Noriyasu Oguma

Crack Initiation Mechanism of Bearing Steel in High Cycle Fatigue

In the last decade, fatigue behavior of metallic materials in very high cycle regime has been coming out as an important subject to guarantee the safety of mechanical structures during the long term service. The authors had developed special fatigue testing machines to perform the fatigue tests effectively in both rotating bending and axial loading in gigacycle region. By using these testing machines, long life fatigue tests were carried out for several kinds of metallic materials including the high carbon chromium bearing steel(JIS: SUJ2).

Effect of stress ratio on fatigue lifetime and crack growth behavior of WC–Co cemented carbide

Two types of fatigue tests, a rotating bending fatigue test and a three- or four-point bending fatigue test, were carried out on a fine grained WC–Co cemented carbide to evaluate its fatigue crack growth behavior and fatigue lifetime. From successive observations of the specimen surface during the fatigue process, it was revealed that most of the fatigue lifetime of the tested WC–Co cemented carbide was occupied with crack growth cycles. Using the basic equation of fracture mechanics, the relationship between the fatigue crack growth rate (da/dN) and the maximum stress intensity factor (Kmax) was derived. From this relation, both the values of the threshold intensity factor (Kth) and the fatigue fracture toughness (Kfc) of the material were determined. The fatigue lifetime of the WC–Co cemented carbide was estimated by analysis based on the modified linear elastic fracture mechanics approach. Good agreement between the estimated and experimental fatigue lifetimes was confirmed.

Fatigue Lifetime and Crack Growth Behavior of WC-Co Cemented Carbide

It is well known that WC-Co cemented carbides have excellent wear resistance. However, information about their fatigue crack growth behavior and fatigue properties is limited. In the present study, rotating bending fatigue tests were carried out on a fine grained WC-Co cemented carbide to evaluate its fatigue lifetime and crack growth behavior. From observations of the micro-notched specimen surface during the fatigue process, it was revealed that most of the fatigue lifetime of the tested WC-Co cemented carbide is comprised of crack growth cycles. Using the basic equation of fracture mechanics, the relation between the rate of fatigue crack growth da/dN and the maximum stress intensity factor Kmax of the WC-Co cemented carbide was derived. From this relation, both the threshold intensity factor Kth and the fatigue fracture toughness Kfc of the material were determined. Fatigue lifetime of the WC-Co cemented carbide was estimated based on the fatigue crack growth law.

Long Life Fatigue Fracture Induced by Interior Inclusions for High Carbon Chromium Bearing Steels Under Rotating Bending

The characteristics of the long life fatigue fractures induced by interior nonmetallic inclusions for two types of high carbon chromium bearing steels were experimentally examined under rotating bending. Specimens were machined from the round bars with different diameters. One type was cut from the central area of a thin bar; the other type was cut from the medium area of the radius of a thick bar. As a result of the fatigue tests, the fatigue life of the latter type specimen was longer in the long life region. Crack initiation sites on fracture surfaces of all the failed specimens were observed by means of a scanning electron microscope. The number of alumina inclusions was decreased, and the average size of titanium nitride inclusions on fracture surfaces was smaller for the latter type specimens. And a characteristic rough surface of fine granular area (FGA) was observed in a circle around the inclusion on the fracture surface. The size of FGA tended to increase with an increase of the fatigue life and a decrease of the stress amplitude at the position of the inclusion. From a view point of fracture mechanics, the stress intensity factor range at the front of FGA, ⊿KFGA, was calculated for each specimen by using √area model. Values of ⊿KFGA of both type specimens were almost constant regardless of the stress amplitude at the site of inclusion, and the average value was about 5 MPa√m. That is, it seems that the fatigue life of the specimens cut from the thick bar was improved by downsizing the inclusion area on the fracture surface.

Construction of Electronic Factual Database on Very High Cycle Fatigue Properties for Structural Metallic Materials

As a joint project of Committees on Fatigue and Reliability Engineering in the Society of Materials Science, Japan (JSMS), an electronic database on fatigue strength of metallic materials fabricated in Japan had been constructed and published in 1996. Book style of the same data compilation had been published at the same time by the JSMS and Elsevier. About twenty years have passed since the above publications of database and databook. Thus, a lot of new fatigue test data have been obtained during such a long period including many data on the very high cycle fatigue such as gigacycle regime. Based on such a circumstance, the JSMS has organized a new project to construct an electronic database on very high cycle fatigue. A lot of numerical data obtained by fatigue tests would be compiled together with many photographs of fracture surfaces. In this paper, fundamental view and compilation concept of the database are briefly introduced by showing a typical example of proto-type database constructed from domestic data in Japan.

