numero25

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 25 (2013) 138-144; DOI: 10.3221/IGF-ESIS.25.20 140 Grain size The grain size measurement is made according to ASTM E112, using software Simagis Live and following the procedure described in [16]. The obtained grain size values are summarized in table 1 Deformation [%] Grain Size [mm] 0 0.066 18 0.394 14 1.41 11 3.46 8 9.74 Table 1 : Grain size obtained in each thermo-mechanical treatment Testing machine Once the thermo-mechanical treatment is completed and the desired microstructure obtained, a circular notch is machined on each specimen, and the samples are subjected to push-pull fatigue loading. Several combinations of notch and microstructural sizes have been tested. A RUMUL resonant testing machine (Testronic 100 kN) was used for fatigue testing. All tests were load-controlled, with R=0.1 (pull-pull) and maximum stress between 45 and 95 MPa. Resonance frequencies for the loading conditions and geometry of specimens are between 75 and 90 Hz. Crack propagation results in a decrease on the resonance frequency. After several trials, it was established that a frequency decrease of 0.7 Hz was a good indicator of impending failure. A larger decrease results in complete failure, where the crack faces rapidly separate and the remaining section is plastically deformed, making post mortem analysis impossible. Crack Follow-up An experimental procedure for measuring crack length and growth was developed in order to observe crack propagation and interactions with the microstructure. Two low magnification optical microscopes showing real time video images were connected to a computer via USB. They are easily mounted onto a platform and, as shown in figure 2, they are set to cover the surface of both sides of the specimen simultaneously. The magnification is adjusted by regulating the distance between the magnifier and the specimen. Figure 2 : Crack Follow up Set up. Since the materials used have low mechanical properties and strains are always kept in elastic region, the movements produced during specimen load and unload are very small; as a result, the recorded video is quasi-static and there is no need to stop the test, measure the crack length and resume. This facilitates the crack growth analysis, and largely reduces the amount of time needed for the test in contrast with other crack follow up experimental techniques, such as replica with acetate. This technique makes it possible to keep a record of every test and of any possible potential crack on both