МОДЕЛИРОВАНИЕ И УСКОРЕННЫЕ ИСПЫТАНИЯ В УСЛОВИЯХ ТРЕНИЯ КАЧЕНИЯ И КОНТАКТНО-МЕХАНИЧЕСКОЙ УСТАЛОСТИ

In rolling contact fatigue (RCF), cracks are usually initiated from inclusions under the surface and propagated to the contact surface. In the present study, RCF cracks was observed using synchrotron radiation computed laminography (SRCL) in SPring-8. For observation of crack propagation, ex situ rolling fatigue test and SRCL were applied to same sample. Laminography is suitable for the observation of sheeted material and sheeted sample is useful for operation of rolling fatigue tests. The specimen with flaking damage was measured by SRCL. It was found that two types of cracks, vertical crack and horizontal crack under the contact surface, which is typically rolling fatigue crack, can be measured by SRCL. For observation of RCF crack propagation, vertical crack which initiate from inclusion and propagated in the direction perpendicular to the contact surface, could be detected. SRCL and fatigue tests were conducted three times. As results, crack propagation could be observed between first and third measurements, while the different part of crack was detected in second measurement. Residual compression stress is considered to affect the observation of RCF crack. SRCL is concluded to be effective for evaluation of initiation and propagation behavior of crack in the rolling fatigue.

The effect of diamond-like carbon coatings on the rolling fatigue and wear of M50 steel

Rolling fatigue tests were carried out on hot isostatically pressed silicon nitride bearing balls. At three load levels of the maximum Hertzian contact pressure pmax=5900, 6740 and 7110MPa, 13 balls each were tested, and the life distribution was studied. It was found that the rolling fatigue life followed approximately a 2-parameter Weibull distribution with the shape parameter of α≅1 at each load level. The relationship pmaxκL50=const (κ≅10) was found between pmax and the median life L50. Microscopic observation of flaking was also performed, and two types of flaking were found. One is flaking with the depth of the order of 100μm, and the other is a shallower one. The depth of the former type was found to coincide approximately with the depth at which the maximum shear stress occurred. At lower load levels, the former type was dominant. Frequency of appearance of the latter type increased with an increase in load level. Comparing the shape parameter of α≅1 and the exponent of κ≅10 mentioned above with those for the cyclic bending fatigue of plain specimens of silicon nitride, it was suggested that the mechanism of rolling fatigue of silicon nitride was different from that of cyclic bending fatigue of the same material.

A rolling fatigue tester of simple construction, well suited to multiple-rig data gathering but also possessing considerable versatility and ease of variable control, is described. Its kinematic and elasto-hydrodynamic relationships for two commonly used test specimen configurations are also presented. Ten such testers were employed to evaluate the rolling fatigue lives of two typical aviation gas turbine lubricants. A total of 100 tests were conducted with each lubricant, comprising 10 tests on each of the 10 testers. The test conditions were such that the operation was definitely in the boundary lubrication regime. The rolling fatigue lives of the two lubricants under these conditions were found to be nearly the same at 90 percent confidence level, despite their difference in viscosities. Using the results obtained, an analysis was made of the statistical implications and economic considerations involved in multiple-rig rolling fatigue testing. Results presented include the effects of tester-to-tester variation, the number of tests, and the extent of test suspension. Precautions and techniques in multiple-rig rolling fatigue testing are discussed.

A report is made in the present paper of a series of tests that were conducted by devising a testing apparatus of a rolling cylindrical body. Special attention was paid to both the line contact and the point contact. It was planned to obtain quantitative information ragarding the chracteristics of friction in the rolling contact of the rolling body in the main shaft bearing of aircraft engines, as well as to measure its lifetime. The testing apparatus was designed on the following bases: (1) accurate measurement of the rolling frictional torque and (2) automatic detection of the lifetime of the rolling body by means of an oscillograph. In the present tests the slip ratio was nearly zero.The present tests were performed under static load, and under conditions of dry rolling, water lubrication and #60 spindle oil lubrication. The following conclusions have been obtained from the present tests.(1) From the tests of the line contact of the rolling cylindrical body under static load, the equipment has been found to be satisfactory for performing rolling fatigue tests, and specially for the automatic detection of the lifetime of the rolling body.(2) There are two inflexion points on the characteristic curve of the rolling friction coefficient, which are considered to show respectively the strength of the oil film and the rolling fatigue limit.(3) The three regions thus divided by these two inflexion points are respectively the region of rolling wear, the region of rolling fatigue damage and the region where remarkable evidence points to its complete freedom from rolling fatigue.