Fatigue Crack Initiation and Propagation Behaviors in Rotating Bending of SNCM439 Steel in Very High Cycle Regime

In order to ensure the long term durability of mechanical structures, the fatigue property of structural components should be clarified in the long life region such as the gigacycle regime. The rotating bending fatigue tests in very high cycle regime were carried out for a nickel chromium molybdenum steel for structural use of machines (JIS Material Code: SNCM439) in this study. Based on the initiation site of the fatigue crack, fracture modes were classified into the following typical three modes: (1) usual surface fracture, (2) surface defect-initiated fracture and (3) interior inclusion-initiated fracture, respectively. In S-N diagram, experimental data in the usual surface fracture mode appeared at higher stress levels with fewer loading cycles, whereas the data in the other two fracture modes appeared at lower stress levels with more loading cycles. Thus, the duplex S-N property was confirmed for this steel in the very long life regime. In order to clarify the fatigue mechanism of the interior inclusion-initiated fracture, the quantitative evaluations were made by applying the stress intensity factor range. The fatigue crack initiation and propagation processes in the interior inclusion-initiated fracture were divided into four stages: formation of the fine granular area (FGA) due to initiation and coalescence of micro-debondings, formation of the fish-eye due to penny-shape crack propagation, crack propagation as surface crack and final catastrophic fracture.

Period of Fine Granular Area Formation of Bearing Steel in Very High Cycle Fatigue Regime

In order to examine the period of fine granular area (FGA) formation of bearing steel in very high cycle fatigue regime, rotating bending fatigue tests were carried out at the stress amplitude 1100 MPa below the fatigue limit. The tests were interrupted at the cumulative damage values ranging from 0.1 to 0.5 with an increment of 0.1 to charge hydrogen to the specimens. After the charge, the rotating bending tests were continuously carried out. The crack origin areas on all fracture surfaces were checked by a scanning electron microscope (SEM), and it was discovered that FGA was not formed in some of them. From a view point of fracture mechanics, the stress intensity factor ranges of FGA areas, ΔKFGA, were calculated by using Murakamis area model. The ΔKFGA values increase with the increase of the cumulative damage values. Furthermore, ΔKFGA values in this study were smaller than 5 MPam which was obtained from usual fatigue testing. Therefore, we conclude that the stable crack growth stage starts when the threshold stress intensity factor range decreases due to hydrogen embrittlement in the middle of formation of FGA.

Estimation of Fatigue Limit in Interior Inclusion Induced Fracture Mode for Bearing Steel in Rotating Bending

The fatigue life forming fine granular area (FGA) is expected to occupy a large fraction of the total fatigue life. In order to examine the commencing time of the FGA forming and estimate the fatigue limit in the interior inclusion induced fracture mode, rotating bending fatigue tests were carried out by using SUJ2 specimens with and without the hydrogen charge. Especially, the hydrogen charge time was set in the wide variety of the cyclic loadings to confirm the FGA forming process supposing the effect of the hydrogen charge on the fatigue crack propagation behavior. From experimental and analytical results, it is concluded that the FGA formation is already started at early stage as 5 % of the fatigue life. Another finding is that we have a clear correlation between the stress intensity factor range and the FGA growth ratio. Based on this aspect, the critical value of the stress intensity factor range in which the FGA is not formed around the inclusion was given as 2.65 MPa.

Effect of Cobalt Content on Fatigue Lifetimes and Short Fatigue Crack Growth Behavior of Tungsten-Cobalt Cemented Carbides

Fatigue lifetimes and crack growth behavior of the cemented carbides with different Co content were investigated. Effect of the Co content on the fatigue lifetimes is found to be minimal within the Co content, 13 –18 wt% tested in the present study. At the high Kmax region, the crack growth resistance for the WC-18 wt%Co is higher than that for the WC-13 wt% Co. In the lower Kmax region, the reverse trend, i.e., the crack growth resistance for the WC-13 wt% Co is higher than that for the WC-18 wt%Co, was expected, but this point remain obscure due to the lack of the experimental data.