Carbon Fiber Polymer Matrix Composites Research Articles

The Role of the External and Internal Reinforcing on the Structural Integrity of Industrial and Transporting Steel Pipelines

The lifetime management of different engineering structures and structural elements is one of the important technical-economic problems nowadays. On the one hand, the aim of our research work is to develop an integrity management plan for pipelines and pipeline systems, and afterwards a Pipeline Integrity Management System. Material databases play important role both on the integrity management and on the engineering critical assessment of the pipeline systems. On the other hand, the aim of our research work is to establish the Pipeline Integrity Management System with different data, frequently with experimental data. The direct purpose of the paper is to present the role of the external and internal reinforcing on the structural integrity of industrial and transporting steel pipelines, based on own examinations. External and internal reinforcement was developed using carbon fibre and glass fibre polymer matrix composites, respectively. Fatigue and burst tests were performed on pipeline sections containing natural and artificial metal loss defects, and girth welds including weld defects. Both unreinforced and reinforced pipeline sections were examined. The burst pressures belonging to the unreinforced and the reinforced pipelines, and belonging to the passed and not passed girth welds were compared.

On the electrical resistance of carbon fiber polymer matrix composites

This paper studies the response of carbon fiber polymer matrix composites subjected to DC electric currents. A new fully automated experimental setup that enables one to measure electric field characteristics (current, voltage, resistance) and temperature at the surface of electrified composites in real time has been developed. The experimental procedure ensured a low contact resistance between the composite and electrodes and low resistance heating. An extensive experimental study on the electrical characterization of AS4/3501-6 and IM7/977-2 carbon fiber polymer composites of different lay-up and thickness has been conducted. The effect of the resistive heating was studied using experimental analysis as well as the finite element modeling.

Use of the mar-lin criteria to determine the influence of porosity on the iosipescu and short beam shear properties in carbon fiber polymer matrix composites

To address a critical aspect of the fast growing use of composites in aircraft and aerospace industry, the influence of the porosity on the shear strength of composites property was investigated as a mean for determining the critical values of porosity. Acid digestion techniques were applied to determine the void volume ratio of two families of carbon epoxy laminates (8 and 16 plies). Ultrasonic inspections revealed the corresponding attenuation coefficients. The void morphology was investigated by optical microscopy. Results from Interlaminar shear and Iosipescu shear tests were correlated with the attenuation coefficient to determine critical values of porosity using a modified Mar-Lin fracture criteria. It has been shown that the shear strength decrease with the increase of void volume ratio and the effects are more significant in thicker laminates. This work showed that by using the Mar-Lin criteria the singularity order, which is an indicative of the sensibility to voids in composites, is dependent of type of loading and void distribution.

Compression Response of 2D Braided Textile Composites: Single Cell and Multiple Cell Micromechanics Based Strength Predictions

This article is concerned with the development of a finite element (FE) based micromechanics model for the prediction of compressive strength and post-peak compression response of 2D triaxial braided carbon fiber polymer matrix composites (2DTBC). This micromechanics based study was carried out on a series of single and multiple representative unit cell (RUC) 3-D FE models. The uniaxial compressive response, including unstable equilibrium paths, was studied using an arc-length method in conjunction with the ABAQUS commercial FE code. In the reported study, explicit account of the braid microstructure (geometry and packing) and the measured inelastic properties of the matrix (the in-situ properties) are accounted for via the use of the FE method. This enables accounting for the different length scales that are present in a 2DTBC. The computational model provides a means to assess the compressive response of 2DTBC and its dependence on various microstructural parameters. The model provides a means to compute the compression strength allowable for a 2DTBC structure. In particular, the dependence of compressive strength on the axial fiber tow properties and axial tow geometrical imperfections is discussed and shown to be significant in capturing the mechanism of damage development. Results are presented for 1, 4, 9, and 16 RUC representations of the 2DTBC, enabling to examine the dependence of compressive strength (or lack thereof) on the size of the region that is modeled. The predicted results are found to compare favorably against experiment.

A statistical model of electrical resistance of carbon fiber reinforced composites under tensile loading

The electrical resistance of carbon fiber polymer–matrix composites under tensile loading was measured using both the two-probe and four-probe techniques. Unlike most existing tests, which show an exponential or power relation between the electrical resistance and the tensile strain due to fiber breakage and/or electrical percolation under tensile loading; our results exhibit a more complex response. When the strain is increasing, the electrical resistance increases with a decreasing rate at the beginning, then decreases, and eventually increases exponentially up to the breaking point. To explain this interesting phenomenon, a statistical model based on Weibull distribution function is introduced. Three stages of the electrical resistance versus tensile strain curve have been identified. At the primary and secondary stages, the conductivity of the composite follows a Weibull statistical distribution function. This is mainly due to the increasing number of contact points of the fibers during loading. At the tertiary stage the usual exponential function is obtained, which is controlled by fiber breakage. The results obtained from the theoretical model agree well with the experiment.

