Issue34

K. Nowak, Frattura ed Integrità Strutturale, 34 (2015) 507-514; DOI: 10.3221/IGF-ESIS.34.56 511 Single central crack The next structure analysed is that with a single central crack (parameter H =0 in Fig. 2c). The time of first damage occurrence and ultimate time to failure are: t 1c =94.1 h and t 2c =715 h. The differences to the previous case are very small - 5% and 10% respectively. The critical length of the crack is 67mm, so it is also very close to critical length of edge crack. P LATE WITH TWO CENTRAL CRACKS he structure with two central cracks (Fig. 2c) exhibits the presence of shielding effect. In elastic regime for infinite plate the SIF is going to SIF of single crack when H is going to infinity (c.f. e.g. [9, 16]); for smaller value of H the structure becomes more safe due to shielding. The analysed values of parameter H are from 1 to 25mm with step 1mm for H <5mm, and 5mm - for larger H . The cracks in first two cases ( H =1mm and H =2mm) are joining together producing single central crack (see Fig. 4). The structure becomes unstable due to cutting off the central part. Obtained times to failure are of the order of time to failure of single central crack t 2c (see Fig. 5). The intermediate cases ( H from 3 to 15mm) produce straight cracks and greater times to failure (about 800h). For the larger distance ( H >15mm) the times to failure are smaller and do not differ much from the time to failure of single central crack t 2c . So the shielding effect is the most pronounced for H in its medium range. For larger distance the long term behaviour is similar to behaviour of single crack so crack interaction can be dealt as negligible. Slightly different situation is for time of first failure occurrence t 1 (see Fig. 6). Except the case H =0mm the time t 1 monotonically decreases with growing H . This situation corresponds to growing SIF in elastic regime as the time t 1 depends partly on elastic stress distribution in contrast to the time t 2 . The critical length of crack, defined as the length of crack with rate growth going to infinity, is almost constant and is equal to about 67mm (see Fig. 7). Figure 4 : Damage distribution close to final failure for the quarter central part of the plate for central crack H =1mm. 0 200 400 600 800 1000 1200 0 5 10 15 20 25 H [mm] t2 [h] edge central multiple 0 20 40 60 80 100 120 0 5 10 15 20 25 H [mm] t1 [h] edge central multiple Figure 5 : Time to failure. Figure 6 . Time to first failure occurrence. T