Multipoint measurements for resin flow during VaRTM process are carried out by using embedded fiber optic sensors to evaluate the effect of a resin distribution medium which is incorporated on a fiber preform as a surface layer on resin impregnation behavior. By simultaneously infusing epoxy resin into two glass fiber preforms with or without a distribution medium that are separated by a plastic film, resin amount impregnated through in-plane flow and out-of-plane flow via the distribution medium are compared. It has been indicated that resin amount impregnated through the out-of-plane flow via the distribution medium is found to be dominant especially for thinner fiber preform and be constant at each evaluated region of the resin flow. Numerical calculation for resin impregnation during the VaRTM process is also performed using coefficient of permeability both in in-plane and through-the-thickness direction in the fiber preform. The simulated resin amount impregnated through out-of-plane flow via the distribution medium is found to be constant during the impregnation process due to a constant slope angle of flow front, and be comparable with the experimental results.
Sandwich panels are widely used in building construction, housing and transportation equipment. The core is normally used low density material such as honeycomb structure or cell-structured thermoplastic foams. However, they have difficulties molding complex shaped structures and being weak in shear. A new stiff and lightweight carbon fiber reinforced foam (CFRF) was manufactured by focusing on spring back effect of monofilament dispersed discontinuous CFRTP (Carbon Fiber Reinforced Thermoplastic). Specific bending modulus of polypropylene matrix CFRF was changed from 2.20 to 5.30 depending on spring back ratio. Press molding for complex shaped CFRF including curved surface and deep drawing structures was successfully obtained by one-step procedure. Furthermore, sandwich structure having solid polyamide skin was also obtained in the same method.
The tensile tests of the unidirectional CFRP laminate composites (CF/PA6) with fiber discontinuity were performed, and mechanical properties were obtained. The laminate stacking sequence of the specimens is , and the specimens have fiber discontinuity of 6, 12 and 18 layers. And, the damage growth behaviors were investigated by microscopic observation and acoustic emission (AE) monitoring. Furthermore, the delamination onset stresses were predicted by the simple analysis model. As a result, the stress-strain curves showed nonlinear behavior, and nonlinear starting points became early with increase of the fiber discontinuity layers. It is found that the initial damage of the specimen was the crack between fiber discontinuity end/resin interface by microscopic observation and AE monitoring. And, the delamination between continuous/discontinuous ply around the resin occurred. After that, the resin at the fiber discontinuity was broken at the time of maximum applied stress, then the delamination grew extensively in the specimen, finally the specimens was broken. In addition, the delamination onset stresses were predicted by analysis model using energy release rate, and these stresses were almost agree with experimental values.
Carbon fiber reinforced plastics (CFRP) pipes are expected to substitute for steel drive shafts to improve motorcar’s fuel efficiency and driving performance. The static torsional strength of CFRP pipes formed by a modified simultaneous multi ply winding method is 20% higher than that of CFRP pipes formed by a filament winding method owing to few initial flaws. As the results of static torsional tests regarding [90/-45/+45]6 pipes, it was revealed that the delamination from the prepreg end occurred in the innermost layer and propagated in the interlaminar area of the -45°/+45° plies before the final failure. It is expected to design stacking sequence for preventing the delamination. In this study, effects of stacking sequence on the static torsional properties of the CFRP pipes were investigated. [902/-45/+45]6 and [90/-45/90/+45]6 pipes were formed to investigate effects of lamination angle difference between adjacent plies. Maximum lamination angle difference of the [90/-45/90/+45]6 pipe is smaller than that of the [902/-45/+45]6 pipe. In case of the [90/-45/90/+45]6 pipes with small lamination angle difference, the initiation of the delamination was delayed because the interlaminar stress was reduced. Furthermore, [(90/-45/90/+45)6/90] pipes were formed to investigate effects of an application of a 90° layer on the innermost layer. The delamination from the prepreg end did not occur before the final failure by the application of a 90° layer on the innermost layer since the applied load on the prereg end was reduced. Finally, the static torsional strength of the [(90/-45/90/+45)6/90] pipes was 25% higher than that of the [902/-45/+45]6 pipes due to improvement of delamination resistance.
