Very uniform nanocomposite materials (PHS-NCs) were prepared by in-situ polymerization of silicon alkoxide in a phenolic resin matrix. The transparency and morphology of the PHS-NCs could be varied by controlling the resulting silica size. SEM and TEM observations reveal that the PHS-NC shows a distinct fine morpholgy which is very different from that of conventional phenolic resin/silica glass composite. The PHS-NC exhibits excellent mechanical improvements in modulus, strength, strain at break and impact strength. It was found that the PHS-NC did not show noticeable hysteresis nor acoustic emission before the fracture in the flexural test, which is different from that of the conventional glass fiber reinforced composite. The PHS-NC also exhibit high surface hardness and smooth fracture surface. Furthermore, the tribological properties such as friction and wear characteristics are improved in the PHS-NC by incorporation of in-situ polymerized silica in phenolic resin matrix.
The effect of interfacial properties on damage behaviors of woven cloth fabric reinforced composites has been investigated by the edge cross-sectional observation of laminate under the tensile load in SEM. Test specimen used is vinyl ester resin matrix laminated composite reinforced by glass woven cloth. It is obvious that the surface treatment conditions of the fiber have affected the appearance of microscopic damages, such as the interfacial debonding and matrix cracking into the fiber bundle, or the generation of AE event. In the case of the fiber treated with the silane-coupling agent suitable for the matrix resin, only a few cracks occur into the fiber bundle just prior to the maximum load, and AE events generate fewer. In the case of chemical-mismatch treated fiber or non-treated fiber, however, much more cracks occur into the fiber bundle at the lower tensile load, but the generation behavior of AE events differs with each other. It is found by in-situ SEM observation that the fiber surface treatments have affected the initiation and progression of microscopic damage under the tensile load. Thus, it can be concluded that in-situ SEM observation with measuring both tensile load and AE event count is a useful evaluation method for understanding the effect of interfacial property on microscopic damage behavior.
Effect of fiber surface treatment on delamination fatigue under mode II loading was investigated for unidirectional CF/Epoxy laminates. Two types of laminates were made from surface-treated-carbon fiber or surface-non-treated fiber, and a common epoxy matrix. Tests were carried out using end notched flexure (ENF) specimens. Stabilized mode II static tests showed that the fracture toughness of the surface-treated CFRP was 30% higher than that of non-treated CFRP. Fatigue crack growth resistance of the surface-treated CFRP was higher than that of non-treated CFRP at higher crack growth rate. However, the effect of fiber surface treatment was negligible near the threshold region. At higher growth rate, the interfacial fracture occurred prior to the matrix fracture near the crack tip for the non-treated CFRP. Then, the fracture mechanism was controlled by the interfacial fracture. On the other hand, the resin fracture with plastic deformation occurred prior to the interfacial fracture near the crack tip for the surface-treated CFRP. Then, the fracture mechanism was controlled by the resin fracture. Near the threshold region, the ratio of the resin fracture was rather large without respect to the fiber-surface-treatment. The main fracture mechanisms near the threshold region was only controlled by the matrix resin. This fact was well correlated to the fact that the threshold value was insensitive to the fiber surface treatment.
The embedded single fiber transverse tensile test has been proposed as a new experimental evaluation method on composite interfaces. The effect of surface treatment on microscopic damage, such as the interfacial debonding and matrix cracking around a fiber was investigated by in-situ SEM observation, when the transverse tensile load to the fiber longitudinal direction was applied to an embedded single fiber composite. The experimental result revealed that the surface treatment condition of fibers has an influence on the initiation and propagation of interfacial debonding and matrix cracking around a single fiber. In addition, the effect of interfacial properties on microscopic damage was investigated by a finite element analysis. The analysis was carried out by using a three layers model, which was modeled separately, fiber, matrix and interphase into the test specimen. The property for the interphase was assumed to be orthotropic, i.e. both tensile and shear moduli of the interphase are dealt independently to express the role of interphase for stress bearing and stress transfer separately. The effect of interphase properties was investigated by changing the tensile and shear moduli of the interphase. The reasonable elastic properties of the interphase were obtained by comparing the computational results with experimental ones. The Hoffman's failure criterion was used for judging of the damage in the interphase and the matrix. The validity of this analytical method is verified by comparing the analytical results with the experimental ones.
