PSI - Issue 17

Moritz Hartweg et al. / Procedia Structural Integrity 17 (2019) 254–261 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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compensation, but it delivers no additional information on the crack propagation behavior. In the model presented above an increase of the specimen temperature influences all three potential measurements. Since the model bases on differences between the individual potential values, an increase of the temperature has only a minor effect on the detection of cracks using this model. Another advantage is the increased sensitivity of the method. The sensitivity of the PDM rises with the crack length and a detection of short fatigue cracks affords a filtering of the measured potential data (Bär and Tiedemann (2017)). The proposed model allows a detection of small cracks having an area of less than 2 % of the cross section of the specimen. A further development of this method with the target to enhance the detectability will be undertaken in the future. Moreover, this method will be transferred to other specimen geometries as CCT- and SEN-specimens. 1. The use of multiple potential probes allows an earlier detection of cracks in notched specimens during fatigue experiments. 2. A geometrical model, which allows the determination of the position of the crack initiation site in notched round bars, is proposed. 3. First experiments have shown that the position of the crack initiation site can be localized with an uncertainty of less than 10° even for small crack sizes. 4. Possibly the decrease in the scatter of the calculated angle may be used as a criterion for crack initiation, even in specimen with multiple crack initiation sites. However, this must be confirmed in further experiments. 5. For an early detection and localization of cracks in fatigue experiments, the use of multiple potential drop probes seems to be promising method. 6. The decrease of the scatter (standard deviation) of the calculated angle may be used to develop a criterion for crack detection. Based on the introduced model, in further investigations the use of the multiple potential drop measurement will be extended to other specimen geometries. ASTM E647; 2013. Test Method for Measurement of Fatigue Crack Growth Rates. DOI: 10.1520/E0647-15E01 Bär, J.; Tiedemann, D.; 2017. Experimental investigation of short crack growth at notches in 7475-T761, Structural Integrity Procedia 5 , 793-800. DOI: 10.1016/j.prostr.2017.07.171. Bauschke, H.-M.; Schwalbe, K.-H.; 1985. Measurement of the depth of surface cracks using the Direct Current Potential Drop Method. Zeitschrift für Werkstofftechnik 16 , 156–165. 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, International Journal of Fatigue 124 , 167-187. DOI: 10.1016/j.ijfatigue.2019.02.045. Doremus, L.; Nadot, Y.; Henaff, G.; Mary, C.; Pierret, S.; 2015. Calibration of the potential drop method for monitoring small crack growth from surface anomalies – Crack front marking technique and finite element simulations, International Journal of Fatigue 70 , 178-185. DOI: 10.1016/j.ijfatigue.2014.09.003. Halliday, M. D.; Beevers, C. J.; 1980. The D.C. Electrical Potential Method for Crack Length Measurement. In: C. J. Beevers (Hg.): The measurement of crack length and shape during fracture and fatigue. Warley, West Midlands: EMAS, 85–112. Hartman, G. A.; Johnson, D. A.; 1987. D-C electric-potential method applied to thermal/mechanical fatigue crack growth. Experimental Mechanics 27 , 106–112. DOI: 10.1007/BF02318872. Johnson, H. H.; 1965. Calibrating the Electric Potential Method for Studying Slow Crack Growth, Materials Research and Standards 5, No. 9, 442– 445. Van Stone, R. H.; Richardson, T. L.; 1985. Potential Drop Monitoring of Cracks in Surface Flawed Specimens, Automated Test Methods for Fracture and Fatigue Crack Growth, ASTM STP 877, W. H. Cullen, R. W. Landgraf, L. R. Kaisand, and J. H. Underwood, Eds., ASTM, 148–166. Verpoest, I.; Aernoudt, E.; Dedruyttere, A.; 1981. An Improved A.C. Potential Drop Method for Detecting Surface Microcracks during Fatigue Tests of Unnotched Specimens. Fatigue and fracture of Engineering Materials and Structures 3 , 203-217. 5. Conclusions References