Spinnable carbon nanotubes enable us to produce carbon nanotube assemblies such as yarn and sheet without binder. Untwisted carbon nanotube yarn is one of the assemblies. Untwisted carbon nanotube yarns are composed of unidirectionally aligned carbon nanotubes along with the yarn axis, and thus untwisted carbon nanotube yarns are an ideal preform for composite fabrication with high mechanical performance. In this study, untwisted carbon nanotube yarns with varied densities were prepared by using spinnable carbon nanotube forests, and the tensile properties were examined. Furthermore, an analytical model of tensile properties for untwisted carbon nanotube yarns were proposed based on the shear-lag model. The proposed model can predict the tensile behavior of an untwisted yarn from the tensile modulus and strength of a carbon nanotube, the volume packing fraction of carbon nanotubes for the untwisted yarn, and shear stress exerted on the slipped region of a carbon nanotube. Then, the proposed model was applied to estimate the elastic modulus and tensile strength of a carbon nanotube from the tensile properties of untwisted carbon nanotube yarns. In our case, the estimated tensile elastic modulus of a carbon nanotube was about 200 GPa, and the estimated tensile strength of a carbon nanotube was about 1.9 GPa.
This paper deals with characterization of interfacial properties of bonded dissimilar materials to fabricate biocompatible composites and functionally graded materials(FGMs) with high mechanical performance. To develop biomaterials with conflicting properties, such as hardness and toughness, for artificial bone, much attention has been paid to the composites and FGMs consisting of biocompatible ceramics and metals. Their mechanical properties are influenced by interface of dissimilar materials, and the interfacial properties should be evaluated. Hence, this study aims to investigate the influence of material combination on interfacial strength and toughness of bonded dissimilar materials consisting of four types of biocompatible materials: titanium, type 316L stainless steel, partially stabilized zirconia, and alumina. The bonded dissimilar materials were fabricated via spark plasma sintering technique, which is a powder metallurgy technique utilizing uniaxial load and pulsed direct current in vacuum. The interfacial strength and toughness of the materials were evaluated via compression-bending testing and indentation testing, respectively. The distributions of elements near the interfaces due to atomic diffusion during sintering were evaluated, and the influence of material combination on interfacial properties was discussed based on the distributions. As a result, it was found that the mechanical properties of all interfaces became lower than those of the monolithic materials, and the amount of reduction in mechanical properties was dependent on the material combination. If the atom diffusion occurred on both sides of the interface, the interfacial toughness and strength tended to be relatively high.
A thermal barrier coating (TBC) system consists of a substrate, a bond-coat (BC), and a top-coat (TC: TBC). The coatings are deposited by thermal spraying. The interfacial fracture toughness between TBC and BC is important mechanical property because it is necessary for damage assessment. The TBC system suffers from thermal exposure, and the interfacial fracture toughness is changed by the thermal exposure as well as the TBC toughness. However, there are few studies on the relationship between the interfacial and TBC toughnesses due to the difficulty of accurate comparison. The purpose of this study is to investigate the relationship between interfacial and TBC toughnesses. For this purpose, we evaluated the two toughnesses of thermally exposed TBC specimens using an indentation technique with an identical specimen and identical evaluation theory. Our results revealed that the interfacial fracture toughness was higher than the TBC fracture toughness for both the coatings as-sprayed and exposed at a moderate temperature. On the other hand, the TBC toughness became to be close to the interfacial toughness for the coatings exposed at high temperatures. Additionally, the convergence rate became to be high by higher treatment temperature. The interfacial cracks occurred not at the strict interface but at the TBC near the interface. These results suggested that interfacial toughness was essentially identical to coating toughness and the changes in the two toughnesses were caused by sintering of the TBC.
In order to find the alternative tungsten alloys to the radioactive Thoria-Tungsten alloy (ThO2-W), the carbide added tungsten alloys (HfC-W and TaC-W) were fabricated by the sintering process. Emission current density was measured to evaluate thermal electron emission property. The thermal electron emission current density of HfC-W was the highest among TaC-W, ThO2-W and W. Tensile tests were carried out at room temperature and elevated temperature of 1573 K and 1773 K. Particles added tungsten alloys showed higher strength compared to tungsten alloy without particles. Grain growth might be suppressed by adding particles, particularly in carbides. Carbides, HfC and TaC distributed among grain boundary could effectively suppress grain growth during sintering and high temperature testing. The highest tensile strength with higher stability of mechanical properties at elevated temperature was obtained in HfC-W among TaC-W, ThO2-W and W. Therefore, it could be concluded that HfC-W would be one of the most promising thoria-free tungsten alloys. Thermal electron emission property and high temperature tensile property were also evaluated in sintered ThO2-W alloy and forged ThO2-W alloy which experienced forging process followed by sintering process. Electron emission current density and tensile strength at room and elevated temperatures were almost comparable in the ThO2-W alloys regardless forging process.
