The effects of fine particle peening (FPP) on the torsional fatigue properties were investigated in the case of a transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel that has potential for use in precision gears. FPP resulted in a significant increase in the torsional fatigue limit of the TBF steel, compared with that of a quenched and tempered SNCM420 steel. An increase in the torsional fatigue limit was larger than that in the rotating bending fatigue limit. In addition, the crack growth rate and the threshold stress intensity range of this steel were lower and higher, respectively, than those of its SNCM420 counterpart. These differences resulted from the TBF steel having a higher (1) Vickers hardness, (2) compressive residual stress, and (3) volume fraction of untransformed retained austenite in the surface hardened layer, which suppressed crack initiation and extension.
The effects of material hardness on the maximum size of a slit, which was rendered harmless by shot peening, were investigated. Specimens of two types of hardness were made of spring steel (SUP9), and a semi-circular slit was introduced by using an end mill. After the shot-peening process, bending fatigue tests with a stress ratio of R = 0.05 were carried out in air. The maximum depth of slit rendered harmless by shot peening for 270 HV and 350 HV were α0p = 0.2 and 0.3 mm, respectively. Considering the experimental results of the present and previous works, the hardness did not have a large influence on the size of α0p. In order to predict the size of α0p, the range of the stress intensity factors at the semi-circular crack caused by the residual stress and applied stress during the fatigue test were analyzed. The calculated result corresponded well with the experimental result.
The influence of shot peening on the fatigue strength in an extruded magnesium alloy AZ80 treated with T6 was investigated and its anisotropic behavior was discussed. The specimen was machined from different orientations in as-extruded rod. One was loaded in the transverse direction (T specimen). The other was loaded in the longitudinal direction (L specimen). The arc height value of shot peening was set to 0.15 and 0.40 mmN. The specimen surface became rough and hardness became high with an increase in the arc height value. Irrespective of specimen orientation, shot peening at low arc height value improved the fatigue strength. However, the shot peening at high arc height value did not improve the fatigue strength. The same tendency appeared in the case of T4-treated sample, suggesting that precipitation did not affect the fatigue strength degradation by hard shot peening. To investigate the influence of surface roughness formed by shot peening, fatigue tests were conducted using some specimens polished after shot peening. In the L specimen, the polishing after shot peening improves the fatigue life significantly. Therefore, the surface roughness formed by shot peening is one of the important factors in fatigue strength of shot-peened specimen. On the other hand, the fatigue life of T specimen decreased by polishing after shot peening. It was suggested that microstructural damage induced by shot peening decreases the fatigue strength in T specimen.
Plain fatigue and fretting fatigue strength tests of aluminum alloy (JIS A7N01) specimens with surface treatments (fine particle peening, burnishing and solid lubricant film) were carried out using an electromagnetic fatigue testing machine. Fretting fatigue limit of untreated specimen was significantly low, which was about one fourth of the plain fatigue strength. Fretting fatigue strength of the specimen surface-treated with fine particle peening was slightly higher than that of the untreated specimen. The fretting fatigue strengths for solid lubricant film and burnishing have been remarkably increased compared to that of the untreated specimen. However, the fatigue limit for solid lubricant film was almost same as that of untreated specimen. From the in-situ SEM observation of fretting fatigue process, it was found that burnishing with smooth surface and high compressive residual stress effectively delayed the fretting fatigue crack nucleation and crack propagation. It was also found that solid lubricant film with low frictional coefficient effectively delayed them.
