Graphite-epoxy Composites Research Articles

Delamination resistance behaviour in unidirectional graphite-epoxy composites

According to the energy criterion for fracture, crack growth occurs when the crack driving force or the energy release rate is at least equal to the crack resistance which is de®ned as the energy consumed in crack propagation. It is well known that, for a cracked metal structure under the plane-strain loading condition, the crack resistance is independent of the crack length. In the case of plane stress, the crack resistance varies with the crack increment. Although a considerable amount of work has been performed to characterize the resistance curve for isotropic materials [1±3], the application of the resistance curve to composites has only recently been performed. Ratwani and Deo [4] applied the resistance curve approach to characterize the delamination growth of AS4=3501-6 composites under mode I loading. They used double-cantilever-beam (DCB) specimens and obtained the delamination growth resistance curve over a wide range of delamination extensions. They found that the resistance increased monotonically with increasing delamination extension, and the delamination growth resistance curve was path independent. Mai and Hakeem [5] noted a resistance-type fracture behaviour of cellulose ®bre cements. They used grooved DCB specimens and observed that the resistance increased monotonically with increasing crack length to a maximum and then dropped to a steady-state value. Foote et al. [6] developed a theoretical model for the crack growth resistance (KR) curve for a strain-softening material with a power-law stress± displacement relationship. They applied the model to a cellulose±asbestos-®bre-reinforced motar and obtained comparable results with the experimental data. Currently, the delamination resistance curve plays an important role in characterizing the delamination growth for unidirectional laminated composites particularly under mode I loading. However, the delamination resistance behaviour of graphite±epoxy composites is not fully characterized at present. In the present work, delamination resistance curves of unidirectional graphite±epoxy laminated composites were constructed as a function of delamination increment, and the effect of initial delamination length on the resistance curve was investigated. DCB specimens made by using graphite±epoxy prepregs (CU125NS) manufactured by Hankuk Fiber Group were used for the resistance curve tests. The DCB specimen was 08 unidirectional and 24 plies thick (3.8 mm) in which delamination was located at the midplane. Initial delaminations applied were 50 and 90 mm, and the specimen length was 230 mm. The edge of the specimen was painted with a white typewriter correction uid and was marked regularly in order to measure the delamination increments. The delamination length was measured using the travelling microscope. Aluminium blocks were adhesively bonded to the ends of the specimen arms to provide the loading points. The experiments were conducted under room-temperature dry environmental conditions, and a displacement-controlled loading of rate 3.5 mm miny1 was applied. After the initial delamination increased by a certain amount, the applied displacement was reduced and then increased to cause delamination growth. This process was repeated at least ten times until the delamination increased to a length of 170 mm. For each specimen, curves of load, P, against displacement, a, during slow delamination growth were obtained where the delamination length was marked against each loading and unloading line. A typical loading±unloading P±a diagram of initial delamination length 50 mm is shown in Fig. 1. As shown in the ®gure, the fracture behaviour is linear elastic. However, it was found that unloading curves do not return to zero displacement although the amount of residual displacement is very small. The reason is that ®bre bridging occurred behind the delamination tip during delamination growth, which prevents the delamination surfaces from closing up

Performance Limits for Stiffness-Critical Graphitic Foam Structures. Part I: Comparisons with High-Modulus Foams, Refractory Alloys and Graphite-Epoxy Composites

Graphitic foams offer exciting potential for mass savings in structures. The present investigation seeks to identify the potential mass savings and corresponding dimensional tradeoffs for application of such materials to simple, stiffness-critical structural elements, in support of an in-house materials development program. A hypothetical family of graphitic foams having ligament stiffness properties identical to those of P120 graphite fiber is considered; the stiffness properties of the hypothetical foams are viewed as realistic upper bounds for the performance of this class of materials. A representative refractory alloy, AS4/3501-6 unidirectional and [0/90], composites, and commercially available SiC and amorphous carbon foams are compared to the graphitic foams for application to plates in bending and buckling, beams in bending and buckling, and tension bars, in analogy to Gordon [1] and Ashby [2,3]. The mass and thickness ratios of the foams to the comparison materials for the chosen applications depend primarily on the Young's modulus of the foam, which is modeled according to a well-established relation due to Gibson and Ashby [4]. The hypothetical foams are found to offer stiffness properties leading to structural masses ranging from competitive to dramatically improved versus the described comparison materials. Increased structural dimensions are often associated with the savings in mass. Strength properties have not been considered in the present modeling, as processing issues still need to be resolved and the resulting defect distributions are unknown; foams of substantial promise are, however, being fabricated in small quantities.

