Ultrasonic fatigue tests were conducted with the specimens cut from LL and TT orientations of alloy tool steel. All fracture origins were defects inside the materials or very near surface. The S-N curves were monotonously decreasing slope as Nf increases. The fatigue strength of SKD11-L was greater than that of SKD11-T, because the defects size of fracture origin of SKD11-L was smaller than that of SKD11-T. This difference of defects size occured due to the rolling direction. Facet region were observed around the defects of the fracture origin for both materials beyond 106 cycles. However, for the specimens which did not break up to 109 cycles, the facet region was not observed on the fracture surfaces after broken under about 105 cycles by retest at higher stress. This suggests that the fatigue limit for internal fracture would exist. Furthermore, by using the root area parameter model the modified S-N diagrams were represented in narrow scatter band. However, to predict the border fatigue strength between broken or not-broken for the present material, the value of the root area parameter model is needed the correction coefficient as multiplid by 0.75.
A high strength steel shows fish-eye failure in very long life fatigue. In this region, the fatigue crack growth in ODA (Optically Dark Area) controls the fatigue process. However, ODA's formative mechanism is unknown. Therefore, to predict fatigue life of fish-eye failure, it is important to know ODA's formative mechanism. So, the present study examined for the influential factors on ODA in the fatigue process through uniaxial push-pull fatigue tests in vacuum as well as computer simulation. In experiment, vacuum environment is employed to simulate fish-eye type crack growth on surface of specimens becase of shutting off supply of air. The test results in vaccum showed that the fatigue life property of surface failure has corresponded to fish-eye fatigue life. And the site of inclusion which has fracture origin size determines whether surface failure or internal failure in vacuum. The results of fractographic observation of crack initiation showed that the rough area of surface fracture in vacuum was formed by mechanism of ODA type crack. Furthermore, the fatigue life of numerical simulation in vacuum showed that the fatigue life which was necessary to form ODA on specimen surface corresponded experimental life.
The effect of hydrogen on high cycle fatigue properties of a martensitic stainless steel was investigated. In the Fuel Cell (FC) system, many components are exposed to high pressure hydrogen environment under cyclic loading. In this study, hydrogen was artificially charged into specimen of a martensitic stainless steel, 0.44C-13Cr-0.13N steel, and the fatigue properties were compared with those of the specimens without hydrogen charge. Fatigue strength and fatigue life decreased with increasing hydrogen content. It implies that the fatigue threshold of the microstructure which contains high hydrogen content is much lower than that of the as-heat-treated microstructure. Specimens having a longer fatigue life had a particular morphology named ODA (Optically Dark Area) besides the inclusion at fracture origin. The fracture origin of hydrogen charged specimens showed smaller ODAs than as-heat-treated specimens. The upper bound of the critical hydrogen content level resulting ODA lies between 1.0ppm and 2.0ppm. The condition for the critical size of ODA for the start of conventional fatigue crack growth was analyzed by the fracture mechanics. The critical stress intensity factor range ΔKODA for the critical size of ODA of as-heat-treated specimens can be correlated with the threshold stress intensity factor range ΔKth expressed by the √area parameter model for small cracks.
Effects of hydrogen charge on fatigue behaviour of two carbon steels, JIS-S10C (SAE1010) and JIS-S45C (SAE1045) were investigated. There was no hydrogen effect in the cyclic stress-strain hysteresis loops of S10C hydrogen-charged with 0.2 ppm. On the other hand, the strain amplitude was decreased in S45C hydrogen-charged with 0.8 ppm. The delayed yielding and the decrease in the saturated value of the strain amplitude were observed in the hydrogen-charged specimen (H : 0.5 ppm) of S45C under the constant stress amplitude tests. It is supposed that the degree of influence of hydrogen on cyclic stress-strain properties depends or material structure and/or hydrogen content. The effect of hydrogen charge (H : 0.5 ppm) on the fatigue life, the fatigue limit and the crack growth curves of S45C were not remarkable, while there was a distinct difference in the morphology of the slip bands between the hydrogen-charged and uncharged specimens. The localized slip bands were observed in the hydrogen-charged specimen of S45C. Therefore, it is presumed that the decrease in the strain amplitude in hysteresis loop by hydrogen charge is caused by the localization of slip bands. More crack initiations from ferrite grains were observed in the hydrogen-charged specimen (H : 0.5 ppm) of S45C. This phenomenon also corresponds to the localization and the formation of slip bands.
