Issue 30

M. Da Fonte et alii, Frattura ed Integrità Strutturale, 30 (2014) 360-368; DOI: 10.3221/IGF-ESIS.30.43 362 Cu Zn Mg Mn Cr Si Fe Ti Al 1.267 5.66 2.72 0.05 0.197 0.156 0.228 0.21 Bal. Table 1 : Chemical composition of 7075 aluminium alloy. Yield strength (MPa) Ultimate tensile strength (MPa) A (%) 513 575 10.9 Table 2 : Monotonic mechanical tensile properties of 7075 T6 aluminium alloy. Figure 1 : Specimen used for fatigue crack growth data. Testing procedure Experiments for fatigue crack growth were performed in a rotating bending fatigue testing machine, Fig. 2, which was designed and constructed to model the real conditions of axles and shafts in service [9]. Figure 2 : Schematic of testing machine for rotating bending with or without steady torsion. The fatigue testing machine can perform rotary bending with or without steady torsion under load control either in bending or torsion. Due to the load capacity of the machine presented in Fig. 1, when higher torsion levels were required, the specimen diameter must be reduced, therefore two specimen diameters, were used, respectively 10 mm and 12 mm. Rotating bending fatigue tests were carried out for nominal bending stresses from 140 MPa up to 190 MPa and nominal steady torsion from 60 MPa up to 127 MPa. Tab. 3 presents the specimen diameters and loading conditions used on fatigue crack growth tests, representing a wide range of bending stresses and steady torsion. These testing conditions refers to multiaxial loading, with a cyclic normal stress, σ, and a static shear stress, τ , therefore it is helpful to full characterize the stress state during a cycle loading. Tab. 3 also presents the ratio between the maximum normal and the maximum shear stress achieved in each loading cycle as well as the maximum and minimum principal stresses, S I,max and S III,min , the maximum shear stress τ max and the maximum equivalent (von Mises equation) stress σ equiv,max . The detection of fatigue crack initiation from the notch and surface crack length measurements were carried out with an optical microscope at a magnification of 80X. Measurements of crack length were obtained at constant intervals, interrupting the tests, till the crack depth reaches approximately 3/4 r. In some selected specimens the crack front shape Dynamometer