Fiber Reinforced Epoxy Matrix Composites Research Articles

Compressive failure of unidirectional hybrid fibre-reinforced epoxy composites containing carbon and silicon carbide fibres

The compressive failure of unidirectional hybrid fibre-reinforced epoxy matrix composites containing carbon (C) and silicon carbide (SiC) fibres has been investigated. In contrast to the case of flexural testing previously investigated by the authors, no significant increase in compressive strength, elastic modulus, or work of fracture was noted for the case of composites containing a mixture of C and SiC fibres. The specific compressive strength and elastic modulus generally decreased with increasing SiC fibre content due to the higher density of these fibres. Failure modes of tested specimens were classified into two main groups, namely compressive shear and compressive crushing, with the presence of fibre kinking and longitudinal splitting being noted in both cases.

Application of prompt gamma activation analysis for the determination of water content in composite materials

Prompt gamma activation analysis (PGAA) is a common technique for the identification of hydrogen and the measurement of hydrogen concentrations within a material. Carbon fiber reinforced epoxy matrix composites are materials that are known to absorb water. The experiments described herein demonstrate the feasibility of using PGAA to detect water absorption and desorption and show that PGAA measurements of hydrogen concentration are more sensitive to water absorption than gravimetric measurements. Hydrogen concentration measurements are made using PGAA after calibration with a boric acid solution sample. Relative changes in PGAA measurements generally agree with values expected based on gravimetric measurements.

The effect of interfacial properties on damage evolution in model composites

This study explores the effect of interfacial properties on damage evolution and damage mechanisms in model glass fiber reinforced epoxy matrix composites. The composite properties were varied by changing the interfacial bond between the fiber and the matrix. Double-edge-notched specimens were tested in tension and evaluated using thermoelastic stress analysis (TSA) to observe damage initiation and evolution, and to identify the operative damage mechanism. Changing the interface properties was found to change the operative damage mechanism. This study conclusively demonstrates that the damage mechanism in composites can be controlled by judicious changes in the interface properties. POLYM. COMPOS. 26:241–246, 2005. © 2005 Society of Plastics Engineers.

Fatigue crack propagation and local crack-tip strain behavior in TiNi shape memory fiber reinforced composite

Active enhancement of fatigue resistance of shape memory alloy TiNi fiber reinforced epoxy matrix composite was studied in this paper. Fatigue crack propagation behaviors were investigated by using fine-grid method, which was engraved on the epoxy surface by photo-etching. The effects of prestrain in TiNi fiber on fatigue crack development and suppression in the local domain region around the embedded TiNi fibers were investigated in detail. The retardation of fatigue crack propagation is attributed mainly to the compressive stress in the matrix caused by shrinkage of the pre-strained TiNi fibers above A f temperature. The local crack-retardation at the interface region very near the TiNi fiber is experimentally 0 confirmed by the change of local crack-tip strain behavior. Crack-tip strain hysteresis in one load cycle indicates that the compressive stress induced in the matrix and the following crack closure play key roles to suppress the crack-tip stress intensity and fatigue crack propagation. Fractographic study verified that suppression by shape memory alloy (SMA) fiber improves fatigue fracture toughness.

X-ray diffraction and X-ray photoelectron spectroscopy studies of Ni–P deposited onto carbon fiber surfaces: impact properties of a carbon-fiber-reinforced matrix

In this work, the Ni–P coating on carbon fiber surfaces was carried out in order to improve the impact resistance of carbon fibers–reinforced epoxy matrix composites. The fiber surfaces and the fracture behaviors of composites were measured in terms of X-ray diffraction spectrometry (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM), and falling weight impact testing. From the XRD and XPS measurements, it was observed that Ni–P coating of carbon fibers led to an increase in two phases, i.e., microcrystalline and amorphous, mainly due to the increase of NiP 2, Ni 3P, and Ni metal. Energy adsorbed by composites through the various fracture mechanisms was seen to be the characteristic distinguishing between nontreated and treated fiber-reinforced composite systems. The Ni–P alloy technique to improve the impact resistance of the composites was shown to be the modification of fiber–epoxy resin interfaces.

