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B. Moreno et alii, Frattura ed Integrità Strutturale, 25 (2013) 145-152; DOI: 10.3221/IGF-ESIS.25.21 148 Figure 3 : (LHS) Schematic of the setup used for electro-spray technique. (RHS) Speckle pattern produced by electro-spray. The diameter of the centre hole is 0.5 mm. The free charges of the liquid transform the shape of the liquid located at the ending of the capillary from spherical shape to conical shape. The thereby created cone is called Taylor cone [8]. The high charge of the drops induces repulsion forces between them. The smaller drops appear in the outer area of the cone and the heavier and larger drops tend to fall in the central axis of the spray [9]. There are two main parameters governing the electro-spray process: (i) the electric current flowing through the cone and (ii) the size of the spray drops. The diameter of the drops ranges between dozens of nanometres to hundreds of microns. Both parameters can be controlled by the flow rate and the properties of the fluid, namely electric conductivity, surface tension, viscosity, density and electric permittivity [10, 11]. To a first approximation, the fluid conductivity allows controlling the order of magnitude of the drop diameter. The flow rate allows a smaller range once the conductivity is fixed. The size of the drops should be small enough so that each of the interrogation windows in which the image is divided can be indentified uniquely by the cross-correlation algorithm. Drop sizes of 3 × 3 pixels are typically recommended to obtain good results. For the magnification level employed with the current long-distance microscope, the resolution is approximately 1 µ m/pixel. Accordingly, the size of the drops should be around 3µm. Black and white inks were chosen for better contrast. The black ink of was made of dimethylformamide (DMF), Vulcan carbon with xylene and polyethylene vinyl acetate (PEVA). The addition of PEVA improved the adhesiveness of the ink to the metal surface. This adhesiveness is much required since once the speckle is applied onto the surface, the paint can be detached during the handling of the specimen. This includes the drilling of the hole, placing the specimen in the grips of the loading rig and placing the extensometer in the sample. In order to increase the conductivity of the solution, antistatic agent Stadis was also added. This was the black solution that was used as background on the metal surface. White speckles were created with a solution made of methylene chloride and methanol with titanium oxide (TiO 2 ). The solvent was chosen so that flow rates between 0.04 and 2 ml/hour and voltage of 60 kV can be used. The flow rate and voltage values were given by the pump and the voltage source respectively. An example of the speckle pattern generated is shown in Fig. 3 RHS. S TRAIN RESULTS IN UN - CRACKED SPECIMENS n order to validate the experimental setup described in the previous sections, DIC strain data were compared with strain information measured with a biaxial extensometer. Cylindrical shaped hollow un-cracked specimens were used. These were subjected firstly to uniaxial tension-compression cyclic load and secondly to biaxial tension- compression-torsion in-phase cyclic load. Tests were made with load and torque control mode on the loading rig at a 0.2 Hz frequency. This frequency allowed the acquisition of 65 images by the digital camera, providing enough information to describe a cycle. The reference image for DIC was the one giving zero strain with the extensometer. Since no stress concentrator was introduced in these specimens, the displacements both in the axial and the angular direction were linearly proportional to the distance from the grips. Therefore, uniform distribution in axial and angular strain fields is expected in the calibrated length of the specimen. Fig. 4 LHS shows a comparison of the axial strain measured by both DIC technique and extensometer. These were obtained under tension-compression uniaxial test with loads ranging between ± 15 kN. Fig. 4 RHS shows a comparison of the angular strain measured by both DIC technique and extensometer. Tension-compression-torsion loads were applied during the test shown in Fig. 4 RHS, with axial loads being ± 15 kN and torsion torque ± 70 Nm. Similar agreement between DIC and extensometer were obtained in biaxial I