Carbon Fiber Reinforced Plastic Composites Research Articles

Surface treatments and adhesion bonding between concrete and a CFRP composite

Applications of fibre reinforced plastics to repair worn-out civil structures rely on comprehensive understanding of strengthening mechanisms, especially adhesion bonding between composites and concrete. A special testing procedure was developed to investigate adhesion bonding between concrete and a carbon fibre reinforced plastic (CFRP), mainly under normal loading conditions. As concrete surface treatments were expected to influence bond strength, different abrasive procedures were investigated; in addition, the application of a silane primer on the concrete surface was studied, using an epoxy resin or a vinyl ester resin as adhesive. An approach based on fracture mechanics was applied to evaluate debonding fracture energy. Microscopic examinations were conducted to identify fracture mechanisms. Crack propagation was found to be dependent on surface treatments as well as the type of adhesive. Applying the silane primer as coupling agent, bonding strength was clearly improved for poorly treated surfaces. The vinyl ester resin as adhesive was found to be inefficient for achieving high bonding strength between the CFRP and the concrete.

A study on residual fatigue bending strength and damage behavior of CFRP composites subjected to impact loadings

This paper evaluates the static and fatigue bending strengths of CFRP (carbon-fiber reinforced plastic) laminates having impact damages, e.g., foreign object damages (FOD). Composite laminates used in this experiment are CF/EPOXY and CF/PEEK orthotropy laminated plates with two-interfaces [0°4/90°4]s A steel ball launched by an air gun collides against the CFRP laminates to generate impact damages. The damage growth during a bending fatigue test is observed by a scanning acoustic microscope (SAM). When the impacted side is compressed, the residual fatigue bending strength of CF/PEEK specimen P is greater than that of CF/EPOXY specimen B. On the other hand, when the impacted side is in tension, the residual fatigue bending strength of CF/PEEK specimen P is smaller than that of CF/EPOXY specimen B. In the case of impacted-side compression, the fracture is propagated from the transverse crack generated near the impact point. In the case of impacted-side tension, however, the fracture develops toward the impact point from the edge of interface-B delamination.

Effect of absorbed water in CFRP composites on adhesive bonding

This paper reports the results of wedge testing adhesive-bonded joints made from dry, and water-immersed and dried, carbon fibre-reinforced plastic (CFRP) laminates based on Ciba Fibredux 914. Five groups of specimens were tested. Those bonded with Hysol EA 9394 adhesive were tested on dry, and immersed and dried, CFRP adherends using mechanical abrasion as the surface preparation. Hysol EA 9390 repair resin was tested on abraded dry samples and on dry, and immersed and dried, peel ply surfaces. Only the initially dry specimens that were also dried before bonding and given the mechanical abrasion treatment gave cohesive failure (now considered to be matrix failure) after 1388 days' immersion. All the peel ply surfaces — whether initially dry, or immersed and then dried — gave interfacial failure. Unfortunately, in the case of the immersed and dried specimens, the two lowest temperature drying cycles were both given to the specimens which were prepared by mechanical abrasion and the two highest temperature drying cycles were given to the peel ply samples. All the peel ply samples gave interfacial failure but the immersed and dried peel ply surfaces gave the same result, in terms of fracture energy and durability, as the initially dry ones, indicating that the higher temperature drying cycles were adequate. One strange and unexpected result was that the fracture energy obtained with Hysol EA 9394 on the immersed and dried surfaces was higher than on the dry surfaces by a large amount. This suggests that, for this particular adhesive, a small amount of moisture actually improves the cure and the fracture toughness, even though a cohesive failure was not obtained. In view of the extensive use of peel plies in industry it would seem that further research is required to achieve more durable bonds. From a repair point of view, where mechanical abrasion is the most common method of surface preparation, it was encouraging to find that in some cases repair bonds could actually be more durable than those made by the manufacturer if peel plies are used.

