Carbon Fiber Strands Research Articles

Mechanical Characterization of Advanced Random Discontinuous Carbon/Epoxy Composites

Compression molding is a closed mold process, the most common choice for high-volume composite parts and often associated with SMC (Sheet Molding Compound) materials. The aim of this work is the experimental investigation of the level of isotropy of the mechanical properties, such as stiffness, strength and elongation to rupture, of a material system composed of randomly oriented discontinuous carbon fiber strands in a polymer matrix after consolidation by the compression molding.

도로교 RC 상판 보강을 위한 탄소섬유 기초 carbon fiber sheet와 carbon fiber strand sheet의 역학특성

Recently, a lot of interest has been shown in structural maintenance managements of civil infrastructures. Many researchers have been conducted on various maintenance techniques and repair materials. Among other fiber materials the carbon fiber materials are especially focused on maintenance management of Highway Bridges. Extensive work has been done on Carbon Fiber Sheet (CFS). Nevertheless, Carbon Fiber Strand Sheet (CFSS) is a newly developed material, on which limited work has been done until now. Therefore, in this study bonding the CFSS to RC slab specimen and fatigue resistance evaluation has been conducted. The results demonstrated an increase of 25.3 times more reinforcement of RC slab compared to non-reinforced RC slab. Moreover, compared to CFS-bonded RC slab, The CFSS-bonded RC slab showed 1.2 times greater reinforcement.

Carbon fiber sheet 및 carbon fiber strand sheet 접착보강한 RC 상판의 내피로성과 파괴거동과의 상관관계

According to the results of Highway Bridges Long Life Repair Plan. The most serious damage to RC slabs is caused by fatigue deterioration, which results from the driving loads of large-sized vehicles, and aging of materials. In response to this, adhesion reinforcement using carbon fiber sheet is being adopted. In addition, carbon fiber strand sheet that holds the same material characteristics as CFS, but has superior workability, has been developed as a new reinforcement material. However, almost no studies have been conducted on CFSS in relation to fatigue resistance evaluation through fatigue tests under running wheel loads, with the exception of a few by some organizations. Therefore, in this study, specimens with front CFS adhesion reinforcement on the bottom surface of the RC slab and specimens with grid-type CFSS reinforcement were manufactured. Then, fatigue tests under running wheel loads were conducted, and thus fatigue resistance was evaluated using the specimens.

The fabrication and mechanical properties of novel composite lattice structures

A range of lattice structures, based on a carbon fiber reinforced epoxy composite, have for the first time been manufactured using a lost-mold technique. Here, core structures for potential use in sandwich panels were prepared by drilling holes in a well-defined pattern through either a high quality wax block or a machined salt slab. Continuous carbon fiber strands were then inserted through each of the holes in the perforated array, ensuring that one continuous tow extended through all of the elements within a given core structure. Two threading techniques were used to prepare the composite columns. Initial attention focuses on vertical truss cores and this is then extended to consider more complex structures.Following infusion, using the VARTM manufacturing procedure, individual specimens were removed from the resulting sandwich panels in preparation for subsequent mechanical testing.Compression showed that the strength of individual struts and the corresponding cores increases with strut diameter and fiber volume fraction. Smaller diameter struts failed in buckling, whereas the larger diameter columns failed in a crushing mode involving high levels of energy absorption. Finally, the properties of the various lattice structures considered here were predicted using finite element modeling techniques.

Comparative Study of Unbonded Carbon Fiber and Steel Strands in Posttensioned Pier Caps

AbstractCarbon fiber strands offer a viable alternative to corrosion-prone steel strands in posttensioning applications. This study focused on a side-by-side comparison of two types of carbon fiber strands with different elastic moduli against steel strands in a 5½∶1 scaled model of a typical interior hammerhead pier of the San Antonio Downtown Y project with two identical cantilever overhangs. Two different strand arrangements were used for posttensioning, with eight and six strands, respectively representing an overdesign and a slight underdesign relative to the factored demand. The model was tested under service and factored flexure and shear loads. The study confirmed the feasibility of using carbon fiber strands in unbonded posttensioning of pier caps. Given the strength-based design of the pier cap, the elastic moduli of different carbon fiber strands did not seem to have affected the serviceability performance with respect to either cracking or deflection. Considering both serviceability and overlo...

