PSI - Issue 28

Bahador Bahrami et al. / Procedia Structural Integrity 28 (2020) 829–835 Bahrami et al./ Structural Integrity Procedia 00 (2020) 000–000

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specimens. The only remained parameter in Eq. 4 is the critical distance r c . The traditional approach for determining r c is to use the equation proposed by Schmidt's as: � � 2 1 � �� � � � (6) Although the Schmidt’s equation is simple, it has been proved that it can provide good results (Ayatollahi and Ghouli (2019); Ghouli et al. (2018)). However, herein another relation that is specifically designed for the GSED criterion is proposed to calculate the value of critical distance r c . This relation is obtained by equating the value of W c in Eq. 3 with the toughness of the specimen that is the area under the stress-strain curve of the dogbone sample of the material, i.e. � � � � � ��2� , which after simplification reads: � � 2 1 �2�1 � ��� � �� � � � (7) 4. Results and discussion As explained in the experimental section, fracture tests were performed for 4 different mode mixities with 3 repetitions in each configuration. In this section, the effectiveness of the SED and the GSED criteria are compared against each other to depict the mixed-mode (I/II) fracture behavior of PMMA dental material. Fig. 2 shows the experimental results together with the predictive curves of the SED and the GSED criteria. Note that two different critical distance relations were used for the GSED criteria as can also be seen in Fig. 2.

Fig. 2. Comparison between fracture predictions of the SED and the GSED criteria. Fig. 2 shows that the conventional SED criterion cannot accurately predict the experimental fracture loads especially when the contribution of K II in the stress field around the crack tip increases. On the other hand, the GSED criterion which considers the T-stress term provides significantly better predictions either with the critical distance relation proposed in this paper (Eq. 7) or with the Schmidt’s relation. Nevertheless, it can be seen that considering the critical distance devised specifically for the GSED criterion in Eq. 7 has ameliorated the predictions.