Issue 37

C. Brugger et alii, Frattura ed Integrità Strutturale, 37 (2016) 46-51 DOI: 10.3221/IGF-ESIS.37.07 46 Focussed on Multiaxial Fatigue and Fracture Ultrasonic fatigue testing device under biaxial bending C. Brugger, T. Palin-Luc Arts et Metiers ParisTech, I2M, CNRS, Universite de Bordeaux Esplanade des Arts et Métiers, 33405 Talence, France, P. Osmond, M. Blanc PSA Peugeot Citroën A BSTRACT . A new fatigue testing device has been developed to test specimens under biaxial loading at 20 kHz. A flat smooth specimen with a disc geometry is placed on a torus frame and cyclically loaded at the center of its upper face. Disc bending generates a biaxial proportional stress state at the center of the lower face. Any positive loading ratio can be applied. A cast aluminum alloy (used to produce cylinder heads) has been tested under biaxial bending using this device in order to determine its fatigue strength at 10 9 cycles under high hydrostatic pressure. Self-heating is moderate but macroscopic fatigue cracks after testing are very long. First results in VHCF regime are consistent with literature results obtained under similar stress state but in HCF regime and at 20 Hz. K EYWORDS . Gigacycle fatigue; Biaxial loading; Bending; Cast aluminum alloy; Ultrasonic testing machine. I NTRODUCTION any components in several industries are loaded in VHCF regime, either at high frequency (wheels of high speed trains, blades in aircraft turbojet engines, etc.) [1] or at low frequency during decades (artificial heart, mooring chains for off-shore petroleum platforms, etc.) [2, 3]. Testing specimens up to 10 9 or 10 10 cycles in a realistic time requires to use a very high loading frequency (20 or 30 kHz). Several devices using the ultrasonic testing technique have been developed all over the world since Mason’s work in the 1950's [4]. A significant effort in this field has been done since the end of the last century [1, 5]. Specimens can be tested under tension (R=-1 or R>0), torsion (R=-1 or R>0) or bending (R>0), either at room, low or high temperature, in air or in liquid environment [1-5], but there is no machine for testing specimens under multiaxial loading. However, it is known that during their life many industrial components are submitted to multiaxial loadings that may lead to a number of cycles close to one billion or more. Fatigue cracks may initiate in areas experiencing multiaxial stress states. That is the reason why a new fatigue testing device has been developed to test specimens under biaxial loading at 20 kHz. After presenting a brief state of the art on ultrasonic fatigue testing machines, the principle of a new biaxial bending device and the associated stress state are presented hereafter. The first results obtained on a cast Al-Si alloy in VHCF regime are then compared with those obtained in literature under similar stress state but in HCF regime and under lower frequency. M