Fiber Volume Percentage Research Articles

Effect of Thermal Residual Stress on the Mechanical Properties of NiAl-Based Composites

ABSTRACTNiAl-based hybrid composites containing 5, 15 and 30 volume percent of A12O3 fibers in addition to AIN dispersion particles were fabricated via a powder metallurgy route. The strengthening mechanisms associated with the short A12O3 fiber in NiAl-(A1N)dispersion,-(A12O3)fiber, composite were studied with an emphasis on the effect of thermal residual stress on the compressive strength at room temperature and 1300K. At 300K, the yield strength of the composites was predicted using a linear superposition of the monolithic yield strength, direct fiber strengthening and tensile matrix residual stresses. The prediction shows good agreement with the measured data. The majority of the room temperature strengthening is attributable to the residual stress. At 1300K, the strengthening was achieved only by the load sharing of the fibers and there is no direct influence from the process-induced thermal residual stresses since they were completely relaxed at this temperature.

Residual strains and stresses in tungsten/Kanthal composites

Neutron diffraction was used to measure thermal residual strains in tungsten/Kanthal metal matrix composites containing 10, 20, 30 and 70 vol.% fibers at room temperature. In addition, thermal residual strains were measured in the 20 and 70 vol.% composites as a function of temperature. The study was conducted over the temperature range 20–800°C. The results were compared with those estimated using finite element modeling. As the matrix has a higher coefficient of thermal expansion than the fiber, the measured bulk axial strain at room temperature in fibers is compressive and varies from about −0.0020 to −0.0004, whereas for the matrix, the strain is tensile and increase from 0.0003 to 0.0027 as the amount of tungsten fibers in the composite increases from 10 to 70 vol.%. For a given volume percent of fibers, the sum, of the absolute values of axial strains measured in the fibers and the matrix in the composite, averaged over 2.5×25×25 mm samples sectioned from 2.5×25×200 mm as-fabricated composites bars, is about 0.0030 or less. Based on the temperature drop from the hot, press temperature to room temperature, the total axial strain should be about 0.0050. This indicates that the thermal residual stresses relax at high temperatures and that the stress-free temperature is lower than the hot, press temperture (1065°C). This agrees with the measured stress-free temperature of 600–650°C. Based on the finite element model, it is estimated that the residual axial strains in tungsten fibers are compressive and are about −0.0020 and −0.0005 in 20 and 70 vol.% composite samples, respectively. The corresponding strains in the Kanthal matrix are about 0.0010 and 0.0022 (tensile). The calculated residual axial stresses in tungsten fibers are about −1015 and −258 MPa and in the Kanthal matrix are about 251 and 602 MPa in 20 and 70 vol.% composite samples, respectively. For the 70 vol.% composite, the equivalent stress in the matrix is about 400 MPa, which is less than the 530 MPa yield stress of Kanthal.

Strong and Tough Hi‐Nicalon‐Fiber‐Reinforced Celsian‐Matrix Composites

Strong, tough, and almost fully dense BN/SiC‐coated Hi‐Nicalon‐fiber‐reinforced celsian‐matrix composites have been fabricated by impregnation of the fiber tows with the matrix slurry, winding on a drum, stacking the prepreg tapes in the desired orientation, and hot pressing. The monoclinic celsian phase in the matrix was produced in situ, during hot pressing, from a mixed oxide precursor. The unidirectional composites having ˜42 vol% of fibers exhibited graceful failure with extensive fiber pullout in three‐point bend tests at room temperature. Values of stress and strain at the proportional limit were 435 ± 35 MPa and 0.27 ± 0.01%, respectively, and an apparent ultimate strength of 900 ± 60 MPa was observed. Young's modulus of the composites was 165 ± 5 GPa.

A comparison of as-cast and heat treated properties in short fiber reinforced Al-Si piston alloys

In recent years, there has been a surge of interest in the use of discontinuously reinforced Metal Matrix Composites (MMCs) as structural materials. Reduction in the cost of constituent materials coupled with improvements in the processing methods have resulted in the ability to produce these materials at a competitive cost, when compared with more traditional structural metals. Most mechanical property data for discontinuously reinforced MMCs have been determined in tension and a relatively small amount of data has been obtained in compression. This is surprising since compression testing has certain advantages over tensile testing. The aim of this work is to investigate the effect of short alumina (Saffil) fibers to the hardness and compressive strength in the Al-Si (LM 13) piston alloy. Particular attention is given to the influence of the heat treatment and the fiber volume percent on the properties.

