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Strain monitoring of resin during cure process is an effective method to manufacture high-quality FRP products because it is known that residual stress generated during molding decreases strength of final products. Although both cure and thermal shrinkage occur during cure process, it is difficult to measure cure shrinkage of FRP products due to dramatic changes in mechanical properties of resin by cure reaction. Among cure monitoring methods, it is proven that fiber optic embedded strain sensors can measure cure shrinkage of resin. However, quantitative evaluation of cure shrinkage strain measured by optical fiber strain sensors has not been conducted. In the present paper, we monitored curing shrinkage of epoxy resin by an embedded EFPI sensor. Mechanical and thermochemical properties of the resin during cure process were experimentally obtained by several methods. Volume shrinkage of resin during cure process was also measured by a dilatometer. In this paper, a viscoelastic model during cure process was driven and the model was used for FEM analysis of cure shrinkage strain generated in the embedded EFPI sensor. From the results, it appeared that the calculated cure shrinkage strain of the EFPI sensors agreed well with the measured strain especially when the degree of cure was high. Therefore, it was concluded that an embedded EFPI sensor can measure cure shrinkage strain quantitatively.

In the present paper, the impact responses and overall damages of Fibre-Metal Laminates based on titanium alloys and glass fibre-reinforced polymers (Ti/GFRP) as Ti/FRP laminate system were evaluated. Low-velocity impact tests using a drop-weight tower were conducted for the cross-plied GFRP laminates and the Ti/GFRP laminates, and the impact responses during impact loading were obtained. Impact damages such as cracks in titanium layer and delamination of titanium-GFRP interface or matrix cracks and interlaminar delamination in GFRP layer were observed from the impact direction. From the experimental evidence, it was found that the Ti/GFRP laminates showed same impact damage modes as of other types of Fibre-Metal Laminates. Internal damages in the Ti/GFRP laminates were restrained by the reinforcing effect of titanium and adhesive layer compared to the cross-plied GFRP laminates. The Ti/GFRP laminates showed two patterns of the impact responses and damages with the threshold impact energy of about 4.8J. With higher impact energy than this threshold, single crack was presented in titanium layer at non-impacted side. The interlaminar delamination area in GFRP layer increased sharply due to the occurrence of this crack. Numerical analysis model that represent the impact behaviour of the Ti/GFRP laminates using finite element method was suggested based on the damage observations. The impact responses obtained by the dynamic analyses agreed well with the experimental results. The calculated area of interlaminar delamination in GFRP layer as a function of impact energy showed same behaviour as seen in the experiments. As a consequence, it was shown by the analysis as well that the drastic increase in the internal damage area of the Ti/GFRP laminates was induced by the fracture in outer titanium layer under the out-of-plane impact loading.

This study proposes a real-time monitoring method for degree of cure (DOC) of curing resin by refractive index measurement using a single-mode optical fiber based on the Fresnel reflection at the fiber end. First, we particularly present an approach for measuring refractive index of resin in real time without the effects of unexpected backward reflection. We applied this approach to measurement of refractive index of an epoxy resin during cure process, and experimentally confirmed that the change of the refractive index was stably measured in real time. Next, we proposes a new approach to calculate DOC from change of refractive index by eliminating effect of temperature change on refractive index of curing resin. This approach can quantitatively evaluate the DOC in real time, which is not obtained by conventional methods, and provide information in the small region at the fiber end. The experimental results of monitoring cure process of the epoxy resin showed that the DOC could be stably measured by this approach without effect of the temperature change. Furthermore, the DOC obtained by this approach was compared with the DOC curve by the thermal analysis using differential scanning calorimeter. These experiments revealed that the DOC by the present approach represented the curing condition of the resin regardless of the cure pattern and was easily translated into the DOC by the thermal analysis. The present approach of real-time quantitative evaluation of the DOC can be applied to fast cure reaction as well as slow reaction demonstrated in this paper.

** Two pitch based high modulus type carbon fibers with oxidative surface and sizing treatments and without any treatments are used in the present work. These fibers are made under the same process derived from the same pre- cursor. Unidirectional carbon fiber reinforced carbon composites (C/C composites) are prepared from two kinds of these carbon fibers and four kinds of carbon matrix precursors, two thermosetting resins (phenolic and furan resins) and two pitches, (coal-tar and petroleum pitches). The tensile test of the strand specimen of the composites heat- treated at different temperatures is carried out by following JIS R7601. Effects of oxidative surface and sizing treat- ments of carbon fiber on tensile properties of the composites derived from different matrix precursors are discussed. In the case of high modulus type pitch based carbon fiber, it is found that there is almost no effect on the strength of composites by the surface oxidation and sizing treatment of the carbon fiber, except for showing different fracture morphologies of graphitized composites derived from the thermosetting resins between the treated and the untreated fibers.

Recently real-time monitoring technique by using built-in sensors of FRP is expected to reduce the cost and enhance the reliability. In the present paper, built-in FBG fiber optic sensors were applied to cure and health monitoring of filament winding (FW) glass/epoxy pipes. FW pipes were wound by ± 45° helical ± 5° parallel winding methods. The FBG sensors were embedded between helical and parallel layers along axis direction of the pipes. Internal strain monitoring of the FW pipes was conducted during cure and fatigue tests. From the results of the cure monitoring, it was found that FBG sensors could be used for measurement of the curing shrink as well as the thermal residual strain. The results of the fatigue tests showed that strain from the FBG sensors had very good agreement with that from the attached strain gauges. Young's modulus of the pipes measured by the FBG sensor was almost constant during the fatigue tests although transverse cracks occurred in the parallel layers. However, the spectral shape reflected from the FBG sensor was affected by existence of the fatigue cracks. In order to evaluate degree of damages near the sensor, correlation coefficient calculated from the spectral shape was introduced. The experimental results showed that the correlation coefficient was useful for evaluation of damages.

