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P. Lorenzino et alii, Frattura ed Integrità Strutturale, 25 (2013) 138-144; DOI: 10.3221/IGF-ESIS.25.20 144 In the case of large microstructures, the grains traverse the entire specimen thickness (2 mm.). This fact allows studying the crack propagation through the DIC analysis on one side of the specimen and by the image analysis method explained above on the other side of the specimen. Figure 8 shows a comparison between the microstructure analysis and the Digital Image Correlation analysis. It is possible to observe how the left crack and its plastic zone is arrested at a grain boundary and a new plastic zone is generated not in the contiguous grain but in another neighboring grain that it is probably better oriented according to the loading direction. We are currently working on refining the speckled pattern in order to measure with more accuracy the spreading and width of the deformation bands. C ONCLUSIONS he experimental set-up needed for crack follow-up is very simple and the tests can be developed easily. As a result, a large number of samples and test conditions can be analyzed in a relatively short period of time. As the studies have shown, the behaviour of the very long “short” cracks observed in the materials with the very big grains procedure is similar to that observed when “normal”, smaller microstructures are considered [7, 8]. As the grain size increases, cracks presenting sizes commonly associated with specific behaviours according to MFLE, begin to behave largely in accordance with the principles of microstructural fracture mechanics. Also, it was observed that for small radii of the notch and large grain sizes there is a wide region along the notch surface where the likelihood of initiation is equally high. The smaller the notch radius, the wider the possible region of initiation and the more it deviates from the horizontal symmetry plane. This experimental technique may be useful for understanding the behaviour of short cracks growing from small stress concentrators (inclusions, bubbles, pits, etc.) existing in materials with smaller grain sizes but presenting similar notch- grain size relation. ACKNOWLEDGEMENTS he authors wish to thank the Ministry of Education and Sciences of Spain for providing the financial support through project DPI2008-01100 and DPI2011-27019. R EFERENCES [1] Lorenzino, P., Navarro, A., Krupp, U., Naked eye observations of microstructurally short fatigue cracks. Submitted for publication to Int. J. of Fatigue (2013). [2] Lorenzino, P., Fatiga en componentes con concentradores de tención bajo carga en modo I, Ph.D. Thesis, University of Seville, Spain, (2012). [3] Taylor, D., Geometrical effects in fatigue: a unifying theoretical approach, Int. J. of Fatigue, 21 (1999) 413–20. [4] Taylor, D., The theory of critical distances: a new perspective in fracture mechanics, Elsevier, (2007). [5] Peterson, RE, Notch sensitivity. In: Sines G, Waisman JL, Eds. Metal Fatigue, McGraw-Hill, (1959) 293–307. [6] Neuber, H., Kerbspannungslehre. Springer Verlag; (1937). Translated into English, Theory of Notches, Edwards, J. W., Ann Arbor, MI, (1946). [7] Herbig, M., King, A., Reischig, P., Proudhon, H., Lauridsen, E. M., Marrow, J., Buffière, J-Y, 3-D growth of a short fatigue crack within a polycrystalline microstructure studied using combined diffraction and phase-contrast X-ray tomography, Acta Materialia, 59 (2011) 590–601. [8] Krupp, U., Düber, O., Christ, H.-J., Künkler, B., Köster, P., Fritzen, C.-P., Propagation mechanisms of microstructurally short cracks factors governing the transition from short- to long- crack behavior, Materials Science and Engineering A, 462 (2007) 174–177. T T