PSI - Issue 28

G. Clerc et al. / Procedia Structural Integrity 28 (2020) 1761–1767 Gaspard Clerc / Structural Integrity Procedia 00 (2019) 000–000

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1.3. Stability of the crack propagation Once the energy release rate is reached the crack will propagate. If the amount of energy released is higher than the amount needed for the crack to propagate, the crack will propagate in an unstable way. This is described in equation 10: � 0 (10) For the 3-ENF sample (left) and the 3-CCF sample (right), the following conditions for a stable crack growth are obtained: � � � √ 3 � ≅ 0.7 � 2 (11) 2. Material and Methods Beech wood (Fagus sylvatica L.) was used for manufacturing the samples and glued with an 1C-PUR adhesive (Loctite HB110, Henkel). The dimensions of the 3-CCF sample are shown in Fig. 2. Steel supports were used for the 3-CCF to prevent a compression failure of the wood under the supports. A detailed description of the 3-ENF sample manufacturing method can be found in Clerc et al (2019). The testing was performed on a universal hydraulic testing machine with a 20 kN load sensor with a load and displacement accuracy of at least 1% of the measured value.

Figure 3: Difference between the calculated and measured rupture force for the 3-CCF sample with different height

3. Results In the figure 3, the measured force needed to break a 3-CCF sample with different height is compared with the rupture force calculated according to equation 9. Only for a sample height of 50 mm did the tested and calculated force coincide. With increasing height, the rupture force increases. This should, however, not be the case as shown in equation 9, with increasing height the rupture force should decrease for constant G-value as depicted in Figure 3. It