Reduction ofK I andK II by the shape-memory effect in a TiNi shape-memory fiber-reinforced epoxy matrix composite

Shape-memory TiNi fiber-reinforced/epoxy matrix composites have been fabricated, and the suppression of crack-tip stress intensity and the change in fracture toughness have been systematically investigated. Stress-strain data for these composite specimens with notches at various angles and different crack lengths in the transverse direction have been measured in tensile tests. The stress intensity factor at the crack tip is experimentally determined from photoelastic fringe patterns. The decreases inK values are attributed to the compressive stress field in the matrix induced when the pre-strains of the TiNi fiber contract to their initial length upon heating above the austenitic final temperature. We present the influences of the pre-strain of TiNi fibers and the compressive domain size between a crack tip and fiber on theK value.

Optically transparent continuous glass fibre-reinforced epoxy matrix composite: fabrication, optical and mechanical properties

Unidirectionally aligned continuous glass fibre-reinforced epoxy matrix composites have been fabricated using specially prepared glass fibres with three diameters of 18, 37, and 50 μm. The fibre volume fraction is from 0.25 to 0.45. The microstructure, light transmittance and mechanical properties of the composites are evaluated. Optically transparent composites are successfully fabricated and 60–85% of the light transmittance of the epoxy matrix in a wavelength range from 600 to 1100 nm is obtained. The light transmittance of the composite decrease with increasing the fibre volume fraction and decreasing the fibre diameter, respectively. The longitudinal Young's modulus and tensile strength of the composite increase with an increase of the fibre volume fraction, i.e., the opposite tendency to the light transmittance. Possible range of the combination between the light transmittance and the mechanical performances of the composites is demonstrated.

Effects of stress waveform and water absorption on the fatigue strength of angle-ply aramid fiber/epoxy composites

The influences of stress waveform and water absorption on the tension–tension fatigue fracture behavior were investigated in ±45° angle-ply laminates of aramid fiber reinforced epoxy matrix composite. For dry specimens, the fatigue strength under negative pulse waveform was higher than that under the positive pulse waveform. Rotation of fibers to the longitudinal direction, which resulted from creep deformation caused by the cyclic loading superimposed on the maximum stress hold time, decreased the compliance, thereby increasing the fatigue life under the negative pulse waveform. Water absorption degraded the fiber/matrix interfacial strength and caused the swelling of the matrix, which resulted in decreases in the static tensile strength and fatigue strength. Although the strength under the negative pulse waveform was slightly higher than that under the positive one, the influence of stress waveform on fatigue strength was smaller in wet specimens.

Application of photon emission technique to the determination of micro-fracture behavior in glass fiber-reinforced epoxy matrix composite

A photon emission behavior in a notched plane woven fabric glass fiber-reinforced epoxy matrix composite has been examined through tensile testing of a single-edge-notched composite specimen. Emitted photons during the test were detected and a spectroscopic analysis was also performed on the detected photons to determine the source of the emission. Direct observation of the fracture process of the composite reveals that bundle unit fiber fracture occurs from a notch tip and the fracture accompanies characteristic load drop in the load-displacement curve. Photons are detected about 15–30 ms after the onset of the load drop, which corresponds to the photon emission occurring at the beginning of a bundle unit fiber pullout from the matrix. The emission mechanism is determined to be gaseous ionization discharging of nitrogen molecules, which are contained in ambient air, at a debonded interface. Application of DC potential to the specimen enhances photon intensity and the technique allows photon imaging. Detected photon imaging clearly shows the area of interfacial frictional sliding. It is concluded that the photon emission technique is effective tool to determine interface debonding and sliding behavior in glass fiber-reinforced epoxy matrix composite.

