Search

Fretting fatigue tests have been made about the effect of clamping pressure, p0, and hardness of materials on the initiation of non-propagating cracks and also complete fracture under the conditions where the relative slip is fully controlled. The main results obtained are as follows. (1) Fretting fatigue strength based on the initiation of fatigue cracks, σw1, decreases linearly with the increase of p0. (2) Fretting fatigue strength based on fracture, σw2, decreases gradually with the increase of contact pressure, and reaches a critical strength when p0 is beyond a certain value. (3) These results in (1) and (2) are analogous to fatigue phenomena of notched specimens, when the stress concentration caused by fretting is considered. (4) When the relative slip of fretting is relatively large, σw1 is affected by the susceptibility of materials to wear.

In the present paper, the construction and features of a fretting fatigue testing machine designed by the authors are explained, and the experimental results obtained by using the testing machine are stated concerning mainly the relative slip which is considered to be one of the most significant factors affecting the fretting fatigue strength. The following are known from this investigation. In fretting fatigue, many shallow micro-cracks which seem not to propagate to a complete fracture are formed and develop somewhat obliquely from the surface of specimen because of the effect of frictional force due to fretting. Fatigue strength is reduced significantly by the effect of fretting, and the strength reduction factors, concerning both nucleation of fatigue cracks and fatigue fracture, are greatly influenced by the magnitudes of relative slips. The fatigue strength is most significantly reduced when the relative slips are about 0.015∼0.020 mm, and reduced to less than 1/5 of its strength without fretting, and if the relative slips could be kept less than 0.005 mm, a marked improvement would be expected.

The fact that the fretting fatigue strength is significantly influenced by the relative slip amplitude was confirmed experimentally and reported in the previous paper. In the present paper, in order to make clear why the relative slip has a marked influence upon the fretting fatigue strength, the relation between the frictional force and the relative slip is investigated experimentally. As the result, the alternating stress amplitude, σfw1, which is the fatigue limit based upon the initation of fatigue cracks under the condition of fretting, could be expressed in the following equation as a function of relative slip, S, σfw1=σw1-2μp0{1-exp (-S/κ)}, where σw1 is the fatigue strength of plain specimen, μ is the coefficient of friction, and k is a constant.

Although the significant reduction in fatigue strength produced by fretting of the press-fitted axle assemblies has been well known, the causes of this phenomenon have not been fully clarified. The purpose of this study is to clarify the influence of fretting on the fatigue strength of press-fitted axle assemblies where the minute relative slips at the end region of the hub on the axle surface occur due to the alternate elongation and contraction of the axle fibers. In the present paper, the authors have focused their attention on the amplitudes of the relative slips near the end of the press-fit, and investigated them quantitatively. The effects of various factors, such as size, shape, clamping pressures, magnitude of nominal stresses, number of cycles and frequency of alternating stresses, on the amplitudes of relative slips are evaluated extensively.