PSI - Issue 37

M. Ajmal et al. / Procedia Structural Integrity 37 (2022) 964–976 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

974

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Table.1: ∆CTODp and da/dN values calculated at different stages of the crack propagation

Measurement Stages

N(accumulated) cycles

∆ CTODp (µm)

da/dN (µm/cycle)

70 , 105 90 , 105 150 , 105 160 , 105

8.63311 ×10 -7 1.20267 ×10 -6 1.70336 ×10 -6 3.13355 ×10 -6

1.16295 ×10 -7 2.34231 ×10 -7 5.38571 ×10 -7

1 2 3 4

1 . 45 ×10 -6

Fig.13: Plot of da/dN vs ∆CTODp for the data in Table.1.

6. Conclusion This work presents the development of an experimental tool to extract parameters from CTOD versus load plots, obtained experimentally using DIC. The characteristic points of the CTOD versus load curve are those corresponding to minimum loads, crack opening and closure loads, and transition from elastic to elastoplastic regimes. From these points different interesting parameters were drawn out, namely the crack opening and closure levels, the elastic load ranges, the elastic CTOD ranges, ΔCTOD e,L and ΔCTOD e,U , the loading and unloading plastic CTOD ranges (ΔCTOD p,L and ΔCTOD p,U , respectively). Some interesting relationships between three important parameters of fatigue process have been observed from Fig.8 to all up to Fig.13 such as: • FCGR is proportiona l to ∆CTODp which is assumed to be a measure of plastic deformation at the crack -tip. • There is inverse relation between FCGR and crack closure. However, there were some important points which were addressed in the program when the parameters need to be extracted from the real data. • The crack opening load is sometimes not very clear as the data is not very smooth. To handle this issue, it is proposed that if the four consecutive values of CTOD are increasing just after zero, then the first of the four can be considered as point 2 in Figure 5(b). • If the data points for CTOD extracted from DIC are low in number then the data for more than one cycle should be considered by merging /overlapping to have better results. • Sometimes CTOD max does not correspond to Load max values in CTOD vs Load cycle, so the selection loading and unloading parts of the cycle is done manually. The da/dN- ∆ CTOD p relation can used to predict fatigue crack propagation as long as the basic assumption that the plastic deformation at the crack tip is quantified by ∆ CTOD p , remains valid. Fig.13 shows that the range of plastic