Issue 38

C. Gourdin et alii, Frattura ed Integrità Strutturale, 38 (2016) 170-176; DOI: 10.3221/IGF-ESIS.38.23 172 Figure 2 : View of the spherical bending device and Technical view of the fatigue cell. To ensure well-defined experimental conditions, various measuring means are located symmetrically at the two half-shells • Pressure sensor with a measuring range between 0 to 100 bars • Type K thermocouple to measure the temperature of the fluid inside the fatigue cell • Displacement sensor (LVDT) to measure the deflection at the center of the spherical bending specimen. This sensor has a 5mm range. Realizations of surface observations after the fatigue test show that the contact between LVDT and specimen is negligible (no fretting). No crack initiation is also observed directly under the LVDT. • Two visualization windows on each half-shell, oriented at 45° with a diameter of 20 mm. The constitutive material is borosilicate glass with a permissible operating pressure of 100 bars. The fatigue cell is built under European Security directives (Machines 2006/42/CE, Pression 97/23/CE). T HE EXPERIMENTAL PROTOCOL he experimental protocol is the following: • Implementation of the spherical bending specimen, with a slight overpressure to ensure a first purge, • Several blocks of 50 cycles with an increasing displacement loading. The aim of these steps is to ensure proper implementation of the components under the effect of pressure, and to ensure the best purge is possible. • Beginning of the fatigue test at the chosen deflection with “slow” cycles every 500 cycles allowing taking photographs through the windows. The objectives of these particular cycles is to taking into account the good value of the residual strain at no pressure due to the elasto-plastic behavior of the specimen. The “center” of the elasto-plastic loop behavior of the specimen can be estimate and the range of deflection is adjusting within this information. The spherical bending fatigue test is stopped when cracks have propagated outside the central zone. Calibration Tests with the new experimental fatigue device are conducted with imposed displacement or deflection. In order to properly connect the strain level in the central area with the measured deflection, we need to define an experimental curve which represents the measured deflection versus the corresponding strains in this zone. A calibration phase is necessary to obtained the appropriate curve [∆  r;  deflection ] directly from experimental results, and with these methods, the obtained curve is taking into account the real mechanical behavior of the specimen. Two experimental calibration methods have been carried out in the LISN laboratory with the FABIME2 device. Specimens used are in stainless austenitic steel type 316L [8]. T