Uncertainty and Resolution of Speckle Photography on Micro Samples

The efficient development of new materials with defined properties requires fast methods of testing low volumes of material,such as a high-throughput investigation of spherical metallic micro samples with varying compositions and structuring treatments.A classical material testing method for macro samples,the tensile test cannot be employed for analyzing the mechanical properties of spherical samples with diameters below 1 mm since there are currently no methods for holding and stretching them.A combination between the incremental electrohydraulic extrusion as stress actuation unit and the speckle photography as strain measuring method is presented for obtaining the required mechanical characteristics.Positive longitudinal strain is generated at stepwise extrusion through<1 mm wide forming channels using a liquid punch and the deformation is observed in situ after each forming step at the interface between the micro sample and a transparent window integrated into the forming die.The occurring local strain fields with a lateral extension down to 100 pm and high gradients require displacement measurements with high lateral resolution over a large range of local dislocations between 0.1 and>10 μm.It is unknown,whether the speckle strain measuring method is able to provide the necessary low uncertainty for the required resolution in the whole measuring range.Therefore,theoretical and experimental investigations of the deformation measurability using the speckle correlation method are presented,showing that local displacements up to 10 μm can be measured with a spatial resolution between 3 and 10μm depending on the displacement size.The dominant effect influencing the measurement uncertainty for displacements at this high spatial resolution is the speckle noise,resulting into measurement uncertainties of less than 100 μm.Hence,speckle photography is shown to be applicable for tensile test on micro samples.