Issue 33

S. Beretta et alii, Frattura ed Integrità Strutturale, 33 (2015) 174-182; DOI: 10.3221/IGF-ESIS.33.22 182 C ONCLUSIONS atigue crack growth in Haynes 230 was investigated at room temperature. Digital image correlation was employed to calculate crack opening levels. Two different approaches were applied to evaluate crack opening levels: it was found that crack closure measurements based on regression algorithms are strongly dependent on the extension of the regression areas. A more accurate estimation of crack closure effects was obtained by applying two points virtual extensometers. It was found that crack opening levels are changing along the crack profile: in order to obtain a more accurate closure measurement, opening levels were evaluated by placing a virtual extensometer 50 µm behind the crack tip. This distance was found to be the closest capable of providing noiseless results. The accuracy of the measurements was verified in terms of crack growth: the calculation of the effective stress intensity factor ranges, based on experimental opening levels, removed the dependency of crack growth rates on the applied stress ratios, underlining the fact that DIC can be successfully applied to fatigue crack growth. Similar measurements are being carried out on single crystals of the same material, in order to compare the closure levels at the same ∆K levels, between single and polycrystals. R EFERENCES [1] Elber, W., Fatigue crack closure under cyclic tension. Engineering Fracture Mechanics, 2(1) (1970) 37-45. [2] Williams, M.L., On stress distribution at base of stationary crack. American Society of Mechanical Engineers - Transactions - Journal of Applied Mechanics, 24(1) (1957) 109. [3] Riddell, W.T., et al., Determining fatigue crack opening loads from near-crack-tip displacement measurements. ASTM STP, 1343 (1999) 157. [4] Sutton, M.A., et al., Local crack closure measurements: Development of a measurement system using computer vision and a far-field microscope. ASTM STP, 1343 (1999) 145. [5] Elber, W. The significance of fatigue crack closure. in Damage Tolerance in Aircraft Structures: A Symposium. In: Seventy-third Annual Meeting American Society for Testing and Materials, Toronto, Ontario, Canada, 21-26 June 1970.. ASTM International. (1971). [6] McNeill, S.R., W.H. Peters, and M.A. Sutton, Estimation of stress intensity factor by digital image correlation. Engineering Fracture Mechanics, 28(1) (1987) 101. [7] Carroll, J., et al., Investigation of fatigue crack closure using multiscale image correlation experiments. Engineering Fracture Mechanics, 76(15) (2009) 2384. [8] Pataky, G.J., H. Sehitoglu, and H.J. Maier, High temperature fatigue crack growth of Haynes 230. Materials Characterization, (2012). [9] Rabbolini, S., et al., Fatigue crack growth in Haynes 230 single crystals: an analysis with digital image correlation. Fatigue & Fracture of Engineering Materials & Structures, (2014). [10] Forman, R.G., Mettu, S.R., Behavior of surface and corner cracks subjected to tensile and bending loads in Ti-6Al-4V alloy. (1990). [11] Pataky, G.J., et al., Full field measurements of anisotropic stress intensity factor ranges in fatigue. Engineering Fracture Mechanics, 94 (2012) 13-28. [12] Tada, H., P.C. Paris, Irwin, G.R., The analysis of cracks handbook.: New York: ASME Press, (2000). [13] Schijve, J., Fatigue crack closure: observations and technical significance, in Mechanics of fatigue crack closure, American Society for Testing and Materials Philadelphia, PA., (1988) 5-34. [14] Allison, J.E., R.C. Ku, Pompetzki, M.A., A comparison of measurement methods and numerical procedures for the experimental characterization of fatigue crack closure. Mechanics of fatigue crack closure, ASTM STP, 982 (1988) 171-85. [15] Rabbolini, S., S. Beretta, Sehitoglu, H., Fatigue crack growth in Haynes 230: a comparison between single crystals and polycrystal effective curve, in 15 th International ASTM/ESIS Symposium on Fatigue and Fracture Mechanics: Anaheim, CA, USA (2015). F