Issue 41

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 41 (2017) 31-39; DOI: 10.3221/IGF-ESIS.41.05 36     1 cos sin 1 , , , 1 cos 1 FEM FEM FEM n FEM FEM FEM FEM r e rr r r FEM n FEM FEM FEM FEM rr r r du du n u u b a n d d I n d D D u u n                                                                                              (3) where ij   - dimensionless stress components, i u  - dimensionless displacement components, , r  - polar coordinates. The distributions of the elastic and plastic SIF along the initial crack front for both alloys and three temperatures are plotted in Fig. 8. The constraint parameters are plotted against the normalized coordinate RR. In this plot RR = 0.0 is the specimen free surface, RR = 1.0 is the mid-plane of the hollow specimen thickness. It can be observed, that all constraint parameters essentially changed along the crack front from the free surface toward to mid-plane. 2 4 6 8 0 0.2 0.4 0.6 0.8 1 RR elastic SIF [Mpa*m^0.5] ‐60°C +23°C +250°C D16T, B95AT 0.3 0.4 0.5 0.6 0.7 0.8 0 0.2 0.4 0.6 0.8 1 RR plastic SIF D16T +250 ° C +23 ° C ‐60 ° C 0.3 0.4 0.5 0.6 0.7 0.8 0 0.2 0.4 0.6 0.8 1 RR plastic SIF ‐60°C +23°C +250°C B95AT Figure 8 : Elastic and plastic SIF distributions for initial crack front. Fig. 8 gives a clear illustration of the necessity to take into account the plastic properties of the material in the interpretation of the characteristics of the material resistance to crack propagation. The distributions of elastic SIF are the same for both tested materials, because elastic properties of tested materials approximately the same (Tab. 1). Contrary to that, the plastic SIF shows very useful effect of the sensitivity to the plastic properties of the tested materials. It can be seen from Fig. 8 that the plastic SIF gradually increases by increasing the test temperature conditions. The data presented very obvious advantages of using the plastic SIF to characterize the material's resistance to cyclic crack growth. Numerical data for the elastic and plastic SIF behaviors accounting for the material properties and temperature conditions will be used to interpret the characteristics of the material resistance to crack propagation. E XPERIMENTAL RESULTS AND DISCUSSION he first part of experimental results includes the data of direct measurements of the objective parameters such as the crack length b and the CMOD on the free surface of specimens for all considered alloys and temperature conditions. Fig. 9 represents the surface crack growth rate db/dN versus CMOD on the hollow cylindrical specimens. It is found that the crack growth rate along the external surface direction as a function of CMOD described by a various curves with a small scatter band of the experimental results for both tested aluminum alloys. Also, looking at Fig.4b, 4c and considering changes in the general durability of the specimens in low/high temperature test, significant differences in the crack growth rate in the depth direction a and on the free surface b of hollow specimens under the above temperature conditions are expected. The second part of the experimental data relates to the interpretation of the surface crack growth rate for aluminum alloys at different temperature conditions with the involvement of the numerical results for elastic and plastic SIF's distributions presented in the previous section of this paper. Based on the interpretation of experimental fatigue fracture diagrams in terms of traditional elastic SIF it is found that there are three separate diagrams for each temperature on the free surface of the hollow cylindrical specimen (Fig. 10a). In T