Shin Ichi Nishida

Research of Fatigue Limit and Stress Concentration on Structural Steel with Double-Notches

The fatigue limit of parts and components that have the multi-notches is important data for the design and manufacture of machinery and traffic equipment which are operated under the high speed or pressure. In this paper the rotating bending fatigue tests have been carried out to investigate the fatigue limit of specimen with double-notch that is constructed of step and blind hole, and analyzed the effect of stress concentrations at the double-notched bottoms on the fatigue limits, using three-dimensional elastic finite element method. Firstly, the fatigue tests of 8 group specimens have been performed for examining the of fatigue limits of the single-notched specimen and double-notched specimen, respectively. Additionally, the stress field interactions between two stress fields by the blind hole notch and step are discussed using three-dimensional elastic finite element method. The main results obtained in this study are as follows: The fatigue limit of the double-notched specimen are down comparison with the fatigue limit of the single-notched specimen; the fatigue limit of the double-notch specimen is insensitive to distance between the blind hole and step for the low carbon structure steel with better ductility; for the high-strength steel, superposition and intensification of the stress concentration by the blind hole and step mutually may be avoided so that their adverse effects on the fatigue strength may be become to minimize, as take appropriate distance between the blind hole and step. The results are significant for the design of engineering design of the multi-notched parts, and the study of fatigue strength.

Tensile Pre-Strain Effects on Torsional Fatigue Properties of Medium Carbon Steels

This study was focused on the effects of pre-strain on the torsional fatigue properties of three kinds of medium carbon steels, including the fatigue strength, surface hardness, microstructure, and the crack initiation and propagation behaviors. The effect of pre-strain on the non-propagating cracks was also discussed. The main results obtained in this test are as follows: 1) the fatigue limits increase with the increasing of tensile pre-strain ratio for all kinds of the test materials; 2) under certain stress amplitude, with the increasing of tensile pre-strain ratio, the fatigue crack initiates a little earlier and propagates faster; 3) the length of non-propagating crack decreases with the increasing of tensile pre-strain ratio.

The Effect of Surface Treatment of Ti-6Al-4V Alloy Specimens

As though titanium and its alloy have specific tensile strength, excellent corrosion resistance, ect., they have not been widely used until now mainly due to their extremely high processing cost. Therefore, it is considered that the above barrier will become smaller if it is possible to improve the mechanical properties of Ti-alloys by applying surface treatments on properties, especially fatigue strength of about 6kinds of surface treatment Ti-6Al-4V alloys. This is, surface treatments by 2kinds of plasma, 3kinds of DLC (diamond-like carbon) coating and the electroness plating method, have been studied for Ti-6Al-4V alloy . This study presents an analysis of fatigue behavior from the viewpoint of micro-hardness and adhesion work through a new technique in Ti-6Al-4V alloy. In addition, the specimens surface has been successively observed by the replica method during tests and it seems clear that the fatigue strength is improved by the DLC treatment in comparison with that of the conventional Ti-6Al-4V alloy by about 20% (fatigue limit improvement by 70MPa). Though all the hardness value of these specimens increased, the fatigue limit improved only in the case of two kinds of specimens by DLC coating.

Evaluation of Fatigue Strength of Double Notched Specimens

Fatigue properties of double-notched specimens have been evaluated using O no-type rotating bending fatigue testing machine. In this test, the double notch was consisted of a step or V-notch with a drill hole. The fatigue limit of the single-notched s pecimen with a V-notch and that with a drill hole was estimated using the fatigue notch factor K f proposed by JSME. According to the result of this study, the fatigue limit of a double-notched specimen c ould be practically estimated by calculating the product of fatigue notch factor for each relative s ingle notch. In addition, theoretical consideration has been performed applying FEM (Finite Element Method) analysis for the notched portion to be examined and compared with the experimental results, respectively. Introduction One of the authors has reported that more than 90% of failures of stee l-structural components have eventually risen from stress concentrated part such as notch. In addit ion, more than 90% of failure cases are caused by fatigue directly or indirectly[1]. Also, eve n th damage from double-notched part is often admitted, however, the law evaluating the above has not been presented. Recently, the design of a machine part tends to be more complicated. Th problem on fatigue strength of double-notched specimen will be very important. Nevertheless , th re are few reports on this double-notched problem so far. Therefore, it is necessary to study the effect of multi-notches on the properties of a component. Experimental procedure Specimen and test condition. The material used in this test is a round bar ( 20mm in diameter) of plain carbon steel (JIS-type S15C). Table 1 lists the chemical c ompositions and mechanical properties. After normalized for two hours at the temperature of 900 , all of the specimens were cut out from the round bar. Fig.1 and Table 2 show the shapes and dimensions of spec imens and specimen designation. After polishing the notched part with emery paper, all of the specimens were annealed in vacuum condition for half an hour at the temperature of 600 , and then were electro-polished to remove the work hardening layer to the depth of 50 m. Fatigue tests had been performed using Ono-type rotary bending fatigue testing machine with 98N m capacity under the Table 1 Chemical composition and mechanical properties. C Si Mn P S Al Yield stress MPa Tensile strength MPa Reduction of area 0.16 0.22 0.50 0.017 0.006 0.037 284 442 71.1 Key Engineering Materials Online: 2003-10-15 ISSN: 1662-9795, Vols. 251-252, pp 19-24 doi:10.4028/www.scientific.net/KEM.251-252.19

Microbial Control on Lamina Formation in a Travertine of Crystal Geyser, Utah

Travertines – carbonates mostly precipitated in situ from calcareous hot-spring water – often show stromatolitic lamination which has been interpreted as daily banding (Folk et al. 1985; Guo and Riding 1992; Pentecost 1994). Prevailing interpretation for the daily rhythm has tied up with photosynthesis of cyanobacteria and algae living on the travertine surface.

