PSI - Issue 39

Branko Nečemer et al. / Procedia Structural Integrity 39 (2022) 34 – 40 Author name / Structural Integrity Procedia 00 (2019) 000–000

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The CT specimens presented in Fig. 1 were fabricated using the abrasive water jet cutting technology from a 3 mm thick sheet made of aluminium alloy 7075-T651. The experimental tests were performed on the pulsating testing machine Smith weld 1405. To determine the appropriate fatigue load levels, three quasi-static tests were performed previously for both auxetic CT specimens. The experimental results of the quasi-static tests are presented in Fig. 2 .

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Fig. 2. The results of the quasi-static test: (a) Re-entrant auxetic structure; (b) Rotated re-entrant auxetic structure

The results shown in Fig. 2 represent the maximum load at which the first three cell struts of the analysed auxetic specimens are broken. Based on the static curves, the LCF loading levels were selected. The fatigue tests were performed at load control at the load ratio 0.1 in tensile. In the LCF tests, five loading levels were selected and at each loading level, at least two tests were performed. To attach the specimens, the special clamping device was designed for that purpose ( Nečemer et al (2019)). The experimentally observed crack paths for the re-entrant and rotated re entrant auxetic specimens are presented in Fig. 3.

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Fig. 3. Fatigue crack path: (a) Re-entrant auxetic structure; (b) Rotated re-entrant auxetic structure

As can be seen from Fig. 3, the fatigue failure paths of the analysed auxetic specimens are different. The fracture of cell struts appears in the corner of the individual unit cell. The experimentally measured fatigue lives obtained from the LCF tests are summarised in Fig. 4, where the number of load cycles to failure represents the number of cycles to failure for the first three cell connections. From the diagram in Fig. 4, is evident that the fatigue-life curve of the re entrant and rotated re-entrant structure are very close. Therefore, it can be concluded that the orientation of the unit cell has not an influence on the fatigue life of analysed auxetic specimens but only on the crack path.