Gigacycle fatigue behavior of a cast aluminum alloy under biaxial bending: experiments with a new piezoelectric fatigue testing device

To C. Bathias memory Many components in several industries are loaded in VHCF regime, either at high frequencies (wheels of high speed trains, blades in aircraft turbojet engines, etc.) or at lower frequencies during decades (artificial heart, mooring chains for off-shore petroleum platforms, etc.). Testing specimens up to 1E9 or 1E10 cycles in a realistic time requires to use a very high loading frequency (20 or 30 kHz). Several devices using the ultrasonic testing technique are described in [1]. 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 (up to 600°C), in air or in liquid environment, but there is no machine for testing specimens under multiaxial loading. That is the reason why a new fatigue testing device has been developed to test specimens under biaxial loading at 20 kHz. This new machine is able to test flat smooth specimens under biaxial bending. The specimen, with a disc geometry, is placed on a circumferential frame. The load is applied at the center of the upper face, so that the stress state on the opposite face is a biaxial proportional stress state. Several positive loading ratios can be applied. This machine is interesting - for instance - to test under biaxial loading a cast aluminum alloy (used to produce cylinder heads) in order to determine its fatigue strength at high hydrostatic pressure. The principle of this machine and the first results obtained in VHCF regime on such aluminum alloy are presented and compared with those obtained in literature under a similar stress state in HCF regime [2, 3]. The crack initiation is investigated, as two competing parameters play a role: the biaxial stress state which is maximal on the surface, and the possible presence of subsurface defects. [1] Bathias C, Paris PC. (2005) Gigacycle Fatigue in Mechanical Practice. New York USA: Marcel Dekker Publisher Co. [2] Koutiri I, Bellett D, Morel F, Augustins L. and Adrien J. (2013) High cycle fatigue damage mechanisms in cast aluminium subject to complex loads. International Journal of Fatigue, Vol. 47: 44–57. [3] Koutiri I., Morel F., Bellett D. and Augustins L. (2009) Effect of high hdrostatic stress on the fatigue behaviour of metallic materials,12th International Conference on Fracture ICF-12, Vol. 5: 3694-3703