Torsional-tensile, dynamic torsional pendulum oscillation and static rotary tests of carbon mono-filament were carried out. Two high performance carbon fibers，a PAN-based and a pitch-based, were chosen for these tests. From static rotational behavior of carbon and glass series connected filament, shear stress-strain diagram of carbon fiber is determined. On the diagram, the PAN-based carbon fiber exhibits a very linear shear stress-shear strain relationship up to the torsional fracture. On the other hand, the pitch-based carbon fiber shows the linear relationship up to a point. Beyond the point, increasing shear strain, slope of the τ/γ (torsional stiffness) is decreased. However, for each carbon fiber, it does not show any plastic torsional deformation, that is, it is perfectly elastic until fracture. In other words, the torsional deformation is completely recovered when the torque is reduced at every torsional deformation stage. The mono-filament of carbon fiber is tensioned until fracture with fixing torsional strain. The torsional-tensile strength of the PAN-based carbon fiber shows that increasing torsional strain, the tensile strength decrease by insensible degrees with the small torsional strain and rapidly decrease coming up to the torsional fracture strain. The torsional-tensile strength of the pitch-based carbon fiber does not change until a torsional strain, and after then decreased rapidly. Tensile rigidity is gradually decreased, increasing torsional strain, and decreased sharply at closing torsional fracture strain. The ratios of anisotropic measure of strength by the tensile strength to the torsional strength are smaller than the ratios of elasticity by the tensile modulus to the torsional modulus. This torsional-tensile test method is useful in order to evaluate strength and structural anisotropy of fiber materials.
The purpose of this study is to investigate the proper temperature condition in extracting carbon fibers as the reinforcement of composite molded by injection method. Recycled carbon fibers were extracted from the wasted CFRP (Carbon Fiber Reinforced Plastic) by pyrolyzing the epoxy matrix of CFRP under air atmosphere under temperature condition at 400, 600 and 800℃ in this study. Recycled carbon fiber/polypropylene (RCF/PP) pellets were prepared using twin screw extruder to mold the RCF/PP specimens with dumbbell shape by injection machine. The mechanical properties of RCF/PP specimen were determined by tensile tests, and the fiber length and fiber orientation in RCF/PP specimen were measured, while the fracture toughness of RCF/PP specimen was also investigated. The tensile strength of recycled carbon fiber was decreased with rising pyrolysis temperature from 400 to 800℃. However, superior mechanical properties of tensile strength of RCF/PP specimen (with 30wt% of reinforcement) were obtained when the extracting temperature was 600℃. Almost fibers in RCF/PP specimens were aligned in the mold filling direction and the largest fracture toughness of RCF/PP the specimen was also obtained, in this study, when the extracting temperature was 600℃. The temperature condition at about 600℃ should be selected in extracting carbon fibers as the reinforcement of RCF/PP composite molded by injection method, for current material system.
Carbon Fiber Reinforced Plastics (CFRP) are expected to be used for the structural parts of automobiles and aircraft because of their light weight and superior mechanical properties. Since the mechanical properties of CFRP are affected by the interfacial properties of the reinforcing fiber and matrix, it is necessary to control their interfacial properties. The CF/matrix interfacial properties are reported to be improved by grafting carbon nanotubes (CNT) on carbon fibers. When CNTs are grafted at high temperature, however, the strength of the carbon fiber itself is reported to be decreased. We have been developing the CNT grafting technology using Ni as the catalyst that enables CNT grafting at low temperature. However, the effects of CNT deposition length and deposition density on the interfacial properties of CF/matrix have not been clarified. In this study, the influence of CNT deposition length and deposition density on the interfacial properties of CNT grafting carbon fiber/matrix was evaluated by single fiber pull-out tests. The longer the Ni plating time became, the more the density of Ni particles on carbon fiber became and higher interfacial share strength of CF/matrix was obtained. Also, the longer the grafting time became, the longer CNT grew and resulting higher interfacial share strength of CF/matrix was obtained.
Carbon Fiber Reinforced Thermoplastics (CFRTP) have been attracting attention in various fields due to their excellence in specific strength, specific modulus, productivity and recyclability. A film stacking method is one of the molding methods of CFRTP laminates. In this method, laminated sheets of matrix films and reinforcing fiber sheets are hot pressed. This method needs a long time under high temperature and high pressure in order to obtain good impregnation of the matrix resin into fiber bundles. To improve the productivity of CFRTP, a non-woven stacking method, in which non-woven fabrics and reinforcing fiber sheets are alternately laminated and molded under compressive stress, has been proposed as an alternative means of the film stacking method. Non-woven fabric can be heated faster than the film because of its larger contact area to the heated air. Therefore, productivity is expected to be improved by using non-woven fabric as a matrix of CFRTP. In this study, CFRTP were molded using non-woven fabrics and films for the matrix, and the bending tests of CFRTP were conducted to clarify the effect of resin supply form on mechanical properties of CFRTP. From the observation of melting form, non-woven fabrics melted into individual fibers while films melted into a single mass. For resin supply from, non-woven fabrics showed good impregnation property compared to films. The interlaminar shear strength and the bending strength of CFRTP using non-woven fabrics for the matrix are higher than CFRTP using films with molding time of 240 seconds or less.
