The purpose of this study is to investigate the influence of the condition of the length of added Cellulose Nano Fibers (CNFs) on the fatigue life of plain woven CFRP plate. Four types of CNFs of which the length were different (about 6, 22, 68 and 126 µm), respectively, while the diameter of those were almost same, were prepared in order to investigate the effect of the fiber length on fatigue lives of the specimens of the CFRP plates under tension-tension cyclic fatigue loading. The apparent interfacial shear strengths between modified matrix and single carbon fiber or two carbon fibers were also evaluated. Changes of energy release rates for crack propagation around the interface between carbon fiber and epoxy resin were also simulated by using FEM. The experimental results showed that the fatigue life of CFRP was not improved by adding comparatively short CNF compared to that of original specimen. In contrast, when comparatively long CNFs were added into the matrix, fatigue lives of CFRPs were improved. The fractured surface observation suggested that interfacial adhesion strength between carbon fiber and epoxy resin was improved by adding comparatively long CNFs. The apparent interfacial shear strength between two carbon fibers and matrix was significantly improved when the well mechanical fiber bridging of CNFs occurred. The calculation results showed that the energy release rate due to de-bonding between carbon fiber and matrix was reduced when CNFs were mechanically bridged between two carbon fibers and matrix.
Carbon fiber reinforced thermoplastics (CFRTPs) are expected to be used for the structural parts of automobiles due to their properties, such as high recyclability and ability of secondary process. They are studied for the purpose of improving productivity and reducing costs. Among thermoplastics, PA6 and PP are expected to be used. PA6 is relatively good in adhesion to carbon fiber among thermoplastics, whereas PP is inferior in adhesion to carbon fiber because it has no polarity. As a fiber modification method for improving the fiber matrix interfacial shear strength (IFSS), oxidizing of carbon fiber has developed. In this study, single fiber pull-out tests of CF/PA6 and CF/PP were conducted to clarify the effect of air oxidation of carbon fiber on the IFSS. The effect of temperature on PA6 and the temperature holding time at specimen preparation for PP on the IFSS were also clarified. The IFSS of CF/PA6 was improved by oxidizing carbon fiber, and the IFSS of CF/PP was improved by oxidizing carbon fiber and using PP with MAPP for matrix. For PA6, the test specimen that was prepared at the higher temperature has higher IFSS. For PP, the test specimen that was prepared with longer temperature holding time has higher IFSS.
Important concerns in Carbon Fiber Reinforced Thermoplastics (CFRTP) molding include resin impregnation into carbon fiber bundles and fiber matrix interfacial adhesion. Generally, thermoplastic resins have a higher viscosity at the time of molding than thermosetting resins, and it is difficult to impregnate matrix resin into a reinforcing fiber bundle. In addition, commercially available carbon fibers are normally coated with a sizing agent for epoxy matrix, compatibility with thermoplastic resin is not necessarily good. Therefore, in order to improve the mechanical properties of CFRTP, it is important to develop a new method for improving the impregnation property and the fiber and matrix interfacial adhesion. In recent years, a method for coating the surface of carbon fiber with amorphous carbon has been developed. It has been reported that CFRTP laminates using amorphous carbon coated carbon fiber or amorphous carbon coated carbon fiber with silica particles as reinforcing fibers have high bending strength. The reason is considered to be the improvement in the interfacial adhesion between carbon fiber and thermoplastic resins by the amorphous carbon coating and the improvement in the impregnation property due to the maintenance of the spread state of carbon fiber bundle by silica particles. However, the influence of amorphous carbon and silica particles on the actual fiber matrix interfacial strength and impregnation characteristic has not been clarified. In this study, amorphous carbon coated carbon fibers with and without silica particles were used as reinforcing fibers; and the fiber matrix interfacial strength with PA6 were evaluated by single fiber pull-out test. In addition, the wettability of amorphous carbon coated plain woven carbon fiber with and without silica particles and the mechanical properties of their laminate were evaluated. By coating the carbon fiber surface with amorphous carbon, the fiber matrix interfacial strength is increased due to the increase of the oxygen-containing functional groups on the fiber surface. By coating the carbon surface with amorphous carbon, carbon fiber plain woven fabric has improved wettability with resin and addition of silica particles spreads fiber bundles and improves wettability. CF/PA6 laminates using amorphous carbon coated carbon fiber with silica particles have lower void content and higher flexural strength.
This study aims to reveal the ultrasonic spot welding behavior of carbon fiber reinforced thermoplastics (CFRTP) using polymer based energy director. The materials used for spot welding are woven-CF/PPS laminates and energy director consisting of 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 displacement of ultrasonic horn during welding process were precisely controlled by using an electric servo press machine. The effects of welding load and thickness of energy director on ultrasonic welding behavior and joining strength were investigated to reveal the welding behaviour. The temperature of the joining part during ultrasonic welding was measured by a ultrafine thermocouples, and the welding load and displacement of the horn was measured by the servo press machine. The welding part was evaluated by microscopic observation and cross-sectional observation using microscope and SEM. From the experimental results, it was revealed that the thickness of the energy director was significantly affects the ultrasonic welding behavior. From the result of single lap tensile shear test, it was found that the there was proper thickness of energy director to increase the joining strength.