Enhancing the thermal conductivity and compressive modulus of carbon fiber polymer–matrix composites in the through-thickness direction by nanostructuring the interlaminar interface with carbon black

Heat dissipation from aircraft is important. Carbon fiber polymer–matrix structural composites have high in-plane thermal conductivity, but low through-thickness conductivity. A nanostructuring method involving carbon black at the interlaminar interface was developed to improve the through-thickness conductivity. Ethylene glycol monoethyl ether (EGME) was used for dispersing the carbon black and to partially dissolution of the epoxy resin on the fiber-epoxy prepreg surface. EGME evaporated from the prepreg surface prior to composite fabrication. The optimum carbon black content in EGME for attaining high through-thickness conductivity was 0.8 wt.% for both unidirectional and crossply configurations. Applying EGME without carbon black improved the conductivity by up to 36%, but in the case with carbon black, the improvement was up to 210%. For the same interlaminar interface modification (except for EGME with 1.2 wt.% carbon black), the conductivity and its fractional increase were higher for the crossply configuration than the corresponding unidirectional configuration. The through-thickness compressive modulus and the flexural modulus were increased by up to 14% and 11%, respectively by using EGME with carbon black. The average thickness of the interlaminar interface increased with increasing carbon black content, but it was decreased by the use of EGME alone.

On the impact response of electrified carbon fiber polymer matrix composites: Effects of electric current intensity and duration

In this work, an investigation of the effects of an electric current on the low velocity impact response of carbon fiber polymer matrix unidirectional and cross-ply composites carrying an electric current is presented. Preliminary experimental results show that a short-term application of the DC electric current leads to an increase in the impact resistance of composites, whereas a prolonged application of the current induces a significant heating in the electrified composites and has rather a detrimental effect. The analysis focuses on elucidating the roles of current intensity, duration, and associated current-induced heating in alteration of the composites response. The contributions of Joule heat and contact resistance heat are particularly highlighted.

Damage detection using self-sensing concepts

Self-sensing refers to the structural material sensing itself. Real-time self-sensing of damage in carbon fibre polymer-matrix composites by electrical resistance measurement is reviewed. The resistance changes irreversibly upon damage, as shown for damage inflicted by flexure, tension, fatigue, and impact. Delamination increases the through-thickness resistance. Fibre breakage increases the longitudinal resistance. The oblique resistance, as measured at an angle between the longitudinal and through-thickness directions, is particularly sensitive. Minor flexural damage causes the oblique resistance in the unloaded state to decrease. Current spreading enables the sensing of localized damage by measurement away from the damage, though it reduces the spatial resolution of the sensing. The resistance method is more sensitive than the potential method. Two-dimensional sensing is complicated by the anisotropic spreading of the current. Thermal damage and through-thickness (fastening) compression effect are indicated by the contact resistivity of the interlaminar interface. The through-thickness compression effect is alternately indicated by the longitudinal volume resistivity. The condition of a composite fastening joint is indicated by the contact resistivity of the joint interface.

Modeling and analysis of the electrical resistance measurement of carbon fiber polymer–matrix composites

The self-sensing damage detection method based on the electrical resistance measurement of carbon fiber polymer–matrix composites has been investigated for a decade. In order to eliminate the effect of contact resistance when using the two-probe method, the four-probe methods, which include the resistance, potential and voltage change methods, were proposed in literature. However, the basic problems involved in the four-probe methods remain unclear, i.e., the validity range and the applicability of the four-probe methods. In this paper, beam-type specimens with and without delamination damage are used to carry out numerical analyses for the above-mentioned problems. It is found that the four-probe resistance method is valid only when the through-thickness conductivity is comparable to or larger than the longitudinal conductivity. For the potential method, which measures directly the voltage values between the voltage contacts, the present results show that the percentage change in damage-induced voltage between a pair of voltage contacts is not consistent with the percentage change in resistance. The underlying reason is that the damage-induced voltage change depends on the location of the applied current, while the resistance change does not.

