In this study, fatigue crack propagation tests were conducted on thin Cu–Sn–P copper alloy strips, which are required to have both mechanical and electrical reliabilities for use in electric devices. For measuring crack lengths during fatigue tests, an unloaded elastic compliance method is usually used. However, it is difficult to apply this method to thin plates such as 0.15 mm thickness plates used in this study. Therefore, an image processing method and a DC potential difference method were applied to measure the crack length, and the accuracies of crack length measurements were compared. Moreover, the effects of rolling direction on crack growth characteristics were investigated. As a result, the image processing method gave similar results as the visual measurement method in longitudinal direction (LD) specimen, but it is difficult to measure the crack lengths for transverse direction (TD) specimen. The DC potential difference method can measure the crack length at the Region IIa in the fatigue crack propagation, which was difficult to obtain clearly by the visual measurement method, and this method is appropriate for the crack length measurement of thin copper alloy strips. For the effects of the rolling direction on the crack propagation, TD specimens have higher fatigue characteristics than LD specimens in the Region IIa and IIc of the fatigue crack propagation. However, LD specimens have higher fatigue characteristics than TD specimens in the Region IIb of the fatigue crack propagation.
Periodic TiB-reinforced titanium, which is defined as Ti-matrix surrounded by a network of TiB structures, was developed via powder metallurgy using TiB powders. The TiB composite was obtained by combining the CP titanium with 59 vol% TiB2 powders, and then mechanical milling was performed using a planetary ball mill to fabricate TiB powders. In this study, in order to investigate the near-threshold fatigue crack propagation in a periodic TiB-reinforced commercially pure titanium, stress intensity factor, K, 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 to discuss the mechanism of fatigue crack propagation. In the periodic TiB-reinformed titanium, fatigue cracks were found to propagate preferentially through the network of TiB, which results in higher crack growth rate, da/dN. In contrast, the effect of microstructure on the threshold stress intensity range, ΔKth, in the TiB-reinforced titanium fabricated by TiB powders was disappeared by eliminating the crack closure phenomenon.
High entropy alloys (HEAs), which have five or more principal elements with an equiatomic composition, exhibit multiple excellent mechanical properties. Superior fatigue properties would be required to use HEAs as structural components in engineering fields. The purpose of this study is to develop HEAs having high crack propagation resistance via melamine reduction-nitridation and powder metallurgy. The HEA with a bimodal structure was fabricated by sintering nitrided HEA powders. Stress intensity factor, K, decreasing tests were conducted under the force ratios, R, from 0.1 and 0.5 in air at room temperature to investigate the near-threshold fatigue crack propagation in CrMnFeCoNi alloys with a bimodal structure. Threshold stress intensity range, ΔKth, of nitrided HEAs compact tested at R = 0.1 was lower than that of the un-nitrided one, whereas ΔKth of nitrided HEAs compact tested at R = 0.5 showed almost the same as the un-nitrided one and effective threshold stress intensity range, ΔKeff,th, was higher than that of the un-nitrided one. After testing, crack profiles were observed by scanning electron microscopy, and microstructures around crack profiles were analyzed using electron backscatter diffraction and electron prove micro analysis to discuss the mechanism of fatigue crack propagation. In some areas, fatigue crack paths in nitrided HEAs compacts were influenced by a bimodal structure.
Porous metals, which include small pores inside metals, are promising materials due to their material and structural characteristics. Although they generally exhibit low strength because the pores behave as defects, it is expected that porous metals achieve high specific strength due to their ultra-lightweight characteristic. This paper deals with a feasibility study on the fabrication of porous steels for developing unique metals with high specific strength. Porous steels were fabricated via powder metallurgy-based space holder technique. Alloy tool steel, SKD11, and sodium chloride, NaCl, were used as a scaffold metal and spacer material, respectively. Mixed powders of SKD11 and NaCl were sintered via spark plasma sintering technique. Each sintered compact was re-heated in an argon atmosphere to remove NaCl and densify the scaffold in the compact. Then, each compact was quenched and tempered. As a result, open-cell porous steels with the porosities of 60% and 70% were successfully fabricated. The heat treatment refined the microstructure of the scaffold without changing the pore shape, porosity, etc., resulting in the improvement of their strength property, irrespective of their porosity. Furthermore, the specific proof strength of heat-treated porous steels was comparable to that of dense pure aluminum.