The present study attempted to model dynamic viscoelastic properties of polyurethane block copolymers to understand the effect of molecular properties on shape change functions of the materials. For this purpose, we proposed dynamic viscoelastic simulations to obtain stress response in frequency domain against oscillating strain history, using molecular mechanics models consisting of hard and soft segments. The simulations employed coarse-grained bead-spring model based on the force fields given by bond-stretch potential for covalent bonding and Lennard-Jones potential for intermolecular interaction. The rigidity and aggregability of hard segments (HS) were represented by varying the molecular mechanics parameters for these force fields. The proposed simulation could reproduce a clear glass transition from glassy state to rubbery state in frequency domain. The rigidity of HS increased the stiffness at glassy state and tanδ as an indicator of energy dissipation as well, which led to the sharpening of glass transition range. The aggregability of HS increased the stiffness at rubbery state and it also shifted glass transition point to a higher frequency and broadened glass transition range by changing the mobility of soft segments (SS). Finally, a spherically aggregated HS domain was found to degrade shape change functions from the viewpoint of storage modulus. We concluded that the shape change functions of the materials were closely related to energy storage ability of HS and mobility of SS, and that the aggregability of HS yielded by hydrogen bonding in polyurethanes plays an important role in shape change functions.
Because mechanical properties of carbon fiber reinforced thermoplastic composites (CFRTPs) are strongly affected by fibers state, e.g., fiber length, fiber orientation and fiber dispersion, and such fibers state is modified with resin flow in injection molding, detection of fiber state in molded composites is inevitable for assuring strength of injection molded products. In order to develop simple and rapid monitoring method for fiber state in molded parts it was studied how heating of carbon fibers by induction varied depending on their length and orientation. In this study MHz-band frequency was employed to heat efficiently the fibers dispersed separately in injection molded specimens and temperature rise on the surface was monitored by a pyrometer. Variation in temperature rise was observed between specimens whose mean fiber length were 0.12mm and 0.16mm each. It was also observed that temperature rise differed depending on specimen settings, where the cross angle between fiber orientation and the direction of magnetic field changed. Moreover, it was proved possible to extract weld part by using a differential image between the images of specimens with and without weld.
This study aims to reveal the induction heating behavior of CFRTP composites by high-frequency induction heating method. The material used is unidirectional CF/PPS (UD-CF/PPS) and satin weave CF/PPS (woven-CF/PPS) laminates. The effects of geometry of induction coils, coil height, heating time and the carbon fiber reinforcement of CFRTP laminates on heating behavior of CFRTP composites in induction heating were investigated. The experimental results revealed that the surface temperature of UD-CF/PPS and woven-CF/PPS laminates was increased with increasing the heating time. Moreover, the surface temperature was increased with decreasing remarkably the coil distance. It was also confirmed that the carbon fiber reinforcement of CFRTP laminates has an effect on induction heating behavior.
Recently, integral molding technology of the carbon fiber reinforced plastics have been developed. However, the geometry of composite products is complicated, it is problem that molding defect (such as local wrinkle) occur during molding. While, shaping simulations have been developed to improve the molding accuracy by predicting the occurrence of wrinkles. In this simulation, short analysis time and high accuracy analysis are required, and the introducing of the experimental data (such as shear response) is needed. In this study, the pull-out tests of CFRP laminate were performed at room temperature and under curing conditions. The specimen observations and cure extent measurement by DSC test were performed, and the shear stress-displacement behavior were evaluated. As a result, the shear stress-displacement curves showed nonlinear behavior, and it was found that these are affected by applied pressure and tensile velocity. In the pull-out test at temperature under curing conditions, low shear stress was caused by resin softing in early on the tests, and rapid shear stress increase was caused by resin cure, that is, the shearing deformation were affected by the curing extent and resin state.