criteria. Tubes with plies of circumferentially oriented fibers in the outermost and innermost positions in the wall proved superior in strength compared with the other cases considered.
The performance of fiber reinforced composites is strongly dependent on the behavior of the fiber/matrix interface. The fiber pull-out test has been widely used to determine interfacial properties, from which fiber debonding/pull-out behavior in the composite can be deduced. Pull-out test results reported in the literature are almost always obtained under the condition of zero far-field lateral stresses. However, in many practical applications, crack bridging fibers can be under significant lateral compression (e.g. splitting cracks, shear cracks) or tension (cracks at the bottom of plates under biaxial bending). In this investigation, a novel experimental set-up is developed to study the effect of lateral stresses on fiber debonding and pull-out. Steel fiber reinforced mortar specimens were tested to provide an example. With lateral compression, both the initial interfacial friction and the effective interfacial shear strength are found to increase. A higher lateral compression, however, also results in a more rapid decrease in the interfacial friction during fiber pull-out. Therefore, while lateral compression can significantly increase the peak pull-out load, the energy absorption capacity (denoted by the area under the pull-out curve) does not increase to the same degree. Qualitatively, lateral tension imposes opposite effects to lateral compression. Quantitatively, a small lateral tension can result in changes in interfacial properties comparable in magnitude to those caused by a much higher lateral compression. Therefore, although the lateral tension that can act on a fiber is limited by the low tensile strength of the matrix, it may still impose a noticeable effect on the fiber debonding/pull-out behavior.
