Papers by Keyword: Rotating Bending

Abstract: 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.

Abstract: 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.

Abstract: In order to investigate very high cycle fatigue properties of Zr₅₅Al₁₀Ni₅Cu₃₀ bulk amorphous alloy, rotating bending fatigue tests were conducted at room temperature by using 5 different series of Zr₅₅Al₁₀Ni₅Cu₃₀ alloy. The differences of material processing conditions are manufacturer, product year, material size and minimum diameter of specimen. Although all specimens fractured from surface, duplex S-N characteristics were observed for each series of the material. Time strength distributions at N=10⁴ for short life region and at N=10⁷ for long life region were well approximated by normal distribution. The entire S-N property accepting the normalized stress amplitude by the time strength at N=10⁴ or N=10⁷ has shown more clearly duplex S-N characteristics. In addition, P-S-N properties were also estimated from the standard deviation of time strength distributions at N=10⁴ or N=10⁷. Based on the observation of fracture surface by scanning electron microscope (SEM), it is confirmed that every fracture surface was consisting of typical three regions such as multi-facet region, stable crack growth region and instantaneous fracture region.

Abstract: In the fabrication process of medicine tablets, working speed of the tablet compressing is an important factor to realize the high fabricating efficiency together with the low cost. Thus, a number of loadings would be applied with very high frequency to tips of a couple of compressing punches. Sometimes, the tablet compressing speed exceeds 150 tablets per second. Due to such a circumstance, the very high cycle loadings are applied to the tips of the compressing punches making medicine tablets. The high strength steel of KNS-ES was specially designed and fabricated for the particular use as the compressing punches. In this study, very high cycle fatigue tests were performed in the loading type of rotating bending in order to obtain the fundamental S-N property of this steel. Based on experimental results, the S-N property in giga-cycle regime was discussed including the effect of the residual stress on the S-N properties. Consequently, the duplex S-N curves were clearly found, but the surface-induced fractures were often found in the fatigue data belonging to the second S-N curve in the longer life region.

Abstract: The paper contains results of a study aimed at exploring possibilities of use of direct current potential drop (DCPD) method for evaluation of depth and profile of cracks occurred under or near press fitted hubs in a full-scale axle during severe rotating bending loading. DCPD method was applied on section of an axle after fatigue by a specific way and potentials were measured in several circumferential areas of the axle section, when direct current passed longitudinally. Results are analysed and discussed. It was indicated that unlike previous use of the method for crack measurement of different components, even large and complicated, described in the literature, when potentials corresponded to ratio of cracked and uncracked area, in this case of approximately circumferential crack, measured potentials correspond better to relative crack depth in the relevant point of the circumference.

Abstract: 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, ΔK_FGA, were calculated by using Murakamis area model. The ΔK_FGA values increase with the increase of the cumulative damage values. Furthermore, ΔK_FGA 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.

Abstract: Based on the observation of fracture surfaces for a low-alloy high strength steel under rotating bending in very high cycle regime, a newly defined method about inspection plane under rotating bending was developed in this paper. By using the statistics of extreme values (SEV) method, the maximum sizes of inclusion and FGA corresponding to the control volume of specimen are evaluated to be about 41.38 μm and 58.51 μm on the basis of this newly defined inspection plane, and the corresponding values of fatigue limit are evaluated to be 690 MPa and 654 MPa, respectively. It should be noted that fatigue design based on former involves a certain amount of risk in very high cycle regime.

Abstract: Effect of high humidity on growth mechanism of a fatigue crack was investigated for an extruded bar of an age-hardened Al alloy 7075-T6 which had the marked texture of plane (111). Fracture in high humidity occurred by the growth of a shear mode crack under high stress levels, though a crack propagated in a tensile mode under low stress ones, macroscopically. Many voids and slip planes were observed on the fracture surface yielded by the shear mode crack. Especially most of the fracture surface was occupied by voids where the crack was small. The void percentage decreased with increasing in the crack depth. Fracture surface yielded by the shear mode crack was a plane (100). The growth direction of the shear mode crack to the cross section of specimen was about 55° corresponding to the angle composed by this plane and the texture of plane (111). These results suggest that the shear mode crack was related to microstructure, stress and environment. The growth mechanism of the shear mode crack assisted by hydrogen was proposed based on the results of the acceleration of crack growth and the formation of voids in high humidity.

Abstract: In railway transportation, the press fit of wheel and axle is typically causing fretting damage and it is also an important issue relative to the safety of railway. To examine the fretting damage of the interference fit of wheel and axle the fretting fatigue test under 4-point rotating bending loads is carried out based on samples, as well as the fretting fatigue damage rule is studied. The test results show that the fretting causes the fatigue limit to decrease significantly and all of the fracture sections occur at the contact edges of the fitting zone with the sleeve.

Abstract: In order to investigate the effect of humidity on fatigue strength of an extruded and age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out using plain specimens in environments of controlled relative humidity of 25%, 50%, 75% and 85% and distilled water. The cross section of the alloy has a marked texture of (111) plane. Although fatigue strength was decreased by high humidity, the decrease by high humidity was very small when the humidity was lower than about 60% -70% and fatigue strength was largely decreased over the humidity. Both of initiation and propagation of a crack were accelerated by high humidity. In high humidity, a crack propagated in a shear mode macroscopically and it was ductile in company with many glide planes and voids microscopically. That is, the propagation was not a tensile mode with brittle facets even in water. The shear mode propagation inclined about 35° to the extruded direction and fracture surface was (100) plane, meaning that the shear mode propagation of a crack was mainly caused by the marked texture of the alloy. The propagation mode of a crack was affected by not only environment but also stress level.