PSI - Issue 17

Meike Funk et al. / Procedia Structural Integrity 17 (2019) 183–189 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Discussion

The measurement with two independent potential probes on SEN-specimens gives more detailed information about the crack propagation, even for short fatigue cracks. The difference as well as the ratio between the two potentials are helpful parameters for describing the form of the crack front and the differences in the crack propagation behavior. The use of the differences ΔP and the ration P 1 / P 2 between the measured potentials has an additional benefit: changes of the specimen temperature are compensated and therefore the sensitivity of the measurement is enhanced, especial in the case of long term measurements. As shown in the measurements the use of two independent potential probes allows the determination of the crack initiation site. This was already demonstrated by Campagnolo et al (2019) on notched steel and titanium bars. In their work the differences in the potentials allowed a determination of the region were cracks were initiated even in an early stage of crack propagation. The investigations, presented in this work, not only the crack initiation site can be determined, but also information about the subsequent propagation of short and long fatigue cracks can be obtained. This first investigations revealed the nigh potential of this method concerning the crack propagation behavior of metallic materials. First experiments have shown that the sensitivity of the measurement can be increased by changing the positions of the potential probes on the specimens. Further experiments with the aim to further develop this method will be undertaken. 1. Fatigue crack propagation experiments with two independent potential probes on SEN-specimens showed distinct differences in measured potential values. 2. The difference between the measured potentials give information about the incline of the crack front. 3. The ratio between the two measured potentials allow the determination of the crack initiation site and give information about the propagation of short and long fatigue cracks. 4. The method has a high potential and will be improved in further investigations. Clark, T., Woodley, R., De Halas, D., 1962. Gas-Graphite Systems, in “Nuclear Graphite”. In: Nightingale, R. (Ed.). Academic Press, New York, pp. 387. Allery, M. B. P., Birkbeck, G., 1972. Effect of Notch Root Radius on the Initiation and Propagation of Fatigue Cracks. In “ Engineering Fracture Mechanics” 4, pp. 325–331. ASTM E647, 2000. Guideline for electrical potential difference determination of crack length. DOI: 10.1520/E0647-15E01. Bär, J., Tiedemann, D., 2017. Experimental investigation of short crack growth at notches in 7475-T761. In “ Procedia Structural Integrity ” 5, pp. 793–800. DOI: 10.1016/j.prostr.2017.07.171. Campagnolo, A., Bär, J., Meneghetti, G., 2019. Analysis of Crack Geometry and Location in Notched Bars by Means of a Three-Probe Potential Drop Technique. In “ International Journal of Fatigue ” 124, 167-187. DOI: 10.1016/j.ijfatigue.2019.02.045. Funk, M.; Bär, J., 2018. Influence of crack initiation on short crack propagation and cyclic lifetime of AA 7475-T761. In “ Structural Integrity Procedia ”, ECF22 - Loading and Environmental effects on Structural Integrity. DOI: doi.org/10.1016/j.prostr.2018.12.047. Johnson, H. H., 1965. Calibrating the Electric Potential Method for Studying Slow Crack Growth. In “ Mat. Res. Standards ” 5. Payne, J., Welsh, G., Christ Jr., R. J., Nardiello, J., Papazian, J. M, 2010. Observations of fatigue crack initiation in 7075-T651. In “ International Journal of Fatigue ” 32 (2), pp. 247–255. DOI: 10.1016/j.ijfatigue.2009.06.003. Suresh, S., Ritchie, R. O., 1984. Propagation of short fatigue cracks. In “ International Metals Reviews ” 29 (6), pp. 445–475. Shanmugham, S., Liaw, P. K., 1996. Detection and Monitoring of Fatigue Cracks. In “ ASM Handbook 19 ”, pp. 210–223. DOI: 10.1361/asmhba0002363. Zhixue, W., 2006. On the through-thickness crack with a curve front in crack-centered tension specimen. In “Engineering Fracture Mechanics” 73, pp. 2600-2613 References 4. Conclusions