PSI - Issue 12

Chiara Morano et al. / Procedia Structural Integrity 12 (2018) 561–566 C. Morano et al. / Structural Integrity Procedia 00 (2018) 000–000

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3. Results and discussion

The results of the finite element analyses are reported in Fig. 3. The normalized energy release rate ( G I / G 0 ) is given as a function of the crack advance ( ∆ a), which has been normalized with respect to the channels wavelength ( λ ). The available rate of energy release rate varies periodically as a function of crack position. The energy which is

C

1

2

1

2

S

1

2

3

2 1

3

BH

1

2

3

2

3

1

Fig. 3. Normalized energy release rate obtained using the VCCT and corresponding schematic of the subsurface substrates geometry. C: circle; S: square; BH: buttonhole.

supplied to the system by the remote applied loading is either absorbed by the material around the tip or is available to extend the crack: depending on the location of the crack tip, the material near the interface will absorb or release energy. In the latter case, an extra energy, beyond that supplied remotely, is available to propagate the crack. The finite element simulations carried out on the configuration C indicate that the material around the crack tip is releasing energy whenever the crack passes under the pillar ( 1 ). In this case the energy flows at the crack tip and the crack can propagate. On the other hand, energy is absorbed when the crack is between the pillars ( 2 ) with the occurrence of a crack trapping e ff ect. As a result, there is an increase in the system compliance for cracks located between to consecutive pillars, which is quite beneficial because it enhances the crack trapping ability A ff errante and Carbone (2011). It is noted that for the geometries denoted S and BH the behavior is similar, although the sharper increase in the system compliance promotes higher fluctuations in the driving force. The obtained distribution of the energy release rate has been correlated with the global load-displacement response recorded during the DCB tests. Figure 4 shows selected portion of the load-displacement curves from the fracture tests of DCB samples of both type C and S. Also shown in the figure are a few selected snapshots of the samples taken during crack propagation. The snapshots are referred to a ROI around the crack tip and were deployed to correlate the position of the crack front with the observed load fluctuations and the energy release rate. Notice that the sample geometry named BH has not been explored experimentally herein and is the subject of follow-up work. In general, after the crack starts propagating, the load varies periodically due to the presence of the channels. In particular, the crack shows limited slow (stable) crack extension when the tip is within the compliant region. In this phase the load increases almost linearly, until the crack snaps through the entire single cell and reaches the next pillar. The snap-through process occurs in conjunction with a sudden drop of the remote applied load, from which an increased work of separation is originated. Interestingly, similar behavior was observed during cracking of layered composite materials with a crack perpendicular to the