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. 21. Cumulative Energy [Left] and Normalized Cumulative Counts [Right] coming from the AE sensor on the chain link for different Peak Amplitude (PA) filters.

7. Calculation of K after the test

7.1. General

The calculation of the fracture toughness value of the chain link in the environment was carried out in two stages. The first stage was breaking the chain link leg region open after the loading and examining the fracture faces. The 2 nd stage was carrying out FEA to determine the critical value of K that caused crack propagation.

7.2. Fracture faces examination

Photographs showing the post-test appearance of the fracture surface are given in Fig. 22 to Fig. 26. From these, it is apparent that the fatigue pre-crack profile is slightly uneven across the width, but that there is clear extension ahead of the fatigue pre-crack tip, prior to the region of brittle fracture created in breaking open the sample post-test. In some regions, the extension possibly appeared to consist of two bands differentiated by the differing levels of surface corrosion. This could correspond to crack extension under two different step loads (i.e. cracking started at step load 3 and continued in step load 4).

7.3. FEA calculation of critical K

Elastic analyses were performed to determine the critical K value when the pre-cracked chain link was loaded under tension. The effective geometry was observed after testing. The stress intensity factor values calculated from these fracture analyses were added to the compressive (negative) stress intensity factor value due to residual stress, estimated from the proof loading and notching model. The model creation methodology was the same as described in section 3.2. The main difference was in the geometry of the crack. The actual crack shape that was observed after the examination of the fracture surfaces was used in the model (see sketch in Fig. 27). The stress intensity factors for step loads 2 (7035kN), 3 (8109kN), and 4 (15250kN) were calculated using FEA.