Issue 29

L. Zhao et alii, Frattura ed Integrità Strutturale, 29 (2014) 410-418; DOI: 10.3221/IGF-ESIS.29.36 415 Distributions of stress and strain ahead of crack fronts Crack tip strain ε ct is the main mechanical parameter affecting SCC growth rate in Ford’s model [15], which is widely used in analyzing SCC behavior of key materials in PWR components. Because it is difficult to directly obtain ε ct , plastic strain ε p was used to replace ε ct at a characteristic distance r 0 ahead of crack tip in FRI model [16]. Thus, the critical issue is to estimate r 0 when SCC growth rate is predicted. Considering that the mechanical field near crack front is pertinent to SCC growth, r 0 is regarded to be smaller than the plastic zone size. Strain and strain rate quickly decrease with the increasing r 0 . On the other hand, the error in numerical calculation is very big at the crack tip because of the stress-strain singularity near the crack tip. Considering the above factors and comparing the crack tip plastic zones where the equivalent plastic strain is 0.2% with different crack depths, the reasonable distance from crack front is designated as 60μm in this paper. In Fig. 8 and Fig. 9, the opening stress and normal plastic strain at a characteristic distance ( r 0 =60μm) ahead of crack front are observably small when the crack depth is 5mm. The difference between yield stresses of interface materials lead to the jump of opening stress along crack fronts in Fig. 8 could be explained that for 7.5 and 10 mm cracks the crack passes Alloy 182 buttering, where contains much higher yield stress. The opening stress and normal plastic strain for 7.5 and 10mm crack make little difference. These results suggest that the normal plastic strain ahead of deeper crack front has a smaller gradient. The stress and strain are symmetrically distributed ahead of crack front when the crack depth is 5mm. For the partial crack in Alloy 182 buttering, the opening stress ahead of the crack front ( a =7.5mm and θ =150º) increases sharply, whereas the strain decreases. As the crack grows, the opening stress ahead of the crack front ( a =10mm and θ =120º) becomes larger. The effect of crack depth on the stress ahead of crack fronts is more dramatic than it on the strain. At the interfacial region between Alloy 182 buttering and Alloy 182 weld, the stresses ahead of crack fronts suddenly increase while the strains decrease due to different material yield stresses of the two sides. Under the same loading condition, the equivalent stress intensity factor ahead of crack front increases with crack length. The increment of stress ahead of crack front, which increases with the applied loads, is connected with the yield stresses of interface materials. 0 30 60 90 120 150 180 300 400 500 600 700 800 a=5mm a=7.5mm a=10mm Crack angle,  ( o ) Opening stress,  zz (MPa) 0 30 60 90 120 150 180 0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 a=5mm a=7.5mm a=10mm Crack angle,  ( o ) Normal plastic strain,   p Figure 8 : Opening stress ahead of crack fronts ( r 0 =60μm). Figure 9 : Normal plastic strain ahead of crack fronts ( r 0 =60μm). Cracks are mainly located in the high residual stress region when the crack depth varies from 5mm to 10mm. Thus, residual stress has a great influence on stress and strain ahead of crack front. The local stress-strain field of shallow surface crack is dominated by RS, not the welded mechanical heterogeneity. The effect of residual stress on crack local stress and strain will become smaller with deeper crack. Normal plastic strain rates and J-integrals Normal plastic strain rates and J -integrals ahead of the crack fronts can be seen in Fig. 10 and Fig. 11, respectively. Located in the middle of a homogeneous material Alloy 182 weld, the stress, plastic strain, plastic strain rate and J -integral ahead of crack front are minimal and of the same trend when the crack length is 5mm. The normal plastic strain rate is higher at the deepest point than each end of the crack front. Correspondingly, the growth rate at the deepest point of the