Comparison of the Electrical Resistance and Potential Techniques for the Self-sensing of Damage in Carbon Fiber Polymer-Matrix Composites

The electric potential method, which involves measuring the potential at a distance from the line of current application, is effective for damage sensing of carbon fiber polymer-matrix composites when the distance is sufficiently small, such as 1.0 mm (8 laminae) in the through-thickness direction. It is ineffective when this distance is 2.1 mm (16 laminae). The electrical resistance method, which involves measuring the potential on the line of current application, has no thickness limitation. However, it suffers from current path distortion upon damage and the consequent reduced sensitivity for damage. A minor phenomenon of the measured resistance decreasing with increasing damage is observed for the regions of the composite not containing the point of impact. The major phenomenon of the resistance increasing with damage overshadows this minor phenomenon for the region containing the point of impact.

Comparative evaluation of the electrical configurations for the two-dimensionalelectric potential method of damage monitoring in carbon fiber polymer–matrixcomposite

The effectiveness of the two-dimensional electric potential method of damage sensing in aquasi-isotropic carbon fiber polymer–matrix composite depends on the electricalconfiguration, i.e., the current direction relative to the surface fibers and theelectrical contact scheme. Oblique current application in any direction provideseffective damage sensing, as shown by using electrical contacts on the oppositein-plane surfaces. In-plane current application through the cross section in anydirection also provides effective damage sensing, as shown by using electricalcontacts that are either on the edge surfaces or in holes through the composite.In-plane surface current application is effective when the current is perpendicular tothe surface fibers (due to the low resistivity in the direction of the fibers) andis ineffective when the current is parallel to the surface fibers (due to the highresistivity in the direction perpendicular to the fibers). The oblique configuration isrecommended for practical implementation. In general, the potential method is reliablewhen (i) the resistance between the electric current line and the nearly parallelelectric potential gradient line is sufficiently low, as attained when these lines aresufficiently close, and (ii) the resistance between the current line and the damagelocation is sufficiently low, as attained when the distance of separation is sufficientlysmall.

Sensitivity of the two-dimensional electric potential/resistance method for damage monitoring in carbon fiber polymer-matrix composite

The two-dimensional electric potential/resistance method is much less sensitive than the one-dimensional resistance method for damage monitoring in carbon fiber polymer-matrix composite. In the two-dimensional method, the resistance measurement is more sensitive than the potential gradient measurement. The sensitivity of the potential method is enhanced when the potential gradient line is close to the current line.

Effect of through-thickness compression on the microstructure of carbon fiber polymer-matrix composites, as studied by electrical resistance measurement

Compression in the through-thickness direction (as in fastening) resulted in reversible and irreversible changes in the microstructure of continuous carbon fiber epoxy-matrix composites, as shown by electrical resistance measurement during dynamic compression. The extent of fiber-fiber contact across the interlaminar interface was increased, with partial irreversibility even at a low stress amplitude of 1 MPa. Within a lamina, fiber squeezing in the through-thickness direction and fiber spreading in the transverse direction occurred upon fastening compression, with partial irreversibility at a stress amplitude of 100 MPa or above. For a single laminae beyond 400 MPa, the lessening of fiber squeezing in the through-thickness direction during unloading dominated over the fiber spreading in the transverse direction during loading.

Limiting the permeability of composites for cryogenic applications

The correlation between thermal fatigue-generated networks of transverse micro-cracks and the permeability of carbon fiber polymer matrix composites (PMCs) was the focal point of this effort. Epoxy and bismaleimide matrix PMCs were cycled between liquid nitrogen temperature (−196 °C) and elevated temperature. The samples were then optically examined for micro-cracks and evaluated for their tendency for leakage in a fluid containment application. Composite coupons were tested for the rate of flow of low-pressure helium gas in the through-thickness direction at room temperature (RT) and liquid nitrogen (LN 2) temperature as a function of the applied cycles. The choice of lay-up drastically influenced the permeability. Samples with [0/90] 2S and [0/90/45/−45] S lay-ups resisted leakage at least 100 cycles more than the [0/45/−45/90] S samples. The temperature of the elevated portion of the thermal cycle also had a large impact on the initiation and propagation of transverse cracks and the eventual creation of complete through-thickness gas flow paths that were the source of permeability. For example, for the [0/45/−45/90] S lay-up of IM7/5250-4 a relatively moderate 57 °C reduction in the elevated hold temperature decreased permeability by up to 100 times.

