Under low cycle fatigue condition, small cracks initiate on the specimen surface, and then they grow and coalesce with each other. The coalescence of small cracks is an important factor in evaluating fatigue life based on the fracture mechanics approach. However, surface observation of the coalescence is not sufficient in order to understand the mechanism and the influence on the fatigue life. Present study performed numerical simulations of the crack coalescence using the S-FEM based on the linear elastic fracture mechanics. The results revealed that the fatigue life was reduced when the main crack coalesces with a crack of comparative size, while the effect of fatigue life reduction was small when the main crack coalesces with a crack of less than half size. Experimental results were compared with the simulated results.
In the very high cycle regime over 107 cycles, a fatigue crack often initiates from subsurface of materials. In subsurface fractures, a unique fracture surface with a fine concavo-convex pattern called Optically Dark Area (ODA) is known to exist around the fracture origins. This type of fracture surface has been observed not only in high strength steel, but also in Ti alloy. To reveal the formative factors of the fine concavo-convex pattern, this study focuses on a unique environment around subsurface cracks. Subsurface cracks are not surrounded by atmosphere; therefore, the environment around them can be similar to vacuum. Based on this idea, fatigue crack growth tests using high strength steel (SNCM439) and Titanium alloy (Ti-6Al-4V) were conducted in high vacuum under various conditions, and fracture surfaces were investigated by SEM analyses. As a result, the fine concavo-convexo morph similar to ODA in subsurface fractures was observed on the fracture surfaces obtained in the crack growth tests. In addition, it was clarified that the unique fracture surface was not formed during crack growth process but formed by the long term repeating contacts of already initiated fracture surfaces in high vacuum. The fine concavo-convexo morph can be formed regardless of materials when the following two conditions are satisfied ; one is a vacuum environment and the other is a long term repeating contacts of fracture surfaces over about 107 cycles.
To determine the influential factors on the formation of initial crack propagation region in the very high cycle fatigue (VHCF), uniaxial fatigue tests and K-decreasing tests were conducted with Ti-6Al-4V alloy. The test data indicated that the crack growth rate in vacuum was much slower than that in air, which corresponded to longer fatigue lives of sub-surface fractures. The fracture surface of sub-surface fracture and that obtained in the crack propagation test in vacuum showed similar features that are characterized by a unique granular concavo-convex microasperity. Since the granular fracture surface was never observed in surface-initiated cracks, the granular concavo-convex pattern was considered as clues to an elucidation of the initial crack propagation mechanism in the sub-surface fracture. The granular features also resembled so-called ODA (Optically Dark Area) in high strength steels. Based on this observation, a common mechanism of sub-surface crack growth regardless of materials was discussed for the VHCF phenomena.
Rotating bending fatigue tests were carried out for a squeeze cast Al alloy, AC4CH-T6, to investigate the effects of solution treatment time on microstructure, static mechanical properties and fatigue strength. The solution treatment conditions selected were treatment times of 15, 30, 70, 240 and 480min at the same temperature of 545 °C. With increase in the solution treatment time, a eutectic Si particle was spheroidized and dispersed. Although 0.2% proof strength and tensile strength were increased by change in the treatment time from 15min to 70min, the effects of solution treatment time over that time on these strengths were hardly observed. Fatigue strength was slightly increased with increase in the treatment time. That is, effect of the treatment time on fatigue strength was very small in comparison with the change in microstructure by the treatment time. A crack initiated in α phase or from a eutectic Si particle irrespective of the treatment time. The increase in fatigue strength was mainly caused by the increase in the crack initiation life which was related to changes in the hardness of α phase and the shape of a eutectic Si particle by the treatment time. The suitable treatment time at the temperature of 545°C was around 70min from the view points of mechanical properties and practical application.
In order to improve the fatigue strength of a nickel base super alloy, Alloy 718, by nitriding, and make clear the crack initiation site in the nitrided alloy, fatigue tests of the plain specimen were carried out under rotating bending. The suitable treatment condition of radical nitriding was investigated from the view points of thickness of the compound layer and strength of the base alloy in combination of nitriding with aging conditions. The selected condition was the combination of treatments of nitriding at 570°C for 20h after aging at 720°C for 8h and then at 620°C for 4h in consideration of practical application. By this treatment, compound layer was formed on the specimen surface with the thickness of about 10μm without softening of the base alloy. Crack initiation was markedly suppressed by the compound layer, causing the increase in fatigue strength. A crack initiated at the base alloy beneath the compound layer. The initiated crack propagated gradually to both directions of the compound layer and the base alloy, though the crack in the compound layer showed a brittle manner.
