PSI - Issue 28

Yifan Li et al. / Procedia Structural Integrity 28 (2020) 1148–1159 Author name / Structural Integrity Procedia 00 (2019) 000–000

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A comparison of predicted fatigue life from the proposed method and experimental results is shown in Fig. 11, very good agreement is observed. To validate the proposed method, the comparison between the experimental fatigue lives of individual struts in triangular lattice and the predictions by the proposed method is shown in Fig. 12. In this figure, the abscissa is for the measured fatigue lives obtained from the experiments and the ordinate is for the predicted fatigue lives by the proposed method. The thick solid diagonal line in Fig. 12 signifies a perfect prediction, and the two dotted lines represent the factor-of-two boundaries in triangular lattice plates. It can be seen from Fig. 12 that life prediction with the proposed method achieves a good agreement with the experimental data under cyclic loading.

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Experimental results (cycles)

Fig. 12. Comparison of individual strut fatigue lives between experiments and predictions.

5. Crack growth rate of lattice plates Paris related the stress intensity factor range (∆ K ) to the crack growth rate under a fatigue stress regime (Paris and Erdogan 1963): � � � � � �� � (7) �� � ��� � ��� (8) where C and m are constants that depend on the material, a is crack length and N is the number of fatigue cycles. The fatigue crack growth rate and SIF range were calculated according to the secant method suggested in the ASTM E647, which are shown as: � � � � � � ��� �� � � ��� �� � (9) �� � � � �� � � � ��� � (10) Although the fatigue load used is perpendicular to the pre-crack, in a lattice the fatigue crack may propagate in inclined direction, so mixed mode I/II fatigue will occur in triangular lattice plates. The effective stress intensity factor K eff is frequently used to study mixed mode fracture and fatigue (Biner 2001; Ayatollahi and Aliha 2007). ��� � � � � � � � � (11) In this work, the stress intensity factors of lattice plates are calculated by using FEA to homogenize the triangular lattice into a continuous plate with effective mechanical parameters (Gu et al. 2018). The boundary conditions are the