Emissivity measurements of reflective surfaces at near-millimeter wavelengths

We have developed an instrument for directly measuring the emissivity of reflective surfaces at near-millimeter wavelengths. The thermal emission of a test sample is compared with that of a reference surface, allowing the emissivity of the sample to be determined without heating. The emissivity of the reference surface is determined by one's heating the reference surface and measuring the increase in emission. The instrument has an absolute accuracy of Δε = 5 × 10(-4) and can reproducibly measure a difference in emissivity as small as Δε = 10(-4) between flat reflective samples. We have used the instrument to measure the emissivity of metal films evaporated on glass and carbon fiber-reinforced plastic composite surfaces. We measure an emissivity of (2.15 ± 0.4) × 10(-3) for gold evaporated on glass and (2.65 ± 0.5) × 10(-3) for aluminum evaporated on carbon fiber-reinforced plastic composite.

Reliability approach to the tensile strength of unidirectional CFRP composites by Monte-Carlo simulation in a shear-lag model

A numerical technique based on the Monte-Carlo method in a shear-lag model is developed to simulate the tensile strength and fracture process for a unidirectional carbon-fiber-reinforced-plastic (CFRP) composite. The technique improves on the conventional approach by using an r min method that can determine the stresses working on the fiber and matrix elements as the damage progresses. The r min method is based on tracking the incremental ratio of the strength of an element to its stress. The present model includes the effect of the sliding frictional forces around fiber breaks caused by debonding between the fiber and matrix. Statistical properties of the tensile strength were obtained through simulation runs involving 100 samples for each value of the frictional force parameter. Also studied was the size effect in composite strength with increasing numbers of fibers, N, where it was found numerically that mean strength varies linearly as 1 [l n( N)] 1 2 and coefficient of variation varies linearly as 1 l n( N) , as suggested from a simple theory.

Fracture of CFRP containing impregnated fibre bundles

The fracture behaviour of carbon-fibre-reinforced plastic composites (CFRP) has been studied by using specimens containing fibre bundles which were impregnated with polymer before being incorporated in an epoxy matrix. The results on CFRP with impregnated bundles show that the impact fracture toughness, the crack growth resistance in terms of stress intensity factor, and the specific work of fracture are enhanced by more than 100%, depending on the impregnant material and fibre volume fraction, as compared to the control composites without bundle impregnation. These beneficial effects are attributed to the increased stability of cracking associated with the bridging of fracture surfaces by the impregnated bundles which act as single fibres of huge diameter effectively increasing the fibre pull-out length. Statistical analysis also demonstrates that bundle impregnation can improve the tensile strength of the bundle as well as the uniformity of strength distribution.

Fatigue-induced damage mechanisms in carbon fibre-reinforced plastic composites

A study has been made of the damage mechanisms introduced by the fatigue process in two high-performance carbon-fibre-reinforced plastic laminates, XAS/914 and T800/5245. A series of experiments has been carried out to evaluate systematically the fatigue damage mechanisms that occur at different cyclic stress levels during repeated tension loading. The state and development of damage were mainly investigated by means of edge replication. Microscopy studies were used in support of the non-destructive evaluation methods. Important information relating to the onset of various types of fatigue-induced damage is presented in the form of schematic “damage maps”. When the differences in strength of the two materials are taken into account, it appears that the XAS system exhibits fatigue resistance superior to that of the T800 system.

Adherent metallization of carbon-fibre-reinforced plastic composites using a combined vacuum/electrochemical deposition process

Carbon-fibre-reinforced plastic composites (CFCs) were metallized using plasma etching and plasma deposition methods in combination with a conventional electrodeposition process. This new approach yields highly adherent, thick metallic layers of Cu on CFCs at economic rates. In addition, the use of environmentally hazardous chemicals can be reduced to a minimum. Dry plasma etching was applied to pretreat the CFC samples and proved to yield superior adhesion results compared to conventional wet chemical etching or mechanical treatment. Electroless-deposited copper, sputtered copper and arc-ion-plated Cu and Cr were investigated as an adhesive intermediate layer between the CFC base material and the electrodeposited Cu top coating. Arc-ion-plated Cr-Cu two-component layers and multilayers were developed, which simultaneously provide high coating adhesion for the CFC matrix and exhibit a high electric conductivity for a subsequent electrodeposition process. The adhesion was measured using a z-axis pull test and a 90° peel test.