Wave propagation in graphite epoxy composites : Mal, A.K.; Lih, S.-S. Enhancing Analysis Techniques for Composite Materials, Atlanta, Georgia (United States), 1–6 Dec. 1991. pp. 89–102. Edited by L. Schwer, J.N. Reddy and A. Mal. NDE-Vol. 10. ASME (1991)

Wave propagation in graphite epoxy composites : Mal, A.K.; Lih, S.-S. Enhancing Analysis Techniques for Composite Materials, Atlanta, Georgia (United States), 1–6 Dec. 1991. pp. 89–102. Edited by L. Schwer, J.N. Reddy and A. Mal. NDE-Vol. 10. ASME (1991)

Environmental Durability of F/A-18 Graphite/Epoxy Composite

An environmental durability trial is being conducted on F/A-18 graphite epoxy composites at five tropical test sites in Australia and Malaysia. Two types of specimen have been exposed in the external tropical environment for periods of up to 9 years and measurements have been made of moisture absorption levels, fatigue durability and residual strength. The maximum level of moisture absorption recorded has been 1.0 wt%. Sandwich beam specimens representative of an aircraft structure, containing different types of damage or simulated repairs, have withstood over 106 fatigue cycles at close to nominal design limit load in the tropical environment. This result indicates that F/A-18 composite structure should be very durable under long term tropical conditions.

Environmental Durability of F/A-18 Graphite/Epoxy Composite

An environmental durability trial is being conducted on F/A-18 graphite epoxy composites at five tropical test sites in Australia and Malaysia. Two types of specimen have been exposed in the external tropical environment for periods of up to 9 years and measurements have been made of moisture absorption levels, fatigue durability and residual strength. The maximum level of moisture absorption recorded has been 1.0 wt%. Sandwich beam specimens representative of an aircraft structure, containing different types of damage or simulated repairs, have withstood over 106 fatigue cycles at close to nominal design limit load in the tropical environment. This result indicates that F/A-18 composite structure should be very durable under long term tropical conditions.

Theoretical and Experimental Investigation of Failure and Damage Progression of Graphite-Epoxy Composites in Flexural Bending Test

Predicting failure stress and failure modes in composite laminates is very difficult. The choice between failure criteria is complex and there is a lack of experimental study to validate the results obtained. In this paper, a theoretical and experimental study of damage progression and failure modes of graphite-epoxy laminates in three points bending tests is presented. A quasi-isotropic [(± 45/9010)]5 graphite-epoxy composite is investigated. C-scan method and microscopic sectioning permit to monitor damage progression and failure modes during the experiment. Specimens at different failure levels are used to determine damage progression and the effect of geometrical parameters on the successive failures and on failure modes is studied. The progression of damage has been followed experimentally and identified in detail. The theoretical study is based on the classical laminate theory in the case of in-plane loads. A software program has been elaborated for post-failure treatment and experimental results are compared with numerical predictions.

Automated analytical biosystem for urea monitoring

The present work describes the construction and automation of a flow injection (FI) system for on-line monitoring of urea. The FI system comprises a nylon column on the wall of which the enzyme urease is covalently immobilized. Urea is hydrolyzed to ammonium and hydrogen carbonate ions in the presence of urease. The final measurement is done by an ammonium ion sensitive electrode. This tubular shaped electrode is a potentiometric device that features a solid internal contact made of a graphite-epoxy composite. Interferents are separated from the analyte by means of a gas diffusion cell. The FI system is automated using electronic units that sample, electrically isolate, amplify and process the electrode signal. This hardware also controls the movements of the injection valve. The entire system is controlled by a computer program that governs the analytical process from sampling to signal processing. The urea analyzer was optimized following two approaches: firstly, signal and sensitivity maximization keeping the dispersion small, and secondly optimization when working with haemodialysis effluents. The first approach produced sensitivities ranging from 54 to 56 mV/decade and linear ranges from 3 × 10 −5 to 3 × 10 −3 M. The second approach produced 50–52mV/decade sensitivities and linear ranges between 1 × 10 −3 and 2 × 10 −2 M. The lifetime of the continuously working reactor was more than 30 days. The analyzer was applied to urea monitoring in haemodialysis processes both discretely and continuously, without any significant differences at 95% confidence levels when compared with classical measuring methods.