Effect of silica fume and steel fibers on some properties of high-strength concrete

The main disadvantage of high-strength concrete is its highly brittle behavior and this can beovercome by adding fibers to the concrete. This would also improve some other mechanical properties of high-strength concrete such as tensile strength and compressive strength. These properties are not very well established for high-strength steel-fiber reinforced concrete (HSFRC) yet. In this study the influence of silica fume on the properties of HSFRC were investigated by using silica fume of two different percentages and three different hooked-end fibers namely, 30/0.50, 60/0.80 and 50/0.60 length/diameter (mm/mm). Fibers were added to concrete in three different volume percentages of 0.5, 1.0 and 2.0 by volume of concrete. The results indicated that there is a linear function between splitting tensile strength ( F splt) and volume percentage of fibers ( V f) [i.e. F plt = A( V f) + B, where A and B are correlation coefficients] as well as between splitting tensile strength ( F splt) and compressive strength ( F c) of plain series A concrete [i.e. F splt = C (√ F c) + D, where C and D are correlation coefficients]. These relations can describe the development of splitting tensile strength of HSFRC containing no silica fume, 5% silica fume and 10% silica fume by weight of cement. On the other hand, although silica fume has an effect on compressive strength, volume percentage and aspect ratio of steel fibers has little effect.

Acoustic emission in fiber reinforced concrete

This investigation is aimed at diagnostic studies on the behavior at different interfaces in fiber reinforced concrete: between hardened cement paste and sand, between mortar and coarse aggregate and between concrete and fibers. Other types of internal failure include aggregate crushing, fiber rupture, and so on. Only cylindrical specimens, with varying volume percentage and aspect ratio of fibers, have been tested in compression. For quite some time additives like latexes, plasticizers, fibers and silica fume are incorporated in concrete and mortar to improve one quality or the other of concrete, but one single common objective is delaying or arresting interface cracks. This investigation attempts to relate external behavior to internal signals of distress through acoustic emission. This helps to identify critical internal distress vis-a-vis external level of stress. Further, spectral analysis of acoustic signals has been attempted in frequency domain in order to establish which types of interface failure are predominant at different stages of stress. Such understanding could enable material scientists to decide on the parameters of additives with greater confidence when developing composites with desired external response. Of interest are instrumentation and on-line data processing in order to record (a) cumulative acoustic activity and (b) spectral analysis in frequency domain at different stages of progressive loading.

Fabrication and Mechanical Properties of an Optically Transparent Glass Fiber/Polymer Matrix Composite

A composite with substantial optical transparency was produced by rein forcing poly(methyl methacrylate) (PMMA) with unidirectional borosilicate glass fibers. Optical transparency of the composite was achieved by matching the refractive index ( n D) of the glass fiber and polymer matrix to within ±0.002. The optical transmission of the composite decreased with increasing volume percentage of glass fibers. Composites con taining 30 volume percent of unidirectional 12 μm diameter fibers had a transmission at 589.3 nm of 20 percent measured through a 10 mm thickness. Composites containing 30 volume percent of 56 μm diameter fibers had an optical transmission of 45 percent. Type written text could be clearly read through the composite when it was laid directly on a printed page. Composites with 40 volume percent fiber had a flexural strength of 600 MPa which is six times greater than that of pure PMMA. The flexural modulus for the same composite was 18 MPa which is nine times greater than the 2 MPa measured for pure PMMA. The work of fracture was 110 kJ/m 2 or more than double the 45 kJ/m2 for pure PMMA. Suggestions are made for further improvements in the optical properties.

Creep Model for Metallic Composites Based on Matrix Testing

An anisotropic creep model is formulated for metallic composites with strong fibers and low to moderate fiber volume percent (< 40%). The idealization admits no creep in the local fiber direction and assumes equal creep strength in longitudinal and transverse shear. Identification of the matrix behavior with that of the isotropic limit of the theory permits characterization of the composite through uniaxial creep tests on the matrix material. Constant and stepwise creep tests are required as a data base. The model provides an upper bound on the transverse creep strength of a composite having strong fibers embedded in a particular matrix material. Comparison of the measured transverse strength with the upper bound gives an assessment of the integrity of the composite. Application is made to a Kanthal composite, a model high-temperature composite system. Predictions are made of the creep response of fiber-reinforced Kanthal tubes under interior pressure.