Effect of Fiber–Polymer Interactions on Fracture Toughness Behavior of Carbon Fiber-Reinforced Epoxy Matrix Composites

The effect of anodic oxidation on high-strength polyacrylonitrile-based carbon fibers has been studied in terms of fiber surface energetics and fracture toughness of the composites. According to contact angle measurements based on the wicking rate of a test liquid, anodic oxidation leads to an increase in surface free energy, mainly due to the increase of its specific (or polar) component. For the carbon-fiber-reinforced epoxy resin matrix system, a direct linear relationship is shown between the specific component and the critical stress intensity factor measured by the single edge notched beam fracture toughness test. From a surface-energetic point of view, the anodic treatment may be suitable for carbon fibers incorporated in a polar organic matrix, resulting in an increased specific component of the surface free energy. Good wetting plays an important role in improving the degree of adhesion at interfaces between fibers and matrices of the resulting composites.

Determining the interphase thickness and properties in polymer matrix composites using phase imaging atomic force microscopy and nanoindentation

In polymer matrix composites, the interface between the reinforcing phase and the bulk phase is paramount to the overall performance of the composite as a structural material. This interface is now thought to be a distinct, three-dimensional phase surrounding the reinforcing phase called the interphase. The developments of the atomic force microscope and nanoindentation devices have facilitated the investigation of the interphase. Previously, force modulation atomic force microscopy (AFM) and nanoindentation were the primary methods used to determine the size of the interphase and its stiffness relative to the bulk phase. The present investigation utilized phase imaging AFM and nanoindentation to examine the interphase in a glass fiber-reinforced epoxy matrix composite. Nanoindentation experiments indicated that the relatively stiff fiber might have caused a gradient in the modulus across the interphase region. Specifically, the modulus next to the fiber approached that of the fiber and decreased to that of the bulk polymer as the distance away from the fiber increased. Once the fiber was removed by chemical etching, this gradient reversed itself; hence, nanoindentation, due to the fiber bias, was not found to be adequate for measuring actual interphase properties. It was found that phase imaging AFM was a highly useful tool for probing the interphase, because it involves much lower interaction forces between the probe and the sample than force modulation or nanoindentation. The interphase in the model composite investigated was found to be softer than the bulk phase with a thickness of 2.4-2.9 μm, and was independent of fiber silane pretreatment, for silane pretreatments between 0.1% and 5.0% (initial aqueous concentration).

Effect of fiber strength on tensile fracture of unidirectional long carbon fiber-reinforced epoxy matrix composites

Carbon fiber-reinforced epoxy matrix composites, with a tensile fracture stress of either 3.5 or 5.5GPa (designated as 3.5 and 5.5GPa CF/RE composites, respectively), were studied to determine the effect of fiber strength on the tensile fracture behavior. The smooth and notched tensile specimens were loaded in the direction parallel to the fibers. The 5.5GPa CF/RE composite exhibited a higher smooth tensile fracture stress but a significantly decreased notch strength ratio compared with the 3.5GPa CF/RE composite. For the smooth tensile specimens of the 3.5GPa CF/RE composite, a zigzag fracture approximately perpendicular to the loading direction occurred, and the fracture process involved a brittle fracture, or pull out, of the fibers; whereas, for the 5.5GPa CF/RE composite, the fracture was approximately parallel to the loading direction, and fiber-matrix interfacial fracture was observed. For the notch tensile tests, fracture occurred at the fiber-matrix interfaces independent of the composite type. The results are described and discussed.

Curing of carbon fibre reinforced epoxy matrix composites

The curing kinetics and viscosity of two carbon fibre reinforced epoxy systems, Fibredux 914C and Fiberite HYE 1248M2F, were measured using differential scanning calorimetry (DSC) and torsion rheometry. Fibredux 914C is a high temperature (175°C) cure system and Fiberite is a low temperature (120°C) cure system. Values of kinetic parameters were obtained from dynamic and isothermal DSC scans using an nth order reaction model. Viscosity measurements as a function of time were made for both systems and fitted to an expression relating viscosity to degree of cure.

