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. 9. Predicted number of fatigue cycles for growing the crack from 15.5mm to 18mm, assuming partial crack closure (R<0.5) or full crack opening (R>0.5) using fatigue crack growth rate data from BS 7910.

4. Estimation of step loads

4.1. General

The principle of step loading is to identify a threshold between no crack initiation and a load where crack initiation starts. Four step loads were selected to span a set of loads expected to show no cracking at step 1 and to show crack growth at step 4. The estimation of the loads for testing was completed in two stages. The first stage included FEA to estimate the applied K due to the externally applied load. The second stage included the calculation of K applied after taking into account the presence of residual stresses. The results from these calculations were used to estimate the step loads during the chain link test. In the analysis of the fracture test, the tensile load was assumed to be applied through cylindrical pins 200mm in diameter and modelled as rigid surfaces. Symmetry boundary conditions were applied in a similar way as for the fatigue pre-cracking model. Fig. 10 shows the variation of stress intensity factor with load obtained from the model, where the assumed initial crack was supposed to be 18mm deep. The figure shows the slight deviation from linearity due to plasticity at the crack tip when the load is above 12000kN. Below this value, the variation of K is linear, with a slope equal to 0.532MPa √ mm/kN. 4.2. FEA for critical load during testing