Issue 37

C. Brugger et alii, Frattura ed Integrità Strutturale, 37 (2016) 46-51 DOI: 10.3221/IGF-ESIS.37.07 50 Discussion For maximum stress levels equal or lower than 140 MPa, the first results obtained with the new ultrasonic biaxial testing device are in good agreement with the results by Koutiri et al [12, 14]. In our data, there is only one specimen broken very early, for a reason to be clarified. The median fatigue strength at 10 9 cycles is close to 63 MPa (corresponding to a maximal stress equal to 140 MPa). Figure 4 illustrates, in a Dang-Van diagram [15], both the experimental median fatigue strengths at 2.10 6 cycles obtained by Koutiri [12] on smooth specimens made in the same cast Al alloy and the threshold line identified from torsion (R=-1) and tension (R=-1) data. Furthermore, the loading paths corresponding to existing ultrasonic fatigue testing machines are shown: torsion (R=-1), tension (R=-1), tension or 3 points bending (R>0). The loading path corresponding to the specimens tested at the stress level corresponding to 1.10 9 cycles with the new device presented here is illustrated. It is clear that this device allows questioning the Dang-Van criterion for higher hydrostatic stress states. The same conclusion is valid for the Crossland criterion too [16]. It should be noted that the results obtained for maximum stress levels equal or greater than 150 MPa cannot be directly compared with literature results due to different stop criteria. Indeed, the test results presented in this paper were stopped when the resonance frequency decreased from about 19,900 Hz to 19,500 Hz. Such frequency drop is due to rigidity loss associated to a very large macroscopic fatigue crack propagation. Indeed, macroscopic cracks are either unique or branched but always extended almost to the frame ring when the test stops (Fig. 5). Additional investigations are needed to quantify the number of cycles associated with this propagation, but first observations indicate it might exceed 10 7 cycles. However, one can note that 10 7 cycles represent 1% only of 10 9 cycles. Figure 4 : Dang Van diagram for the cast Al-Si alloy: loading paths associated with existing ultrasonic fatigue machines and new ultrasonic biaxial bending device. -a- -b- Figure 5 : Macroscopic fatigue crack after testing a disc specimen under biaxial bending, a) lower face of the specimen and b) after breaking it under monotonic quasi-static loading.