Fiber Polymer Matrix Composites Research Articles

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.

Can water cause brittle fracture failures of composite non-ceramic insulators in the absence of electric fields?

It was postulated by J. Montesinos et al. (see ibid., vol.9, p.236-43, 2002), based on experimental evidence, that brittle fracture failures of composite (non-ceramic) HV insulators could be caused by water and mechanical stresses. It was also claimed therein that the brittle fracture process was more likely to happen with water than acids. This postulation could be of major importance as its ramifications might affect the entire composite insulator technology and, in particular, the usage of glass fiber polymer matrix composites in HV applications. Such an important statement should not be left without an independent verification. Therefore, attempts have been made in this research to initiate this process in unidirectional E-glass/modified polyester and E-glass/vinyl ester composites, used in non-ceramic insulators, by subjecting them to water under four-point bending conditions. This was done to independently verify the main conclusion of J. Montesinos et al. that water may be more damaging to unidirectional E-glass/polymer composites than acids. It has been clearly shown in this work that water, in the absence of electrical field, cannot cause stress corrosion cracking of unidirectional E-glass/polymer composites and thus brittle fracture of composite non-ceramic insulators. Thus the main results of J. Montesinos et al. could not be independently reproduced.

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.

OS09W0052 Monitoring of damage and strain in carbon fiber polymer-matrix structural composites by electrical resistance methods

Polymer-matrix composites containing continuous carbon fibers are the dominant lightweight structural materials. The monitoring of the damage is important for structural health monitoring and hazard mitigation. The monitoring of the strain is relevant to structural vibration control and security. This paper reviews the use of the composites themselves to sense their own damage and strain, thereby removing or reducing the need for embedded or attached sensors, which suffer from high cost and poor durability. Both damage and strain affect the volume electrical resistivity of a composite, thereby allowing the monitoring of damage and strain in real time by volume resistivity measurement. The contact resistivity of the interlaminar interface (i.e., the interface between the laminae) provides another attribute for monitoring. A configuration involving two crossply laminae provides a two-dimensional array of sensors and an x-y grid of electrical interconnections, thereby allowing spatial distribution sensing.

Effects of the temperature, humidity, and stress on the interlaminar interface of carbon fiber polymer-matrix composites, studied by contact electrical resistivity measurement

The interlaminar interface (i.e., the interface between laminae) of continuous carbon fiber polymer-matrix structural composites was monitored in real time during dynamic changes in temperature, humidity and stress by measurement of the contact electrical resistivity of the interface. The stress was compressive, in the direction perpendicular to the interlaminar interface. Temperature, humidity and stress were all found to have reversible effects on the resistivity, due to the effect of temperature on the probability of the jump of an electron from one lamina to the adjacent one, and the effects of humidity and stress on the extent of contact between fibers of adjacent laminae. In addition, due to damage, temperature caused the resistivity to increase whereas stress caused the resistivity to decrease.

Decreasing the electromagnetic observability of carbon fiber polymer–matrix composites by using epoxy-coated carbon fibers

The electromagnetic observability of epoxy–matrix composites containing continuous carbon fibers was decreased by using epoxy-coated carbon fibers. The attenuation of electromagnetic waves at 1.0–1.5 GHz upon reflection was increased from 1.3 to 1.7 dB, while the attenuation upon transmission was decreased from 30 to 24 dB. The effect is attributed to the decreased transverse electrical conductivity of the composite due to the epoxy coating. The epoxy coating caused the tensile strength and modulus of the composite to decrease by about 10%, while the elongation at break was not affected.

Thermoelectric property tailoring by composite engineering

The tailoring of the thermoelectric properties (the sign and magnitude of the absolute thermoelectric power) was achieved by composite engineering. The techniques involved the choice of the reinforcing fibers (continuous or short) in a structural composite and the choice of the particulate filler between the laminae in a continuous fiber composite. The tailoring resulted in thermoelectric structural composites, including continuous carbon fiber polymer-matrix composites and short fiber cement-matrix composites. In addition, it resulted in thermocouples in the form of structural composites. The choice of fibers impacted the thermoelectric behavior in the fiber direction of the composite. The choice of interlaminar filler impacted the thermoelectric behavior in the through-thickness direction.

Thermal Fatigue in Carbon Fibre Polymer-Matrix Composites, Monitored in Real Time by Electrical Resistance Measurements

Thermal fatigue and temperature of a continuous carbon fiber epoxy-matrix composite were simultaneously monitored by measurement of the contact electrical resistivity of the interlaminar interface. A temperature increase caused the resistivity to decrease reversibly within each thermal cycle, while thermal fatigue caused the resistivity to increase. The increase took the form of a spike increase at the maximum temperature of a cycle in an early stage of fatigue, and took the form of an abrupt increase of the baseline in a later stage of fatigue.

