Susumu Miyakawa

High Cycle Fatigue Strength of Spring Steel with Small Scratches

When the compression coil springs are subjected to high cyclic stress, fatigue crack sometimes initiates and propagates from small scratch or defect produced in the manufacturing process. However, there are a limited number of studies on the fatigue behavior of high-strength spring steel in the presence of small scratch. In this study, the fatigue tests were conducted to examine the uniaxial fatigue behavior of a high-strength spring steel (JIS G 3561, SWOSC-V) in the presence of small circumferential notches. Based on \(\sqrt{area}\) parameter model, the sensitivity of hard steels, including a high-strength spring steel, to small scratches was qualitatively and quantitatively examined.

Microscopic observation of shear-mode fatigue crack growth behavior under the condition of continuous hydrogen-charging

Rolling bearings facilitate the smooth motion of moving mechanical transmission devices substantially reducing the energy loss. However, flaking failure caused in bearings by rolling contact fatigue can crucially deteriorate the integrity of an overall mechanical system composed using bearings. It is understood nowadays that this phenomenon is intimately associated with shear-mode (Mode II and III) fatigue crack growth caused by cyclic shear stress in the presence of large compression. Further, a problem has become of importance that the premature flaking in ball bearings could be caused by the assistance of the penetration of hydrogen into material during the operation. In this study, torsional fatigue test of a bearing steel (JIS SUJ2) was performed by using a newly-developed method of continuous hydrogen-charging. Based on the microscopic observation, it was found that continuous hydrogen-charging had an influence on the shear-mode fatigue crack growth behavior in the high-cycle fatigue regime and reduced the threshold level.

A new testing method for investigating the shear-mode fatigue crack growth behavior in hydrogen environment

Ball bearing is widely used in a variety of machines including the transportation equipments of the automobiles and airplanes. Flaking failure is a common problem for ball bearing and it is caused by shear-mode fatigue crack growth under cyclic shear stress. Further, it is known that the premature flaking is attributed to the combined effect of hydrogen penetration into the material and cyclic shear stress during the operation. Therefore, in order to ensure the integrity of ball bearing, it is necessary to clarify the effect of hydrogen on shear-mode fatigue crack growth behavior, in particular, the threshold behavior. The evaluation of the shear-mode crack growth behavior is not easy because mode I crack branching occurs easily. Our previous studies revealed that it is required to apply static compression in the direction of specimen axis to attain a stable shear-mode fatigue crack growth. In addition, successive hydrogen supply to the specimen is essential for the evaluation of hydrogen effect on the fatigue threshold because hydrogen emits from the specimen during the fatigue test. In other words, the hydrogen-precharging method, commonly used for the research on hydrogen embrittlement, is not appropriate for the evaluation of fatigue threshold. In this study, to solve these problems, we have developed a novel, easy-to-use experimental method to evaluate the threshold behavior of shear-mode fatigue crack in the presence of hydrogen. The fundamental principle of the method is introduced in this paper.

OS0517 Homogenized Elastic Stiffness of Fin Layer in Tube-Fin structures : Verification for bending loading

The authors recently developed a homogenization method for fin layers in tube-fin structures composed of flat tubes and wavy fins. In this paper, the resulting, homogenized elastic stiffnesses of fin layers with designed and real shapes are applied to finite element (FE) analysis of tube-fin layered plates subjected to bending. By comparing the fin-homogenized and full-scale FE analysis results, the homogenized elastic stiffnesses of fin layers are shown to be valid even under bending although the homogenized method is of first-order. Bending experiments of tube-fin layered plates are moreover performed to examine the homogenized elastic stiffnesses of fin layers. It is shown that the bending strains are predicted well if the real shapes of outer and inner fins are considered in the fin-homogenized FE analysis. It is also shown that the Bernoulli-Euler assumption is not satisfied in the homogenized outer fin layers in the analysis.

Study on the Fatigue Fracture Surface of Highly Hardened Material by Microbeam X-ray Techniques

ABSTRACT Tensile and compressive fatigue testing was performed under three standard mean stress conditions on high-speed tool steel, and an examination was made of the effect of mean stress on fatigue strength as well as of the fracture mechanics involved. The results showed the effect of mean stress on high-speed tool steel to be greater than that indicated by the Soderberg diagram, and showed that a great deal of caution is called for in the use of highly hardened material and that brittle fracture occurred on the fractured surface, which included fish-eyes at K fc = K IC after the crack that originated from a non-metallic inclusion had propagated radially. Further, the stress distribution in the vicinity of the crack in a 1/2 CT specimen, as well as the residual stress distribution on the fracture surface of a specimen subjected to tensile and compressive fatigue testing, were measured by microbeam X-ray stress measurement equipment, and a model is proposed for the generation of residual stress on the fracture surface.

Behavior of crack propagation in the test piece of circular section with "Fish eye".

The behavior of crack propagation in the complete reversed tensile and compressive fatigue test piece of circular section with Fish eye of the heat treated high-speed steel was investigated by using an acoustic emission technique. Moreover, the property of crack propagation in the 1/2CT test piece was investigated.The results obtained are summerized as follows;(1) The number of stress cycles NAE for crack initiation and propagation and the number of stress cycles Nf for fracture were accurately estimated by measuring continuously AE total ringdown counts during the fatigue process.(2) The fatigue life was good accord with the value of Nf estimated by the acoustic emission technique. The life of fatigue crack propagation (Nf-NAE)Lab. was 25-50% of the fatigue life Nf and decreased with decreasing stress amplitude.(3) The life of fatigue crack propagation (Nf-NAE)cal. calculated from the property of crack propagation in the 1/2CT test piece showed good agreement with the value of (Nf-NAE)Lab. obtained by the acoustic emission technique.(4) The behavior of crack propagation in the complete reversed tensile and compressive fatigue test piece of circular section showed good corrrelation with the behavior of crack propagation in the 1/2CT test piece, so that it is suggested that the initiative mechanism of residual stress on the fracture surface with Fish eye can be discussed by using the 1/2CT test piece.

Rapid estimation of mean value and standard deviation of endurance limit using acoustic emission.

The behavior of AE total ringdown counts during the stepwise increment of oscillating stress under various conditions (loads, microstructures and materials) was investigated and the following conclusions were obtained.1) Various factors influencing AE total ringdown counts during the stepwise increment of oscillating stress were clarified.2) The mean value and standard deviation of stress giving rapid AE increment corresponded to those of the endurance limit given by 14S-N testing method under various conditions (loads, microstructures and materials). Thus, the mean value and standard deviation of the endurance limit can be estimated precisely from the stress giving rapid AE increment.3) The stress giving rapid AE increment corresponded to generation of slip bands during the stepwise increment of oscillating stress. It was 0.9 times of the endurance limit given by 14S-N testing method.