FABRICATION OF A Ni3Al/Al2O3UNIDIRECTIONAL COMPOSITE BY PRESSURE CASTING

ABSTRACT A gas pressure liquid metal infiltration technique for producing fiber reinforced Ni3Al/Al2O3 and Ni3Al/Al2O3 composites has been described. Composite bars of 9.5 mm diameter, 150 mm long were produced. It was found necessary to add 0.5 at.%Ti to both Ni3Al and NiAl alloys to obtain adequate wetting. There was a strong tendency for liquid metal infiltration along one side of the fibers causing a shift of the fibers to the opposite side. No interface reaction zone was observed at optical magnifications up to 1000X. A composite of Ni3Al containing 33 volume percent of FP fibers fractured at 890 MPa which is approximately 3 times the yield strength of the matrix IC-50(21.73A1-0.34Zr-0.IB-Balance Ni at.%) + 0.5 at. %Ti.

Muscle involvement in pyruvate dehydrogenase complex (PDHC) deficiency

Muscle biopsies from a 13-month-old female infant with a delay in developmental milestones, lactic acidosis and visual disturbance, and a 6 year-old female with frequent epileptic fits are described. Biochemical studies of biopsied muscles and skin fibroblasts demonstrated markedly decreased pyruvate dehydrogenase complex (PDHC) activity to about 16% of normal value. Muscle histochemistry in both patients showed disorganized intermyofibrillar networks containing large diformazan granules on NADH-TR, small angulated fibers with high nonspecific esterase (NSE) activity and basophilic fibers. Ragged-red fibers and increased lipid droplet accumulation were absent. Patient 1 had increased numbers of type 2C fibers (11.3%) and mild fiber type grouping. On electron microscopy, most mitochondria were nearly normal. There were focal aggregates of mildly enlarged mitochondria in the subsarcolemmal areas in both patients. Morphometric study showed that the mean mitochondrial size and the mitochondrial percentage of fiber volume were not significantly different amongst patients and normal controls.

Minimum Weight Design Problems of Fiber-Reinforced Beam Subjected to Uniform Bending

Problems considered here are that of minimizing the weight of beams, which are subjected to a uniform bending moment and reinforced by the fibers distributed in the direction of beam axis. The beam is simplified as a multilaminate structure, of which the fiber volume percent Vfi of each lamina is considered as the design variables. To formulate this design problem the bending theory of multilaminate beam and the law of mixture for the composite material strength are applied. Furthermore, the sequential linear programming and the sequential unconstrianed minimization techniques are used to obtain the design solutions numerically.

A morphometric analysis of human muscle fibers with relation to fiber types and adaptations to exercise

Muscle biopsies were obtained from the vastus lateralis of 14 male subjects: 3 long distance runners, 2 world class power lifters and 9 active, although not highly trained, individuals used as controls. The fibers were investigated by electron microscopy and the mitochondrial volume percent, lipid volume percent and Z-line width were analyzed morphometrically. With the combined data a direct correlation was found between mitochondrial volume percent and lipid volume percent, lipid volume percent and Z-line width and mitochondrial volume percent and Z-line width. The muscle fibers were classified as slow-twitch oxidative (SO), fast-twitch-oxidative-glycolytic (FOG) and fast-twitch-glycolytic (FG) based on relationships found in the data and well established properties of muscle fiber types. Although no distinct patterns emerged, a good approximation of fiber type characteristics was obtained, and values for volume percent of central mitochondria, volume percent lipid and Z-line width are reported. The fibers classified as SO were characterized by having wide Z-lines, a high mitochondrial volume percent and high lipid volume percent. The fast-twitch fibers (fibers with narrow Z-lines) were separated into 2 groups, those with high mitochondrial volume percent (FOG) and those with low mitochondrial volume percent (FG). No distinction could be made between the fast-twitch subgroups with regard to Z-line width. The fibers from distance runners differed from those from controls by exhibiting a greater capacity for aerobic activity as evidenced by the increased volume percent of mitochondria and lipid in both slow- and fast-twitch fibers. The high strength, anaerobic activity of the world class power lifters was reflected by the low mitochondrial volume percent of many fast-twitch fibers (FG) and the decreased lipid stores in all fibers.

Filler effect on the recoil kinetics of drawn polystyrene

AbstractRecovery experiments above Tg have been performed on glass bead and short glass fiber polystyrene composite sheets obtained from extrusion and subsequent hot drawing. A shift procedure has been applied to recoil data using the Williams‐Landel‐Ferry (WLF) equation with the same values of C1 and C2 used for unfilled polystyrene. The data reported show that an improvement of the dimensional stability of a drawn thermoplastic material can be obtained by adding very low volume percentage of fibers. Moreover, all the recoil data have been reduced to a unique generalized master curve which takes into account the draw ratio, the filler content, and geometry, using a fractional distance from equilibrium as a measure of the recovery. These results indicate that the presence of the filler does not affect the recoil kinetics of the polymeric matrix.