Titanium alloy (Ti-6Al-4V) having a bimodal “harmonic structure”, which consists of the coarse-grained structure surrounded by the network structure with fine grains, was fabricated by mechanical milling (MM) and spark plasma sintering (SPS) to achieve high strength and good plasticity. The microstructure of the MM-processed powder and the sintered compacts were characterized using a micro-Vickers hardness tester, scanning electron microscope (SEM) and an electron backscatter diffraction technique (EBSD). Harmonic structure was created in the sintered Ti-6Al-4V compacts prepared from the MM-processed powders having fine grains at its surface. The Ti-6Al-4V alloy with harmonic structure exhibited high tensile strength and good plasticity. The effects of the harmonic structure on the 4-points bending fatigue properties of Ti-6Al-4V alloy was investigated under the stress ratio R = 0.1 in ambient air without any controls of the temperature. The compacts with harmonic structure exhibited higher fatigue strength compared to the conventional coarse-grained material prepared from as-received initial powders. This was because the Ti-6Al-4V alloy with harmonic structure had higher tensile strength and hardness. Moreover, fatigue fracture mechanism of the Ti-6Al-4V alloy with harmonic structure was discussed from viewpoints of fractography and crystallography. As results of observing and analyzing the fracture surfaces, the Ti-6Al-4V alloy with harmonic structure failed from the coarse grain in the harmonic structure in the surface fracture mode.
A complex stress occurs on a mechanical surface of transport machineries in service such as automobiles and railway vehicles. The authors have developed a fatigue testing machine enable to reproduce such a complex stress. The surface stress on mechanical structures can be described as in-plane structural element deformations in transverse, longitudinal and diagonal directions. The testing machine is able to reproduce stress states working on surfaces of mechanical structures by applying three independent loads to the test specimen using actuators which apply loads in the 0, 45 and 90 degree direction. The testing machine was developed with consists of a fixed frame which forms 45-degree axis and two movable frames which form 0-degree and 90-degree axes. Evaluations were conducted on the test specimen with the complex stress data obtained from an actual operating transport machinery, further it was found that the measured stress and the emulated stress corresponded with an error range of less than 10%. When additional fatigue tests were performed on a test specimen with a cylindrical projection, it was found that fatigue failure occurred at stress concentration point in the root of projection.
The measured fatigue life of mild steel under random loading is compared with the fatigue lives predicted in the time and frequency domains, and the factors that reduce the accuracy of fatigue life prediction in the frequency domain is identified in this study. Several random stress and strain histories for the comparison of the fatigue lives were generated by conducting the numerical analysis of an artificial structure. To obtain the measured fatigue life, the smooth and notched round bar specimens made of mild steel were tested so that the generated random stress or strain histories could occur in the cross section of the specimens. In addition to the fatigue tests, the fatigue lives of the specimens were predicted in the time domain using the rainflow cycle counting method and the Palmgren-Miner cumulative damage rule given the SN curve of mild steel and the generated random histories. The fatigue lives were also predicted in the frequency domain using Dirlik’s method with the SN curve and the random PSDs that were generated using fast Fourier transformation of the random histories or the numerical analysis. As the result of the comparison, the relation between the measured fatigue life and the fatigue life predicted in the frequency domain was changed depending on the crest factor of the random stress and strain histories. Since this tendency of the fatigue life prediction in the frequency domain is explained by considering the feature of the PSD and the cycle counting method used in the frequency domain, the fatigue life under random loading can be accurately predicted in the frequency domain by adjusting the stress or strain range distribution obtained using Dirlik’s method.
This study was aimed at showing the validity of the strain intensity factor for predicting crack growth of low-cycle fatigue regime. First, by reviewing the fatigue test results of Type 316 stainless steel, roles of the stress and strain ranges for determining the fatigue life and validity of the strain intensity factor for predicting the crack growth were investigated. The test results obtained using cold worked specimens revealed that fatigue life correlated better with the strain range rather than stress amplitude. Crack initiation and growth investigations using replica technique showed that the fatigue life was almost equivalent to the cycles for a small crack of few tens micrometers growing to the critical size. The crack growth rates of small cracks correlated well with range of the strain intensify factor. It was concluded that the strain intensity factor is valid for predicting the crack growth rate because the fatigue life was correlated well with the strain range and was almost equivalent to cycles for a small crack growing to critical size. In order to discuss the closure effect on the crack growth rate, plate specimens were subjected fully reversed cyclic loading. Then, the effect of single overloading on crack opening strain and growth rate were investigated. It was shown that the crack growth was accelerated more by compressive overloading than by tensile overloading. The change in the growth rate was reasonably assessed by considering change in the effective strain intensify factor.
