Issue 37

D. Angelova et alii, Frattura ed Integrità Strutturale, 37 (2016) 258-264; DOI: 10.3221/IGF-ESIS.37.34 262 Steel A. The graphical presentation of the applied PLM from Eq. 1 to the experimental data at Δσ = 1400 MPa can be seen in Fig. 2a. In this case the basic model is applied only to the major crack and is marked as version L of it. The secondary cracks have not been taken into consideration. At the same time the graphical presentation of the applied PLM from Eq. 1 to the experimental data at Δσ = 1200 MPa is shown in Fig. 2b in three versions, and is very different from that in Fig. 2a Stage D 1200 MPa 1400MPa 1 2 3 4 5 6 L version M version N version L version PSC D 4 -2.2E-05 -1.3E-05 -9.9E-06 -7. 5E-05 1 2 3 4 5 6 D 5 1.4E-02 8.5E-03 6.2E-03 4.5E-02 MSC D 1 -2.3E-06 -5.1E-06 -3.2E-06 -9.1E-06 D 6 -2.0E+0 -1.2E+0 -8.8E-01 - 6.5E+00 D 2 7.3E-04 1.3E-03 9.7E-04 2.7E-03 LC D 7 2.0E-06 2.7E-06 1.7E-06 6.5E-05 D 3 -1.7E-02 -2.3E-02 -2.0E-02 -6.5E-02 D 8 1.6E+00 1.6E+0 1.7E+00 1.2E+00 Table 1: Values of coefficients D in Eq. 1 at Δσ = 1200 & 1400 MPa, Steel A. The version L presents crack growth rate only of the major crack. The version M shows a different behaviour of the major crack while influenced by the secondary cracks merging with it. Δσ, MPa a 0 , μm d 1 , μm d 2 , μm a f , μm N 0 , cycles N f,exp , cycles N f,m , cycles ( N f,m – N f,exp )100/ N f,exp , % 1200 L version 30 220 385 4395 20790 37840 49654 31.2 M version 41867 10.6 N version 44149 16.7 1400 L version 30 250 360 5835 4840 17380 19310 11.1 Table 2: Fatigue characteristics of Steel A for MSC, PSC, LC growth and some numbers of cycles during specimen fatigue lifetime. The version N presents the major crack propagation involving all the secondary cracks. The three versions L, M and N are analytically described by Eq. 1 and Tabs. 1, 2. It is important to mention that for modeling different stages of the major crack propagation (MSC, PSC, LC), there are used only those data da/dN, a of the secondary cracks initiated or developed in the corresponding regimes of the major crack growth. The comparison between the three versions (L, M, N) of the basic PLM at 1200 MPa (curves L, M and N), shown in Fig. 2b, can be used for usefulinteresting discussions. The curve M shows acceleration of crack growth rates for MSC and LC regimes in comparison with L and N curves, due to interaction and merge of the secondary cracks with the major crack. However in PSC regime the curve M is located below the curve L although very close to it. It is connected with the multitude of secondary cracks initiated easily due to the observed surface roughness and helping with plasticity exhaustion of Steel A in PSC regime of the major crack; also it is connected with the specific events taking part in PSC and described in 3.3. Transition regime PSC for Steels A and B. So, we can conclude that at 1200 MPa, the version M of the basic PLM most realistically represents the fatigue behaviour of Steel A. At 1400 MPa there are registered only a couple of secondary cracks which merge with the major crack, Figs. 1a, 2a; their influence is exerted during the third stage (LC) and practically does not affect the major crack growth behaviour; in this case the version L of PLM takes place. The versions L, M, N at Δσ = 1200 MPa and version L at Δσ = 1400 MPa are shown together in Fig. 3a. The highest crack growth rates correspond to the highest stress range. At the same time the final length of the major crack at Δσ = 1200 MPa is 75% from the length of the major crack at Δσ = 1400 MPa. This can be explained as a result of both, a full and quick consumption of specimen plasticity at Δσ = 1200 MPa and a high degree of energy dissipation, due to initiation and development of many and more secondary cracks at that stress range, interacting and merging with the major crack. Steel B. The graphical presentation of the applied PLM from Eq. 1 to the experimental data of Steel B (subjected to TF in- air and corrosion environment) can be seen in Fig. 4, and the version M of PLM is shown by thick line. The original model of Murtaza (having 4/2 microstructural barriers of type d 1 characterizing TF for in-air/corrosion environment, and