CFRP with thermosetting resin as a matrix cannot be welded to metals because thermosetting resins have crosslinked structure and cannot be melted. We have developed a new welding method known as Friction Stir Spot Welding (FSSW) in which by inserting the thermoplastic resin PA12 is inserted between Al alloy and carbon fiber reinforced epoxy matrix composite. ISO 14272 and 14273 joint strength evaluation of resistance spot welding specify tensile shear tests and cross tensile tests. In order to ensure the reliability of welding, it is necessary to evaluate the strengths of each tests and clarify the fracture mechanism of CFRP/Al welded by this method. In this study, CFRP and Al were welded by FSSW using polyamide as an adhesive, and the tensile shear test and cross tensile test were performed to evaluate their strength. Since PA12 and epoxy resin decomposed and disappeared under the welding tool, CFRP and Al are not welded at this area, and the welded strength is maintained at the outer area of this decomposed area. For the tensile shear test, the strain distribution on the specimen surface was measured using a non-contact 3D displacement and strain measurement system. Since Al occurs plastic deformation earlier than CFRP, a crack initiated from the Al side to the center of welded area, and the maximum load was reached followed by sudden fracture. In the tensile shear test, a strength of 7.9 ± 0.1 kN, which was higher than the standard value of resistance spot welding of steel was obtained. In the cross tension test, 1.4 ± 0.1 kN of cross tensile test strength was obtained.
During vacuum heat treatment, the cooling rate of workpiece at gas quenching decreases in proportion to time. It's difficult to express with cooling rate (ex. K/s) the relation between the metallurgical change and the cooling intensity of workpiece. The purpose of this paper is to examine the cooling behavior of workpiece in vacuum heat treatment using lumped capacitance method. As the temperature difference between center and surface of column of workpiece was small at nitrogen gas quenching, heat transfer coefficients were determined around 40 W/(m2・K) at 0.067MPa and around 75 W/(m2・K) at 0.197MPa. The dimensionless temperature, the cooling time and the time constant had an exponential relation. The time constant consists of physical properties and is also defined as the time at that the dimensionless temperature equals to 1/e. The agreement was confirmed by both of experiment and theoretical calculation. It is concluded that the time constant was suitable for the explanation of cooling intensity at gas quenching under vacuum heat treatment.
This paper deals with the material and fracture surface examination of T-type joint for piping that caused hydrogen leakage in a high pressure hydrogen facility (Service pressure: 99 MPa). T-type joint is made of precipitation hardening martensitic 17-4PH stainless steel. The design fracture pressure of T-type joint is as high as 1050 MPa. Nevertheless, T-type joint caused hydrogen leakage in 150 MPa hydrogen gas whose pressure was 14 % of the design fracture pressure. Chemical composition analysis, microstructure observation and Vickers hardness measurement proved that 17-4PH of T-type joint was sound. Fracture surface observation showed that semi-elliptical crack surface was covered with quasi-cleavage and that peel-like defects were formed on taper surface. It was presumed from these points that hydrogen-leakage of T-type joint was caused by hydrogen-induced crack growth from peel-like defect in 150 MPa hydrogen gas.
This paper deals with the fracture mechanics-based analysis of T-type joint for piping that caused hydrogen leakage in a high pressure hydrogen facility (Service pressure: 99 MPa). T-type joint is made of precipitation hardening martensitic 17-4PH stainless steel (corresponds to JIS-SUS630). The design fracture pressure of T-type joint is as high as 1050 MPa. Nevertheless, T-type joint caused hydrogen leakage in 150 MPa hydrogen gas whose pressure was 14 % of the design fracture pressure. According to fracture toughness tests of JIS-SUS630, fracture toughness, KIc, in air at the displacement velocity of V = 5.9×10–3 mm/s was 83.2 MPa・m1/2 and hydrogen-induced crack growth threshold, KI,H, in 115 MPa hydrogen gas at V = 2.0×10–5 mm/s was 7.4 MPa・m1/2 which was 9 % of KIc. Fracture surface in hydrogen gas was covered with quasi-cleavage. Fracture mechanics-based analysis using KI,H showed that hydrogen-leakage of T-type joint was caused by hydrogen-induced crack growth from peel-like defect in 150 MPa hydrogen gas.
Creep damage gradually progress during operation especially at weldment parts of 2.25Cr-1Mo steel boiler pipes. In creep damage assessment of the weldment parts based on stress analysis by a finite element method, creep deformation property of constituent materials in the welded joint has to be obtained. Additionally, it is desired to develop a creep damage assessment method by using a miniature specimen. In this study, in order to clarify influence of a long-term operation on creep strength property, creep tests were performed in inert gas condition using miniature specimens with 1mm diameter taken from each portion of constituent materials of a welded joint in a long-term used boiler header. Although minimum creep strain rates of a base metal and a fine grain are similar to those in a new welded joint, those of a weld metal and a coarse grain are much higher than those in the new welded joint indicating significant reduction of creep strength in the weld metal and the coarse grain occurred during the long-term operation. Creep rupture times of the miniature welded joint specimens are almost equal to those of the standard welded joint specimens. Therefor creep rupture time of the standard welded joint specimen may be predicted by a creep test result of the miniature welded joint specimen. From creep test results of the miniature welded joint specimen taken from creep damaged standard welded joint specimen, it was demonstrated that creep damage greater than medium level can be evaluated by using the miniature welded joint creep specimen.
This report was investigated about accuracy of phase composition of cement and cement hydrates in mortar samples when the heavy liquid separation was applied as sample preparation and then the samples were determined by XRD/Rietveld analysis using the internal standard method. Amount of amorphous phase as a part of hardened cement pastes in mortar samples with the heavy liquid separation were compared to mortar samples without the heavy liquid separation and hardened cement paste samples. As a result, it is found that the heavy liquid separation improved accuracy of phase composition of cement and cement hydrates mortar samples.