In order to improve the fatigue properties of SCM435H steel, a surface treatment system was developed that combines high-frequency induction heating (IH) with fine particle peening (FPP). In this IH-FPP system, a compressed air spray from the FPP nozzle rapidly cools the specimen surface, which is pre-heated by the IH system. The specimen surface can be simultaneously modified by plastic deformation and quenching. The IH-FPP process was performed at temperatures ranging from 400-750°C. Vickers hardness and residual stress distributions were measured in order to examine the characteristics of the surface-modified layer created by the developed process. Surface microstructures were also observed using an optical microscope. As a result, the developed processes from 650-750°C created a surface with a high hardness and an extremely fine-grained microstructure. The fine-grained microstructure was created due to dynamic recrystallization. In order to clarify the effects of the IH-FPP treatment on fatigue strength of notched SCM435H steel with a stress concentration factor of Kt = 2.36, fatigue tests were performed at room temperature using a rotational bending fatigue testing machine. The specimen treated by IH-FPP process at 700°C exhibited the highest fatigue strength. This was because micro crack initiation and propagation were inhibited by the surface modified layer with high hardness and fine-grain. This result suggests that the IH-FPP treatment process is highly effective in improving the fatigue strength of steel.
Rotary bending fatigue tests have been conducted using DLC-coated alloy steels, SKD11, SKD61 and SNCM439, with different hardness and carbide or inclusion size in order to investigate the effect of DLC film on the fatigue behavior. DLC films with the thicknesses of 1μm and 15μm were evaluated. The effect of DLC film on fatigue strength was found to depend on substrate hardness, carbide or inclusion size and film thickness. DLC-coated SKD11 and SKD61 showed the similar fatigue behavior. In SKD11 and SKD61 with lower substrate hardness, the thin DLC film exerted little influence on the fatigue strength, but the thick DLC film enhanced the fatigue strength due to the transition of crack initiation from surface to subsurface. In SKD11 and SKD61 with higher substrate hardness, the subsurface crack initiation occurred at the higher applied stress at which cracks were generated at surface in the substrate material, resulting in the improved fatigue strength. In those materials, coarse carbides or nonmetallic inclusions invariably acted as the crack origin. In SNCM439 with the lowest substrate hardness, the fatigue strength was not affected by the thin DLC film, but was improved by the thick DLC film. With increasing substrate hardness, the subsurface crack initiation occurred at nonmetallic inclusions whose sizes were much smaller than carbides and nonmetallic inclusions in SKD11 and SKD61.
The acceleration mechanism of crack growth after applying overload was investigated under baseline stress ratio R = -1. An overload was applied during fatigue crack growth test with constant stress amplitude by using carbon steel. Not only the overload level but also the cyclic stress condition affected the crack growth behavior after overloading. The effects of an overload on the rate of fatigue crack growth at constant amplitude have been of interest for some time, and much information has been gathered through which the crack growth was retarded. However there is relatively little information concerning the effects of an overload through which the crack growth was accelerated. Where the cyclic stress level was higher, the crack growth rate after overloading was kept higher level depending on the experimental conditions. Also, the acceleration of the crack growth was observed in some testing under plane stress conditions. Where the acceleration of crack growth was observed, the crack tips were blunted by applying overload. Thus the conditions of tensile overload affected zone were related to the acceleration of crack growth. It is discussed from the experimental results that the crack growth acceleration after overload was caused by the tensile residual stress developed in front of the crack tips due to applying compressive cyclic stress of which value exceeded a critical level.
A simplified method for estimating the fatigue limit and the fatigue life under two-step loading for the stress ratio R = 0 based on the results for R = -1 was proposed. This method required the fatigue test under constant amplitude loading and two-step loading for R = -1. In this paper, fatigue tests were carried out using high-strength aluminum alloy. In two-step loading for R = -1, two stress amplitude 1eve1s were used as the primary stress amp1itude and three cycle ratios were combined. The cycle ratio was defined as the ratio of cycles at the primary stress amplitude to the corresponding fatigue life. The fatigue limit and the fatigue life under the secondary stress amplitude tended to lower, as the cycle ratio under the primary stress amplitude increased. This tendency did not depend on the magnitude of the primary stress amplitude and the results were approximated with the same line. Hence, the method that had proposed for carbon steel under two-step loading could be applied to high-strength aluminum alloy. Then, the results of fatigue test for R = 0 were discussed using the equivalent stress amplitude, which was estimated by considering the stress amplitude and the mean stress. Under constant amplitude loading, the results for R = 0 approached to R = -1 by using the equivalent stress amplitude. For R = 0, the fatigue limits under two-step loading estimated by the simplified method were lower than the experimental ones, whereas the estimated fatigue life was in good agreement with the experimental results.