A beta-titanium alloy, which has good biocompatibility and low Young’s modulus, is expected to use for biomedical applications. In this study, in order to investigate the near-threshold fatigue crack propagation in a beta-titanium alloy (Ti-29Nb-13Ta-4.6Zr; TNTZ) with low Young’s modulus, stress intensity factor decreasing tests were conducted under the force ratios from 0.1 to 0.8 in air at room temperature. After testing, crack profiles were observed by scanning electron microscopy, and microstructures around crack profiles were analyzed using electron backscatter diffraction to discuss the mechanism of fatigue crack propagation. The crack growth rate in the solution-treated TNTZ followed by aging were constantly higher at comparable stress intensity range levels, and its threshold stress intensity ranges were lower compared to the only solution-treated TNTZ. This is attributed to the reduction of the opening stress intensity factor resulting from the formation of the alpha-phase by aging. However, the effect of microstructure on fatigue thresholds in TNTZ was disappeared by eliminating the crack closure phenomenon.
The present study investigated the effect of a compressive mean stress on the fatigue strength and crack growth behavior using smooth and notched specimens, respectively, for aluminum alloys of 6061, 6066 and 7075 in dry and humid air environments by an ultrasonic fatigue testing machine. Very high cycle fatigue strength of the smooth specimen of 6061 and 6066 alloys were insensitive to the relative humidity of the air, while it was much reduced in humid air than in dry air for 7075 alloy. The influence of the compressive mean stress on the fatigue strength was small for all the alloys regardless of the relative humidity. Crack growth characteristics tested by the notched specimen with precrack revealed that the crack began to grow under stress intensity factor range, ΔK, constant condition when the tensile component of ΔK, i.e. maximum stress intensity factor, Kmax, exceeded the threshold value of effective stress intensity factor range, ΔKeff,th, obtained by crack growth tests at high stress ratio. Crack growth life was predicted by the fatigue crack growth characteristics obtained by the latter tests. Also, it was compared with the fatigue strength characteristics and the small crack initiation and growth behavior obtained by the former tests. The results suggested that the crack closure point reduced to be compression side by the compressive mean stress. Therefore, a fatigue life under compressive mean stress could be evaluated conservatively by a full load range including compressive component.
For utilizing fuel cell systems, it is important to establish a design standard for safety. For this purposes, it is necessary to accumulate design data, such as fatigue propagation properties of candidate materials for fuel cell systems equipment. However, fatigue crack propagation tests in a high-pressure H2 gas are difficult, because it is not able to apply crack length measurement instrument such as a microscope as a direct measuring and/or strain gages and a clip gage as an indirect measuring. In this study, a new fatigue crack propagation test with piston displacement aided compliance technique have been proposed for that carried out in high-pressure H2 gas. For verifying its usefulness, fatigue crack propagation properties of SCM435 under 115-120 MPa ultra high-pressure H2 gas were evaluated. It was found that the fatigue crack in H2 gas grew faster about 30 times than that obtained in ambient air.
Lap-shear joints of aluminum alloy A6061-T6 sheets were fabricated by a friction stir spot welding (FSSW) technique using a conventional tool with a screwed probe (probe tool) and a scroll-grooved shoulder tool without a probe (scroll tool). Tensile-shear fatigue tests were conducted using the fabricated FSSW joints and non-destructive observation using X-ray computed tomography (CT) scan was performed on the fatigued FSSW joints to investigate the fatigue crack initiation and propagation behavior around the nugget. The failure modes were dependent on the applied load and the tool geometry, in which shear failure through the nugget at higher load and base metal failure of the lower sheet at lower load occurred in the joint fabricated by a scroll tool (scroll tool joint). On the other hand, plug failure at higher load, base metal failure at middle load and plug failure combined with shear fracture at lower load occurred in the joint fabricated using a probe tool (probe tool joint). Fatigue crack initiated in the lower sheet at the edge of nugget and propagated along the nugget and in the thickness direction in the scroll tool joint. The fatigue crack initiation life was about 33~44% of the total fatigue life. In the probe tool joint, fatigue crack initiated in the upper sheet around the nugget in the early stage of fatigue life, and subsequently the other fatigue crack initiated in the lower sheet at the edge of the nugget. The fatigue crack initiation life in the lower sheet was about 55~66% of total fatigue life.
