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: S_{deform. relief} - surface area with deformation relief; S_total - 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:

N_remaining = 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.