Techniques for compounding glass fiber‐reinforced thermoplastics

AbstractThe processing characteristics of various glass fiberreinforced thermoplastics (FRTP) and the basic approaches for their production are discussed. Experimental work on twostage production‐scale continuous kneaders was conducted to define the most important process variables in compounding FRTP materials. Results demonstrate how processing conditions and screw design influence the temperature profile in the kneader and the glass‐fiber length in finished products. Feed steek appearance (i.e., pellet shape and size), melt viscosity, and volume percentage of fibers are basic material variables. High quality products can be achieved through a careful, layout of the continuous kneader in order to minimize thermal degradation of the base polymer and to effect adequate dispersion of the glass fibers in the matrix.

Carbon-Felt, Carbon-Matrix Composites

Carbon-felt, carbon-matrix composites were prepared by the chemical vapor deposition of carbon within carbonized viscose-rayon substrates. The thermal and mechanical properties of these carbon-carbon composites were investigated over a wide range (9-46 v/o) of fiber volume percent. In creases in fiber content resulted in: (1) decreases in both total porosity and pore size, (2) filament and matrix reorientation, and (3) reduction of the pyrolytic carbon matrix sheath thickness. Fiber content, and its inter related effects, are correlated with significant changes in flexural and ten sile strength and modulus, thermal conductivity and linear thermal expansion.

Buckling of Microfibers

Most composites are fabricated at elevated temperatures and cooled to room temperature. The difference in coefficients of thermal expansion of the fiber and matrix may cause buckling of the slender-shaped microfibers. This would greatly reduce the initial strength of the composites. The primary objective of this paper is to explain qualitatively the basic phenomenon of buckled microfibers in a composite and to present some numerical results in certain ranges of elastic parameters. Our analysis is based on assumptions that (a) the volume percentage of fibers is small so that the mutual interference of fibers is negligible and the matrix surrounding a fiber may be considered as infinitely large; (b) the diameter of the fiber is very small in comparison to its length so that the fiber may be treated as a linear fiber with infinite length; (c) the constituents are homogeneous and isotropic and the classical linear theory of elasticity may be applied; and, (d) in the process of cooling, the fiber is subjected to compression but not twisting moment by the surrounding matrix. The analysis leads to a relation between the pertinent elastic and thermal characteristics of the matrix and microfiber and the compressive force along the axis of the microfiber at the critical moment of incipient buckling. With the knowledge of that relation, it is now possible to know in advance whether or not buckling is to be expected and to avoid such manufacturing methods in which a threat of having buckled microfibers is present.

Elastic properties of fiber-reinforced shells.

The applicability of classical laminated thin-shell theory to the calculation of elastic coefficients of fiber-reinforced shells is demonstrated. It is assumed that currently available techniques can be used to calculate the elastic coefficients of a unidirectional layer in terms of fiber and matrix elastic constants and volume percent of fiber content. The unidirectional system is then treated as being homogeneous and anisotropic. Considering a multidirectional composite as being constructed of unidirectional layers having specified directions of fiber orientation allows laminated thin-shell theory to be applied to the determination of gross elastic properties of the material. The results of this approach often differ drastically from results obtained by treating reinforced shells as simply being homogeneous and anisotropic. This difference is emphasized through use of numerical examples and experimental data. In particular, it is shown that the modulus of elasticity in bending and the modulus of elasticity under plane deformations can be quite different. The magnitude of this difference is shown to be a function of unidirectional properties, order of stacking of the individual layers, and number of layers.

Role of medial changes in the pathogenesis of the diffuse intimal thickening and in atherogenesis: An approach based upon quantitative histological changes in the media of the ascending aorta

A quantitative histological study in the tunica media of the ascending aorta of 40 patients ranging in age from birth up to 71 years of life was reported. Half of the aortas presented an intimal thickening, focal at the beginning, and the other half presented no such thickening. The tunica media of the former aortas, in comparison with the latter, revealed a decrease in the volume percentage of muscle cells and elastic elements and an increase in the volume percentage of metachromatic substance and collagenous fibers. An index has been established to indicate these changes. The results have reinforced the old idea of T HOMA of the primary role the tunica media plays in atherogenesis.