On fretting maps

Crack tip shielding phenomena, whereby the “effective crack-driving force” actually experienced at the crack tip is locally reduced, are examined with reference to fatigue crack propagation behavior in metals, composites and ceramics. Sources of shielding are briefly described in terms of mechanisms relying on the production of elastically constrained zones which envelop the crack (zone shielding), on the generation of wedging, bridging or sliding forces between the crack surfaces (contact shielding) and on crack path deflection and meandering. Examples are taken from the fatigue behavior of high strength lithium-containing aluminum alloys, aluminum alloy-aramid fiber-epoxy laminate composites, and zirconia ceramics. It is shown that, whereas crack tip shielding can provide a potent means of enhancing “resistance” to crack growth, such extrinsic toughening mechanisms can result in the apparently anomalous behavior of “small cracks” and to the susceptibility of brittle materials to fatigue failure.

/pdf/mechanisms-of-fatigue-crack-propagation-in-metals-ceramics-52goujidjf.pdf

Mechanisms of fatigue crack propagation in metals, ceramics and composites: Role of crack tip shielding☆

Several aspects of the mechanics of cracks originating at sites of fretting are considered. It is argued that the problem may be distilled into three separate parts: the contact problem itself in full or partial slip, the initiation of a crack from a surface suffering severe distress, and the propagation of a crack under combined contact and bulk loading. The first of these may be solved by either a classical or numerical means, whilst the last merely requires the careful use of fracture mechanics. However, it is the second element which remains elusive to quantify, and the influence of the intrinsic length scales in the problem, including contact length, surface roughness and amplitude of relative tangential displacement on initiation conditions, is discussed and explored.

Mechanics of Fretting Fatigue

Mechanics of fretting fatigue crack formation

https://www.ideals.illinois.edu/bitstream/handle/2142/318/1060.pdf?sequence=1

Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite – Part I: Manual infiltration

Initiation and propagation of fretting fatigue cracks

The relation between tangential force and relative slip displacement under fretting fatigue is analyzed by using a model of a spring system, and is compared with the experimental results. And the correspondence of the damaged layer of a carbon steel due to fretting fatigue with the stress conditions near the contact surface is studied. When slip amplitude Δy is small, it is proportional to tangential force T because of elastic adhesion which exists within contact surfaces partly. Both the wave forms of T and Δy are sinusoidal. When Δy becomes so large that macro-slip occurs, T shows a peak value and then decreases with progress of cycles. The wave form of T is trapezoidal and that of Δy is not sinusoidal, agreeing well with the analytical results. The frequency effect appears under Δy showing macro-slip, coefficient of friction being smaller under higher frequency due to the lubricating action of more oxidized debris. The depth of the damaged layer due to fretting fatigue is measured in several tens of microns. This depth is nearly equal to the depth where the combined maximum repeated shearing stress reaches the fatigue strength of materials.

Behaviors of Frictional Force in Fretting Fatigue

Numerical analysis of a simplified crack model has been conducted on the wedge action of the viscous fluid in the cracks subjected to the bending moment, and rotating bending fatigue tests have been carried out on carbon steel specimens in paraffin oils of high and low viscosity. The crack rate decreases when the product of the viscosity and the cycle frequency, ηf, is increased in agreement with the analysis. However, the crack rate increases when ηf is greater than about 105 owing to the poor penetration of oil into the fissures. The protective effect of oil, which limits the access of oxygen to freshly created surfaces, is hardly found. Neither the effect of the viscosity nor that of the cycle frequency is observed in silicone oil. This may be due to the small pressure index and to the weaker adhesion to the crack wall of silicone oil, which is improved by activation with stearic acid.

Fatigue Crack Propagation of Steel in Oil

Effects of environment on fretting fatigue

The bending and twisting fretting fatigue tests of carbon steels are carried out and the effects of cycle frequency are studied in comparison with the corrosion fatigue. The behaviors of the frictional force with progress of fretting cycles are observed. The results obtained are as follows : The fretting fatigue strength is decreased with lower frequency. The fretting damage is saturated in the early period of the total fatigue life, and the subsequent crack propagation period constitutes the most part of the life. The lower the frequency is, the shorter the saturation period. The frictional force increases initially and then decreases after showing a peak value. These changes at various frequencies well correspond to the extents of the fretting damage. Consequently, this shows that the fretting damage is attributed to cracks initiated under the frictional stresses at the initial stage simultaneously applied with the repeated stresses in the surface layers. The fretting corrosion without the repeated stresses hardly reduces the fatigue strength though the depth of wear scars is the same as that under fretting fatigue. The greater reduction of the fatigue strength by fretting under bending than under twisting may be also explained with the combined stresses.

Kichiro Endo

Papers

Initiation and propagation of fretting fatigue cracks

Behaviors of Frictional Force in Fretting Fatigue

Fatigue Crack Propagation of Steel in Oil

Effects of environment on fretting fatigue

Effects of Cycle Frequency on Fretting Fatigue Life of Carbon Steel