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Digital image correlation (DIC) is a common software tool to measure displacement and strain fields on the surface of a specimen during mechanical testing. Due to its low requirements on the experimental setup and the simple usability it is widely spread in the community of applied mechanics. Nevertheless, the real noise of the method often stays uninvestigated. Therefore, this work shows different approaches to measure errors in DIC. The present contribution proves lighting as the main error source and it shows that a series of static images allows to estimate the error level of the DIC before performing the experiment. To give a proof of concept of the improved DIC setup, three‐point bending tests on single struts of aluminum foam are performed and evaluated.

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and the Young's modulus E. As the Poisson's ratio approaches its theoretical limit for an isotropic material of À1, the shear modulus goes to infinity. Other macroscopic effects of the negative Poisson's ratio are an increased thermal shock resistance, [3] a higher indentation resistance, [4][5][6] and a higher fracture toughness. [7,8] Due to that, auxetic structures offer higher energy dissipation [9,10] and a higher impact resistance, [4][5][6] which qualifies them as a suitable material for crash absorber elements in cars or as a core material for ballistic and blast protection. [11,12] Further applications of auxetic materials include press-fit fasteners, which take advantage of the contraction under compression to get the fastener inside. When trying to pull it out, the fastener resists by expanding due to its auxetic behavior. [13] Rivets can also be designed using the same mechanism. With the same concept, it would be possible to enhance biomedical components such as stands or arterial prostheses. [14] However, the first artificially created material with a negative Poisson's ratio was a converted polyurethane (PU) foam manufactured in 1987 by Lakes. [5] He converted a conventional opencell PU foam through triaxial compression and heating. This also marks the starting point for the investigation of tailored auxetic materials and as then many auxetic materials have been produced in the main material classes such as polymers, metals, ceramics, and composites.Auxetic behavior in general is easiest explained in the 2D case. In 1985, Almgren [15] described the re-entrant honeycomb (Figure 1a) as a possible microstructure which displays that property.Another geometry which exhibits auxetic behavior is based on the method of rotating squares [16] (e.g., Figure 1b). The squares are connected at the edges and can rotate freely. Under tension in one direction, this structure will expand in the perpendicular direction. It is also possible to connect rectangles, triangles, parallelograms, and so on, to get the same behavior. With these two mechanisms, the negative Poisson's ratio of many materials can be explained. For example in α-cristobalite, the rigid SiO 4 tetrahedrals are

This paper deals with experimental investigation into a strain‐rate dependent function of Poisson's ratio of three auxetic structures subjected to compressive loading. The missing rib, the 2D re‐entrant honeycomb, and the 3D re‐entrant honeycomb lattices printed using selective laser sintering from powdered SS316L austenitic steel are investigated. The samples are subjected to uni‐axial compression under quasi‐static conditions and dynamic conditions using the Split Hopkinson Pressure Bar (SHPB). The deforming specimens are optically observed in order to apply a digital image correlation for evaluation of the in‐plane displacement and strain fields. From the calculated strain fields, the function of Poisson's ratio is calculated for each experiment using different methods taking specific regions of interest of the specimen microstructures into account. The obtained functions of Poisson's ratio are plotted for each microstructure and strain‐rate. The analysis of the results shows that the strain‐rate has a significant influence on the deformation characteristics of all the investigated microstructures yielding differences in the magnitude of the minima of Poisson's ratio and the differences in the maximum overall compressive strain, where the lattices are still auxetic.