Statistical Creep Failure Time of Unidirectional CFRP

A prediction method for the statistical creep failure time of polymer composites using the statistical static strengths of polymer composites measured at various temperatures is proposed based on Christensen’s model of viscoelastic crack kinetics. Then the static strengths at various temperatures under tension loading along the longitudinal direction of unidirectional CFRP are measured experimentally and statistically using numerous resin impregnated carbon fiber strands (CFRP strands). The creep failure time of unidirectional CFRP is predicted statistically based on the prediction method using statistical static strengths at various temperatures. Finally, the creep failure times of unidirectional CFRP at a constant load and a temperature are measured experimentally and statistically using many CFRP strands for comparison with predicted values. Results show that the statistical creep failure time of unidirectional CFRP is predictable using the statistical static strengths of unidirectional CFRP, because the experimental creep failure times agree well with the predicted ones.

Calculation of the robot trajectory for the optimum directional orientation of fibre placement in the manufacture of composite profile frames

This article deals with theissue of calculating the trajectory of the end-effector of an industrial robot in the manufacture of composites. In the introduction to the article we describe the basic approaches used in the manufacture of composites. Robots are used to define the winding orientation of carbon fibre strands on an uneven polyurethane 3D core. The core is attached to the robot-end-effector and is led through a fibre-processing head according to a suitably defined robot trajectory during dry carbon fibre winding on the core. The model of a passage of the polyurethane core through a fibre-processing head is described in the article. The placement of the fibre-processing head is defined in the basic Euclidean coordinate system E3 of the robot. The core is specified in the local coordinates of the Euclidean coordinate system E3, the origin of this local system is in the robot-end-effector. The positioning of the local system in the basic system of the robot is entered using the “tool centre point” of the robot. A matrix calculus is used when calculating the trajectory robot-end-effector to determine the desired passage of the core through the fibre-processing head. Gradually, the required rotation and translation matrices of the local coordinate system of the robot-end-effector relative to the basic system are calculated and subsequently the Euler angles of rotation are determined corresponding to the transformation matrices. This is used to determine the sequence of values of the “tool centre point” for defining the desired trajectory of the robot-end-effector. The calculation for the trajectory was programmed in the Delphi development environment. The article also solves practical tasks of the polyurethane core passage through the fibre-processing head. The calculations of the trajectory of the robot-end-effector were used as input values for the graphic software simulator and at the same time winding of carbon strands on the polyurethane core was verified for the calculated trajectory of the robot-end-effector in the experimental laboratory.

Progressive failure analysis of the fiber metal laminates based on chopped carbon fiber strands

In this study, we propose a new fiber metal laminate based on unidirectionally arrayed chopped strand (UACS) reinforced aluminum sheets, referred to as UACS/Al laminate. UACS is made by introducing slits into unidirectional carbon fiber-reinforced plastic (CFRP) prepreg. Due to the presence of discontinuous fibers, the microstructure of the UACS/Al laminate is much more complicated than the conventional fiber metal laminate, which also results in a failure progression that is more complicated. Tensile failure of the UACS/Al laminate might occur as combination of intra-laminar damage at the slits, inter-laminar damage at the interfaces, in-ply damage of the CFRP, and plastic deformation of the aluminum plies. Fabrication and tensile tests of UACS/Al laminate specimens were performed. A two-dimensional finite element model was developed with intra-laminar cohesive elements inserted into the slits of the UACS plies and with inter-laminar cohesive elements inserted into the interfaces between all laminas in the modelled UACS/Al laminates. A numerical study is conducted to investigate the influence of the shape of the cohesive laws on the FEA predictions. The combined experimental and numerical studies provide a detailed understanding of the failure progression of UACS/Al laminates under tensile load.

