Investigation of the fatigue behavior of structures with negative poisson's ratio (auxetic)

Auxetic structures are the structures with a negative Poisson's ratio. This property causes the structures to expand horizontally when subjected to a vertical tensile force. Similarly, a vertical compressive force will cause the structures to contract horizontally. This behavior provides auxetic structures with properties such as superior energy absorption and good impact resistance. In this thesis, re-entrant, arrowhead and newly designed octagonal auxetic structures are compared with each other and with a conventional structure with a positive Poisson's ratio in terms of fatigue strength by conducting experiments and computer-aided analyses. As a result of fatigue tests carried out at a frequency of 2 Hz and an amplitude of 0.5 mm, the conventional structure showed an average fatigue life of 1600 cycles. The re-entrant, arrowhead and octagonal auxetic structures experimentally showed 34%, 49% and 1263% longer fatigue life compared to the traditional structure, respectively. In addition, in tensile tests, it was seen that the unfatigued re-entrant, arrowhead and octagonal auxetic structures could withstand maximum failure forces of around 1462 N, 1655 N and 773 N, respectively, when pulled until failure. Following the experimental studies, fatigue tests were simulated in the nCode DesignLife module of the ANSYS program, which is a computer-aided analysis program, and the results obtained from the simulations verified the experimental studies. According to the computer-aided analyses, the conventional structure showed a fatigue life of 1838 cycles. The re-entrant, arrowhead and octagonal auxetic structures showed 37%, 48% and 962% longer fatigue life according to the analysis results, respectively. Both experimental and computer-aided studies concluded that the octagonal auxetic structure designed within the thesis exhibited superior fatigue strength. The rounded corners of the octagonal auxetic structure prevented stress concentrations and resulted in a significantly longer fatigue life. On the other hand, since the octagonal auxetic structure does not have a unit cell pattern as dense as other auxetic structures, the maximum failure force it can withstand was measured to be lower.