Issue 35

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 35 (2016) 114-124; DOI: 10.3221/IGF-ESIS.35.14 122 a) b) Figure 11 : Crack growth rate on free surface of (a) cruciform specimens and (b) bending plate versus COD. а) b) Figure 12 : Crack growth rate as a function of (a) elastic and (b) plastic SIFs under bending for different crack front points. 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 plate tested under bending loading. The left picture in Fig. 12 depicts the behavior of the da/dN and dc/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 SIFs for two main points of the crack front, namely, the free surface a and mid-plane section c , was used the distributions represented in Fig. 9. Looking at Fig.12 it should be noted that a significant reduction 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 bending specimens in terms of the elastic and plastic SIFs. 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 alloy characterized by E =76 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 the tested aluminum alloy D16 in the terms of the elastic and plastic SIFs.