Silver nanoparticles (NPs) / chitosan nanofibers (ChNF) composite material was fabricated with a high-pressure wet-type jet mill. A mixture comprising an aqueous silver nitrate solution and a ChNF suspension was prepared as a raw starting material. The mixture was then processed with the high-pressure wet-type jet mill at a discharge pressure of 100 MPa. An X-ray diffraction analysis of the obtained samples revealed the presence of not only chitosan crystallites, but also silver metal crystallites. According to field-emission scanning electron microscopy observation, many nano-sized silver particles were immobilized and well-dispersed on the surface of ChNF. The silver particles had a spherical shape with an average particles size of about 20 nm regardless of the number of jet milling cycles. The silver content in the composite materials increased slightly with the number of jet milling cycles. As a result of investigating the antibacterial property of the silver NPs /ChNF composite material processed five times with the high-pressure wet-type jet mill at a discharge pressure of 100 MPa, it was found that the minimum inhibitory concentration (MIC) of that for E. coli was 4.2 mg/L, although a raw ChNF had no antibacterial property.
This study aims to reveal the ultrasonic spot welding behavior and joining strength of carbon fiber reinforced thermoplastics (CFRTP) using carbon fiber reinforced energy director. The materials used for spot welding are woven carbon fiber reinforced poly phenylene sulfide (woven-CF/PPS) laminates and energy director consisting of spread carbon fiber and PPS polymer. The ultrasonic oscillator for the ultrasonic spot welding process has an oscillation frequency of 40 kHz and a maximum output of 600 W. The welding load and push-in amount of ultrasonic horn during welding process were precisely controlled by using an electric servo press machine. The effects of welding load and carbon fiber volume fraction in energy director on ultrasonic welding behavior and actual tensile shear strength were investigated to reveal the welding behaviour. The welding load and push-in amount of the ultrasonic horn was measured by a data logger. The welding part was evaluated by image analysis and cross-sectional observation using microscope. From the experimental results, it was revealed that the carbon fiber volume fraction in the energy director was significantly affects the ultrasonic welding behavior. According to the results of single lap tensile shear test, it was found that the actual tensile shear strength was increased by the addition of spread carbon fiber.
It has been reported that the tensile strength of CFRTP becomes lower at high temperatures due to the decrease in the fiber matrix interfacial strength at high temperatures. This is due to the relaxation of the fiber-tightening pressure of the resin due to the thermal expansion of the resin at high temperatures. Therefore, it is important to suppress the thermal expansion of the resin. In this study, we focused on the addition of SiO2 particles to the matrix resin as a means of suppressing the decrease in the fiber matrix interfacial strength at high temperatures, and conducted single-fiber pull-out tests at room temperature and 80 °C using carbon fibers and PA6 resin with different amounts and surface treatments of SiO2. Up to 5.0 wt%, the higher the amount of SiO2 added to PA6, the lower the coefficient of thermal expansion, but when 10 wt% was added, the coefficient of thermal expansion became higher due to agglomeration of SiO2. The frictional force at the fiber matrix interface determined from the microdroplet test, which is considered to be the fiber-tightening pressure of the resin, corresponds to the fiber matrix interfacial strength determined from the single-fiber pull-out test at room temperature and 80 °C. The higher the fiber-tightening pressure of the resin, the higher the fiber matrix interfacial strength. The decline rate in strength from room temperature to 80 °C is lower for resins with a lower coefficient of thermal expansion. The addition of water-treated SiO2 shows the lowest coefficient of thermal expansion and the lowest decrease in the fiber matrix interfacial strength from room temperature to 80 °C.
Recently, a modified CrMoV forging steel was developed with superior creep strength property by adding tungsten to conventional CrMoV forging steels which are widely used for high temperature components. Generally, creep damage proceeds at stress concentration portion under multiaxial stress gradient. It is important to clarify creep rupture property and damage evolution process under the multiaxial stress states for the modified CrMoV forging steels to achieve life extention. In this study, creep tests were conducted using plain and four kinds of circular notch specimens with tip radius in 0.1 mm(R0.1), 0.5 mm(R0.5), 2.0 mm(R2.0) and 4.0 mm(R4.0) of the modified and a conventional CrMoV forging steels as well as finite element creep analyses of the notch specimens. It was found that creep rupture times of the modified CrMoV forging steel are three times longer than those of the conventional one showing notch strengthen effect. Rupture times of the notch specimens of the modified one are longer as the tip radius decreases. Although the distribution tendency of creep void number density at the notch root sections is similar in both CrMoV forging steels and it corresponds to the distribution tendency of the maximum principal stress, number of the creep voids in the modified one is smaller than that in the conventional one. The difference of creep rupture property and void number density between the modified and the conventional ones may be caused by the difference of initial dislocation density and contents of Laves phase. It was demonstrated that creep rupture times of both the modified and conventional ones were precisely predicted based on the area average damage concept considering effective damage region with the principal creep strain as a criterion.
