DEFORMATION AND FAILURE BEHAVIOR OF FEE46 0 AND ALMG4.5MN UNDER MULTIAXIAL PROPORTIONAL LOADING WITH CONSTANT AND VARIABLE AMPLITUDES

To calculate the fatigue life-to-crack initiation of engineering compo nents under combined cyclic loading, experimentally secured knowledge on the cyclic deformation and failure behaviour of the materials used under the certain multiaxial cyclic stress and strain conditions are r equired. To obtain this, strain-controlled fully reversed experimental tests at tensional, torsional and combined loading with constant and variable amplitudes have been conducted using thin-walled tube specime ns of FeE460 and AlMg4.5Mn. Experimental tests on standard uniaxially loaded hourglass specimens have also been conducted to study specimen form effects. Cyclic deformation behaviour can be uniformly described by the stabilised cyclic sigma-epsilon-curve, if stresses and strains are expressed as equivalent values according to the von Mises criterio n. Failure behaviour at constant and variable amplitude loading is cha racterized by the initiation and growth of short cracks at right angle to the direction of the greatest principal stress (mode I) in the cas e of tensional or combined loading and by short crack growing in both shear stress directions (mode II+III) in the case of torsional loading . At fully reversed constant amplitude loading, all three types of loa d can be described by one constant amplitude strain life-to-crack init iation curve. At variable amplitude loading (notch strain simulation w ith gaussian spectrum, H-0 = 10(5)), the experimental fatigue life-to- crack initiation values are lower than estimated values based on Miner -calculations using an equivalent stress-strain supported P-SWT-N-curv e. The question of mean stresses and their evaluation is discussed.