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. 10. Increase of stress intensity factor (SIF) at the centre of the crack tip when the link is loaded in tension for fracture. The effect of plastic yielding at the crack tip can be observed only when the load is higher than 12000kN.

At the point of fracture initiation (beyond the fatigue pre-crack), the total elastic K, which in this case is considered to be the fracture toughness (expressed in terms of K 0.2 ) is the sum of stress intensity factor due to applied load, and stress intensity factor due to the residual stress. The following equation can be established at fracture initiation:

P K K K   0.2

RS

(2)

where RS K is the stress intensity factor due to the residual stresses. The average of the two available fracture toughness values from small scale tests K 0.2 = 2372N/mm 1.5 was used (see section 2). The through-thickness residual stresses at the flaw location were measured using the deep-hole drilling (DHD) technique on a similar chain link from the same batch as the chain link examined here. Only the relevant results of the DHD measurement method are given in this paper. Fig. 11 shows the measurement location and Fig. 12 presents the through-thickness residual stresses measured at the uncracked leg region opposite to the weld. Comparison between the measurements in this figure and the predicted residual stress profile in Fig. 7 shows that the residual stress profile in the region of the crack is well predicted. However, the measured profile is symmetrical about the centre of the cross section, whereas the predicted residual stress shows three different slopes. This difference is due to the residual stress in the link generated by quenching, prior to proof loading, not being taken into account. Martinez-Perez et al. (2017) demonstrated that in large chain links, the residual stress profile is dominated by the effect of quenching, rather than proof loading, as seen in smaller links. The axial stresses measured from the intrados, were used to calculate the Mode I stress intensity factor using a weight function from Annex Q in BS 7910. The residual stress profile in Fig. 13 was generated by using the best-fit polynomial equation below: P K is the stress intensity factor due to the applied load and