The objective of this study is to clarify the effect of hydrogen gas environment on fretting fatigue strength of the materials, which will be used for the mechanical system for hydrogen utilization. It is important to take fretting fatigue into account for strength design, since many fatigue failure accidents have occurred at joints or contact parts between components. The test materials were austenitic stainless steel SUS 304 and corrosion resisting aluminum alloy A6061. Fretting fatigue tests and also fretting wear tests were done in hydrogen gas, nitrogen gas and air. Fretting fatigue strength in hydrogen gas environment was lower than that in air for both materials. Tangential force coefficient in hydrogen gas was larger than that in air. The appearance of fretting pit and microcrack was quite different among environments. Absorption of hydrogen was detected in contact pad of fretting wear test fretted in hydrogen gas environment. Fretting fatigue life of the stainless steel was decreased due to hydrogen pre-charge.
The effects of crack size, stress ratio and absorbed hydrogen on the fatigue crack propagation characteristics of high strength steel were studied near threshold regime. The test material was SCM440H low alloy steel. The material was quenched in vacuum and tempered at low temperature to achieve high strength and to lower the initial content of hydrogen of the material. The effects of crack size and stress ratio on the fatigue crack propagation rate and threshold stress intensity factor were investigated in air at first. These data offered a baseline as a reference. The effect of absorbed hydrogen was examined using a material in which hydrogen was introduced by cathodic charging. It was verified that the absorbed hydrogen in high strength steel affected the high cycle fatigue crack propagation characteristics. The fatigue crack propagation rate near threshold region was accelerated and ΔKth was lowered. The concentration of about 1ppm hydrogen caused 20% reduction of ΔKth. Since absorbed hydrogen has an influence even on high cycle fatigue crack propagation, it should be taken into account in the evaluation of defect of hydrogen utilization machine.
The objective of the present study is to understand the fatigue behaviour and fracture mechanisms of cast irons with spheroidal vanadium carbides dispersed within martensitic matrix microstructure. At first, rotary bending fatigue tests have been performed using smooth specimens of cast irons with two different matrix microstructures: fully martensitic microstructure and martensitic/bainitic microstructure. It was found that there existed a considerable scatter, but the former showed higher fatigue strength than the latter. Fatigue cracks invariably initiated from casting defects in both cast irons, and thus their fatigue strengths depended strongly on casting defects. Then, additional fatigue tests have been conducted using a cast iron with fully martensitic microstructure whose castability was improved by varying chemical composition. Fatigue cracks were still generated from casting defects, but the fatigue strength was significantly high with a small scatter because the casting defect sizes were very small compared with the cast irons described above. The √area parameter model was applied to predict the fatigue limit of those cast irons. Although the maximum casting defect sizes estimated from the statistics of extremes were considerably smaller than the sizes of the casting defects from which the crack initiated, the √area parameter model gave a reasonable prediction of fatigue limit.
In order to evaluate the effects of notch and surface roughness on fatigue fracture morphology of high strength steels in long life region, cantilever-type rotary bending fatigue tests have been performed using notched specimens with different stress concentration factors, α, of 1.16, 1.51 and 2.0, and specimens with roughened surface of the maximum height, Rz, of approximately 10μm, 16μm and 19μm. The smooth specimen and the notched specimen with α=1.16 showed subsurface fracture in long life region. However, subsurface fracture was not seen in the notched specimens of α=1.51 and 2.0 because of much higher maximum stress at the notch root. In the surface fracture region, fatigue strength decreased with increasing stress concentration factor, while in the subsurface fracture region, there was no discernible difference in fatigue strength between the smooth specimen and the notched specimen with α=1.16. Furthermore, it was indicated that the high strength steel studied had a very high sensitivity to notch. All specimens with roughened surface exhibited subsurface fracture, where the transition stress at which fracture mode changed decreased in the specimens with rougher surface. In the surface fracture region, fatigue strength decreased with increasing surface roughness, while in the subsurface fracture region, it did not depend on surface roughness. The √area parameter model gave a reasonable estimation of the transition stress of the surface-roughened specimens.
