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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3.4. Fatigue testing of lattice plates Six identical triangular lattice plates divided into three testing groups were used to test the fatigue life. Three lengths of edge initial crack were introduced into the plates. The crack was cut through 2, 4, and 6 cells in the plates, respectively. The maximum loads of three groups lattices was 3000N, 2700N and 2400N. Test conditions for all the triangular lattices were similar as for the single struts except that the servo-hydraulic testing machine (Instron 8872) used a maximum load capacity 10 kN sensor. The ends of the plate were left solid with some holes so that the sheet could be attached to the test machine grips using pins. The experimental setup for triangular lattices is shown in Fig. 8.

Fig. 8. Experimental setup for uniaxial tension-tension fatigue testing of a large 2D triangular lattice plate.

The test machine was stopped when a strut near crack tip broke to record the cycle number, then restarted. The whole fatigue life of plate is the sum of fatigue lives of different individual struts in the crack path. Crack initiation and propagation in the first and second struts is difficult to observe in lattice plates fatigue test, and the lives of these first two struts will also occupy a large proportion of the whole plate’s fatigue life. The fatigue crack path in lattice plates had a specific pattern in all three groups, the crack developed in the direction 30° to the horizontal axis of introduced edge crack in the upper half of the lattice, which is similar to the monotonic fracture (Gu et al. 2018). Fig. 9 shows the relationship between crack length and cycles for different specimen groups under different cyclic loads. The fatigue failure of triangular lattice structures can be described by a three-stage mechanism as used by Yavari et al. (2013). In the first stage crack length propagates very slowly for a long time until the second stage. In the second stage, the crack length has a small increase after a certain number of cycles. While in the last stage, the struts break instantly that causes the crack grows very quickly, and the whole specimen will fail rapidly after a very short time. Therefore, the fatigue life is largely determined by the first and second stages. 4. Method validation and comparison A beam model was used in the Abaqus 2017 FEA suite to simulate individual struts in the lattice plate, considering elastic-plastic properties of the material. The cross-sectional force of the individual struts near the crack tip was obtained from the result. In the 2D beam model, each strut is meshed using two-dimensional, shear deformable, linear beam element B21 with 1 mm seed size to consider the transverse shear deformation of individual struts. The strut length and width in the model were 10 mm and 1.1 mm respectively, based on the measured data from Section 3.1.