Nobuyoshi Ohno

Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength, crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension−compression fatigue tests were carried out with cylindrical smooth specimens. Two types of specimens were machined with the loading axis parallel (L-specimen) and perpendicular (T-specimen) to rolling direction. Monotonic compressive 0.2% proof stress, tensile strength and tensile elongation were similar for both specimens. On the other hand, monotonic tensile 0.2% proof stress of the L-specimen was slightly higher than that of the T-specimen. Moreover, monotonic compressive 0.2% proof stresses were lower than tensile ones for both specimens. The fatigue strengths of 10 7 cycles of the L- and T-specimens were 95 and 85 MPa, respectively. Compared with the monotonic compressive 0.2% proof stresses, the fatigue strengths were higher for both specimens. In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension−compression loading. The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction. The crack growth rate of the L-specimen was slightly lower than of the T-specimen. Consequently, the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.

Bearing Fatigue Life Tests in Advanced Base Oil and Grease for Space Applications

Two synthetic base oils (815Z and 2001A) and two greases (601EF and R2000) used for space applications have been studied at ground level. Rheological tests were performed in order to characterize the behavior of each of the base oils versus the pressure and the temperature. Next, the effect of base oils and greases on ball bearing fatigue life was carried out using thrust ball bearings. The results of L 10 life tests showed a bearing life order from the highest to the lowest of grease 601EF (blended with base oil 815Z), base oil 2001A, grease R2000 (blended with the base oil 2001A) and, the lowest, the base oil 815Z. The general tendency of the base oils and the greases shows that the bearing life increases with the EHL film parameter. However, the anticipated beneficial effect of an apparently high film parameter for the base oil 815Z was not seen due to permanent viscosity loss in the EHL contact. The results showed that the viscosity of the base oil 815Z, which contains the acetal group (-OCF 2 O-), was decreased by mechanical shear at the high shear rate in the EHL conjunction and the EHL oil film was thinner than expected. Hydrogen fluoride is released with the decomposition of the acetal group. As a result, the permanent viscosity loss at high Hertzian pressure and the hydrogen fluoride generation shortened bearing life with the 815Z base oil. In the case of grease 601EF with base oil 815Z, the permanent viscosity loss did not occur in ball bearings and the bearing life is extended.

Friction and Wear Characteristics of Advanced Space Lubricants

Synthetic oils and greases are used for space lubricant. So, it is important to know the performances of these lubricants. The base oil 815Z and 2001A and the greases 600EF, 601EF and 602EF with base oil 815Z and grease R2000 with base oil 2001A were considered as the test lubricants in this study. The highest wear scar has found for base oil 815Z but it showed the lowest coefficient of friction, whereas greases 600EF, 601EF and 602EF showed lower wear scar and considerable friction coefficient. Investigating these phenomena, authors found that the base oil 815Z contains the acetal group (-OCF2O-). At high shear rate in EHL conjunction the viscosity was decreased by mechanical shear. Hydrogen fluoride occurred with the decomposition of acetal group. It increases the wear rate of the contact surfaces. But that decomposition does not occurred in the greases with base oil 815Z and showed better result as space lubricant.

Pressure effects on thermal isomerizations in highly viscous media. The first clear-cut example of viscosity-induced retardation of “slow” thermal reactions

Abstract Kinetic effects of pressure on thermal Z/E isomerizations of 4-(dimethylamino)-4′-nitroazo-benzene (DMNAB) and N-[4-(dimethylamino)benzylidene]-4-nitroaniline (DMBNA) were studied by flash photolysis in three viscous solvents; glycerol triacetate (GTA), 2-methyl-2,4-pentanediol (MPD) and “Traction Fluid B” (TFB). In all cases studied, the pressure effects observed at the beginning of pressurization were qualitatively similar to the ones observed in less viscous solvents. The results strongly suggest that the conventional understandings of the kinetic effects of pressure based on the transition state theory (TST) are valid in these thermal unimolecular reactions. At higher pressures, however, pressure-induced viscosity increase resulted in strong retardation of both of the isomerizations. The diffusion-controlled rate constants obtained by substituting the observed and the TST-expected rate constants to I/kobs = l/kTST + l/kdif showed inverse fractional dependence on the solvent viscosity.

Bulk Modulus of Solidified Oil at High Pressure as Predominant Factor Affecting Life of Thrust Ball Bearings

In a previous paper, it was pointed out that the solidification behavior of mineral oils at high pressures exceeding glass transition points plays an important role on bearing life. Further detailed investigations have been carried out based upon actual observations of solidification phenomena of traction oils and mineral oils. Traction oils characterized by large pressure-viscosity coefficients and high friction result in shorter bearing life, in spite of their excellent separating film formation ability, in particular, under solidified conditions. The general tendency of mineral oils shows that bearing life increases with the film parameter. However, the beneficial effect of high film parameters of traction oils is considered to be disturbed by high friction, which is found to increase linearly with the bulk modulus of solidified oil. In view of this fact that brittleness of solidified oils is estimated by the temperature difference from the viscoelastic solid transition to the initiation of cracks unde...

