PSI - Issue 18

Nikolas Baak et al. / Procedia Structural Integrity 18 (2019) 274–279 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Due to this design, the drill smoothens the surface of the bore and at the same time influences the subsurface area by initiating changes in the microstructure and inducing residual stresses (Baak et al. 2018). The resulting surface integrity is significantly influenced by the process parameters cutting speed and feed rate (Nickel et al. 2018). Varying the feed rate leads to significant changes in feed force F f and torque M d (Sakuma et al. 1981). These values were measured using a piezoelectric dynamometer from Kistler. The specimen were drilled using an Ixion TLF 1004 deep drilling machine. Fatigue tests were carried out using a Rumul Testronic 150 kN resonant testing system with a test frequency f cyc = 75 Hz and a load ratio R = -1 (Fig. 2a). The tests were performed as load increase tests (LIT), starting at a damage-free amplitude of σ a = 100 MPa which was increased by Δσ a = 10 MPa every ΔN = 10 4 load cycles and besides as constant amplitude tests (CAT). The tests were instrumented with an extensometer, thermocouples and an alternating current potential drop (ACPD) measuring setup (Fig. 2b). These instrumentation allows the detection of material reactions at an early stage of the fatigue test. (Starke et al. 2018).

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Fig. 2 (a) Instrumented resonant testing system with accessories; (b) Instrumented and clamped fatigue specimen. (Baak et al. 2018)

The fatigue tests were interrupted for intermittent magnetic Barkhausen noise (MBN) measurements. These measurements were performed with a “FracDim” measuring system provided by Fraunhofer IKTS. To measure the MBN at the crucial inner surface a custom built inner-surface sensor (Fig. 3) was used.

Fig. 3. Inner-surface sensor for the “FracDim” measuring system. (Baak et al. 2018)

This sensor system excites the alternating magnetic field from the outer surface and measures the micro-magnetic properties with a separate sensor-element, which can be placed inside the specimen.