Fatigue Fracture Surface Analysis

Fracture surface analysis was performed by Macek [34 (link),35 ,39 (link)] using an optical 3D test stand that facilitated the acquisition of data sets at a high depth of focus [45 (link),46 (link)]. The failed specimens were observed under 10× magnification using an Alicona G4 InfiniteFocus (Alicona Imaging GmbH, Graz, Austria) as described previously [47 (link)]. Due to the restricted field of view, nine rows by seven columns were stitched together to map the entire fracture area. Each individual micrograph had a vertical resolution of 79.3 nm with a lateral resolution of 3.91 µm. The abovementioned measurement device, exhibited in the upper part of Figure 4, was operated via IF-MeasureSuite software (version 5.1, Alicona Imaging GmbH, Graz, Austria), while the measurement of surface features was conducted using MountainsMap software (version 7.4, Digital Surf, Besançon, France). Alicona (*.al3d) files were imported into the surface metrology software MountainsMap and resampled into height maps at a resolution automatically determined by the software. Surfaces were analysed in relative coordinates (X, Y, and Z axes) with the Z axis in heights from the lowest point by default. No additional filters were used. Fatigue fracture surfaces were measured for local (propagation and rupture) profiles and for total areas. Figure 5 shows examples of the propagation areas and main surface parameters as well as rupture areas and main texture parameters observed in the experiments for the three metal alloys studied.
To check the fracture surface dependency on the fatigue loading history, selected parameters, reported among others in Figure 5, were measured and calculated. Table 4 defines the used parameters according to the ISO 25178 standard.
It is known that the microrelief of fatigue fracture surface is determined by the material properties and the stress intensity factor in the tip of the initial crack; therefore, the parameters of the microrelief depend on the stress amplitude and the fatigue crack length. When testing ductile materials, the height of the fracture profile usually increases with increasing crack length, and at the stage of the fatigue crack propagation, three zones with different roughness are found: the initial zone with a predominant shear microrelief; the zone with striation microrelief; and the zone of accelerated crack growth, in which striations and dimples are observed. With an increase in the stress amplitude, the size of the zones changes, the zone with striations decreases and the zone of rupture grows. When testing brittle or quasi-brittle materials, the height of the fracture profile often decreases with increasing crack length as a result of the formation of facets of cleavage or intergranular fracture [48 (link),49 (link),50 (link),51 (link)]. Overall, there were obvious differences in topography for propagation or rupture, particularly the coarser areas. Ra (Equation (1)) averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points, so that the extreme points have no significant impact on the final results. Sa, as expressed in Equation (2), represents the mean height of the surface, according to the ISO 25178 standard. Their functionality was analysed later in the study.
The Abbott–Firestone curves (see centre of Figure 4) provide important information on the surface properties in a systematic and quantitative approach. In the example chart, the Abbott–Firestone curve shows the cumulative height distribution histogram. The horizontal axis represents the measured scale in depth of the surface, and the vertical axis depicts percentage of the whole population of data. The shape of the curve is distilled into several of the surface roughness parameters [52 ]. The distributions of the surface highlights that the crack initiation region had a smoother surface without asperities.
$Ra=\frac{1}{lr}{{\displaystyle \int }}_0^{lr}\left|z\left(x\right)\right|dx$
$Sa=\frac{1}{A}\underset{A}{{\displaystyle \iint }}\left|z\left(x,y\right)\right|dxdy$