Issue 37

Y. Hos et alii, Frattura ed Integrità Strutturale, 37 (2016) 234-240; DOI: 10.3221/IGF-ESIS.37.31 236 E in MPa R p0.2 in MPa K’ in MPa n’ 210000 378 680 0.11 Table 1 : Mechanical properties of S235 Two different types of crack growth experiments were performed. In the first series of experiments, photos of the specimen were taken during experiment at predefined intervals. (Length of the interval is dependent on the nature of the experiment) Three cameras were installed in front of the starter notch, one facing the notch directly, the second with a 45° angle to the right and the third with a 45° angle to the left. After a certain number of cycles, the experiment was stopped, and a load corresponding to 90% of the maximum cyclic load was applied. During the short hold time, the photos of the specimen surface were taken. The specimen was unloaded again and the cyclic loading continued. The pictures were evaluated after the experiment, with the help of the laser-engraved dot pattern on the surface. Five different loading sequences were used in the experiments: pure tension-compression loading, pure torsion loading, proportional loading resulting from the superposition of these two and out-of-phase loading with phase angles of 45° and 90°. The experiments were conducted under load/moment control, using a servo-hydraulic, four-pillar tension-torsion machine, with frequencies ranging between 0.25 Hz and 2 Hz. A temperature of 21°C and a relative air humidity of 50% were kept constant. The cracks were assumed to be through-wall cracks with a straight crack front. The crack length is defined as the arc length, where the crack starts at the crack initiation location in each test. The scheme of the presentation of the results in references [20] and [22] were taken as a guide, with crack 1 being left and crack 3 being right. In the second series of experiments, the tests were interrupted in the fashion that was described above, only this time, the whole cycles were recorded and analyzed applying the digital image correlation (DIC) technique. More details about DIC can be found in references [19] and [20]. Later, the results of this analysis are utilized while dealing with crack closure in the simulations. E XPERIMENTAL RESULTS Uniaxial Tension-Compression Loading he specimen R-002 was tested uniaxially with a cyclic tension-compression force (F max = 35 kN and R F = -1). Two symmetric cracks grew in the centre cross-section, check Fig.3. The crack growth curve was indicated in Fig. 4. Figure 3 : Cracks in the specimen R-002, pure tension-compression with F max = 35 kN and R F = -1, steel S235. Figure 4 : Crack growth curve of specimen R-002, pure tension-compression with F max = 35 kN and R F = -1, steel S235. T