A series of single-stage true triaxial compression (TTC) tests and multi-stage intermediate principal stress (σ2) loading tests were carried out on three types of brittle rock specimens in this study to examine the influence of σ2 and the validity of obtaining strength parameters by using a multi-stage loading method under true triaxial stress conditions. There exists a critical σ2 at which the rock strength attains its ultimate value under both loading conditions. Under the same σ3 level, the critical σ2 is much smaller under the multi-stage σ2 loading compared with under the single-stage TTC. The test results also show that the strength determined under multi-stage σ2 loading is much smaller than that determined under single-stage TTC. The strength difference can attain 39%. A lower strength is measured under multi-stage σ2 loading because the local fracture plane induced by true triaxial stress is parallel to the σ2, and the strength of the rock specimen containing this type of local fracture plane is not affected by σ2. On the contrary, rock damage accumulates during multiple loading and unloading cycles. The current testing results indicate that the multi-stage loading test is not suitable for determining the strength envelope of brittle rock under true triaxial conditions.

The rolling t*e sts of rail material were carried out under unlubricated condition using a JD-1 wheel/rail simulation facility. The rolling fatigue and wear behavior of rail roller were investigated by examining the microhardness, wear volume and wear scar using microhardness tester and scanning electronic microscopy. The results indicate that the wear volume of rail roller would increase rapidly with axle load increasing and curve radius decreasing. The wear volume of U71Mn rail is bigger than PD3 rail’s. The microcrack on PD3 rail roller has propagated toward depth direction along the angle of 15∼20°. By comparison, it is not found the obvious flaking defects on the wear scar of U71Mn rail and the wear surface is relatively smooth. Furthermore, U71Mn rail material has better fatigue resistance performance. It is concluded from the experimental results that the relationship between rolling fatigue and wear can be seen as competitive and restrictive coupling mechanisms. In other words, while the wear is serious, the fatigue damage is slight. On the basis of the above experimental results, it is found that increasing wear rate using rail grinding can alleviate its fatigue damage. The test results in the field indicate the rail asymmetrical grinding can prevent and alleviate effectively fatigue oblique crack damage of high-speed railway rail.

Experimental study of the effects of temperature and humidity on the wear and damage behavior of U71Mn rail steel

Rolling contact fatigue in lubricated contacts

Influence of loading history on creep behavior of rock salt

This research centres on surface initiated damage on back-up rolls whereby rolling contact fatigue cracks can propagate into the rolls potentially reaching the internal stress fields and leading to catastrophic failure and has sought to establish design criteria for avoiding such failures. The project objectives have been achieved by examining field evidence, determining the loading and tribological conditions at the work roll/back-up roll interface and investigating both theoretically and experimentally the mechanisms involved in rolling contact fatigue in this case. The presented rolling contact, fatigue and fracture mechanics model includes criteria for crack branching either upwards (i.e.relative safety) or downwards (i.e.potentially catastrophic) and the link between these two cases is related, quantitatively, to the properties of the roll material. After linking mechanics to microstructure, the influence of work roll test disc surface roughness on both the surface wear of and the interaction between wear and rolling contact fatigue at the surface of back-up roll test discs has been quantified using the results obtained from experimental simulations carried out on a rolling–sliding testing machine. Finally practical quantitative recommendations are made for the mechanical and microstructural design of bainitic back-up roll materials, back-up roll redressing procedures and the surface roughness of both work rolls and back-up rolls presented to the mill.

Rolling-contact fatigue lives of steel AISI 52100 balls with eight mineral and synthetic lubricants

Strain-controlled fatigue properties of Ti 3Al based alloy : Cao, J., Sun, F., Cao, C. and Gao, Y. Xiyou Jinshu Cailiao yu Gongcheng (Rare Metal Materials and Engineering) (1996) 25, 17–20

The observation of individual asperity interactions in lubricated point contact : T. E. Tallian and J. I. McCool, ASLE Trans., 11 (2) (1968) 176–189; 5 figs., 38 refs.

Subject index of volume 155