We are trying to clarify the metal fatigue mechanism by dividing into "crack initiation" and "crack propagation" of fatigue process. In-situ observation methods are used.
We are aiming to reduce a safety factor on fatigue design by a quantitative prediction of fatigue limit and, furthermore, to improve a fatigue limit under variable loading conditions via simulations. Since several initial defects such as cracks generally exist in mechanical parts and structures, therefore the fatigue limit can be clarified through an analysis of non-propagating fatigue crack behavior. The non-propagating fatigue crack behavior depends on the history of loading condition as well as the material property. Considering the application to various loading conditions and materials, a theory-based analytic approach is more effective than an experimental approach in order to figure out the non-propagating fatigue crack behavior. For the theory-based analytic approach, simulations such as numerical analysis and finite element method (FEM) analysis are used.
Although the problem of the metal fatigue actualized with increase of load about the steel used for the machine structure, about 100 years have been required and solved. We think that we can say that steel can be managed now at last in recent years. Since it will take 100 year before managing when the same approach as steel is taken about the advanced light metal which is a new material, and we are studying about rapid evaluation and management of the materials.
Now, environmental problem, energy problems, and resources problem act as an important subject matter. One method considered as a method to solve these problems is a weight saving of a structure. Although the approach of a structure and material can be considered to a weight saving, the approach of a structure seems to have been all out and it is thought that it is difficult. Then, attentions have gathered for the weight saving of material. Now, a most diffused material is an alloy of iron including steel. In order to perform a weight saving, metal lighter than this is needed.
Although a magnesium alloy is the lightest alloy in the light metal used in practical use, it has the bad point of being easy to burn since the fire point is low. The non-combustible Mg alloy used in this study is a new material which has an improved resistance of being easy to burn by the addition of about 2% calcium (Ca) to the conventional Mg alloy. We are investigating especially the fatigue characteristics of this new magnesium alloy for the purpose of putting this alloy in practical use.
Since the specific strength is excellent and corrosion resistance is also good, the demand of commercially pure titanium as structural materials, such as a chemical plant, is increasing. We are studying the fatigue characteristics of notched commercially pure titanium.
Study for giving the rational choice and the guideline of material development of an aluminum alloy is done. We clarify the fatigue mechanism and the difference of fatigue characteristics (the process of a fatigue crack initiate, existence of a fatigue limit, notch sensitivity). The objects are age hardening aluminum alloy (2000 series, 6000 series, 7000 series) and work hardening aluminum alloys (5000 series etc.).
Recently, we have developed an aluminum alloy which has "distinct fatigue crack propagation limit" by adding excess Mg to the ordinary precipitation-hardened aluminium alloy, JIS 6061-T6.
We are doing study on the clarification of fatigue damage initiation mechanism and extension of life in rolling-contact components. Objects are the rolling mill rolls used for iron making process, the rail used for train, and so on. The roll and the rail obtain the compressive stress of the repetition by different contact from the usual machine element. In these, the metal fatigue phenomenon called "Rolling Contact Fatigue (RCF)" comes out. Compared with the metal fatigue phenomenon generated by the usual tension, the question remains in the mechanism about the rolling fatigue generated with the complicated stress containing compressive stress.
We are doing study on the fatigue strength of the pearlite steel currently used on a rail mostly. The pearlite steel has the characteristic different from usual steel, such as having a different crystalline structure from usual steel, and causing remarkable work hardening. About the reason why the pearlite steel fits the rail, since there are many unclear points, we clarify the reason the pearlite steel is optimum as a rail, and study is advanced for the purpose of considering it as the guideline of material development.
If detailed observation of the rail which the rolling contact fatigue caused is performed, damage in the part of the plastic deformation area is observed. On the other hand, the material before plastic deformation is used for the test for choosing a rail material against fatigue strength. We are studying in order to clarify the inconsistency about this evaluation method and to propose an optimum evaluation method.
Crack propagation rate is quite-variable, which strongly affects fatigue life. Therefore, safety ratio is always contained in structure designs. Since understanding the scatter would allow to reduce the safety ratio, we first suggest a definition of the scatter of fatigue life associated with a small fatigue crack propagation rate. Then, as the final goal, we try to produce new materials without the factors providing the scatter.
Fatigue life is known to depend on volume of materials. Thus, we establish the prediction method of the volume effects through utilizing statistical method.
Fatigue damage grows through a ductile mechanism. However, the mechanism depends on various factors: stress state, strain distribution, microstructure, temperature and frequency. Thus, systematic and quantitative investigations of the fatigue damage evolution are recognized to be very tough. In this study, we try to apply local strain mapping and damage evolution curve analysis to quantitatively elucidate the strain frequency dependence of the fatigue damage evolution.
Fatigue crack propagation limit often dominates the fatigue life in steel; however, fatigue crack in austenitic steels with low carbon content is known to grow gradually even at a low stress level as long as a fatigue crack is initiated. This means the fatigue limit of low carbon austenitic steels is dominated by fatigue crack initiation limit. In terms of crack propagation limit, strain aging has been revealed to stop small crack propagation through hardening the plastic zone at a vicinity of a crack tip. Moreover, I-S (interstitial-substitutional atoms) Interaction has been reported to enhance the prevention of small crack propagation. In this study, we attempt to improve the fatigue limit by using strain aging of Mn-C couples.
In this theme, we evaluate the important of carbon on fatigue crack propagation limit of TWIP steels. Crack propagation limit is practically much more important than crack initiation limit, since crack-like products such as scratch always exist on the material surface in practical situations. By using FIB-notched specimen which is regarded as a small crack, we clarify the influence of carbon on fatigue crack propagation limit of TWIP steels.
Fatigue crack propagation is interpreted by repetition of crack open and closure associated with dislocation slip behaviour. However, the characteristic deformation modes in austenitic steels, extended dislocation glide, martensitic transformation, deformation twinning, have never been investigated to correlate fatigue crack open/closure. In this study, we examine the correlation through in-situ optical and SEM observations.
The variety of microstructure and solute elements in steel provides various unique fatigue properties. In particular, carbon is the most important elements, since the concentration, distribution, morphology are known to affect mechanical properties strongly. In this study, we note the carbon distribution and morphology dependence of fatigue properties, and clarify the fatigue crack propagation mechanisms corresponding to the different carbon states.