Issue 33

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 33 (2015) 335-344; DOI: 10.3221/IGF-ESIS.33.37 343 Fig. 12 shows the typical experimental fatigue fracture diagrams in the coordinates of the crack growth rate versus the values of the stress intensity factors for the hollow cylindrical specimens tested under pure tensile loading. The left picture in Fig. 12 depicts the behavior of the db/dN as a function of the elastic SIF K 1 , whereas the right picture in Fig. 12 gives us the crack growth rate depending on of the dimensionless plastic stress intensity factor K P . To determine the experimental values of the elastic and plastic SIF's for two main points of the crack front, namely, the free surface and midplane section, was used the distributions represented in Fig. 9. Looking at Fig.12 it should be noted that increasing of the crack growth rates is observed in the direction of the deepest point of the crack front with respect to the crack front intersection with the free surface of the hollow cylindrical specimens in terms of the elastic and plastic SIF's. а) b) Figure 12 : Crack growth rate as a function of (a) elastic and (b) plastic SIFs under tension for different crack front points. In contrast to the elastic SIF K 1 , the plastic SIF K P shows very useful effect of the sensitivity to the plastic properties of the tested materials. It can be seen from Fig. 12 that the plastic SIF gradually increases by increasing the crack length and crack depth at fixed elastic properties of the aluminum alloys characterized by E =74 GPa and  =0.3. The data presented very obvious advantages of using the plastic stress intensity factors to characterize the material's resistance to cyclic crack growth. This conclusion is confirmed by the relative position of crack growth curves in Fig.12 for both tested aluminum alloys under pure cyclic tension in the terms of the elastic and plastic SIF"s. C ONCLUSIONS atigue crack growth for an elliptical arc-fronted edge crack with two different initial notch form in hollow cylindrical specimens of B95 and D16 aluminum alloys is studied. Experiments and calculations made under axial cyclic tension with and without cyclic torsion are described. All the experimental and numerical results are shown: - for the same specimen configuration and different the crack front position as a function of cyclic tension and torsion loading, the following constraint parameters were analysed, namely, the non-singular T -stress, z T -factor and the stress triaxiality parameter h in the 3D series of elastic-plastic computations for aluminum alloys different properties; - the governing parameter of the elastic-plastic stress fields I n -factor distributions along various crack fronts was also determined from numerical calculations, this governing parameter is used as the foundation of the elastic-plastic stress intensity factor; - under pure cyclic tension loading, it can be seen that the crack propagation paths differ with diverse initial flaw forms, but converge to the same configuration when the crack depth ratio is larger than about 0.25; - it is found that there is one general relationship between the crack growth rate on the free surface of specimen and COD for both tested aluminum alloys and loading conditions including the case when the torsion loading is superimposed on the cyclic tension; F