Static and fatigue tests under varying load ratio of tension and torsion at room temperature were carried out with short-glass-fiber-reinforced phenolic-resin matrix composites (GF/Phenol) made by injection-molded (S-series) and transfer-molded (P-series) processes. We investigated the short fiber content (V = 0%, 20%, and 50%) and stress ratio α = τ/σ effect on static and fatigue properties. Static strength and elastic modulus in uniaxial loading conditions were higher with increasing short fiber content. Furthermore, sensitivity for short fiber content of the injection-molded process was higher than that of the transfer-molded one. Static strength showed good agreement with the Tsai-Hill criterion. Relationships between the maximum principal stress σp1, max and number of cycles to failure Nf were approximately linear in the whole range of fatigue life. Normalizing σp1, max with the principal stress of static strength σp1, 0 gave S-N curves that depended on V, α and the molding processes. For unified evaluation of multi-axial fatigue life for GF/Phenol, non-dimensional effective stress σ* by the Tsai-Hill criterion was applied. Relationships between the σ* and N onto a double logarithmic chart was presented in the form of Basquin's exponential law without dependence on molding-process, V and α. The material constant n in Basquin's law showed a slope of σ*-N curves of S-series (n = 26.3) was equivalent to P-series (n = 27.0). It has been confirmed the multi-axial fatigue life of GF/Phenol could be predicted by using σ* with unique S-N curve.
In order to ensure the safety of pipe and vessel used in high pressure gaseous hydrogen for fuel cell vehicles and hydrogen stations, material properties should be clarified and the mechanical structure needs to be designed. In this study, a cyclic pressurization fatigue test was conducted using tubular specimens in the high pressure gaseous hydrogen environment. Stable austenitic stainless steel SUS316L showed no degradation of fatigue life in high pressure gaseous hydrogen environment. In contrast, metastable austenitic stainless steel SUS304, precipitation hardening steel A-286 and low alloy steel SCM435 (high and low strength) showed degradation of fatigue life in high pressure gaseous hydrogen environment. Higher strengthened SCM435 showed degradation more significantly. It was clarified that the fatigue cracks grew along the interface between martensite phase and austenite phase in SUS304 with the use of Electron Back Scattering Diffraction Pattern (EBSD). Sub-cracks and intergranular fractures were observed in the fracture surface of A-286 which was tested in hydrogen environment. Intergranular fractures were observed in the fracture surface of high strengthened SCM435 and transgranular fractures were observed in the fracture surface of low strengthened SCM435 which was tested in hydrogen environment.
This paper presented the multiaxial low cycle fatigue test method for cruciform specimens of YH61 nickel base single crystal superalloy . The elastic compliances were obtained in tensile and torsion tests using single crystal bar specimens whose specimen axis was aligned to the crystal growth directions of  and . Elastic finite element analysis was performed to determine specimen shape and relationship between the specimen axis and crystal growth direction. Multiaxial low cycle fatigue tests were performed using the cruciform specimens whose x axis was aligned to the  direction and the y axis to ,  and  directions both x and y axis were aligned to the<110> direction. Mises equivalent strain was not a suitable parameter and Mises equivalent stress was proved to be available for the correlating the multiaxial fatigue lives of the single crystal superalloy.
Environmental awareness leads to an increasing use of screw piles in construction because such piles generate low noise, low vibration and also generate no surplus soil. Also, such piling technique has practical benefit during construction. Such piles are also believed to provide relatively large compressive strength and pullout resistance capacity. In this study, a new kind of ground improving pile is developed wherein the surrounding soil around the pile is also improved by injecting compressed cement grout using non-return valves. Such piles provide larger resistance capacity than conventional screw piles. The ground is improved by injecting grout into the ground in three patterns. The improvement effect and bearing capacity characteristics of each pattern was compared and examined through field tests (plate loading test).
The Swedish weight sounding test machine (SWS test machine) used in the in-situ ground investigation method, is easy to move, shortens test time, is simple to use for testing, and less expensive compared to mechanical boring test. However, this method cannot be used in dense sand layers and stones/cobblestones layers. An in-situ ground investigation machine, NSWS (Nippon Screw Weight System) that enables identification of super soft zones and has the functions for controlling hydraulics and pneumatics of loading, and for segmentation of measuring interval was developed by utilizing the measuring system (using load and rotational penetration resistance) of the SWS test machine. In this paper, we report a case in which the NSWS test (machine) was used for identifying the subsidence locations on the river bank made up mainly of cobble-mixed gravels. Specifically, the NSWS test (machine) was used for cobble-mixed gravel layers in the initial investigation stage with the purposes of identifying the area loosened due to ground subsidence and assessing its impact to the dam's body. In addition to the measurement results of the amount of load, the penetration rate and the number of rotations, the area loosened due to ground subsidence was identified by paying attention to the scuttle frequency considering subsidence frequency specific to a NSWS test (machine) as a parameter. The test result was verified by boring measurement, and the loosened area was relatively small at the subsidence locations. Moreover, at an early stage, it was determined that the impact on the dam's body was not devastating.