Accelerated hygrothermal cyclical tests for carbon/epoxy laminates

The paper deals with the design of reasonable accelerated test conditions to assess polymer matrix composite durability when it is subjected to supersonic flight-cycles. The study is closely linked to novel application of carbon fibre polymer matrix composites in supersonic aircraft primary structures, leading to substantial weight saving and stiffness improvement. A supersonic flight can result in surface temperatures close to 130 °C, inducing severe thermal gradients and drying which are quite new for this type of materials, now used in primary structures of subsonic jets operating at low subsonic flight-temperatures. Therefore, the particular effect of the drying on the long-term behaviour of composites should be investigated. Numerical simulations based on Fick's law confirm that the supersonic flight-cycles induce a material drying on the long-term and a significant moisture uptake occurs during the aircraft maintenance periods. Then, particular accelerated cycles are proposed to approach the effect of the drying and moisture uptake during service life. First experiments showed that the long-term hygrothermal fatigue can induce significant changes in the material properties and a drop in the glass transition temperature of about 20 °C.

Impact damage of carbon fiber polymer–matrix composites, studied by electrical resistance measurement

Abstract Drop impact damage of continuous carbon fiber epoxy–matrix composite laminates, was studied by electrical resistance measurement, which was shown to be more sensitive than the ultrasonic method. The oblique resistance at an angle between the longitudinal and through-thickness directions was more effective than the surface longitudinal resistance in indicating damage, particularly interior damage. The oblique resistance values from longitudinal segments of a specimen were not additive, but the surface resistance values were. In the case of a unidirectional composite, electrical contacts at 45° from the longitudinal direction in the plane of the laminate were more effective than those at 90°. Even minor damage associated with negligible indentation was sensed. The spatial distribution of damage was also studied.

Effects of composite lay-up configuration and thickness on the damage self-sensing behavior of carbon fiber polymer-matrix composite

The lay-up configuration (unidirectional, crossply and quasi-isotropic) and thickness (8–24 laminae) affect the damage self-sensing characteristics of continuous carbon fiber epoxy-matrix composites. The damage is by drop impact directed at the top surface of the laminate. The oblique resistance (i.e., resistance at an angle between the longitudinal and through-thickness directions) is an effective damage indicator for all lay-up configurations and thicknesses. The surface resistance of the bottom surface is an effective damage indicator for thin (8-lamina) composites, though it is less sensitive to minor damage than the oblique resistance. The surface resistance of the top surface is less effective than that of the bottom surface for 8-lamina multidirectional composites. The through-thickness resistance is an effective damage indicator for 16- and 24-lamina quasi-isotropic composites, but is ineffective for 8-lamina composites of any lay-up configuration. In general, effectiveness means a monotonic and significant increase of the resistance with damage extent.

A fiber bridging model for fatigue delamination in composite materials

A fiber bridging model has been created to examine the effects of bridging on Mode I delamination fatigue fracture in a carbon fiber polymer–matrix composite. The model uses a cohesive zone law that is derived from quasi-static R-curves to determine the bridging energy applied in the bridged region. Timoshenko beam theory and an iterative self-consistent scheme are used to calculate the bridging tractions and displacements. After applying the bridging model to crack propagation data the scatter in the data was significantly reduced and clear trends were observed as a function of temperature that were not apparent previously. This indicated that the model appropriately accounted for the bridging in the experiments. Scanning electron microscopy crack opening displacement measurements were performed to validate the model’s predictions. The measurements showed that the predictions were close to the actual bridging levels in the specimen.

Piezoresistivity in unidirectional continuous carbon fiber polymer-matrix composites: single-lamina composite versus two-lamina composite

Piezoresistivity in unidirectional carbon fiber epoxy-matrix composites was studied by measurement of the electrical resistance of single-lamina and two-lamina composites in the longitudinal (fiber) direction during elastic tensile loading and unloading in the same direction. The piezoresistive effect was reversible. The resistivity increased slightly with longitudinal tensile strain for the single-lamina composite, but decreased with strain for the two-lamina composite. The gage factor was around +2 and -6 for single-lamina and two-lamina composites, respectively. The very weak piezoresistivity in the single-lamina composite was probably due to decrease in the extent of fiber-fiber contact upon tension. The relatively strong piezoresistivity in the two-lamina composite was probably due to increase in the degree of fiber alignment upon tension.

Self-sensing of Damage and Strain in Carbon Fiber Polymer-Matrix Structural Composites by Electrical Resistance Measurement

The self-sensing of damage and strain in carbon fiber polymer-matrix structural composites has been attained by DC electrical resistance measurement. This paper reviews the self-sensing behavior, in addition to addressing the electrical contact configurations and providing a model for relating the resistance to the damage.