Fig. 1. Surface relief extrusion/intrusion pattern scheme [5]
The relief parameters were proposed and verified by the numerous
experiments [6-9].
The primary damage parameter considers the intensity of the surface
relief:
,
where: Sdeform. relief - surface area with
deformation relief; Stotal - total investigated
area.
The additional damage parameter is a fractal dimension of the surface
pattern. It describes quantitatively the shape of surface relief
clusters. The fractal dimension Dp/s is based on the ratio of the
perimeter of the clusters to their area:
,
where: Np(δ), Ns(δ) - number of cells with the size δ in
pixels, which cover the perimeter and area of deformation relief
clusters.
It was proved, that the remaining life of the component, made of a metal
exhibiting surface relief could be calculated with an empirical
relationship between the number of cycles to failure and relief
parameters:
Nremaining = f (D, Dp/s).
Some bearing components of the primary structure are not able to exhibit
surface relief after cyclical loading due to their physical properties,
that’s why the attachable fatigue indicator has been proposed as an
alternative to the direct investigation of the surface of the
components.
Indicator (sensor) looks like a micro specimen for fatigue tests (Fig.
2) made of Al-clad alloy.
The strain of the indicator is not constant value along the indicator
axis; it depends on the stiffness, which varies due to the different
cross sections. The required level of strain and stress in the inspected
cross section of the indicator can be achieved by the proper selection
of the indicator’s geometry. This procedure is provided by the
application of Finite Elements Analysis (FEA) using ANSYS software.