Degassed processing (DG) was used for decreasing cast defects such as porosities or micro shrinkage of casting aluminum alloy. So it was very excellent method for improvement of fatigue property on casting aluminum alloy. In this study, for the aim of more fatigue property improvement of the casting alumium alloy AC4CH, the shot peening treatment was applied to DG treated AC4CH alloy, and rotating bending fatigue tests were carried out. As the results of fatigue tests, the fatigue life property of the shot peened DG material is more improved. To investigate the cause of the fatigue life improvement, fracture surface of all specimens were observed with SEM. And surface cracks were observed using replication technique. Comparing the shot peened DG material with the DG material, the crack initiation life of shot peened DG material was shorter than that of non peened DG material, but in the other hand, the crack propagation rate da/dN of shot peened DG material was slower than that of non peened DG material.
In order to evaluate the effect of stress concentration on long life fatigue properties of SUS316NG and SUS304TP, rotating bending fatigue tests were carried out using the specimen with artificial corrosion pits. And then, to evaluate the fatigue crack initiation and propagation properties from the pits, the stress intensity factor range was calculated by the pit diameter at surface and the pit depth which were obtained by optical microscope observations, and also it was calculated by the pit area on fracture surface which obtained by SEM observations. As the results, the surface fatigue crack initiation and propagation properties of both austenitic stainless steels could be evaluated by the value of the stress intensity factor range which calculated using the pit diameter at surface and maximum pit depth by the optical microscope observation.
Fatigue crack growth test under constant amplitude loading was carried out on α-brass. Successive observation of transgranular small fatigue crack growth behavior was performed by means of an atomic force microscope (AFM) equipped with small in-plane bending fatigue testing machine. In the low growth rate region after crack initiation, the inclined fatigue crack grew along one slip plane in contrast with the alternating slip-off crack growth process in a long crack. Twin boundaries of α-brass worked as a constraint against slip deformation, resulted in frequent crack deflection and crack branching. A large number of dislocations were piled up along the activated slip planes due to cyclic strain hardening, which changed the stress state around crack tip, resulted in the activation of slip deformation on the other slip plane. The fatigue crack deflection behavior was investigated by the crystallographic orientation analysis based on the Electron Backscatter Diffraction (EBSD). It was found that the direction of crack deflection did not decided only by Schmid factor. The slip factor considering the slip system and singular stress field at the crack tip was introduced in order to evaluate the easiness of slip deformation instead of Schmid factor. The direction of crack deflection was found to be explained well by the slip factor and the relative location between the preferential slip plane and crack front.
The practical stresses at the critical point of structures or components are often in multiaxial stress states due to the complexity of shapes and external loads. So, in order to evaluate the fatigue strength accurately, we must use an appropriate method considering the multiaxial stress state. In this report, we focused on railway wheel plates being in biaxial stress states by external loads. FEM analyses and uniaxial/biaxial fatigue tests of a wheel steel were carried out. Then we investigated applicability of various multiaxial fatigue strength evaluation methods. As a result, fatigue limits evaluated by uniaxial stresses in biaxial fatigue tests were different from those in uniaxial fatigue tests. And it was clarified that considering a critical plane and crack initiating direction, fatigue limits in biaxial fatigue tests were evaluated accurately by a method using the octahedral share stress.
A simulation of fatigue crack propagation from a hole or crack under combined axial and torsional loading was conducted on the basis of the maximum tangential stress criterion for determining the crack path. The simulation results of the crack propagation path were compared with the experimental results obtained from fatigue tests by using thin-walled tubular specimens made of a medium-carbon steel. Fatigue cracks are nucleated at the position of the maximum of the amplitude of the tangential stress around the hole, and propagated straight away from the hole. The path of fatigue crack propagation from a hole or a crack followed the direction perpendicular to the maximum of the range of the tangential stress, Δσ*θmax, near the crack tip calculated from the stress intensity ranges by considering the contact of crack faces near the minimum load. The crack path predicted from the Δσ*θmax criterion was very close to that calculated from the maximum of the total range of the tangential stress, Δσθmax, calculated by neglecting the crack face contact. The superposition of static mode II shear loading changes slightly the propagation path of a crack propagating under uniaxial tension compression. This deviation is caused by the generation of cyclic mode II component due to the zigzag shape of a fatigue crack.
