Research on effect of auxetic structure parameters on the mechanical performance of sandwich materials

In recent years, the high engineering properties required for shock absorbers have led to increased expectations from auxetic materials. The increasing development of additive manufacturing methods and design tools has also increased the interest in auxetic materials. Auxetic materials have achieved significant success in improving the crash performance of shock absorbers. Studies show that auxetic materials provide better crash performance than conventional materials under equal mass conditions. The use of autectic materials in tubular structure contributes significantly to the improvement of crash performance. There are many studies investigating the crash performance of autectic materials using different cell geometries in tubular structure. However, in these studies, tubular autectic structures are considered as thin-walled and there are very limited studies on the crash performance of thick-walled auxetic structures. In this thesis, the effect of cell geometry, core thickness and shell thickness parameters on the crash performance of thick-walled auxetic sandwich tube is investigated. The sandwich tube was obtained by adding a shell to the inner and outer surfaces of a auxetic tube. Crash performance was investigated experimentally with a drop weight impact test, and Specific Energy Absorption and Initial Peak Force were considered as indicators. For the drop weight impact tests, 3D printed test specimens were used. Material verification was carried out by applying tensile tests to 3D printed specimens of the same material as the drop weight impact test specimens. The Poisson ratio of the sandwich tubes was determined experimentally by the Digital Image Correlation method. Poisson ratio was determined based on the deformation of the tubular materials produced by 3D printer under compressive stress conditions. The levels of the parameters whose effect on the crash performance will be investigated were determined as Double arrow-head, re-entrant circular and silicomb for cell geometry, 16, 20 and 24 mm for core thickness, and 1.2, 1.6 and 2.0 mm for shell thickness. Taguchi experimental design method was used to investigate the effect of sandwich tube parameters on crash performance. According to Taguchi L9 design, a drop weight impact test was applied to 9 3D printed test specimens. From the force-displacement curves obtained as a result of the test, both Specific Energy Absorption and Initial Peak Force were obtained as indicators of crash performance. Both crash performance indicators were evaluated separately by Taguchi method. For the cell geometry and core thickness parameters, the silicomb and 16 mm levels provide the best crash performance indicators, respectively, overlapping for both indicators. On the other hand, 1.2 mm shell thickness provides the best crash performance for specific energy absorption, while 1.6 mm provides the best crash performance for initial peak force. However, when the sinal-to-noise ratio values are compared, it is clearly seen that the shell thickness level that provides the best crash performance for both indicators is 1.6 mm. In addition, ANOVA analysis was applied to investigate the effect levels of sandwich tube parameters on crash performance. For specific energy absorption, the effect ratios of cell geometry, shell thickness and core thickness are 61.62, 10.98 and 20.83, respectively. For Initial Peak Force, the effect ratios of cell geometry, shell thickness and core thickness are 52.33, 38.37 and 8.24, respectively.