PSI - Issue 76

Robin Motte et al. / Procedia Structural Integrity 76 (2026) 74–81

77

Fig. 2. (a,b) Experimental setup with (unpainted) specimen in four-point bending fixtures, with DIC cameras and IR camera, and (c) an example of a thermogram during a fatigue test.

free edges of a crack decreased, the corresponding drop in T 1 was used as an indicator for crack initiation.

α T 0 ρ C p

(2)

T 1 = −

σ kk

DIC is an optical method which relies on a speckle pattern on a specimen’s surface, which is divided into subsets to allow calculating displacement and strain fields. White and black spray paints were used to apply the speckle patterns. A stereo setup was used, consisting of two monochromatic Basler acA4096-40um cameras with a resolution of 9 megapixels (Basler AG), each equipped with a 1 mm extension ring and a RICOH FL-CC7528-2M 2MP lens with a focal length of 75 mm (Ricoh International B.V (2024)). Data acquisition was performed using Correlated Solutions’ VIC-Snap software (v.8), while post-processing was done in the VIC-3D (v.7) software. For every 2000 fatigue cycles, DIC images were captured at 9.8 Hz during a 500-cycle segment to limit data volume. Only the images taken at the moment of maximum load during a fatigue cycle were used for post-processing. For the tests at lower fatigue loading frequencies, the DIC acquisition rate was adjusted accordingly. During the analysis, a region of interest of approximately 10 × 15 mm² around the location of failure was defined. The subset size was adjusted for each individual experiment and ranged between 25 pixels (approximately 0.569 mm) and 45 pixels (approximately 1.248 mm), while the step size was set to around one quarter of the subset size.

3. Results and discussion

3.1. Fatigue life

The S-N data is presented in Table 2 and Figure 3 in terms of nominal stress range ∆ σ nom . Details regarding the number of cycles to crack initiation will be discussed in section 3.2. Ordinary Least Squares Regression (OLSLR) curves according to equation 3 (in accordance with the ISO 12107 standard (International Organization for Stan dardization (2012))) based on the failed samples are displayed in this figure for both types of specimens, with the parameters (i.e. inverse slope m and intercept log( C )) summarised in Table 3.

log( N ) = − m · log( ∆ σ nom ) + log( C )

(3)