Creep-fatigue life evaluation method for HR6W base metal and welded joint was examined based on the results of uniaxial fatigue and creep-fatigue tests using smooth specimens at 700 and 750℃. Regarding the base metal, creep-fatigue life evaluation by linear damage rule was carried out using the creep-fatigue test results with strain holding time up to 10 hours. In the case of time fraction rule, failure criterion with intersection at (Df , Dc) = (0.1 , 0.1) gave conservative prediction even in the case of long holding time of 10 hours. In the case of ductility exhaustion model, on the other hand, life reduction with increase of strain holding time was expressed well including the dependence on strain range. With respect to the welded joint, fatigue life was shorter than that of base metal although cracks were observed at base metal near fusion line under all test conditions. From both the FEM analysis results and DIC (digital image correlation) strain measurement during the test, this was found to be caused by strain concentration due to the longitudinal variation of stress-strain behavior in the welded joint. Fatigue life of welded joint could be predicted well by using the base metal fatigue curve and the strain concentration factor obtained from FEM analysis.
Thermal barrier coating (TBC) generally consists of top coat (TC), bond coat (BC), and substrate. The TBC system is used for the hot section components of gas turbine engines. It is important to know the accurate residual stress distribution in the TBC system. Thin layer-removal method can evaluate the through-thickness stress distribution based on the strain change of substrate when the coating surface layer is sequentially removed. However, this method has not been applied to a three-layered material. In this study, we proposed an analytical formula for the stress distribution of a three-layered model and a stress correction formula taking into account the stress redistribution of removal layers into the remaining piece. The stress correction formula provides the accurate stress even if the coating is relatively thick compared with the substrate, in which case conventional method is inapplicable. Using the proposed formulae, we experimentally evaluated the through-thickness stress distribution of BC system materials, consisting of substrate and BC (CoNiCrAlY) prepared by atmospheric plasma spraying or high-velocity oxygen fuel spraying. TBC system materials, consisting of substrate, BC, and TC (yttria-stabilized zirconia or alumina) were also evaluated. The obtained stress values and distributions were appropriate to the mechanism of stress evolution (deposition and cooling), indicating that the proposed model is effective for the stress evaluation of three-layered materials.
Rotating bending fatigue tests were conducted using A6061 and Al-Si eutectic alloys to investigate fatigue strengths and small fatigue crack growth behavior. Both alloys were fabricated by continuous casting and extrusion, and the effect of fabrication processes on the fatigue properties was investigated. The fatigue strengths of continuously-casted and extruded A6061 alloys were nearly comparable. The specimens, which were sampled parallel to the extrusion direction, exhibited higher fatigue crack initiation resistance than the specimens sampled perpendicular to the extrusion direction. However, the fatigue crack growth resistance was higher in the specimens sampled perpendicular to the extrusion direction. The differences in the crack initiation and growth resistances were attributed to the different microstructures depending on the extrusion direction. The fatigue strength of the extruded Al-Si eutectic alloy was higher than that of continuously-casted alloy, which could be attributed to the higher fatigue crack initiation resistance of the extruded alloy. The continuously-casted Al-Si eutectic alloy had typical dendritic microstructure, in which eutectic Si dispersed heterogeneously. The extrusion process homogenized the dispersion of eutectic Si in the microstructure, which resulted in the higher fatigue crack initiation resistance.
In this study, strength properties of concrete using steel-slag coarse aggregates such as electric-furnace oxidized-slag coarse-aggregate (EFG) or blast-furnace slag coarse-aggregate (BFG) were investigated. In addition, the cracking process when concrete using EFG and BFG had received compression loading was confirmed by acoustic emission (AE) techniques. As the result, the compressive strength of EFG concrete was 1.2 times as large as that of concrete using normal aggregates such as tight-sands crushed-stone (NG), river gravel (RG), and limestone crushed-stone (LG). BFG concrete had approximately the same compressive strength compared with concrete using normal aggregates. In the AE measurement result, the accumulation number of AE hits of EFG concrete increased with normal aggregate concrete. On the other hand, the accumulation number of AE hits of BFG concrete was increased notably from the early stress level. Therefore it was considered that a lot of destruction had occurred inside concrete from an early stage. From relation with stress level and improvement b (Ib) value, and RA value-average frequency(A-FRQ) relation, cracks that occurred inside BFG concrete were hard to progress even if more than stress level 40%. However, in the case of BFG concrete, cracks progressed easily from early stress level because of many voids which exist between coarse aggregate and mortar.