The mechanism of fatigue crack propagation in pure aluminum was investigated. Two types of fatigue crack propagation tests were carried out. One of them was a cyclic torsion test with static tension using a pre-cracked specimen. In this test, the effect of static tension on shear mode crack propagation by cyclic torsion was examined. The other was a push-pull fatigue test of plate type specimens in which a slit was cut in the center section of the test section. The slit direction cut in specimen was perpendicular or a 45 degree inclined direction to the specimen axis. In the former, it was found that the application of some static tension was necessary to continue the shear mode crack growth by cyclic torsion. In the latter, crack propagated by mode I or by a mixed mode, which combined mode I and shear mode, depended on the testing conditions. In these cases, the crack propagation rate da/dN was evaluated with a modified effective stress intensity factor range, which was independent of the crack propagation mode. In the pure aluminum case, the shear stress strongly affected crack propagation rate. This was thought to be related to the crystal structure.
Mode II fatigue crack growth has been investigated for a high strength bearing steel, SUJ2, using a specially prepared fixture developed by Fujii et al. The fixture could apply large T-stress, i.e. compressive stress component parallel to the crack surface, which enabled us to grow the fatigue crack to the pure mode II direction. Detailed experiments for mode II as well as conventional mode I fatigue crack growth were performed, and the characteristics were compared. In order to investigate the net characteristics of mode II fatigue crack growth, deformation fields near the crack tip were measured under mode II loading by ESPI (Electronic Speckle Pattern Interferometry) method and used to derive effective stress intensity factor range using nonlinear least-squires based on Newton-Raphson method. The results revealed that the mutual contact of the crack surfaces reduced the crack driving force and the crack growth rate simultaneously. The mode II fatigue crack growth rate exhibited remarkable scatter due to the crack surface contacts at the deflected path of the mode II fatigue cracks.
In the previous paper, in order to evaluate the strain rate dependence of the dynamic flow stress of aluminum alloys, 6061-O and -T6, high strain rate tests were performed at strain rates ranging from 1 × 103s-1 to 3 × 104s-1, and strain rate reduction tests were also conducted in the strain rate range from about 1 × 104s-1 to 2 × 104s-1, which is the strain rate range before reduction. A steep increase in the flow stress was observed for 6061-O at the strain rate of about 5 × 103s-1. The above phenomenon, however, was not observed for 6061-T6 in the strain rate range where the strain rate reduction tests were conducted and also the strain rate dependence of the flow stress was negligibly small. In this paper, to clarify the difference between the above-mentioned phenomenon for 6061-O and -T6, a simplified model for dislocation kinetics under dynamic plastic deformation is used which can represent a transition in the rate controlling mechanism of dislocation motion from a thermally activated process to a viscous drag. The equation derived from the kinetic model reveals that the steep increase in the flow stress of 6061-O observed at the strain rate of about 5 × 103s-1 is attributed to the rate dependence of the viscous drag on the dislocation motion and furthermore, the increase in the mobile dislocation density lowers a velocity of moving dislocations and shifts the transition region, or the strain rates in which the steep increase in the flow stress becomes to appear, to the higher strain rate side. It is expected that the flow stress of 6061-T6 increases abruptly in the strain rate region above about 2 × 104s-1.
An X-ray stress measuring technique —pseudo-ψ angle changing method— is proposed to measure circumferential stress on a concave cylindrical surface at a narrow part. To avoid X-ray interception by a side wall, X-ray irradiation and detection are conducted in a plane parallel to the plane including the central axis of the cylindrical surface (the configuration of axial stress measurement by the iso-inclination method), and the diffraction angle at ψ=0deg is measured on an inclined surface (part of the cylindrical surface). This measurement is conducted on five or more irradiation positions changed along the circumference of the cylindrical surface by moving a measured object, keeping the attitude of the object fixed, in the direction of the normal of the plane including the X-ray path. The position change causes the shift of diffraction peak since the angle between the normal of the irradiated area and incident X-rays changes according to the position, i.e. the position change is equivalent to the change of ψ angle in the conventional sin2 ψ method. The circumferential stress is obtained from the relation between the irradiation position and the diffraction angle at ψ=0deg. To examine the validity of pseudo-ψ angle changing method, residual stress in the tooth depth direction at the tooth root of a rack was measured using this method and the conventional method respectively. The measured values of the stress by two different methods were in good agreement.