An analytical model for predicting the matrix crack induced stiffness reduction of FRP laminates with off-axis cracked plies is developed. The constitutive equations for a cracking ply are first proposed with the assumption of the equivalence between a cracking ply and a work-softening material. Then they are incorporated into the classical lamination theory to describe the damage evolution in FRP laminates with off-axis cracked plies. The energy release rate in a cracking ply is adopted as the criterion for crack formation, and the number of cracks is predicted by dividing the cumulative released energy in a cracked ply by the released energy due to single crack formation. With the predicted number of cracks, the variation of the laminate stiffness with the crack density is evaluated. The uniaxial tension test is performed on GFRP [0/θn/0] laminates, and the elastic modulus in the loading direction is measured at five different crack density. The experimental results are compared to the corresponding predictions of the elastic modulus for three different center ply angles.
Thermal fatigue tests of an unidirectional SiC fiber reinforced Ti-24Al-11Nb matrix composite, SCS-6/Ti-24Al-11Nb, have been carried out without external load in air and in vacuum, under various thermal cycle conditions. The fiber employed was a β-SiC fiber of 140μm diameter with a carbon rich graded silicon carbide coating on the surface: SCS-6. This paper covers the following characteristics of the material subjected to thermal fatigue, compared with those subjected to long term isothermal exposure: (i) change of the visible crack density with thermal cycling; (ii) change of the fiber/matrix interface morphology by means of EPMA and SEM; (iii) change of the interfacial shear strength by the push-out tests, and (iv) effect of environment on the properties from (i) to (iii). Based on these systematic investigations, special attentions are paid to understand the mechanisms and mechanics of thermal fatigue failure, and how thermal fatigue damage should be assessed.
In recent years, it has been required for the packaging density of PWB to be improved. It is necessary that the holes are made smaller in drilling and that the circuit patterns are made fine and multi-layer. Therefore, laser drilling is paid attention as a new method of the smaller diameter drilling in PWB. This study describes the characteristics of the blind via hole for multi-layers PWB by laser drilling using CO2 laser beam. Specially, aramid fiber reinforced plastics (AFRP) for laser drilling is compared with glass fiber reinforced plastics (GFRP). As results, it is clear that irradiation time has influence on the damage width around the hole for both PWBs. It takes shorter time to drill the blind via holes on PWB for AFRP than GFRP. Additionally, the taper angle of laser drilled hole is an important factor of copper plating of blind via hole after drilling.
N2+ ions were implanted into commercially supplied pure titanium with different doses ranging from 5×1016 to 1×1018ions/cm2, and the corrosion behavior was investigated in 70% sulphuric acid solution. Anodic polarization curves show that the peak anodic current density, passivation current density, and corrosion potential increase with increase in implantation dose when the dose is less than 1×1017ions/cm2. When the implanted dose is 5×1017ions/cm2, the corrosion resistance decreases though it is still larger than no implanted titanium. X-ray diffraction analysis shows the formation of titanium nitride on the implanted surface, i.e., Ti2N is found when the implanted dose is larger than 5×1016ions/cm2 and both Ti2N and TiN when the implanted dose is 1×1018ions/cm2. The aging at 673K after ion implantation largely increases the corrosion resistance, which corresponds to the enhanced precipitation of Ti2N and TiN. AFM observation shows fine particles of titanium nitrides on the implanted surface. It is considered that the precipitation of Ti2N or TiN decreases the effective corrosion area to make the corrosion resistance higher.
A mullite/SiC composite ceramic was sintered. Four point bend tests were conducted according to JIS standard. Semi-elliptical surface crack of 100 or 200μm in diameter was made on the specimen. For four kinds of specimens (asreceived, heal treated, pre-cracked and heal treated cracked), crack healing behavior and high temperature strength were tested systematically. The main conclusions were obtained as follows: (a) Mullite/SiC composite ceramic has ability to heal crack. (b) Best healing condition was found to be 1300°C, in air for 1h. (c) Maximum crack size to be healed is semi-elliptical crack of 100μm in diameter. (d) Crack healed part has enough strength up to 1100°C and most specimen failed outside the pre-cracked zone and healed crack is not sensitive to static fatigue.
