PSI - Issue 19

Wolfgang Schneller et al. / Procedia Structural Integrity 19 (2019) 556–565 Author name / Structural Integrity Procedia 00 (2019) 000–000

560

5

3.3. Assessment of surface roughness To capture and assess the surface roughness of unprocessed specimens is of utmost importance since the surface topography is superior to internal defects and decisive in terms of fatigue [17]. Exemplary, the result of a surface scan with a light optical KEYENCE VHX-5000 microscope is shown in Figure 3. The obtained three dimensional datasets were processed utilizing a user-defined routine, as described in [26]. Applying a second order robust Gaussian regression filter, the roughness profile is evaluated by defining a cut-off wavelength  c of 2.5 mm. The cut-off length was chosen as suggested by the authors in [26]. The results of the surface evaluation are given in Table 4, whereby S t stands for the maximum areal roughness surface deviation.

Fig. 3. Surface roughness profile

Both post treatments reveal a beneficial effect on the surface topography. The maximum surface deviation is decreased by -19.4% for the HIP condition and by -20.6% for the SA condition, compared to the AB test series. For the validation of this routine, the depth of the failure initiating surface roughness is also measured within the fractured surfaces after testing. The comparison of those two methods, proves the applicability of the used optical evaluation method, since the results show only minor deviation regarding the assessment of the surface topography, see Table 4. Since the surface roughness is contributing to the fatigue resistance in a great measure, a mitigation of surface features may result in an improved fatigue behaviour [27]. Therefore, one can conclude that the smoother surface topography for the HIP and SA condition may be beneficial regarding fatigue. This is in sound correlation with the fatigue test results presented in this work.

Table 4. Results of the surface roughness evaluation

Norm. mean S t (frac. surf.) 1.000 (Basis) 0.868 (-13.2%) 0.852 (-14.8%)

Norm. mean S t (optical eval.) 0.926 (-7.4%) 0.804 (-19.4%) 0.794 (-20.6%)

Deviation

Condition

UP-AB UP-HIP UP-SA

7.4% 6.2% 5.8%