Bulk Modulus of Lubricating Oils as Predominant Factor Affecting Tractional Behavior in High-Pressure Elastohydrodynamic Contacts

The phase diagrams corresponding to transition from liquid to viscoelastic solid and that from viscoelastic solid to elastic-plastic solid of Santotrac100 (SN100), mineral oil, synthetic naphthenic oil, polybutene, and tetradecane were first made up by high-pressure density measurements and others. The bulk modulus of lubricating oils under a quasi-static condition was evaluated using a phase diagram. The results indicated that the bulk modulus of lubricating oils is closely related to the oil molecular packing parameter T VE −T (where T VE is the viscoelastic solid transition temperature at pressure p, and T is the oil temperature). The constant values of the bulk modulus in the elastic-plastic range are different depending on the molecular structures of the oils. It has also been shown that SN100, mineral oil, synthetic naphthenic oil, and polybutene converted to amorphous solids at high pressures and tetradecane converted to molecular crystal. Next, the elastohydrodynamic lubrication tractions were measured by a ball-on-disk machine. The results indicated that the maximum traction coefficient is closely related to T VE −T. As a result, the importance of the bulk modulus as a predominant factor for traction characteristics of lubricating oil was pointed out.

Diagrams for Estimation of the Solidified Film Thickness at High Pressure EHD Contacts

This paper deals with diagrams for estimation of the film thickness at point contacts based upon observation by means of optical interference at pressures, sufficiently high to cause solidification of lubricants. The observed minimum film thickness at rolling contact is plotted here in the form the Greenwoods parameter H K min = h min /(αan O u) 2 / 3 R 1 / 3 . This serves to estimate the minimum thickness with sufficient accuracy under wide conditions of circular and elliptic contacts. It is noted that in the solidification ranges VE (viscoelastic) and EP (elastic-plastic) larger thickness is formed. The upper parts of these thickness curves are limited by the boundary of the PR (piezoviscous-rigid) range defined by Houpert.

Behavior of some vegetable oils in EHL contacts

EHL oil film thickness measurements for rape seed oil, camellia oil. olive oil. castor oil and glycerol have been carried out by optical interferometry. The experimental results showed that the central film thickness of castor oil and glycerol under rolling conditions was 0.5-0.9 times thinner compared with the Hamrock - Dowson central film thickness formula. This fact was considered to be attributed to their poor wettability at the liquid/solid interface, which was estimated by observing the contact angle with the aid of a goniometer. Furthermore, the largest contact angle of glycerol was lowered by adding surfactant and brought about an increase in film thickness. It is suggested that the interfacial phenomena near the inlet side of the EHL contact region affects the EHL central film thickness, in particular, in case of vegetable oils and glycerol. Further, comparing oils with equal viscosity grades, vegetable oils with lower pressure-viscosity coefficients show lower traction coefficients than the paraffinic mineral oils. The effect of wettability on the traction coefficient was unrecognized.

Effect of Molecular Weight Distribution of Mineral Oils on Life of Thrust Ball Bearings

In order to investigate the effect of molecular weight distributions of mineral oils on bearing life, the authors carried out systematic tests using thrust ball bearings, type 51104. Besides the general trend of increasing life with viscosity, blended oils characterized by wide-ranged molecular weight distributions exhibited longer life compared with base oils with narrow-ranged distributions corresponding to the same viscosity grade. These results were obtained by estimating states of oil film and oxide film formations, and furthermore, by measuring coefficients of friction, oil temperatures, and viscosity at the glass transition points.

Cyclic Deformation and Fatigue Crack Behavior of Extruded AZ31B Magnesium Alloy

Pseudoelastic behaviors were observed in compressive and tensile loading-unloading tests at room temperature. The large anelastic strains were observed in compressive stress-strain hysteresis loops. The fatigue limit of axial load-controlled fatigue test at 107 cycles was 90MPa at room temperature. The deformation twins were observed in the specimen subjected to the higher stress amplitude of fatigue limit and free deformation twins were observed in the specimen subjected to the lower stress amplitude of fatigue limit. Stress-strain hysteresis loops were linear in tensile and compressive phases at the lower stress amplitude of fatigue limit and the complicated pseudoelastic deformations were observed in tensile and compressive phases at the higher stress amplitude of fatigue limit of axial load-controlled fatigue tests. Compressive mean strain generated by cyclic pseudoelastic deformations at the higher stress amplitude of fatigue limit. Fatigue cracks initiated at the secondary particle/matrix interface or broken secondary particle near the surface. Subsequently, small cracks tended to grow through transgranular.