One of the limiting factors in an operation of a high-speed train is the wave propagation rate in trolley wire, because the current collecting performance is reduced when the speed of a train reaches the wave propagation rate. Therefore, an operation of a high-speed train requires trolley wire with high wave propagation rates. For this purpose, the trolley wire should be tightened up with high tension stress which in turn requires a high-strength property and/or should have less density, although the mechanical properties including wear and (corrosion) fatigue and other properties such as electric properties should be sufficient. In this investigation, fatigue and corrosion fatigue properties have been evaluated in a newly developed high-strength Cu-Sn trolley wire for an operation of a high-speed train. The influence of NaCl solution on corrosion fatigue life is found negligible, because of relief of stress concentration due to initiation of multiple cracks, blunting of corrosion fatigue crack tip by dissolution, and decrease in total crack depth due to general corrosion on the wire. Therefore, the corrosion fatigue strength is determined by mechanical fatigue strength in air. Attention is also paid to crack initiation and corrosion behavior by using scanning electron and atomic force microscopy, and the mechanisms of corrosion fatigue are discussed.
Fatigue crack initiation and growth in titanium alloy composites reinforced with TiC and TiB particles were investigated. The composites were Ti-6wt%Al-4wt%V reinforced with 10vol% of TiC particles and Ti-8wt%Al reinforced with 8vol% of whisker like TiB particles. They were produced in-situ by the vacuum arc remelting process. Fatigue strength and life of the composites in notched specimens are significantly less than that of Ti-6wt%Al-4wt%V alloy, while the static strengths are higher in both composites. On the other hand, fatigue crack growth rate is dependent on the type of the reinforcement. The particulate TiC reinforcement increases the growth rate through their cracking, while the TiB particles decreases the growth rate through the higher crack closure level and the crack deflection. Direct observations in scanning electron microscope by in-situ fatigue tests revealed that the fatigue cracks in composites initiated from the cracking of the reinforcing particles and the early cracking and its growth to matrix reduced the fatigue lives of the composites in low cycle region. At the stress level above the fatigue limits, fatigue cracks in both composites initiated at the matrix or the interface between the matrix and the particles. Degradation in fatigue limit might be caused by the difference in microstructure of matrix. The titanium alloy itself shows higher static and fatigue strength. However, the reinforcements are not so tough as to bear their high shared stress. The reinforcements are eventually damaged due to high applied stresses and those provide occasionally fatigue crack initiation site.
It is known that the strength of FRP laminates decreases due to the delamination caused by impact loading. Although a CAI (=Compression After Impact) test is often performed to evaluate the impact damage of FRP laminates, it requires a relatively large-sized test specimen and a complicated test jig. The objective of the present study is to search a simple method alternative to the CAI test for evaluating impact damage of FRP laminates. For this purpose, we measured the interlaminar shearing strength (=ILSS) by means of a short beam three-point bending test and the bending strength by means of a conventional three-point bending test after impact damage is given to three kinds of FRPs using an air gun type test apparatus. The test results show that a short beam three-point bending test after impact is a promising method for evaluating the impact damage of FRP laminates.
Stress states in adhesive layer of bonded joints are usually three-axial. However, yield and failure behavior under triaxial stress conditions have yet been clarified sufficiently. Two types of adhesive joints are available for investigating yield and fracture characteristics. One is scarf joint which has considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied by changing scarf angle. Another is butt joint with thin wall tube in which considerably uniform pure shear stress state in the adhesive layer can be realized under torsional load condition. Furthermore, stress triaxiality which appeared in the adhesive layer of most of adhesive bonded joint can be reproduced by tensile tests of the scarf joint and torsional tests of the butt joint with thin wall tube. In this study, the effects of stress triaxiality on yield and failure stress were investigated by above mentioned tests using three kinds of adhesive with various ductility. The results showed that the yielding and failure criterion was changed by stress triaxiality and that the fracture mechanism of adhesive layer with homogeneous adhesive was different from the one with heterogeneous adhesive. Based on these experimental results, the method to predict the yielding and failure criteria were proposed by using the triaxiality parameter.