In order to understand the fundamental physics of the heat generation at a defect in Sonic-IR method further, and to discuss the suitable ultrasonic excitation method to improve the detectability of defects, Sonic-IR tests for the strip shape specimens with single edge penetrating fatigue crack were carried out with changing the ultrasonic excitation position, and the relationship between the vibrational modes of the specimen and the temperature rise at the fatigue crack vicinity was investigated. Three specimens with different crack locations were prepared. In all specimens, the temperature rise at the fatigue crack changed periodically with the change of the ultrasonic excitation position. For each specimen, the 2nd, 4th and 6th order torsional natural vibrations were observed, and among these vibrations, there was a torsional vibration which strongly affected the heat generation at the crack. The position of the anti-nodes and nodes of such vibration corresponded well to the position of the peaks and the bottoms of the temperature rise, and the order of the vibration varied depending on the location of the crack. It was found that the torsional vibration that strongly affected the heat generation at the crack had the maximum vibration energy calculated from the relative displacement between the two crack surfaces and the frequency of the vibrations among the three vibrations observed in the specimens. In the conditions of the present study, the presence of a crack at the nodal position could increase the relative vibration between the two crack surfaces due to the vibration of these two crack faces in opposite phases, thus increasing the vibration energy, i.e., the heat generation at the crack.
A new immersion testing method has been developed where the water of the conventional immersion method was replaced with powder. In this method, ultrasonic testing is carried out while compressing the powder couplant between probes and object surfaces. Even though the powder usually prevents the transmission of elastic waves, they can be transmitted through it under pressurized conditions. In addition, the acoustic impedance can be matched to that of the objects being tested by controlling the pressure applied to the powder couplant. In this paper, an ultrasonic Tungsten powder couplant with a high acoustic impedance was applied to testing magnesium alloy specimens with three different surface properties: a smooth surface, a surface with a man-made defect and a surface with V-shaped grooves. The results were that the reflections from the specimen surfaces were eliminated by suitably adjusting the pressure of the powder. The man-made defect located just below the surface could be successfully detected.
In this study, we attempted to nullify the harmful influences of processing marks on the fatigue strength of Ti-6Al-4V alloy by particle collision treatments. As the surface treatments, fine particle bombarding (FPB) and shot peening (SP) were applied to form hardened layers and introduce compressive residual stress. The surface of the objective material was polished to a mirror surface to eliminate the influences of machining on specimens and was then grinded with emery papers (#80) to make uniform processing marks. After the particle collision treatments, the surface conditions, hardness distributions and residual stress were systematically examined, and their relationships with the fatigue strength were considered in detail. On observation of the surfaces, the processing marks were eliminated by the particle collision treatments. At the same time, the surface hardness was increased and high compressive residual stress was introduced. As a result, the fatigue strength was markedly improved by the treatments beyond the level of the material with the processing marks without deterioration of the mechanical properties. The improvement rates of the fatigue strength were high, at 75% by FPB treatment and 58% by SP treatment.
This paper aims to measure the change in corrosion rate over time of reinforcing steel inside concrete, including reinforced concrete (RC) after crack initiation. The corrosion rate is important information for residual life assessment of RC structures. In some previous studies, corrosion rate of RC specimens before crack initiation was evaluated using the polarization resistance method. However, few studies have considered application of the method for specimens after crack initiation, and evaluated the change of corrosion rate over time. In this study, we measured the corrosion rate of reinforcing steel inside concrete after crack initiation and after crack repair every 1-3 months for 2.5 years using the AC impedance method. As a precision test, we removed the steel bar out from specimens to measure the corroded area of the steel surface and corrosion mass loss. Our findings revealed that the corrosion rate could be calculated precisely by setting the measurement area of the steel surface to the corroded area when the steel was corroded locally. In addition, the measurement results showed that the corrosion rate of the steel inside the concrete under a salt environment increased with time after crack initiation, whereas the increase rate of corrosion rate became smaller over time. And an early crack repair can delay corrosion initiation and decrease corrosion rate of reinforced concrete. Based on these results, we proposed the prediction method of corrosion progress of reinforcing steel inside concrete over the entire service period.