Issue34

J. Bär et alii, Frattura ed Integrità Strutturale, 34 (2015) 456-465; DOI: 10.3221/IGF-ESIS.34.51 457 In this work the integral measurement of the heat flow with a peltier element will be combined with space resolved lock-in thermography measurements during fatigue crack propagation experiments under load as well as stress intensity controlled conditions. E XPERIMENTAL DETAILS Crack propagation Experiments he crack propagation experiments were undertaken with S ingle E dge N otched specimens of a high-alloyed steel (X5CrNi18-10, AISI 304). SEN-specimens with a length of 80 mm and a width of 12 mm were produced from a sheet material with a thickness of 4 mm. A starter notch with a length of about 1 mm and a notch radius of 0.25 mm was machined into the specimens. Symmetrical to the notch two pins for potential drop measurement have been mounted into the specimens with a spacing of 4 mm between the pins. The fatigue tests have been performed under fully reversed loading conditions at a frequency of 20 Hz using a servo- hydraulic testing machine with a DOLI EDC 580 controller. The machine is equipped with a specimen chamber and fixed grips to minimize bending forces. The specimen chamber is shown in Fig. 1. On the left hand side the peltier sensor for the heat flow measurement mounted from the backside is visible. On the right hand side a specimen is fitted and the peltier sensor is pressed on the backside of the specimen. A detailed description of the equipment and the test methods are given by Bär and Volpp [8]. Figure 1 : Specimen chamber for the crack propagation experiments. On the left image, the peltier sensor is visible. On the right the equipment with a mounted specimen is shown. For crack length measurement a DC potential drop method was used. Therefore, a constant current of about 4.1 A was conducted through the specimen. The potential drop was measured between the two pins mounted adjacent to the notch of the SEN-specimen using an amplifier of the EDC 580 control electronics. The crack length as well as the stress intensity were calculated for each cycle and therefore it was possible to control the stress intensity during the test. Consequently, experiments with constant stress intensity K max and  K can be undertaken. Thermographic measurements The fatigue crack propagation experiments were accompanied by thermo elastic stimulated lock-in thermography. For cyclic thermal stimulation the thermoelastic effect is used in this tests. The investigations were performed with a Cedip Titanium HD 560 camera (Fig. 2) and the software Altair LI. T