Fractography of multidirectional CFRP composites tested statically

The morphology of fracture surfaces of multidirectional quasi-istropic layups of carbon fibre-reinforced plastic composites has been studied to identify the features characteristic for basic types of static loading. Specimens were tested in tension, compression, three-point bending and in-plane shear. Optical and scanning electron microscopy were employed for the analysis of fracture surfaces. Some problematic observations of the fractography of delamination surfaces are discussed. The macroscopic morphological features of the delaminations are explained from microscopic observations.

Analysis, Design and Development of a Carbon Fibre Reinforced Plastic Knee-Ankle-Foot Orthosis Prototype for Myopathic Patients

A traditional knee-ankle-foot orthosis (KAFO) for myopathic patients has been studied for the assessment of loads and fatigue resistance. Starting from this basis a thermoplastic matrix carbon fibre reinforced plastic composite (CFRP) KAFO has been developed in order to reduce the weight. A finite-element simulation programme for deformation analysis was used to compare the behaviour of conventional and CFRP orthosis. There were no breakages either of the prototype or of its parts. The CFRP orthosis allows a weight reduction of more than 40 per cent.

Residual stresses and the optimum cure cycle for an epoxy resin

Residual stresses in carbon fibre-reinforced plastic composites arise because of the large difference in thermal expansion of the fibres and matrix. It has been suggested that these stresses might be reduced in epoxy resin matrices by curing at a lower temperature. Experiments have been made on five curing cycles, to test this possibility and to study the variations of viscosity and volume changes of the resin under different cure conditions. DSC experiments have also been made to determine the degree of cure at various stages of the cycles. It was found that a cycle with the low peak temperature of 120°C required a prohibitively long length of time to produce a complete cure. Of the other cycles studied, it is predicted that it would be advantageous, in reducing residual stresses, to use a peak temperature of 160°C, rather than 175°C, provided that a dwell of 18 min is included at 160°C to ensure a complete cure.

Prediction of material property parameter of FRP composites using ultrasonic and acousto-ultrasonic techniques

A study has been made of the use of ultrasonic techniques and acousto-ultrasonic techniques to evaluate the material property parameter of the glass fibre reinforced plastic (GRP) and carbon fibre reinforced plastic (CFRP) composites. The theoretical results indicated that the ultimate tensile strength of GRP and CFRP composites can be related to the material property parameter of FRP composites.

The stiffness of unidirectionally reinforced CFRP as a function of strain rate, strain magnitude and temperature

The Young's modulus and shear modulus of a unidirectional carbon fibre-reinforced plastic composite were measured at −40° C, 20°C, 80°C and 120°C over the strain range 0–0.9%. Within the strain range 0.1%–0.9%, a non-linear increase of up to 20% was found in the Young's modulus, which was independent of strain rate. The shear modulus was sensitive to the viscoelastic properties of the matrix; its magnitude decreased with increasing temperature and increased with increasing strain rate. However, this latter change was not very apparent, particularly at strains beyond 0.6%.

Temperature dependence of elastic constants of CFRP

The elastic constants (Young's modulus, shear modulus, and Poisson's ratio) of both uniaxial and balanced biaxial carbon fibre-reinforced plastic composites and the epoxy resin matrix within the temperature range −100° C to 170°C are presented. Comparison is made between the experimental values of the elastic constants and those predicted using micromechanic and macromechanic theories. The sensitivity of the elastic constants to temperature is closely related to the temperature dependent behaviour of the matrix. Below the transition region ( T ⩽ 120° C), the matrix is glassy and the elastic constants of the composite exhibit little change in value with change of temperature. In the transition region (120° C ⩽ T ⩽ 170° C), the matrix changes from a glassy to a rubbery state. The extent to which significant changes in the elastic constants of the composite occurs then depends upon whether the fibres or the matrix dominates its mechanical performance.