Temperature dependence of longitudinal tensile strength in unidirectional carbon fiber-reinforced plastic

This paper is concerned with the characteristics of the temperature-dependent tensile strength along the longitudinal direction of the unidirectional carbon fiber-reinforced plastic. The temperature-dependent tensile strength of the unidirectional carbon fiber-reinforced plastic was evaluated using resin-impregnated carbon fiber strand (carbon fiber-reinforced plastic strand) specimens with highly reliable co-cured tabs developed by the authors. The tensile strength of carbon fiber-reinforced plastic strand was measured at three constant temperatures below the glass transition temperature of the matrix resin for 50 specimens at each temperature. The results showed that the tensile strength of the unidirectional carbon fiber-reinforced plastic has a Weibull distribution at each temperature. The shape parameter of the tensile strength does not change with the temperatures and the scale parameter decreases clearly with increasing temperature. The degradation rate of the tensile strength of the unidirectional carbon fiber-reinforced plastic by the temperature raise against that of the viscoelastic modulus of the matrix resin agrees well with the predicted results based on the Rosen’s strength analysis.

Effect of fiber damage on the overall electrical conductivity of bare carbon fiber strand

The paper focuses on the effect of fiber breakage on the overall electrical conductivity of IM7 carbon fiber strands used to manufacture composite overwrapped pressure vessels (COPV). The goal is to demonstrate the feasibility of performing resistance measurements in bare carbon fiber strands and identify a correlation between the percentage of surviving filaments, local changes in resistance measurements, and strength reduction. A simple model relating fracture load of a single strand to change in the electrical resistance is proposed and experimentally verified. The methodology can be used as a tool for rapid evaluation of composite overwrapped pressure vessels to predict stress rupture.

Reliable test method for tensile strength in longitudinal direction of unidirectional carbon fiber-reinforced plastics

This article is concerned with the reliable test method for measuring the tensile strength in the longitudinal direction of unidirectional carbon fiber-reinforced plastics. A filament-wound resin-impregnated carbon fiber strand (carbon fiber-reinforced plastics strand) was employed as the specimen. The most important improvement for reliable tests is that the bonding strength of the end tabs with carbon fiber-reinforced plastics strand should be higher than the tensile strength of CFRP strand. We improved the bonding strength by co-curing the carbon fiber-reinforced plastics strand with end tabs. The improved method was applied to two kinds of carbon fiber-reinforced plastics strands and the reliable tensile strengths were obtained.

Flexible towpreg preparing carbon fiber‐reinforced phenolic resin composites

AbstractThe current article describes a novel approach to produce the carbon fiber/phenolic matrix composites. The composites are focused on the carbon fibers and interfacial behavior. The washed, oxidized, and flexible towpreg‐treated carbon fiber is investigated by tensile strength. To relate the interfacial behavior with the characteristics of the components, the surface free energy of the carbon fibers is carried out by capillarity. Thereafter, the tensile strength of carbon fiber strands, the interlaminar shear strength, and the flexure strength are measured. The results show that appropriate treatments could effectively improve the mechanical properties. Interfacial adhesion between the carbon fibers with phenolic resins being fractured under deformation and the dispersed resins on the flexible towpreg carbon fiber are, respectively, studied by scanning electron microscopy and optical microscope. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Integrated Monitoring System for Carbon Composite Strands in Cable-Stayed Bridge, Penobscot Narrows, Maine

An integrated monitoring system for carbon fiber composite cable (CFCC) strands recently implemented in the Penobscot Narrows Bridge in Maine is presented. This cable-stayed bridge features a cradle-stay system in the pylons that provides a continuous cable stay between the main span bridge deck anchorage and the back span bridge deck anchorage. The cradle-stay system allowed for replacing epoxy-coated steel strands with CFCC strands. The monitoring system installed in the bridge is composed of a set of sensors and devices that provide complementary structural information: (a) load cells supported on steel anchorage chairs, (b) linear voltage differential transformers (LVDTs) mounted on the steel anchorage chairs, (c) fiber-optic strain (FOS) sensors embedded in composite sleeves, and (d) temperature sensors. The monitoring system was designed to detect variations over time in individual carbon fiber strand force and strains at the anchorage locations and to correlate the measurements with ambient temperature. Laboratory tests were conducted to proof-load the steel anchorage chairs and to calibrate the LVDTs and embedded FOS strain measurement systems. The baseline response of the CFCC strands that was based on load cell, FOS sensors, and LVDTs measurements after the strand stressing operation is presented. The CFCC strands experienced changes in tensile force between +3% and −5% over the first 4 months in service, which were consistent with ambient temperature fluctuations. The FOS measured strand strains and the LVDT deflections of the anchored chair exhibited trends consistent with the variations in CFCC strand force.