Mean torsional stress is considered to less affect torsional fatigue strength of steels, but several experimental results have been recently reported that mean torsional stress caused significant reduction in torsional fatigue strength in the very high cycle region for shot-peened spring steel. To investigate the effect of mean torsional stress on high strength steel, ultrasonic torsional fatigue tests with mean torsional stress were conducted for spring steel and bearing steel, which are used for mechanical components subjected to cyclic shear stress. Torsional fatigue strength up to 109 cycles were obtained for fully reversed torsional loading (R=-1) to pulsating torsional loading (R=0). The results revealed that mean torsional stress caused reduction in fatigue strength in the very high cycle region for both spring steel and bearing steel, and applying higher mean shear stress would result in transition of the fracture origin from surface to an internal inclusion. The reduction in torsional fatigue strength was discussed from the viewpoint of the transition of fatigue origin, and applicability of a √area parameter model was discussed for predicting the reduction in torsional fatigue strength.
GFRP is applied in various fields because of it has excellent characteristics, but there is also a problem that it has low fire resistance. It is necessary to addition particles of flame retardant such as Aluminum hydroxide (Al(OH)3) on matrix resin for apply in fields where nonflammability is required. It is expected that the flame retardant characteristics will be improved by adding many particles and including many small particle sizes. However, the increase in resin viscosity caused by the addition of large amounts of particles is a problem point as a matrix resin of GFRP. Therefore, we focused on suppressing the increase in viscosity due to the use of two-particle diameter particles (2DP) that have both large and small particles. It has been confirmed in various studies that the addition of single particle diameter particles to the matrix resin affects the mechanical properties of GFRP. However, the effect of particles with multiple particle sizes on the mechanical properties of GFRP has not been confirmed. In this study, the effect of the addition of 2DP on the mechanical properties of GFRP was clarified from the viewpoint of the fracture mechanism.
The macroscopic creep properties of negative electrode for lithium-ion batteries and their estimation method have been investigated based on the microscopic structure of the electrode. Tensile tests and creep tests were conducted on the negative electrode consisting of carbon powder and polyvinylidene fluoride (PVDF) binder. The stress-strain curve, the time history of the tensile strain and the creep rupture time were measured in these tests and estimated by using a simple model proposed in this study. The proposed model approximates the alignment of the carbon particles as body-centered cubic (bcc) and face-centered cubic (fcc). The external load on the model is supported by the PVDF binder located between the carbon particles. The test results show that the macroscopic mechanical properties including the creep properties of the negative electrode were affected by the mechanical properties of the PVDF binder. The stress-strain curve and the time history of the tensile strain located between the upper limit and the lower limit of the proposed model. The tensile strength and the creep rupture time agreed with the lower limit of the proposed model.
Ti-6Al-4V is a titanium alloy frequently used in many fields including aerospace industries. However, in the very high cycle regime, this alloy develops fatigue fractures from within the material even at stresses lower than those generated at the surface. To ensure long-term reliability, a fatigue life prediction method that takes internal fractures into account should be established. In this study, the initiation and propagation processes of fatigue cracks in Ti-6Al-4V were modeled based on the non-destructive observations of small internal cracks using synchrotron radiation X-ray computed tomography. The proposed model is based on the competition of surface and internal cracks in a virtual specimen. We considered the statistical properties of fatigue cracks, such as initiation life, crack size, and the growth behaviors of surface and internal cracks. To describe the initiation and propagation of fatigue cracks, a Monte Carlo simulation was conducted. The simulation results explained the trend in the fatigue life and fracture mode of the experimental data. Specifically, the effects of initiation life and origin size on fatigue life, which could not be obtained from experimental data, were investigated. As a result, the initiation life accounted for more than 90 % of the fatigue life in surface fractures, but only 15% to 70% in internal fractures. Crack initiation sites that were larger than the average α-phase became the origin of main cracks in surface fractures, whereas those that were smaller than the average α-phase were found to be those in the internal fractures. These findings were attributed to the fact that the surface and internal fractures are affected by distinct different factors. In other words, surface fractures are dominated by the crack initiation process and internal fractures were by the crack propagation process at an extremely low growth rate.
This paper describes a stress measurement from a welded part of an austenitic stainless steel using synchrotron X-rays. Observing the diffractions from a single crystal, coarse grained material and the welded part, the diffraction pattern of the welded part shows the feature of textured materials with large grains. Difﬁculty measuring the X-ray stress of the welded part is caused by the broadening of the diffraction spot in the radial and circumferential directions. The bending strains of the rectangular bar made of the welded part were measured using synchrotron white X-rays and the double exposure method. However, the results were not enough accuracy to obtain the strains. To improve the energy resolution, monochromatic synchrotron X-ray of 70 keV was used. The diffraction pattern showed the sharp arc like a pattern from texture material. The diffraction proﬁle was obtained from the integral of the diffraction intensity in the direction of the circumference. The diffraction angle was determined using the double exposure method. As a result, the distribution of the residual stresses of the welded part could be measured.