The interface region in glass fibre-reinforced epoxy resin composites: 2. Water absorption, voids and the interface

This paper deals with the influence of voids, the fibre surface coating and the nature of the fibre-matrix interface on water absorption in glass fibre-reinforced epoxy matrix composites. We have found that the rate of water uptake and the maximum level of water absorption in E-glass fibre-reinforced Epikote

The interface region in glass fibre-reinforced epoxy resin composites: 1. Sample preparation, void content and interfacial strength

This paper deals with the influence of the fibre-matrix interface on the properties of glass fibre-reinforced epoxy matrix composites. Interfacial strength, as measured by the interlaminar shear strength, in E-glass fibre-reinforced Epikote

The Effects of Fiber Form on the Mechanical Properties of Glass Fiber Reinforced Composites

Tests were conducted with continuous glass fiber reinforced epoxy matrix composites that had different types of fiber reinforcements. Composites reinforced with either unidirectional, triaxial or random fibers were tested. The tensile and compressive strengths and moduli, the Iosipescu shear moduli, the short beam shear strengths, the mode I critical strain energy release rates, and the damage introduced by impact were measured. The results demonstrate the influence of fiber form on these properties and provide design guides for the use of different fiber forms.

Evaluation of Delamination Induced by Low-Velocity Impact by Scanning Acoustic Microscope and Influence of Water Absorption on Delamination and Compression after Impact of CFRP.

An investigation of water absorption on delamination by low-velocity impact loading, and compression strength and delamination growth behavior after impact by fatigue loading was carried out on quasi-isotropic laminated ([0°/±45°/90°]2s) carbon fiber reinforced epoxy matrix composite. Ply-to-ply delamination was evaluated by using a scanning acoustic microscope. Interlaminar strength was lowered by water absorption, and the impact-load-induced delamination area was increased by water absorption. Compressive strength after impact was evaluated as the sum of delaminated areas of respective ply-to-ply delamination ; when the delaminated area was large, compressive strength after impact was solely determined by the delaminated area, and no influence of water absorption was observed ; however, when the delaminated area was small, compressive strength after impact of the water absorbed specimen decreased. The fracture surface, in particular, topographic features of hackle patterns, was closely examined by a scanning electron microscope, and the fracture mechanism of compressive and fatigue failure mechanism were discussed

Cryogenic Temperature and Water Absorption Effects on Mechanical Properties and Fatigue Strength of Carbon Fiber Reinforced Plastics.

In this paper, we describe the effects of cryogenic temperature at 77 K and water absorption on the mechanical properties and fatigue strength of intermediate-temperature cured carbon fiber reinforced epoxy matrix composites. Tensile and tension-tension fatigue tests were conducted on both dry and wet specimens, which had been preconditioned in laboratory air and in water for two months, respectively. The test results showed that i) an increase in the strength of the fiber/matrix interface and matrix material and in the elastic modulus of the matrix at cryogenic temperature, ii) a decrease in interlaminar strength at cryogenic temperature, and iii) a decrease in fiber/matrix interfacial and interlaminar strength by water absorption severely influenced the mechanical and fatigue fracture behavior of the composites. Internal damage was non-destructively examined with a scanning acoustic microscope, the fracture surfaces were closely examined with a scanning electron microscope, and the influence of cryogenic temperature and water absorption on fracture mechanisms was discussed.

Moisture Absorption Characteristics of Aramid-Epoxy Composites

Tests were conducted measuring the changes in weight of aramid fiber reinforced epoxy matrix composites during exposure to humid air. Composites containing surface treated fibers and composites containing fibers without surface treatment were tested. In addition, for the purpose of comparison, the weight changes of glass fiber reinforced epoxy com posites and pure epoxy were measured. From the weight change data the values of the maximum moisture contents and diffusivities were determined. The effects of fiber surface treatment on the moisture absorption characteristics of the aramid fiber reinforced com posite were assessed. An expression was developed for estimating the transverse diffu sivities of composites from the known resin and fiber properties when fibers absorb a sig nificant amount of moisture.

Effects of Moisture on the Compressive and Interlaminar Shear Strengths of Aramid-Epoxy Composites

The effects of moisture on the compressive and interlaminar shear strengths of aramid fiber reinforced epoxy matrix composites were investigated. The compressive strengths were measured with a Celanese fixture, the interlaminar shear strengths were inferred from the results of three point bend tests. The influence of fiber surface treatment on the compressive and interlaminar shear strengths were also assessed by testing composites containing fibers with their surfaces treated and with fibers without surface treatment.