The non-linear response of quasi-isotropic composite laminates

The stress–strain response of quasi-isotropic composite laminates consisting of carbon fiber strands embedded in urethane resin was tested at several load orientations. It was noted that at sufficiently high stresses the above response exhibited angular dependence and deviation from linearity, especially at load orientations intermediate to the fiber directions. Similar results were reproduced for quasi-isotropic carbon/PPS composites, where failure was associated with a most substantial amount of delaminations. The above angular dependence could not be explained by either of two forms of elaso-plastic models, and is probably attributable to damage.

Carbon fibres and plasma-preserved tendon allografts for gap repair of flexor tendon in bovines: gross, microscopic and scanning electron microscopic observations.

The efficacy of carbon fibres and plasma-preserved tendon allografts for gap repair in the superficial digital flexor tendon in the mid-metatarsal region was evaluated in 12 crossbred calves. Experimental tenectomies were performed, followed by implantation of carbon fibres in group I (12 legs) and plasma-preserved tendon allografts in group II (12 legs). Gross observations in group I showed filling of the defect with granulation tissue with more vascularity on day 7, which was less prominent at day 14. On day 30, the neotendon formed was slightly thicker and comparable to normal tendon in appearance and texture. On day 90, it exhibited all the characteristics of a fully developed tendon. Whereas, in group II increased vascularity at the site and encapsulation of the graft with connective tissue in early periods was observed. The gap between graft and host was filled with fibrous connective tissue. Peritendinous adhesions were maximum on day 7 which were gradually reduced in both groups. Microscopically, an acute inflammatory reaction in the periphery of carbon fibres was observed on day 7. Immature fibroblasts were arranged in a haphazard pattern at this stage. By day 14, numerous newly formed capillaries and comparatively more mature fibroblasts were present in between and around the carbon fibres which were aligning parallel to the longitudinal axis of the tendon. By day 30 the healing tissue exhibited longitudinal orientation of collagen fibres and was at a more advance stage of maturation. By day 90, the neotendon formed simulated the picture of normal tendon. In the grafted tendon group, there was normal healing tissue at the functional sites between host and grafted tendon. The fibroblastic activity appeared to be both extrinsic and intrinsic in origin. The connective tissue had invaded the graft to a variable distance and there was resorption of graft which was replaced by newly formed connective tissue on day 90. Scanning electron microscopic observation revealed formation of neotendon between carbon fibre strands, resulting in thickening of the implant. In later stages parallel collagen fibres resembling normal tendon were observed in both groups.

Relationship between the Carbon Fiber Strands Assembly and the Amount of Attached Microorganism.

Relation between the form of assembly of carbon fiber strands and the amount of attached microorganism was studied by using an artificial waterway set up at the side of the pond AZAMI. A variety of rectangular sail-type carbon samples were prepared by arranging a definite number of the strands (N) in parallel to each other in width of A cm and in length of B cm. These samples were placed on a setting frame with a length of L0, and then immersed in the circulating water pumped up continuously from the pond during the period of each four months in winter and summer. The amount of microorganism attached and proliferated in carbon fiber entanglement network was evaluated by weight increase(Winc). It was found that Winc depends on the relative length, B/L0 and the value of N/A, namely, compactness of the strand consisting of 12, 000 monofilaments. The former value was closely related to easy separation of the strand into monofilaments that gives rise to an increase in effective surface area of carbon fibers for initial attachment of microorganism, while the latter was a function of an effective volume of network in water for proliferation of microorganism. A suggestion obtained was, further, that aerobe microorganism was preferential species within the carbon fiber network perturbed by water flow which leads to an increase in the rate of exchange of water in the network and this probably be related to the high bending modulus of carbon fiber filament. A most preferable result for proliferation of microorganism was obtained at the condition of B/L0=1.4 and N/A=2.7, and the average distance between neighboring carbon monofilaments entrapped in the aggregates of microorganism reached to about 750μm corresponding to ca. 100-folds of the diameter of carbon monofilaments.