Longitudinal and Transverse Moduli and Strengths of Low Angle 3-D Braided Composites

This paper presents the results of an experimental study conducted on a class of three-dimensional braided graphite-epoxy composites as well as the results of a modeling study to relate the microstructural variables such as volume fraction of fibers and braid angles to the anisotropic elastic moduli and strengths of these composites. In the experimental program, composite samples with a range of volume fractions (0.2-0.45) and a range of braid angles (0°-30°) were produced and tested in simple compression in both longitudinal and transverse directions. Such data is currently unavailable in literature on a single material system. Extreme care was taken in processing to minimize porosity and non-homogenieties in microstructure, and in proper strain gaging to ensure the reliability and reproducibility of our measurements. In the modeling study, the measured elastic moduli were compared against the currently employed isotrain and isostress models for these materials. At the present time, it is widely reported in literature that the predictions of isostrain model are fairly accurate. This study revealed that the isostrain model predictions can be in significant error and that a weighted average of the isostrain and isostress model predictions yielded the best comparisons against the experiments. For the strength of the composite in the longitudinal and transverse directions, correlations expressing these properties as a function of volume fraction and braid angle were motivated by micromechanics and experimental observations. The predictive capability of these correlations for the selected class of 3-D braided composites was found to be reasonable.

Quantitative non-destructive characterization of damage in graphite-epoxy composites

We investigated the effect of controlled damage on the residual strength, failure mechanism and acoustic emission (AE) activity of graphite epoxy composites. The controlled damage was introduced by indenting unidirectional tensile specimens using a hardness machine with a diamond indentor. The specimens were loaded parallel to the fiber direction. The damage progression mechanisms operating upon subsequent loading and the failure modes were found to be different for sound and indented specimens. The difference in mechanical behavior affected also the AE, as displayed by the statistical moments of the peak amplitude distribution. Specifically, the variation of moment values with stress level were found to be different for the two groups. This difference can be used to sort sound from defective material during loading. In addition, this approach can in future be used as a means to ratify the soundness of structures, if a reliable database can be generated.

Nondestructive Inspection of Graphite-Epoxy Composites for Heat Damage Using Laser-Induced Fluorescence

Laser-induced fluorescence has been applied to the nondestructive inspection of graphite-fiber-reinforced polymer-matrix composites. Upon curing, the epoxy resin in these materials forms a highly cross-linked network of aromatic sub-units. This result yields a material with significant absorptivity from the ultraviolet through the near-infrared and a fluorescence bandwidth covering 200 to 300 nm. When subjected to temperatures beyond the recommended upper service temperature, these materials experience thermally induced degradation and a reduction in mechanical strength. Furthermore, these heat-damaged materials exhibit significant changes in fluorescence intensity and in the wavelength of maximum fluorescence intensity. These spectroscopic changes can be correlated with the extent of heat damage. An imaging approach has been developed to monitor these fluorescence signatures, allowing rapid nondestructive inspection of large surface areas. This capability provides qualitative and quantitative information about the location and severity of damage.

