PSI - Issue 18

Fokion Oikonomidis et al. / Procedia Structural Integrity 18 (2019) 142–162 Dr Fokion Oikonomidis/ Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 8 shows the linear variation of K at the centre of the crack with load for the three notch/crack conditions. The relationship between the K range Δ K (in MPa√m) and the load range Δ L (in kN) is: ΔK=1.5 ΔL (1)

Fig. 8. Variation of stress intensity factor at the centre of the crack with bending load, for the three different crack geometries for pre-cracking.

Data from BS 7910 can be used to estimate the number of cycles necessary to grow the crack from 15.5mm to 18mm for structural steels in air, for cases where the fatigue ratio is over or below 0.5 (see Fig. 9). Following the FEA estimates of cyclic loading, the minimum bending load of 870kN was chosen to ensure full crack opening during fatigue pre-cracking. Then, using the R>0.5 option in Fig. 9 , the value of Δ K =300N/mm 1.5 was chosen, resulting in a maximum bending load of 1070kN. The chain link was placed on the machine and the cyclic bending load was applied at a frequency of 1Hz. The estimated number of bending cycles for those loads and for achieving crack growth from 15.5mm to 18mm was about 295000 cycles (see Fig. 9). It is worth noting that this estimate was obtained assuming that the 15.5mm crack had initiated from the EDM notch. The number of cycles necessary to initiate the crack from the notch was not determined.