Our previous work demonstrated that selective induction heating of carbon fibers increases the flexural modulus of CFRTP(Carbon Fiber Reinforced Thermo-Plastics) injection-molded products, indicating that selective heating of carbon fibers offers the possibility of repairing interface debonding in CFRTP. In this study, it is demonstrated that selective heating of carbon fibers using a focused halogen lamp can repair interfacial debonding in a monofilament model composite consisting of carbon fiber monofilaments and a polypropylene resin matrix, without requiring external pressure. The stress distribution in the stress recovery region around the fiber end was monitored before and after heating using a micro-Raman spectrometer. The debonding region and shear strength of the rewelded interface were evaluated from the fiber stress profile. Varying the heating conditions revealed that the reweld interface strength recovered to full virgin strength when it was heated for a short period of time at high power. In contrast, the rewelded interface strength was found to decrease after a long period of heating at low power. The shear strength of the repaired interface was also found to vary with the embedding depth of the fibers.
Since thermoplastic resins have a higher viscosity than thermosetting resin during the molding process, it is necessary to develop a method for impregnating resin into the carbon fiber bundle at the molding of carbon fiber reinforced thermoplastic (CFRTP). Recently, a method for grafting carbon nanotubes (CNTs) on the surface of carbon fibers has been developed. It has been reported that CNT grafted carbon fibers have higher fiber/matrix interfacial shear strength, and one of the reasons for this result is considered to be the improvement of wettability. Therefore, it can be expected that grafting CNT on carbon fiber surfaces can be used not only for obtaining high fiber/matrix interfacial shear strength, but also as a technique for ensuring the impregnation property of the matrix resin. In this study, grafting CNT on the carbon fiber surface was focused on as a method for improving the impregnation property. The fiber/matrix interfacial shear strength, wettability of a single fiber and fabric surface, and impregnation property of CF/PA6 laminates were evaluated. By grafting CNTs on the surface of carbon fibers, the single fiber had higher wettability and higher fiber/matrix interfacial shear strength than untreated carbon fiber. CNT grafted plain woven carbon fiber fabrics with CNT deposition times of 30 and 60 minutes had higher wettability with PA6 resin than untreated plain woven fabrics. In the case of CNT deposition time of 30 minutes, CF/PA6 laminates with excellent impregnation were obtained, while CF/PA6 laminates made with CNT deposition time of 60 minutes had partly higher void content since excessive CNTs restricted the inflow of resin into carbon fiber bundles.
In order to measure strain of non-orientated polymers or metals in m order spatial resolution by using microscopic Raman spectroscopy, single-wall carbon nanotube (SWCNT) which has piezo-spectroscopic characteristics was coated with aligned on the substrates. Solution in which CNTs were uniformly dispersed with help of a dispersant of hydroxypropyl cellulose were oriented by spin-coating into film on a PET plate. Orientation of CNT was observed by AFM and the D* Raman band was measured as a function of tensile strain. Relation between microstructure and the strain dependence of the Raman shift were investigated for test pieces with different degrees of orientation. When CNTs are well aligned in one direction and the aligned direction coincided with the polarization direction of the incident laser light, the peak wavenumber of D* Raman band proved to change linearly with applied strain at the slope of about -9 cm-1/%. On the other hand, in-plane uniformity of initial D* Raman band at zero strain was reduced with degree of orientation.