Low-cycle fatigue tests of SUS316NG austenitic stainless steel were conducted for several strain ranges (0.76, 1.26, 2.1, 4.1, and 8.1%) in which the specimen’s surface topography was regularly measured using a laser scanning microscope. The surface topographies obtained were analyzed by frequency analysis to separate the surface relief due to persistent slip bands (PSBs) from that due to crystal grain deformation. The PSBs-induced surface relief evolution and the grain-deformation-induced one were quantitatively evaluated by using arithmetic mean roughness Ra and arithmetic mean waviness Wa, respectively. The ΔRa and ΔWa, the increments in Ra and Wa from the initial values, increased with the usage factor (UF) for each strain range. Moreover, the rates of increase in ΔRa and ΔWa with respect to UF increased with the strain range. ΔWa/ΔRa was larger for higher strain ranges. This tendency of ΔWa/ΔRa indicates that the surface relief due to grain deformation develops more prominently than that due to PSBs for larger strain ranges. The results for Ra and Wa agree with the results of surface topography observation. On the basis of these results, a method was developed for estimating the UF of fatigued material and the applied strain range by using only the measured Ra and Wa.
Three types of free-standing copper thin films were produced by electrodeposition using sulfate solution under constant current: CC made without brightener addition, CC-B with brightener and CC-T with thiourea. The grain size of CC, CC-B, CC-T measured by transmission electron microscopy was 421, 237, and 28.5nm. Those films did not have any strong texture. The effect of grain size on the tensile and fatigue properties was studied. The tensile yield stress of CC and CC-B were slightly higher than the Hall-Petch relation published for bulk wrought copper, while that of CC-T was lower than the relation. The fatigue life increased with decreasing grain size especially for longer fatigue life region. The fatigue strength gets higher with decreasing grain size. The fatigue limits of CC-B and CC-T approximately follow the Hall-Petch relation, while the fatigue limit of CC was lower than the relation. The fatigue crack propagation rate for R = 0.5 was higher than that for R = 0.1 when compared at the same stress intensity range. The threshold stress intensity for fatigue crack propagation increased with increasing grain size. A fatigue crack of CC was accompanied with shear bands and striations were observed on the fatigue fracture surface made at high stress intensities. There was no shear band near fatigue cracks in CC-T, and only few in CC-B. The granular feature was observed on the fatigue fracture surface of CC-B and CC-T, and the size of the granular feature was much smaller in CC-T than in CC-B.
Relationship between microstructure and fatigue limit of a smooth specimen of tool steel SK85 (0.83% carbon steel) was investigated. Also, fatigue limit of a pure copper was evaluated. Push-pull tests were performed under stress ratio R = -1. The initial length of crack which initiate during fatigue process is related to the size of a single crystal in the case of carbon steel, etc. In this study, SK85 was chosen for a testing material to investigate the relationship between the microstructure and fatigue limit due to the following reason. The original microstructure of the material used includes spherical microstructure. However, after annealing or normalizing under some conditions, that microstructure changed to a lamellar microstructure. The fatigue limit of heat treating SK85 could be evaluated by a relation in whom the parameters are the hardness and initial crack length. The crack initiation sizes were related to the microstructure. The evaluation method of fatigue limit was investigated in consideration of initiation size of crack or sizes of microstructure. Also, it was discussed that the crack growth rate of SK85 was related to the condition of ferrite grain distributions. For the purpose of investigation of evaluation method of fatigue limit, experiments of smooth and holed specimens of copper were performed. Furthermore, we investigated the relationship between the Hall Petch relation and evaluation method of fatigue limit in this study.
