Nobusuke Hattori

Application of Roller-Working on Improving the Fatigue Strength of Notched Austenitic Stainless Steel

About 90% of steel structure™s failure was induced from the notched parts due to the stress concentration. Therefore it was very important to research how to improve the fatigue property of notched parts when the stainless steel was used as structural material. Rotating bending fatigue test was preformed to investigate the effect of different notch-machining process (Conventional lathe-machining, Combination of lathe-machining and roller-working) on fatigue strength of the typical austenitic stainless steel SUS304. Fatigue limit of roller-worked specimen was improved to 285% compared to that of lathe-machined specimen. It was found that, work hardening effect induced in roller-working process played an important role to improve the fatigue strength. In addition, the residual compressive stress and the fibriform structure in the specimen surface-layer induced by roller-working were also effective to improve the fatigue strength. Non-propagating fatigue crack was observed in the notch surface of the roller-worked SUS304 for the first time.

Microstructure dependence of fatigue crack propagation behavior in wrought magnesium alloy

This paper deals with the fatigue crack propagation behavior of rolled AZ31B magnesium alloy (grain size: approximately 40 ?m). Fatigue crack propagation tests were performed on single edge notched tension specimens at a stress ratio of R = 0.1 and a frequency of 10 Hz at room temperature. Loading axes were parallel to the rolling direction; fatigue cracks propagated parallel to the transverse direction (L-T specimen), parallel to the short transverse direction (L-S specimen). Loading axis was perpendicular to the rolling direction; fatigue cracks propagated parallel to the transverse direction (S-T specimen). The crack growth rate (da/dN) of the L-S specimen was several times lower than that of the L-T specimen in the examined stress intensity factor range (?K). Fracture surfaces of the L-T and L-S specimens showed many steps parallel and perpendicular, respectively, to the macroscopic crack growth direction. The da/dN of the S-T specimen was higher than that of the L-T and L-S specimens in the examined ?K. The fracture surface was covered by quasi-cleavage facets independent of macroscopic crack growth direction, and the fracture surface roughness at low ?K was larger than that at high ?K.

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.

Fatigue Properties of Austenitic Stainless Steels with Different Nitrogen

The fatigue properties of austenitic stainless steels become inevitably important when using in structural materials. The authors have performed fatigue tests to investigate the effect of nitrogen content on fatigue properties of typical austenitic stainless steels (SUS304) and two kinds of nitrogen-contained SUS304 (SUS304N and YUS170). The main results obtained in this study are as follows; (1) The knee point in S-N curves exists for SUS304 and SUS304N, but does not exist for YUS170. (2) The fatigue limit of SUS304N is higher than that of SUS304. On the other hand, the fatigue strength by 107 cycles of YUS170 is lower than those of SUS304 and SUS304N. (3) For SUS304, the transformation ratio increases with increase in stress amplitude. On the other hand, the transformation ratio of SUS304N is very small and that of YUS170 remains essentially zero.

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

Fatigue Properties Of Pre-strained StructuralSteels

The effect of pre-strain on fatigue properties has been investigated using h\o kinds of typical structural plain carbon steels (S15C and S45C). The specified prc-strain \\as gi\en to the specimens by the Instron-type precise tensile testing machine, and the fatigue test specimens were cut out from the pre-strained specimens without working the testing portion Fatigue damage had been intermittently observed by the successive-taken replica method and fractographic analysis was applied to the broken specimens after fatigue test Though the only plastic slip lines are observed in the boundary area between ferritic grain and pearlite block by the static tensile strain of 5% without crack initiation in the case of §150. some cracks are observed in the pearlite block by the static tensile strain of r, p = 4.8%. The fatigue limit of S15C is somewhat lower at the pre-strain of e p = 2% and becomes equal or larger than that of non-pre-stram (e p = 0%) at the strain of 5 and 8%. On the other hand, the fatigue limit of S54C is remarkably lower than that of non-pre-stram at the prestrain of r, p = 2. 5 and 8% respectively. Transactions on Engineering Sciences vol 19,

Effect of Mean Stress on Fatigue Strength of Spheroidal Graphite Cast Iron

Mechanical properties, especially fatigue strength, of ferritic spheroidal graphite cast iron might depend not only on the graphite size but also on the ferrite grain size, little systematic research has been made on these factors. To clarify the influences of these structural factors as well as loading condition, fatigue tests have been carried out on ferritic spheroidal graphite cast iron with different sizes of graphite nodules and ferrite grains, under the axial loading with mean stress from -70MPa to 240MPa. The results obtained in this study are summarized as follows: (1) The fatigue limits are decreased with increasing graphite nodule and matrix grain sizes. (2) The fatigue limit decreases with increasing tensile mean stress. The fatigue limit at an arbitrary mean stress has been estimated by the modified Goodman diagram as well. (3) The parameter α is evaluated from the experimental results relating to the stress ratio R, for the mean stress acting on the tensile side and on the compressive side separately. For the case of R0, the value of α is 0.324. The result has been applied to the fatigue limit evaluation equation of Y.Murakami et al, the fatigue limit ratio falls within the range of about ±10%.

Bending Fatigue Strength Evaluation of Notched Structural Steel Sheet Repaired by Bolt

The present investigation attempts to evaluate the improvement of working by bolt on the fatigue limit of structural steel sheets with drilling a circular hole. The material used in this study is structural steel (JIS SM400A). And the specimens are identified to two types of the non-worked specimen and the worked specimen. The results obtained in this study can be summarized as follows: (1) The fatigue limit of worked specimens increases than that of non-worked specimen. Futhermore, there has a suitable value of the torque for improving the fatigue limits. (2) The reasons of enhancing the fatigue strength of the worked specimens are due to the elastic deformation, the plastic deformation and the changing of stress concentration part. Our investigations confirm the advantages of this working method which repaired the structural steel sheets by bolt.

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.