PSI - Issue 17

Hayder Al-Salih et al. / Procedia Structural Integrity 17 (2019) 682–689

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Al-Salih/ Structural Integrity Procedia 00 (2019) 000 – 000

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5. Results

5.1. DIC Results

DIC data was collected for the bifurcated out-of-plane crack for each load case. Typical DIC results, visualized for both strain and displacement, are shown in Fig. 3. The majority of the crack is clearly visible in both the strain and displacement images, although the vertical branch is difficult to see. Due to the complex geometry of the crack and multi-axial fatigue loading, displacements were examined in all three principal directions, and resultant differential displacements were calculated for use with the crack characterization methodology. As described above, differential displacements were taken orthogonal to the crack path, and convergence was calculated along the length of the crack path.

Fig. 3. Typical DIC results of the distortion-induced fatigue specimen visualized in terms of a) strain and b) displacement

5.2. Horizontal Branch Characterization Results

Horizontal branch relative displacement along the crack path is presented in Fig. 4a for all load cases. Calculated convergence values are shown in Fig. 4b. In the figures the actual crack length of 75.1 mm (2.95 in.) is represented by a vertical dotted line. It should be noted that displacements caused by load case 1 were extremely small, causing extreme variation in convergence values. Due to this, load case 1 convergence values are not presented. Using 90% and 95% convergence, crack lengths were predicted and compared with the optically measured length. The predicted crack lengths and error, expressed as a percentage, are presented in Table 2. Using a value of 90% convergence under-predicted crack lengths by an average of 27%. The 95% convergence performed much better for the horizontal branch, under-predicting crack length by an average of only 10%.

Fig. 4. Horizontal branch a) Relative displacement and b) Convergence of relative displacement