PSI - Issue 28

F. Conrad et al. / Procedia Structural Integrity 28 (2020) 2195–2205 Author name / Structural Integrity Procedia 00 (2019) 000–000

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2. Focus of investigation, specimen types and loading scenarios Two specimen geometries, a corner-crack specimen for uniaxial and a cruciform specimen for biaxial loading cases, are used for experiments (Fig. 1, left). Corner-Crack (CC) specimens are a commonly used geometry for fracture mechanical experiments to mimic fatigue-crack-growth in aero-engine turbine discs e.g., see Pickard et al. (1983). The geometry is based on the standard given in DIN EN 3878 with a length of 92 mm and square cross section with edge length of 8 mm. In this specimen, crack-growth proceeds from a predefined crack-starter notch perpendicular to the loading axis. As a comparison to the uniaxial loading case, the crack-growth behavior considered to multiaxial loading is examined with the use of cruciform specimen. The specific geometry is depicted in Fig. 1, right side, and was mainly used for numerous high-temperature validation tests successfully applied in industry, Wang et al. (2012), Kong et.al (2015). A defined multiaxial stress state is induced in the center of the specimen through the load introduction arms of axis A and B, where the specimen center section has a diameter of 15 mm and a thickness of 1.8 mm. First fracture mechanical tests have been performed with this specimen and the biaxial servo-hydraulic test-rig situated at the Technical University of Darmstadt, see Giannella, Dhondt and Kontermann (2019).

Fig. 1. Corner-Crack-Specimen for uniaxial loading case (left) and cruciform specimen for biaxial loading case (right). Detail A outlines the orientation of the crack-starter notch.

To simplify crack-initiation at a certain location of the specimen, crack-starter notches were machined through wire electrical discharge machining. For the uniaxial specimen the orientation of the crack starter notch is perpendicular to the loading axis, while the orientation for the cruciform specimen has an incline of 45° against axis A. Prior to the actual fatigue crack-growth experiment, defined pre-cracking procedures were carried out to initiate a fine crack with negligible induced plastic zone effects. First tests were performed on uniaxial and biaxial servo-hydraulic powered test rigs with a frequency of 5 Hz and under force-controlled settings and room-temperature conditions. Table 1 outlines the test-parameters in detail. Next to the developed GPU-DIC system the tests were accompanied with mechanical extensometers (two-point uniaxial and four-point biaxial) and an Alternating Current Potential Drop (ACPD) probe that allows an indirect measurement of crack-depth as a function of load-cycles. A common linear correlation between the ACPD-Signal and the crack-evolution is used. Required calibration data for this procedure were gained with Heat-Tints prior and at the end of the actual experiments.

Table 1. Settings of uniaxial and biaxial fatigue crack-growth experiments, room temperature, test frequency = 5 Hz Specimen, Material Maximum Force, A Direction ( F A,max ) R-Ratio, A Direction ( R A ) Maximum Force, B Direction ( F B,max )

R-Ratio, B Direction ( R B )

Corner-Crack Specimen, Ti6246

25.0 kN 35.0 kN

-1 -1

17.5 kN

-1

Cruciform Specimen, 26NiCrMoV14-5