Microstructure and Damage of the Interlaminar Interface of Carbon Fiber Polymer-Matrix Composites, Monitored by Contact Electrical Resistivity Measurement

AbstractThe microstructure and damage of the interlaminar interface of continuous carbon fiber polymer-matrix structural composites were monitored during dynamic changes in temperature, humidity and stress by contact electrical resistivity measurement. The stress was compressive, in the direction perpendicular to the interface. Temperature, humidity and stress had reversible effects on the resistivity, due to the effects on the interfacial microstructure, such as the extent of contact between fibers of adjacent laminae. These parameters had irreversible effects on the resistivity, due to relaxation and damage of the interface. The effects differed between composites with thermoplastic and thermoset matrices. The reversible effects allow the use of the contact resistivity as an indicator of temperature, humidity and stress.

Interlaminar damage in carbon fiber polymer-matrix composites, studied by electrical resistance measurement

Interlaminar thermal damage in continuous carbon fiber polymer-matrix composites was monitored in real time during thermal cycling by measurement of the contact electrical resistivity of the interlaminar interface. Damage was accompanied by an abrupt increase of the resistivity for a thermoset-matrix composite, and by an abrupt decrease of the resistivity for a thermoplastic-matrix composite. Both phenomena are due to the effect of matrix damage on the chance of fibers of one lamina touching those of an adjacent lamina. The damage involved matrix molecular movement in the thermoplastic case, but not in the thermoset case.

Thermoelectric structural composites and thermocouples using them

ABSTRACTThe tailoring of the sign and magnitude of the absolute thermoelectric power was achieved in structural composites by the choice of the reinforcing fibers and of the particulate filler between laminae. The resulting thermoelectric structural composites included continuous carbon fiber polymer-matrix composites and short fiber cement-matrix composites. In addition, it resulted in thermocouples in the form of structural composites. The fibers and interlaminar filler impacted the thermoelectric behavior in the longitudinal and through-thickness directions respectively.

Tribology of Material Contacts under Dynamic Loading, Studied by Electrical Resistance Measurement

AbstractThe tribology of material contacts under cyclic compression in the direction perpendicular to the plane of the contact was studied by measurement of the contact electrical resistivity of the contact during the dynamic loading. The real-time monitoring allowed observation of both reversible and irreversible effects. The material contacts studied were those involving steel, carbon fiber polymer-matrix composite, cement mortar and graphite, due to their relevance to fastening, concrete structures, electric brushes and electrical pressure contacts. Correlation was made between the contact resistivity and the occurrence of elastic/plastic deformation at asperities. The interfacial structure was found to depend on the stress and the loading history.

Thermal Analysis by Electrical Resistivity Measurement

Thermal analysis in the form of electrical resistivity measurement is reviewed. It is useful for studying phase transitions and electrical conduction mechanisms. The resistivity can be the volume resistivity or the contact resistivity, as illustrated for the case of continuous carbon fiber polymer-matrix composites.

Thermal analysis of carbon fiber polymer-matrix composites by electrical resistance measurement

Thermal analysis conducted on carbon fiber polymer-matrix structural composites by DC electrical resistance measurement provided information on structural transitions, residual stress, composite interfaces, the composite fabrication process, and thermal damage. The composites involved continuous carbon fibers in single fiber and laminate forms, together with thermoplastic and thermoset matrices. The experimental methods and data interpretation are covered in this review.

Prediction of the Layer Longitudinal Compression Strength

This paper presents a new approach to the prediction of the layer longitudinal compression strength of continuous fibre polymer matrix composites. It is known that failure is caused by fibre microbuckling and that the initial fibre waviness and the matrix non-linear behaviour play a major role. A previous model compared very well with experimental results, but required a relatively complex numerical solution procedure. In the present model, the compression strength is obtained by solving a fourth-order polynomial equation. Although based on several simplifying assumptions, the model is in good agreement with the numerical model predictions and with experimental data. Further model simplifications lead to a closed-form expression that seems to be valid for carbon and boron fibre composites.

Interfacial debonding analysis in multiple fiber reinforced composites

Decohesion at multiple fiber interfaces of elastic fiber reinforced composites is modeled by the Voronoi cell finite element model (VCFEM) in this paper. Interfacial debonding is accommodated by cohesive zone models, in which normal and tangential springs tractions are expressed in terms of interfacial separation. Model simulations are compared with results from experiments using cruciform specimens, of single and multiple fiber polymer-matrix composites. An inverse problem is solved to calibrate the cohesive zone parameters. Debonding at fiber-matrix interfaces is simulated for different architectures, volume fractions and boundary conditions, to understand the influence of microstructural morphology and boundary conditions on the decohesion process.