S. Xinhong et alii, Frattura ed Integrità Strutturale, 35 (2016) 441-448; DOI: 10.3221/IGF-ESIS.35.50 442 fatigue failure conditions or predict the fatigue life [2] as the critical plane approach has clear physical meaning. Once the fatigue damage plane is selected, then the stress or strain history on the critical plane is calculated and converted into cumulative fatigue damage. So the research on multiaxial fatigue crack initiation and propagation plane orientation contributes to the selecting of critical plane and determining the fatigue failure mode. Fatigue crack initiation and propagation are affected by material type, structure, size and load paths [3]. Although the effects of mean stress and multiaxial loading paths on fatigue life are considered by many criteria, the study of mean stress on the crack initiation and propagation plane orientation in middle and high cycle fatigue is fewer. Fatigue crack initial generally at the site of the highest stress or the place which has a defective. Then crack propagate into the material, this progress can be divided into two stages: stage I is in the direction of the maximum shear stress, which is dominated by shear stress and controlled by the microstructure within individual grains. After propagating several grains diameters, stage II crack growth begins and propagates perpendicular to the maximum normal stress at the macro point of view, which is controlled by the maximum normal stress. The crack propagates usually 2 to 5 grains diameters in stage I, but contributes to a large proportion of the fatigue life [4]. However, the definition of crack initiation size is not clear. The nucleation of flaws along persistent slip bands is considered as the crack initiation stage by material scientists, while a detectable crack size by engineers [5]. The crack initiation approach consists of microscopic growth and small crack growth up to a length of about 1mm [6]. In this paper the stress state on planes and fatigue fracture surface under tension-torsion loading with different mean tensile stress is analyzed for 2A12-T4 aluminum. Experiment had been carried out in order to study the effect of mean tensile stress on crack initiation and early propagation plane orientation. Optical microscope is used to measure crack initiation and early propagation plane orientation (stage I) on specimen fracture. The predicted plane orientations based on critical plane models are compared with experimental measured angles. E XPERIMENTS Material and specimens he material studied in this paper is 2A12-T4 aluminum alloy, which is usually used in aircraft. The shape and dimensions of specimens used in axial-torsion fatigue experiment are shown in Fig. 1. Figure 1 : Specimen geometry for tension-torsion loading. A PLS-200/1500 servo-hydraulic tension-torsion load frame was used for tension-torsion fatigue experiments. The test system, which has a capacity of 1500N m in torque and 200kN in axial load, is equipped with the electronic control, computer control, and data acquisition. The axial and shear loading are controlled at the same time. Loading frequency was f =3Hz in the experiment. In order to investigate the effect of mean tensile stress, the tension-torsion fatigue experiment is carried out. The waveform of load is as follows: , , , , sin sin x a x m xy a xy m x xy t t (1) The test is conducted at the circumstance of , xy m =0, , xy a =165MPa, so the mean shear stress and shear stress amplitude is fixed. And the phase angle of axial between shear loading =0 , so the effect of non-proportional loading is excluded. The chosen mean tensile stress is , x m =0MPa, 50MPa, 100MPa and 150MPa. Then the macro-fractures of the tested specimen were observed by optical microscope, and fatigue crack initiation and early propagation plane orientations were measured at a magnification about 30 times. The angle between the axial T

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