Effects of Dry Treatment Condition on Mechanical Properties of Carbon Fiber/Ceramic Composite Using Aluminum Phosphate

Preparation of carbon fiber reinforced ceramic composite (CFRC) was tried by low temperature sintering, using a phosphate (ACP) mixture consisting mainly of aluminum phosphate as a matrix material. Carbon fiber (CF) strand (HP, 12K) was used as a reinforcing material. Prepreg sheets of CF impregnated with ACP were dried at 80°C. Laminated prepreg sheets were heated up to 200°C in a hot press, and then heated in N2 at 500°C to prepare the CFRC. In this study, the prepreg sheets were heated under continuously generated steam, and its effects on mechanical properties and structure of the CFRC were investigated. Consequently, mechanical properties of CFRC were improved by heat treatment in moist air, showing a bending strength of 516MPa and a Young's modulus of 55GPa. Observation on its side surface indicated that few voids were left in the CFRC, which leads to the improvement of adhesiveness between prepregs.

Application of Reliability and Fiber Probabilistic Strength Distribution Concepts to Composite Vessel Burst Strength Design

With the expanding use of composite vessels, including space and tactical applications, accurate prediction of performance becomes increasingly important. A criti cal parameter in performance prediction is the burst strength as measured by the delivered fiber ultimate strain-to-failure. This paper discusses a composite pressure vessel design approach that is based on reliability and probabilistic fiber strength distribution concepts.1 The method is based on the fiber strain-strength interference reliability theory. A substan tial part of this paper examines how fiber strength distribution parameters generated by tow tensile tests of impregnated carbon fiber strands can be used in pressure vessel strength design. The fiber statistical strength distribution is analyzed using the Weibull distribution function. It is shown that the bimodal Weibull distribution function fits the impregnated fiber strand strength distribution data much better than the single mode Weibull function. This is particularly important because the bimodal function gave a much more accurate representation of the lower tail of this distribution. According to the strain- strength interference theory, the lower tail of the fiber strength distribution is the most sig nificant in pressure vessel strength reliability calculations. The impregnated fiber strand strength distribution agrees well with the strength distribu tion of 4-in. biaxial composite tubes manufactured from the same carbon fiber lot. Good agreement was also shown with the strength distribution of pooled fiber lot tow tensile data and hydroburst strength distribution of full-scale composite pressure vessel manufactured from these lots. In this comparison, the A-basis fiber strength allowable calculated from the strand data was slightly lower (3%) than those calculated from the full-scale pressure vessel strength data, whereas, the B-basis allowables calculated from the strand and the full-scale pressure vessel strength data were the same. However, because A-basis and/or B-basis allowables require many data points from many material lots, the use of fiber lot qualification data is advantageous because it meets these requirements.

Ligament and tendon substitution with composite carbon fiber strands.

Carbon fiber strands were used experimentally to substitute for tendons in dogs and clinically to reconstruct tendons and ligaments in human. The investigation was carried out to determine the histologic appearance of the new composite structure and its tensile strength in comparison to the natural structure. The histologic picture disclosed a remarkable structure evolved by continuous irritation of the carbon fibers and consisting of two interrelated components: synthetic carbon fibers and biologic collagenous tissue. This carbon fiber composite structure was composed of long cylindrical units containing concentric layers of collagenous fibers and cells enveloping the core of each carbon fiber. After one year of physiologic use in dogs, the average ultimate tensile strength of the composite structure which replaced the quadriceps and triceps was 372 N, or 88% the strength of the natural tendons. One year after implantation the histologic picture of the composite structure in human showed a relatively dense collagenous architecture. However, a significant proportion of the structure was taken up by histiofibroblasts produced by the irritation of the carbon fibers. Thus, the density of the collagen in the composite structure remained relatively deficient in comparison to the nature tendon, and the structural tensile strength continued to depend entirely on the integrity of the carbon fibers.

The influence of various carbon fibre braiding techniques and methods of fixation on the extensibility of ligament prostheses

An alloplastic ligament prosthesis with carbon fibres should not only be biocompatible but should also restore the biomechanical function of a natural system, which includes restoring its elasticity. Tests with new types of braided carbon fibre strands showed their elastic extensibility to increase with increasing number of filament tows and increasing braid angle. The best results were obtained with 32 tows and a 43° braid angle. This strand type was tested in vitro as a medial collateral ligament prosthesis with two methods of fixation in human knees and, in like manner, in vivo in sheep knees. Biomechanical tests in vitro using human cadaver knees and sheep knees after 12 weeks' implantation showed an elasticity similar to that of normal natural ligaments.