Charcoals were prepared under various conditions of carbonization from three kinds of plants: balsa (Ochroma lagopus Sw), giant ipil-ipil (Leucaena leucocephala (Lam.) de Wit) and mousou chiku (Phyllostachys pubescens). The pore structure of the various charcoals was observed on fractured cross-sections under SEM and was evaluated utilizing mercury porosimetry. The sorption capacity of the charcoals for A-grade heavy oil was determined by direct soaking in oil. It was found that the pore structure of the charcoals depended strongly on, not only, the precursor plants but also carbonization conditions, particularly heating rate. Balsa charcoals prepared with a heating rate of 50°C/min. under vacuum gave a relatively high sorption capacity for A-grade oil, approximately 30g/g of charcoal. The other two charcoals, derived from giant ipil-ipil and Mousou chiku, had a considerably lower capacity, approximately 1.5g/g-carbon. The dependence of pore occupancy, the ratio of volume of oil sorbed to total pore volume, on the average pore radius indicated that pores with a diameter above 1μm were largely responsible for heavy oil sorption.

The effect of the orientation texture of cross-sectional graphitic layer planes on transverse compressive properties was studied in pitch-based carbon fibers. The finite element analysis indicated that radially oriented fiber could be expected to display higher transverse modulus and lower transverse compressive strength than randomly oriented fiber. The transverse compressive test was carried out by means of direct pressing of single filaments in which each filament's strength and modulus were measured. The cross-sectional orientation texture was identified not only by observation under scanning electron microscope (SEM) but also by torsional modulus measurement. The transverse modules and transverse compressive strength were found to decrease with increasing crystallite size. It was however not possible to find out clear differences between radially and randomly oriented fiber. These transverse properties cannot therefore be attributed to differences in cross-sectional texture. Bonding between crystallites appears to be the dominant factor in determining the transverse modulus, while the strength in the central area of the cross section is expected to determine the transverse compressive strength of the filament.

The effects of interface control and matrix microstructure on the interlaminar shear strength and mode II interlaminar fracture toughness of 5H satin woven C/C composites were investigated by coating bismaleimide-triazine co-polymer (BT-resin) on the surface of carbon fiber and changing the heat-treatment temperature. Three point short beam flexure tests were carried out for the shear strength. End notched flexure specimens were used for the mode II interlaminar fracture toughness tests. Both the interlaminar strength and toughness decreased by coating BT-resin and increasing HTT from 1600°C to 2500°C. However the influence of the BT-resin coating was much larger for the mode II interlaminar fracture toughness than that for the interlaminar shear strength. This difference of the effect of interface control was discussed on the basis of microscopic observation.

There are two major fracture criteria for notched C/C (carbon/carbon) composite materials. One is that the fracture strength is determined by the fracture toughness criterion, and the other is that it is simply decided by the net section stress failure criterion. In the present study, effects of crack depth, specimen size, and the heat treatment temperature on the strength and fracture mechanism of C/C composites were investigated by using double edge cracked plate tension specimen in order to clarify the confusion on the fracture criterion. For the case of carbonized C/C composites (heat-treatment temperature = 1600° C), the notched strength of wider specimens was smaller than the net section stress failure criterion, showing the notch effect. When the ligament length of the notch was shorter than about 8 mm, the fracture strength of notched specimen was well correlated to the net section stress criterion. On the other hand, the notched strength of graphitized C/C composites (heat treatment temperature = 2500 and 3000°C) was well estimated by the net section stress criterion without respect to the ligament length. The strength of smooth specimen for C/C composites was almost the same without respect to the heat-treatment temperature, while the fiber strength decreased by half with increasing the heat-treatment temperature. Thus, the contribution of the fiber strength to composites strength was much higher for the graphitized C/C composites, The change of fracture mechanism in macroscopic and microscopic scale was well correlated to the change of the fracture criterion.

For carbon fiber reinforced plastics (CFRP) using 6 kinds of carbon fibers with different surface conditions, ILSS was measured from 25°C to 200°C at various cross head speeds (CHS), and the influence of surface structure of carbon fiber on the high temperature ILSS of CFRP was investigated. The following results were obtained.(1) ILSS measured at high temperatures was higher for CFRP with surface treated fibers.(2) The logarithmic values of shift factor aT by adopting time-temperature superposition principle were plotted against 1/T. These Arrhenius' plots showed differences among kinds of fibers and those with or without surface treatment. In every case, plots are aligned on a line that bends at a point.(3) In the lower temperature range than the bending point on Arrhenius' plot, the activation energy were from 140 to 230kJ/mol and it increased with the increment of the amount of COOH group obtained from C1s peak of X-ray photoelectron spectroscopy.

The strength by a short beam method of three point bending was measured for some CFRPs with varying fiber and matrix resin combination and their failure states were examined. The strength of every CFRP decreased linearly with increasing temperature. The strength retention at 200°C depended on the glass transition temperature of matrix resin. The retention for PMR-15 matrix resin was the highest, followed by MY720 resin and then EPIKOTE 828 resin. The failure mode of CFRP with EPI-KOTE 828 matrix resin tested at a cross head speed of 5mm/min was a mixture of shear and compression failure below 150°C, but pure shear failure at 200°C. Pure shear failure was also observed at cross head speeds less than 0.5mm/min, while compression failure in addition to shear failure was observed at cross head speeds more than 5.0mm/min. In CFRP with MY720 matrix resin, only shear failure was observed in the range of testing temperatures. In CFRP with PMR-15 matrix resin, buckling failure in addition to shear failure was observed up to 250°C. These differences in failure modes of different CFRPs were related with the ultimate strain of the matrix resin.