Issue 37

D. Angelova et alii, Frattura ed Integrità Strutturale, 37 (2016) 258-264; DOI: 10.3221/IGF-ESIS.37.34 261 However, the actual onset and extent of crack tip plasticity are functions of the cyclic stress-strain curve, and its well known that normally materials having a high yield strength and exhibiting cyclic strain hardening behaviour have a small plastic zone size at crack tip, and smaller crack growth rate compared with a lower yield strength materials that cyclically soften - originally mentioned by [13]. It has been shown that for a cyclic hardening material, the hardening rate is lower than the critical value which renders a continuously increasing plastic zone size relative to crack length - discussed for the first time by Navarro et al. [14]. (ii) During the PSC regime, microstructural barriers between grains are located in both tangential and normal directions to the advancing crack front. As the crack advances past the first dominant structural barrier it will still be decelerating as it approaches subsequent barriers, but to a lesser extent, before finally accelerating as it escapes the influence of barriers at a > d 2 . In addition to that, at some conditions as increased stress range, rough surface, aggressive environment, the cyclic plastic behaviour of materials can be exhausted quickly, in fact there is not time enough for plasticity realization. The cases of PSC regimes of Steel A at Δσ = 1200 MPa and Steel B at Δτ = 817 MPa and 900 MPa can be treated in the above mentioned way. Steels A and B show a high yield strength and appearance of many secondary cracks initiating in narrow cycle interval in MSC regime (Stage I) of the major crack. Later on these secondary cracks probably help with plasticity exhaustion in PSC regime - Stage II (described by EPFM), influencing the behaviour of the major crack, Figs. 1a, b, c. After the plasticity exhaustion and still in PSC regime, the major crack transforms from shear mode into tensile mode; also do the secondary cracks. (In Figs. 1a, b, c the PSC regime is defined by lines showing barriers d 1 and d 2 .) In LC regime (Stage II) some of the secondary cracks merge with the major crack. Crack growth rates da/dN are calculated and presented versus crack lengths a for Steel A in Figs. 2, 3a, and for Steel B, in Fig. 4. Applying the Parabolic-linear model, PLM, of [12], Eq.1, to these data leads to obtaining of dependences “Crack growth rate, da/dN – crack length a ”. For Steel A the values of coefficients D are shown in Tab. 1, and the values of all the other fatigue characteristics, in Tab. 2. a b Figure 2: New Parabolic-linear model “Crack growth rate, da/dN – crack length a ” for Steel A: (a) L version, Δσ=1400 MPa;.(b) comparison between L, M and N versions, Δσ = 1200 MPa a b Figure 3: Steel A tested at Δσ = 1200 MPa (L, M and N versions) and Δσ =1400 MPa (L version): (a) Comparison between the new parabolic-linear model; (b) Comparison between experimental and predicted fatigue lifetimes and the corresponding error bands in percentages.