PSI - Issue 52

604 Ruben I. Erives et al. / Procedia Structural Integrity 52 (2024) 600–610 Ruben Erives et al./ Structural Integrity Procedia 00 (2019) 000–000 5 experiments can show compliance at the very beginning of the tests. Then, the linear fit of the region between 10% and 80% of 0 needs to be extrapolated. The deviation from linearity is simply obtained by subtracting the measured end-opening at crack onset from the extrapolated linear opening as, ∆ℓ = 0 − ℓ . (4) A schematic of this process is shown in Fig. 2b.

a) b) Fig. 2. Schematic of determination of a) initiation time, 0 , and b) the deviation from linearity, ∆ℓ

4. Results 4.1. Determination of onset time

The onset time of delamination was measured for each of the tested specimens by identifying the transition from slow to fast acoustic emission (the knee mentioned in the previous section) from the cumulative plots, as well as the derivative of the cumulative plot to visualize the transition from a slow slope to a large one. Fig. 3 shows both methods (respectively) using a specimen tested under (near) pure normal opening displacements.