Issue 38

M.A. Meggiolaro et alii, Frattura ed Integrità Strutturale, 38 (2016) 67-75; DOI: 10.3221/IGF-ESIS.38.09 72 These ideas have been implemented in a suitable computer code and used to analyze the results obtained from two challenging tests that involved non-proportional tension-torsion load histories applied on tubular specimens, as well as another idealized bi-axial load history that illustrates well the effects of high mean loads, as discussed next. E XPERIMENTAL RESULTS he improved version of the MRF, proposed in this work to properly consider the difference between the well- known effects caused by tensile and compressive mean loads on fatigue damage, is evaluated using experimental and idealized tension-torsion 2D stress histories. The experiments are performed on annealed tubular 316L stainless steel specimens in a multiaxial servo-hydraulic testing machine. The cyclic properties of this 316L steel are obtained from simple uniaxial tests, using standard procedures. Its Ramberg-Osgood uniaxial cyclic hardening coefficient and exponent are 874MPa and 0.123, with Young’s modulus 193GPa and Poisson ratio 0.3 . This material has been chosen for those tests because it presents a significant non-proportional (NP) hardening effect as well, which cannot be neglected in multiaxial fatigue damage calculations, as discussed below. The two experiments reported below consist of strain-controlled tension-torsion cycles applied to identical tubular specimens, one for the cross and one for the x-shaped paths from Fig. 2, represented in the normal-effective shear strain space  x ×  xy /  3 . Figure 2 : Applied  x ×  xy /  3 strain paths on two tension-torsion tubular specimens, with successively imposed amplitudes  a = 0.2% , 0.4% , 0.6% and 0.8% in each case. Figure 3 : Experimentally measured data points (  markers) from the  x ×  xy  3 stress paths induced by the cross and x-shaped inputs from Fig. 2, and associated outputs from the MRF (solid lines) for a chosen filter amplitude r = 7MPa . T