Playing with the Anisotropy of the Matrix and Fiber for the Improvement of the Strength of Composites

The paper deals with the theory of the coupling of strongly anisotropic fibers along their axis with anisotropic matrices along either the fiber direction or the transverse plane to the direction of the fibers. It is shown that the first arrangement deteriorates, whereas the second improves significantly the mechanical behavior of the composites. Indeed, it is shown that anisotropy of the matrix, increasing its mechanical properties on the transverse isotropic plane of the composite, increases also the transverse-transverse Poisson's ratio, v23, whereas decreases the longitudinal shear modulus, G,2. This results in values of the eigenangle wx approaching the corresponding value Wic for the equivalent isotropic material and caused a deterioration of the strength and toughness of the composite, since the material now has the tendency to develop higher stress concentrations for equivalent loadings [1,2]. On the contrary, a strong anisotropic matrix along the direction of the fibers yields the inverse results for the respective moduli of the anisotropic composite. Finally, the most important result is a perceptible decrease of the ratio E,1/2G,2 which yields values of the eigenangle wc tending to approach the critical value wi for the isotropic material. The decrease of wc indicates the improvement of the quality of the composite, since it develops relatively lower stress concentration factors, which approach their respective isotropic values [2]. Thus, the anisotropic composite material is approaching an equivalent state of quasi-isotropy and therefore improves its strength by reducing considerably the eventual anisotropic stress concentration factors of the respective structural elements [1]. Examples with T300/N5208 graphite-epoxy composites and Borsic-1100 aluminum metal-metal composites indicate clearly the beneficial effect of the anisotropy of their matrices.

Toughened graphite-epoxy composites exposed in near-Earth orbit for 5.8 years

This experiment was designed to measure the effect of near-Earth space exposure on three mechanical properties of specially toughened unidirectional T300/5208 graphite-epoxy cross-ply composite materials. The properties measured are fracture toughness, elastic modulus, and strength. Six toughness specimens and nine tensile specimens were mounted on an external frame during the 5.8-y orbit of the Long Duration Exposure Facility. Three identical sets of specimens were manufactured at the outset: the flight set, a zero-time nonflight set, and a total-time nonflight set. Results indicate that with proper protection from solar exposure and atomic-oxygen degradation, toughened graphite-epoxy composites can provide useful structural service in near-Earth orbit for at least five years.

A study on the influence of fiber-matrix interphase properties on ultrasonic wave behavior using effective elastic property models

In this paper, a three-phase (fiber-interphase-matrix unit cell) micro-mechanics model (Gardner et al. (1993), Comp. Sci. Technol. 46, 307–318) is used to study the influence of fiber-matrix interphase properties on the overall effective elastic constants. Then, the behavior of measurable ultrasonic parameters such as velocities of the three ultrasonic bulk wave modes, and their reflection factor (as a function of wave orientation and frequency) is discussed as a function of interphase properties. Theoretical results are compared with experimental studies on both glass-epoxy and graphite-epoxy composites.

Closed-form solutions of the in-plane effective thermal conductivities of woven-fabric composites

A micromechanics model is developed to predict the in-plane effective thermal conductivities of plain-weave fabric composites based on a thermal-electrical analogy. Closed-form solutions of the effective thermal conductivities in the warp and fill directions are presented. In order to affirm the applicability of these solutions, the in-plane effective thermal conductivities of S-glass, E-glass, graphite, and Kevlar-49 plainweave fabric-reinforced epoxy composites are predicted. Excellent agreement is found between the present predictions and published analytical, numerical, and experimental results.

Fermentation monitoring using a glucose biosensor based on an electrocatalytically bulk-modified epoxy–graphite biocomposite integrated in a flow system

Glucose oxidase (GOD) and metals for the electrocatalytic oxidation of hydrogen peroxide were incorporated in the bulk of a graphite–epoxy composite. An amperometric glucose biosensor based on this biocomposite material made of graphite–epoxy–Au–Pd–GOD was integrated in a flow injection system and used for discrete glucose monitoring of a bioprocess. Stability, reproducibility and flow parameters related to the flow injection system were studied. The analytical system showed a linear response for glucose in the range 10–5–10–2 mol l–1 when a potential of 900 mV was applied with respect to an Ag/AgCl electrode. A solution of 0.1 mol l–1 phosphate and 0.1 mol l–1 KCl buffered to pH 7 was used as the carrier solution. The reproducibility of the signal was below 5% and the sampling frequency was about 20 samples h–1. For the final application, the linear working response range was adapted from 1 to 10 g l–1 of glucose. The results obtained were compared with those obtained with a commercially available glucose sensor and by HPLC.