In order to prevent fractures caused by fatigue or stress corrosion cracking in welded structures, it is important to predict crack propagation for cracks observed during in-service inspections. However, it is difficult to evaluate three-dimensional welding residual stresses non-destructively. Today, it is possible to measure residual stresses just on surface by X-ray diffraction. Neutron diffraction makes it possible to measure welding residual stresses non-destructively even in the thickness direction but it is only available in special irradiation facilities. Therefore, it is impossible to use neutron diffraction as an on-site measurement technique. As non-destructive method of three-dimensional welding residual stresses based on the eigenstrain methodology, the bead flush method has been proposed. In this method, three-dimensional welding residual stresses are calculated by an elastic FEM (Finite Element Method) analysis from eigenstrain distributions which are estimated by an inverse analysis from released strains by strain gauges in the removal of the weld reinforcement. Here, the removal of the excess metal contributes inhibition of crack initiation. Therefore, the bead flush method is a non-destructive technique essentially. However, estimation accuracy of this method becomes relatively poor when processing strains are added on the machined surface. The first author has been developed the bead flush method to be free from the influence of the processing strains. In this method, eigenstrains are estimated not from released strains but from residual strains on surface by X-ray diffraction. In this study, welding residual stresses on the bottom surface in an actual welded plate are estimated from elastic strains measured on the top surface using this method. To evaluate estimation accuracy, estimated residual stresses on the bottom surface are compared with residual stresses measured by X-ray diffraction. Here, eigenstrain distributions not only in the welding direction but also in the thickness direction are considered to improve the estimation accuracy.
Recently, wire electric discharge machining method has been applied to the working and the cutting of parts as well as to die machining. Also in research of strength of materials, in processing of a minute specimen, a wire electric discharge machine will demonstrate great help. Therefore, the data concerning the relationship between the surface layer caused by wire electric discharge and fatigue strength are beginning to be required but we cannot find so many reports of the data concerning on this theme. So, plane bending fatigue tests were carried out on heat treated and annealed 0.45% carbon steel specimens machined by wire electric discharge. The fatigue strength of the heat treated 0.45% carbon steel processed by rough cutting with a wire electric discharge machine decreases approximately 44% from that of the unprocessed material. However, in the annealed 0.45% carbon steel, the decrease of the fatigue strength was only 10%.
To reduce weight of car bodies, which are aiming to obtain better gasoline millage, Carbon Fiber Reinforced Thermoplastics (CFRTP), which have advantages in terms of high-specific strength, high-specific modulus, high production cycle and high recycle efficiency, are expected to be used for automotive parts. For structural components, pipe-shaped members are well used and low-cost and high-speed molding method is required for CFRTP pipe molding. High frequency direct resistance heating of mold has been developed so as to heat only the mold surface rapidly by skin effect. However, uniformity of temperature distribution of the mold is one of the important issues to be solved. It is necessary to obtain the uniform temperature distribution of the mold surface in order to mold products with high-quality and high-efficiency. In this study, the temperature distribution of the mold for pipe-shaped components heated by high frequency direct resistance was analyzed by using FEM model and showing a good agreement with the experimental results. Influence of the shape of electrodes and electric frequency on the temperature distribution of the mold was also revealed by FEM analysis. In addition the mold model that prevents heat radiation to the electrodes was proposed and showing the good temperature distribution within ±4.8 degrees Celsius.
“Yuzen” is a traditional but still popular method of dyeing fabrics in Japan. The products using the Yuzen method and manufactured in Kyoto city are called "Kyo-Yuzen." The dyeing method of Yuzen can be dividing into 10 procedures. A specialized craftsman is in charge of each procedure. During the paste application (Nori-oki) procedure, the expert applies a starch paste or a rubber paste to a fabric in order to resist dyeing. The two resist pastes create different effect on the dyed fabric. At market, the fabric with a starch paste application is perceived to have a higher value than that with a rubber paste. In this study, the difference of the viscosity between two materials was clarified, and specimens which craftsman dyed were observed. Then how two materials put on fabrics, and the structures of them were measured. As a result of this research, it was clarified that rubber paste penetrated into fabrics rather than starch paste.