Issue 41

L. Patriarca et alii, Frattura ed Integrità Strutturale, 41 (2017) 277-284; DOI: 10.3221/IGF-ESIS.41.37 281 (a) (b) Figure 3 : Experimental setup: (a) Deben Micro-Testing Machine, 5 kN dual leadscrew; (b) Specimen geometry and micro-indentation markers. The specimen was initially polished with abrasive SiC paper, diamond suspension and colloidal silica. Successively, a rectangular region of 1 mm x 0.8 mm was selected in front of the notch and marked with micro-indentations. An EBSD map was then performed in the marked region in order to precisely establish the grains orientation and geometry in the specimen region. According to preliminary plastic radius calculations, the selected EBSD region is large enough to contain the strain localization produced around the crack tip. The specimen was successively pre-cracked in compression in order to limit the load history effects produced by the classical pre-cracking methodologies based on tensile loading cycles. The compressive applied load range was 2700 N (between -3000 N and -300 N), this condition ensured a plastic radius at the notch of about 100  m. Fig. 4 shows the outcome of the pre-cracking procedure, a crack with a length of 60  m from the notch was detected. Figure 4 : Crack nucleated at the notch tip after the compression pre-cracking process. Following compression pre-cracking, the specimen was prepared for high resolution ex-situ DIC. A fine speckle pattern was applied to the target surface and enabled to acquire images with a resolution of 0.34  m/pixel. The images were captured before and after deformation in the un-loaded condition by means of a Carl Zeiss Axio Cam A1 optical microscope. The resulting residual strain field were then overlapped with the EBSD grain map using the micro- indentations as reference points. Finally, the test was performed under a Deben Micro-Testing Machine (maximum load 5 kN, Fig. 3) in load control imposing a load/unload procedure to an objective stress intensity factor of about 20 MPa√m. The result of the correlation is shown in Fig. 5; the analysis of the DIC image pointed out the presence of a secondary