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K. Nowak, Frattura ed Integrità Strutturale, 34 (2015) 34 (2015) 507-514; DOI: 10.3221/IGF-ESIS.34.56 512 P LATE WITH MULTIPLE INITIAL CRACKS he multiple crack configuration consists of two edge cracks and two central ones (see Fig. 2d). The numerical analysis is performed for values of H parameter from 0 to 25mm. The crack paths for all examined configurations are shown in Fig. 8. It can be noticed that for small values of distance H , the crack remains straight until the central and edge cracks meet together, and kink to join. For larger values of H , the cracks are bypassing each other, but damage field develops in area between cracks, causing them to join together. In some situations a new crack arises, transverse to main ones. For large values of H the central crack keeps developing in its main direction, as the edge crack stops its straightforward growth and becomes the transversal crack. The area behind the central crack is not stretched and value of damage parameter is close to 0, all the time. The interactions of both cracks is always visible in all examined configurations (also for time t 1 ). So the distance H of not interacting cracks should be greater than 25 mm. This is in contradiction to commonly used ‘rule of thumb’ in elastic conditions, that for distance greater than crack length (18 mm) the interaction of cracks is negligible. 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0 5 10 15 20 25 H [mm] l crit [m] edge central multiple - central multiple - edge Figure 7 . Critical length of cracks. Figure 8 . Crack paths for multiple cracks configurations. Red are initial cracks, dark grey - crack at time to failure, light grey - crack opening area. Comparison of times to failure (Fig. 5) for cases 2c and 2d shows that the edge crack in all configurations shortens significantly the specimen life (except the H =1 and 2 mm cases, but here another failure mechanism determines the time t 2 for central crack configuration). The reduction in time is close to 60%. It is much larger than reduction of time t 1 which is up to 20% (Fig. 6). It can be concluded that the evolution of damage field in time multiplies the effect of cracks interaction. T