Fatigue Strength Analysis of Structures Made of Different Auxetic Geometries

Abstract

Auxetic geometries exhibit a strange behavior of increasing their transverse dimension, when given a displacement along their vertical dimension. This special behavior is explained with a negative valued Poisson’s ratio. Mainly for their incredible energy absorption and lower densities, auxetics structures can be applied on engineering problems that arise in defense, medicine and sports. As fatigue strength is a crucial concept when engineering applications are considered, the less amounts of research in the literature on fatigue strength of auxetic structures can be taken the main reason for this study to be carried on. The aim of this study is to analyze the fatigue strength of structures made from different auxetic geometries with different cross-sectional areas. These geometries are re-entrant hexagonal, arrowhead and octagonal type auxetic structures, and the choice was based on their frequency that they have been studied in the literature. Because the standard specimen design and dimensions for fatigue testing of auxetic structures have not been standardized yet, the test specimens were custom designed in SolidWorks software. Following that, the specimens were given a cyclic tension load with a 0,2 mm amplitude in Ansys engineering simulation software. The material assigned was left custom (structural steel) for all three different auxetic structure types. So, the life, and safety factor was analyzed and compared between the three types of auxetic structures in terms of percentages. Arrowhead type auxetic structure was taken reference for comparisons as it was the structure to show the highest fatigue strength, withstanding 2090 cycle of loading until failure. When compared, octagonal structure had 25,6% shorter, and re-entrant hexagonal had 95% shorter lifetime in terms of cycle amount until failure. Comparing the safety factors with arrowhead structure being the reference again, octagonal structure had 5,56 % lower, and re-entrant hexagonal had 36,62% lower safety factors. In conclusion, Arrowhead and octagonal structures showed similar fatigue strength and the difference is coherent with their different cross-sectional areas. However re-entrant structure showed poor fatigue strength, much below its counterparts. As it can be inferred from contour and probe tools of Ansys software analysis results, this poor fatigue strength of re-entrant structure is due to its design with many sharp edges. Sharp edges created stress concentrations, thus leading to a sooner failure. To further validate these findings, experimental research can be conducted, and the results can be discussed in future studies.