Machine parts operating under contact cyclic loads are subject to a destruction risk in the event of their working surface pitting (destruction). To increase the service life of such parts, various finishing and hardening technologies are used. In the absence of standard methods used to assess the durability of parts operating under contact endurance conditions, they have should be developed in relation to a specific case. The fractal geometry methods' usage in analyzing the surface layer state of parts operating under contact endurance conditions is complicated due to high equipment cost and fine and coarse noise high risk, which negatively affects the quality of generated images. The aim of the work is to develop a simple and accessible method for evaluating the results of contact endurance tests, based on the analysis of the investigated surface images. The developed patented technique consists of several stages. At the first stage, high-resolution photographing of the surface under investigation is performed using a microscope, providing magnification of no more than 1 × 50. At the second stage, the obtained image is analyzed in the cad editor, where the number and area of the investigated defects are determined. At the third stage, wear result numerical assessment is carried out according to the proposed algorithm. Image analysis, high image resolution, contact endurance, cad editor, pitting.
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The working surfaces of parts that perceive concentrated cyclic contact loads usually break down due to fatigue failure. Contact fatigue is the damage accumulation process and the development of the surface layers destruction of a material under alternating contact stresses action, which cause pitting or cracks formation as well as a decrease in durability. Contact fatigue typically causes the failure of gears, rolling bearings, rails and rail wheels, rolling rolls, cam pushers, strikers, worm gear, hypoid, screw, chain and globoidal gears, guides, spline joints with rolling elements, overrunning roller couplings etc.
To increase the durability of machine parts operating under contact fatigue loading, various hardening
methods are usually used (heat treatment, chemical heat treatment, surface plastic
deformation, etc.) aimed at increasing the surface layer hardness. Hardening effectiveness evaluation is
possible only due to experimental tests. At present, R 50-64-30-87 recommendations are known but
there is no standard describing a unified approach to equipment and methods for contact endurance
assessment [
To assess the durability of machine parts, which fail due to contact load action, various parameters are used.
2021 Copyright for this paper by its authors.
Contact fatigue is the ability of the contracting parts' working surfaces to withstand contact fatigue. The contact endurance quantitative assessment is the contact pressure value, which does not lead to pitting for a given number of cycles.
Often machine parts' durability, which fail due to the action of contact stresses, i.e. fatigue life, is assessed by the number of loading cycles during which pitting appears on the surfaces of parts.
Fatigue failure under cyclic contact loading occurs due to the accumulation of stresses in the surface
layer under the action of normal forces periodically applied to the surface, which first promote cracks
initiation and then their development. The emerging fatigue crack can join with adjacent microcracks.
The path, as well its development speed, depend on the orientation of the microcracks encountered near
it, as well as on mechanical properties of adjacent metal sections. Several fatigue cracks can develop in
a part at the same time. After a certain number of loading cycles, the crack found under the conditions
most conducive to its growth reaches the part surface. Then metal pitting occurs (with simultaneous
crack propagation parallel to the part surface), as, for example, in Figure 1. Further part loading leads
to the development of fatigue pitting [
The criteria for contact fatigue fracture assessment are: reaching the diameter of one or several chipping pits of the calculated contact spot minor semiaxis half; contact area reduction to the limiting value according to the static strength criterion, i.e. before the beginning of plastic crushing of the spalling pits boundaries; the pits' area reaches a certain part of the tested surface total area; the chipping pits' appearance along any contact surface generatrix entire length or the achievement of the established pits' limit number at a certain generatrix length; reducing the sample mass until the specified value is reached.
Indirect criteria for contact fatigue tests are: changes in the noise pitch, vibration intensity, and the rotation resistance moment value.
Recently, the surface layer state parameters assessment is carried out on the analysis of images of
the investigated (working) product surface by the fractal geometry methods [
The purpose of this work is to develop an accessible technique for studying contact endurance based on the investigated surface image processing.
The main principles underlying the contact endurance assessment methodology based on image
analysis are: implementation ease, assessment accuracy and instrumental availability [
The investigated surface with traces of wear in the form of pitting is divided into sectors. The size of a single sector is determined by the used microscope optical capabilities. For this kind of research, equipment with 1 × 50 magnification is sufficient. The equipment with such optical characteristics is widely used in laboratories (for example, instrumental microscopes).
Through the microscope objective, high-resolution photographs of the investigated surface selected sectors having wear traces are taken.
The resulting images are transferred to a specialized software package, for example, a CAD system (Compass, AvtoCAD or SolidWorks). In the CAD system, using digital photographs of sectors and their linear dimensions' values, images of sectors are processed at 1: 1 scale. On the images obtained, the sectors' boundaries and the existing contact-fatigue surface pitting are outlined. They are shown in
, ∆ = ∑ 0 0 ∑ ∙
, ∆ ∆ = = 0 0 , , where Smax – maximum pitting area, measured in the investigated sector, mm2.
Resistance to contact pitting ΔI is taken as the main characteristic that allows us to assess the hardened specimens' durability, which makes it possible to qualitatively and quantitatively compare the pitting amount on a hardened surface relative to an unhardened one.
Resistance to contact pitting, ΔI, is a complex parameter that takes into account the change in the size of the tracks of the balls raceway during testing, and the pitting area that occurs on them, which makes it possible to objectively assess the durability of machine parts operating under contact fatigue conditions. (1) (2) (3) (4) (5) where S – total pitting area in the investigated sector, mm2. where Smin – least pitting area measured in the test area, mm2.
∆ = 0 ∙ ∑ 0 ∑ .
The application of this technique made it possible to identify wave strain hardening rational modes,
which allow us to create a heterogeneously hardened structure in the part surface layer, providing an
increase in ΔI by 3-6 times, a decrease in the pitting number by 1.5-2.5 times and a decrease in pitting
size by 2-3 times [