Issue 37

M. Vieira et alii, Frattura ed Integrità Strutturale, 37 (2016) 131-137; DOI: 10.3221/IGF-ESIS.37.18 131 Focussed on Multiaxial Fatigue and Fracture Development of a Very High Cycle Fatigue (VHCF) multiaxial testing device M. Vieira, M. de Freitas, L. Reis, A. M. R. Ribeiro IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal, mario.vieira@ist.utl.pt, mfreitas@dem.ist.utl.pt http://orcid.org/0000-0003-3525-9218 luis.g.reis@ist.utl.pt http://orcid.org/0000-0001-9848-9569 aribeiro@ist.utl.pt http/ /orcid.org/0000-0001-2345-6789 M. da Fonte Escola Superior Náutica Infante D. Henrique, Av. Eng. Bonneville Franco 2770-058 Paço d'Arcos, Portugal manuelfonte@enautica.pt http://orcid.org/0000-0002-2345-6790 A BSTRACT . The very high cycle region of the S-N fatigue curve has been the subject of intensive research on the last years, with special focus on axial, bending, torsional and fretting fatigue tests. Very high cycle fatigue can be achieved using ultrasonic exciters which allow for frequency testing of up to 30 kHz. Still, the multiaxial fatigue analysis is not yet developed for this type of fatigue analyses, mainly due to conceptual limitations of these testing devices. In this paper, a device designed to produce biaxial fatigue testing using a single piezoelectric axial exciter is presented, as well as the preliminary testing of this device. The device is comprised of a horn and a specimen, which are both attached to the piezoelectric exciter. The steps taken towards the final geometry of the device are presented. Preliminary experimental testing of the developed device is made using thermographic imaging, strain measurements and vibration speeds and indicates good behaviour of the tested specimen. K EYWORDS . Multiaxial fatigue; Very high cycle fatigue; Fatigue testing machines; Strain measurements. I NTRODUCTION atigue damage has special relevance on the life span of mechanical components and structures, as it takes responsibility for the majority of the registered structural failures. Although its mechanisms have been the subject of continuous research, the growing need for greater lifespans forced the understanding of the behavior of materials under very high cycle loadings [1], also known as the Very High Cycle Fatigue (VHCF) regime. This field of research, which studies the mechanical behavior of materials for fatigue lives over 10E7 cycles, has recently gained notoriety [2], largely due to the appearance of ultrasonic fatigue testing machines, working at 20-30 kHz and due to the acquisition and control equipment capable of handling signals at such high frequencies. In this context, the results found in the bibliography [1, 2], which usually focus on either axial or torsional fatigue tests, allow us to understand the behavior of materials on the very high cycle region of the S-N curves, remarking the absence, F