Analysis of double cantilever beam specimen

A double cantilever beam (DCB) specimen is analyzed by utilizing a Timoshenko beam supported by an elastic foundation with an extensional stiffness. The deflections and stresses of the DCB specimen are compared with the results based on the other beam models as well as the potential energy release rates. It is found that the present results for graphite-epoxy composites are as accurate as those by Whitney using a higher order beam theory.

Heterogeneous Material Mechanics at Various Scales

Some contemporary problems involving the mechanics of materials will be examined at widely different length scales. The three scales chosen are i) the near atomic scale, ii) an intermediate range of the order of microns, and iii) large scale macro or structural behavior. An additional theme transcending these scales will be carbon based, or carbon like heterogeneous material behavior. At the near atomic scale, examples are chosen from the various forms that elemental carbon can take including diamond, graphite, newly discovered fullerenes, and ideal carbon fibers. Examples at the intermediate length scale feature low density materials, since typical cell sizes are about a micron or somewhat larger or smaller. The effect of microstructure becomes the dominant and determining characteristic of such materials. A hierarchy of microstructures will be revealed which gives a coherent connection between microstructure and mechanical behavior. Finally, at the macroscale, involving structural behavior, a methodology for the life prediction of fiber dominated, graphite epoxy composites will be given. These three classes of problems are intended to show that virtually equal opportunities for meaningful research contribution occur across the spectrum of length scales.

Microradiographic strain measurement using markers

A method for measuring strain on interior planes of some real structural materials is presented. X-ray images are formed of small (10–40 micron) gold markers placed on selected interior planes of optically opaque X-ray transparent materials. The use of well collimated monochromatic synchrotron radiation makes possible high contrast images of the small markers. Images of the particles before and after straining are recorded photographically. Photographs are enlarged 33X and measured using a simple electro-optical setup. In calibration experiments using approximately a 300-micron gage length, the strain measured by this method agreed with extensometer measured strains to within 100 microstrain. Example applications in a graphite-epoxy composite are presented, including measurement of the strain drop off near the free edge, strain concentration around a hole, and the strain field on a particular plane near a hole after local delamination. The technique is currently limited to materials no less X-ray transparent than titanium.

Delamination Growth in Composites under Cyclic Loads

A semi-empirical model was developed for estimating the growth of delaminations inside fiber reinforced organic matrix composites subjected to cyclic loads. At any point on the circumference of the delamination mode I, mode II or mixed mode conditions may exist. The form of the proposed delamination growth model follows Elber's expression for the growth of a mode I crack in an aluminum sheet. By employing dimensional analysis, the parameters were established which are to be included in the model. The expression obtained depends on the material properties (transverse tensile ply strength, transverse tensile ply modulus, longitudinal ply shear strength, critical energy release rates) and on the energy release rate at the location under consideration. Tests were conducted with Fiberite T300/976 and IM7/977-2 graphite epoxy composites under mode I, mode II and mixed mode conditions using double cantilever beam, end notched cantilever beam, and mixed mode bending test coupons subjected to both static and cyclic loads. From these tests, first, the constants needed in the model were determined. Second, data were generated and compared to predictions of the model, and good agreement was found between the measured and predicted delamination growths. Results of the present model were also compared to data reported previously, and the model predictions agreed well with these previous data.

The effects of sorbed moisture on resin-matrix composites

The effects of water absorption have been investigated on the diffusion behavior, moisture-induced expansion, and fracture properties of thermoset and thermoplastic resin-matrix composites. The composite materials were submerged in distilled water at temperatures from 45°C to 90°C for more than 8,000 h. Moisture saturation levels were considerably higher in the thermoset composites than in the thermoplastic composite. Moisture sorption in the graphite/epoxy composite exhibited both classical Fickian and non-Fickian diffusion behavior. Deviant, non-Fickian behavior was related to chemical modification and physical damage in the composite. Moisture sorption in thermoplastic resin composites follows Fickian diffusion rather closely. Anomalous moisture expansion behavior in graphite-epoxy composites was attributed to environmental-induced damage of the laminate. Sorbed moisture reduced the delamination fracture toughness (DFT) of a thermoset composite, whereas moisture sorption had virtually no effect on the DFT of a thermoplastic composite.