Issue 37

M. Vieira et alii, Frattura ed Integrità Strutturale, 37 (2016) 131-137; DOI: 10.3221/IGF-ESIS.37.18 132 for some materials, of the fatigue limit that used to be considered on mechanical design. However, these results only refer to uniaxial loadings when, in real conditions, mechanical components are usually loaded under multiaxial loadings. Because of the axial character of the excitation created by the piezoelectric exciter, only axial, bending or fretting specimen testing were able to be performed up to now. The appearance of torsional piezoelectric exciters allowed for VHCF testing on torsional conditions, as well. But, for multiaxial conditions, no VHCF results have been described on the literature due to conceptual limitations of these devices. Multiaxial loading fatigue has been the subject of intense research for low and high cycle regimes, but not on the VHCF region, due to the inexistence of machines capable of operating on ultrasonic frequencies and submit specimens to multiaxial loadings. For the high cycle regime, the von Mises criterion on biaxial loading has been questioned since experimental data does not correlate well, either for in-phase or out-of-phase loadings [3]. In this paper, the development of a fatigue testing machine for biaxial conditions working at VHCF is presented. The device is comprised of a horn and a specimen, which are both attached to an ultrasonic piezoelectric axial exciter. B IAXIAL FATIGUE TESTINGMACHINE FOR VHCF he present work describes the processes of creation and development of a VHCF testing device for biaxial conditions, using a single axial piezoelectric exciter. The device is comprised of a horn and a specimen, being the latter the component to be tested on biaxial conditions, with a loading that was predefined to have in-phase sinusoidal components of axial and shear stress in R=-1. The horn Since the horn receives a sinusoidal axial displacement from the piezoelectric exciter, and it is intended to induce also torsional loadings on the specimen, the horn has to be responsible for the generation of the rotational movement which will be imposed on the specimen and will promote shear stresses in it. This implies that the horn takes special importance on the behavior of the device, specifically on the relationship between axial and torsional loadings imposed on the specimen. The computational modal analysis made to this geometry proved that a certain dynamic vibrational mode could be achieved in which the horn would vibrate in a hybrid mode composed by the first axial mode and the first torsional mode, where axial and rotational displacements were amplified on the smaller free end. Still, there was a need for a horn that would possess this specific mode on the frequency at which the exciter operates (20 kHz). The iterative process to obtain the final geometry was produced using finite element software, and a schematic representation is shown on Fig. 1: Figure 1 : 2D representation of the developed biaxial horn. The final horn geometry consists of a conical shaped piece possessing two groups of oblique slits responsible for the generation of the rotational character of the horn, which in turn will promote sinusoidal rotations on the specimen that will add to the already existent sinusoidal axial excitation. The specimen Before introducing the final geometry of the used specimen, it might be interesting to analyze the dynamic equation for a generic bar, Eq. 1, [1]: T