Issue 32

N. Bisht et alii, Frattura ed Integrità Strutturale, 32 (2015) 1-12; DOI: 10.3221/IGF-ESIS.32.01 8 of crack offset distance H. There is an amplification effect as far as mode II SIF is concerned which also becomes insignificant for remotely placed cracks. This indicates that when two edge cracks are extremely close to each other (H ≤ 0.5 mm), shielding effect occurs and the normal stress components σ xx and σ yy reduces causing the shear component of the stress to increase. As a result, mode I SIF decreases and mode II SIF increases. Now, as the crack offset distance H increases from H=0.5 mm to higher value, mode I SIF increases and mode II SIF decreases. It is found that for about H ≥ 20 mm, the crack interaction becomes negligible and both cracks behave as a single crack. The variation of normalised K II /K I with H is shown in Fig. 9. This figure reveals that when the cracks are very close to each other mode II SIF plays a significant role in crack growth and contributes about 27% of mode I SIF. Beyond H= 20 mm, the contribution of mode II SIF remains below 2% only. These results indicate that special care should be taken while predicting growth from stress intensity factor, particularly when two or more edge cracks are very close to each other. Figure 9 : Variation of K II /K I with crack offset distance H. Von-Mises Stresses Distribution of von-Mises stresses around the crack tip for various values of H at uniform stress σ = 1 MPa are shown in Fig. 10-11. von-Mises stresses are computed taking midpoint or element stresses. Identical boundary conditions, crack tip element size and full mesh model have been used for all crack configurations. The distribution of equivalent von Mises stresses for two edge cracks configuration for H = 2 mm and H = 20 mm have been shown in Fig.10 and 11. From Fig. 10 and 11 it can be observed that with increasing crack offset distance H the equivalent von Mises stresses increases. This indicates that crack offset distance have greater affect on the state of stress ahead of the crack tip. Analysis of the State of Stress around the crack tip The two dimensional finite element model presented in this work has been used to investigate the state of stress around the crack tip in plane strain condition. The state of stress at a radial distance r from the crack tip is schematically shown in Fig. 12. The results obtained for different crack configurations are presented in Fig. 13 to 15. The stresses computed for double edge cracks are normalized with σ y0 which is the FE solution for single edge crack corresponding to θ=0 0 and r=0.5 mm along the crack plane. The variation of normalised σ xx , σ yy and τ xy (normalised by σ yy for single edge crack= 731.57 MPa) for two edge cracks on the same side of a rectangular plate and separated by offset distance H= 0, 2, 10 and 16 mm are shown in Fig. 13 – 15. The variation of these stresses around the crack tip at a radius of 0.5 mm from the crack tip is shown for the assumed plane strain condition. Identical loading, boundary conditions and mesh arrangements were used for all crack offset conditions. The stresses are taken at the midpoint of each crack tip element. It has been observed that symmetrical