It is important to evaluate the fiber/matrix interfacial shear strength (IFSS) of Fiber reinforced thermoplastics (FRTP) as their mechanical properties are greatly affected by the IFSS. Moreover, IFSS is considered to be affected by chemical bonding and physical bonding occurring by the force with which the resin tightens the fiber. Although single fiber pull out tests have been well studied for evaluating the IFSS, it has not been clarified whether chemical bonding or physical bonding is the dominant factor in IFSS. In this study, single fiber pull out tests were conducted using glass fibers with sizing agent, and glass fibers without sizing agent which are considered to be less capable of chemical bonding. The effects of cooling speed on the specimen at preparation, and the temperature during single fiber pull out tests were revealed. There was no significant difference between the IFSS obtained by the single fiber pull out test under 25℃ using glass fibers with sizing agent and without sizing agent. This means that the effect of chemical bonding does not appear on the IFSS under high tightening force of resin in the radical direction of the glass fiber. Although the IFSS evaluated with specimens prepared through high cooling speed or under high temperature was degraded regardless of the presence or absence of sizing agent on the glass fibers, the decrease of IFSS could be reduced by using glass fibers with sizing agent. This result means that the chemical bonding can be evaluated by single fiber pull out tests under the condition of relatively low tightening force in the radial direction of the resin, such evaluation under high temperature and with specimens prepared through high cooling speed.
Ni based superalloy IN 738 LC is used for the first stage blades of power generation gas turbines and aircraft jet engines. Creep damage occurs preferentially in stress concentration portions. Therefore, it is important to clarify creep damage evolution process under multiaxial with stress gradient on keeping safety operation. In this study, creep tests have been conducted by using smooth and notch specimens which have multiple round notches with notch tip radius of 0.5 mm(R0.5) and 2.0 mm(R2.0) on IN 738 LC. Crystal misorientation parameters of damaged specimens were measured by an EBSD method. Creep rupture times of the notch specimens were longer than those of the smooth specimens showing notch strengthen effect. Voids were observed at the grain boundaries vertical to stress direction in the smooth specimen, whereas voids were also observed at the grain boundaries with a small inclination from the axial direction in the notch specimens. The maximum void length took the maximum value around the notch root in R0.5, and decreased toward center of the specimen. Those results correspond to higher axial tensile stress and axial creep strain yielding at around the notch root in R0.5. From the crystal misorientation measurement, significant difference of GRODave was not observed at the same creep damage level between the smooth and around notch root of the R0.5 specimens. Accumulated axial creep strain around notch root of the R0.5 specimen is almost equal to that of the smooth specimen at the same damage level. This suggested that GRODave corresponds to accumulated axial creep strain regardless of stress states.
In general metallic materials, flow stress during plastic deformation depends on plastic strain, plastic strain rate and temperature. At an extremely high strain rate more than 104 s-1, the flow stress tends to drastically increase with strain rate. Moreover, some researchers reported that the flow stress also increases with temperature at such high strain rate. This indicates that the temperature dependence of the flow stress must be evaluated under a corresponding strain rate condition. However, a conventional split-Hopkinson pressure bar method is difficult to evaluate the temperature dependence due to the measurement principle. In this study, a simple estimation method for the temperature dependence of the flow stress based on the difference between the indentation sizes was proposed. In this method, the temperature dependence is estimated from the difference between the indentation sizes formed at room and high temperatures via a high-velocity impact test, a drop-impact test, and an indentation test with a spherical impactor. The fundamental equation in the proposed method was derived based on the energy conservation during impact process and the expanding cavity model combined with the Johnson-Cook (JC) flow stress model. The equation of thermal conduction was also introduced into the cavity model. The verification with the finite element analysis revealed that the temperature dependence of the flow stress can be accurately estimated under various materials, temperatures, strain rates, and radii of impactor by the proposed method.
One of the common issues related to the ground improvement methods is that it is difficult to properly design and manage the construction process of improved soil body because the condition inside the target ground cannot be directly confirmed (visualized) during the construction. if the target ground can be modeled using a computer and the excavation and agitation can be reproduced by the ground improvement method, it is possible to predict what kind of ground improvement is performed in the target ground. As the result, more efficient ground improvement can be considered. In this study, the process of excavation and agitation is three-dimensionally modeled on a computer, and the behavior inside the target ground and its influence on the surrounding ground are visualized. The purpose of this is to evaluate the excavation and agitation by the DCS blades while confirming the condition inside the target ground. For the purpose of experimentally and visually confirming the condition of excavation and agitation by the DCS blades, an excavation and agitation model experiment by the DCS blades using pellets colored in each color is carried out.