Fatigue Properties Improvement of Notched Structural Steel Sheets

Nearly 90% of failures of machines and mechanical parts are caused at the area of stress concentrated in the structural components. Hence, it is important to investigate the method of improving fatigue strength for notched parts. In this paper, the fatigue tests have been performed to investigate the effect of the plastic-working on fatigue strength of specimens with notch. The main results obtained in this study are as follows: (1) The fatigue limit of notched specimen chamfered by the plastic-working increases by 45% than that of specimen without chamfer. (2) The reason of enhancing the fatigue strength may be to attributed to the work-hardening and compressive residual stress which suppress the fatigue crack initiation and propagation in early stage.

Effects of DLC on Fatigue Properties of Eutectoid Steel

Diamond-like carbon (DLC) is an amorphous hard carbon, which has very high hardness, high resistivity, and dielectric optical properties. Economically and technologically attractive properties have drawn almost unparalleled interest towards the coatings. Eutectoid steel is a kind of material that has been widely used in shafts and various kinds of industrial components. Three kinds of fatigue specimens with different DLC conditions were used in this study. Fatigue test had been performed to investigate the effects of DLC on fatigue properties of eutectoid steel. The fractography was analyzed by a scanning electron microscope (SEM), and surface hardness was also evaluated. The fatigue limits of the DLC coated specimens did notincrease after DLC process, though the compressive residual stress which produced by DLC process can prevent fatigue fracture. According to the results of fatigue test, the optimal DLC method for improving the fatigue properties of eutectoid steel is determined and the relationship between fatigue limits and coating bias are obtained.

Effect of High Tensile Pre-Strain on the Torsional Fatigue Properties of a Structural Steel

This study is developed to investigate the effect of monotonic plastic deformation on the torsional fatigue properties of a structural steel. Five different kinds of tensile pre-strain, 2%, 5%, 8%, 12% and 22%, were applied to the specimens, respectively. And the maximum pre-strain value is near to the necking strain of the test material. The effects of tensile pre-strain on surface hardness, fatigue crack initiation and propagation behaviors, and the behavior of non-propagating cracks. The main results obtained are: The fatigue limits are 145, 160,175, 200 and 215MPa for specimens with tensile pre-strain of 2% 5%, 8%, 12% and 22%, which are improved to 104%, 114%, 125%, 143% and 153% of the fatigue limit for non-pre-strained specimens, respectively. The torsional fatigue limit increases with the tensile pre-strain increasing, until the pre-strain value being near to the necking strain ratio. However, the fatigue limit increase becomes more slowly for high tensile pre-strained specimens than the lower ones. The fatigue cracks of the tensile pre-strained specimens initiated earlier than that of the non-pre-strained specimens, and the propagation is also accelerated, but there is no effect on the fatigue crack initiation point and the branch point. Non-propagating crack length becomes shorter with increasing of tensile pre-strain until the value near the necking strain, and the quantity of non-propagating crack increases at the same time.

Optimum Performance of High Tension Steel Sheets under Various Heat Treatment Process

This study is developed to establish the optimum performance of high-tension steel sheets that are to be used as automotives structures and components. In an automotive industry optimum performance means good combination in safety, durability and stability as well as good press formability for complicated shapes. This condition is measured by mechanical properties such as yield strength, tensile strength, fatigue strength and hardness. The materials which have been used in this study are 4 types of high-tension steel sheets designated as A, Q, T1 and T2 in term of as-rolled sheet, direct quench process and different tempering temperatures. The results of this study show that material A has the highest fatigue limit followed by materials T2, T1 and Q. The tensile strength of A is the highest, followed by T1, T2 and Q. In addition, as far as the surface hardness is concerned, T1 shows the highest value followed by A, Q and T2. In this study fatigue properties are particularly investigated in detail. Fatigue fracture surfaces observed under a scanning electron microscope (SEM), show that most of the fatigue cracks are initiated from an inclusion or a void in the microstructures.

Fatigue Properties of Austenitic Stainless Steel with Circumferential Notch

The effect of stress concentration factor on the fatigue properties of typical austenitic stainless steel SUS304 have been investigated using the circumferentially notched specimens. The notch of specimens has six kinds of radii, i.e. ρ = ∞ (i.e. plain specimen), 2.0, 1.0, 0.6, 0.3, and 0.1 mm with constant notch depth (t=0.2mm). Though the fatigue cracks in the specimens with a blunt notch initiate at one point, those in the specimens with a sharp notch initiate at several points. There exist the slip bands in the surface of the specimen under the stress amplitude of fatigue limit by 1×107 cycles, and do not exist the non-propagating micro-cracks in all kinds of the specimens. Furthermore, it has been found that notch sensitivity of austenitic stainless steels is higher than that of a typical plain carbon steels under the higher stress concentration factor region.