numero25

B. Moreno et alii, Frattura ed Integrità Strutturale, 25 (2013) 145-152 ; DOI: 10.3221/IGF-ESIS.25.21 147 approximately in a linear way as the size of the hole is reduced, up to a size of approximately 0.15 mm. By reducing further the size of the hole below 0.15 mm no change in the fatigue life is observed. A similar behaviour was previously found on other steels [2]. Hence holes of size 0.15 mm and smaller would not be expected to be detrimental to fatigue life. This is also supported by the fact that one of the samples with 0.15 mm hole did not fail at the hole and instead it failed from a crack nucleated elsewhere. Accordingly, drilling a hole of 0.15 mm diameter was chosen to force the crack to nucleate at a particular spot without affecting the fatigue properties of the material. 0 10000 20000 30000 40000 50000 0 0.5 1 1.5 Hole diameter (mm) Fatigue life (cycles) Figure 2 : Fatigue lives for the different sizes of holes drilled investigated. D IGITAL IMAGE CORRELATION TECHNIQUE igital Image Correlation (DIC) allows experimental measurement of displacement and strain fields in mechanical components. Important progress in the last decade in the field of digital photography gave rise to a more robust and accurate technique, in particular in the fields of fatigue and fracture [3, 4]. DIC equipment is relative simple and consists of a digital camera, a computer and specific software to process the images. The technique works by comparing digital images with different strain levels. Each image is subdivided in interrogation windows or subsets, so that the change in position of each of these interrogation windows is identified by comparison with a reference image. The displacement of each interrogation window is determined by minimising a cross-correlation function. Accurate results can be achieved by imaging non-uniform surfaces, so that each interrogation window can be identified uniquely in the deformed image. This non-uniform finish is normally obtained by slightly spray painting the surface with black and white [3-6], although other alternatives have also been developed [7]. In this work DIC technique has been employed to measure experimentally displacement and strain fields in cylindrical hollow specimens such as those described in the previous section, subjected to tension-torsion loads. DIC commercial package Vic-2D was employed for data processing. Images were captured with a long-distance microscope (Questar QM- 100) and a 5 Mega-pixel digital camera with 8 bit dynamic range. Due to the high level of magnification achieved with the microscope, the speckle pattern has to be extremely fine. This cannot be achieved by conventional spray-painting. It is thus the aim of this work to develop a technique capable of producing a pattern with ultra-fine speckles. The possibility of electro-hydrodynamic atomisation by means of electro-spray technique will be studied for this purpose. First, the methodology for generating the micro-speckle pattern is presented. The validity of the methodology is then studied by comparison with standard extensometer measurements. Finally the new procedure is used to characterise crack-tip displacement and strain fields under biaxial conditions (tension and/or torsion). M ICRO - SPECKLE GENERATION BY MEANS OF ELECTRO - SPRAY TECHNIQUE lectro-spray technique consists of extruding a liquid through a needle subjected to an electric field. By introducing solid particles in the liquid, the very fine jet coming out of the needle produces very small drops which get attached to the surface. Once the liquid is evaporated, the drops will give rise to micro or nano- speckles [8]. The most common electro-spray setup consists of a capillary with sharp ending through which a pumped liquid flows. A high voltage is then applied between the sharp edge of the capillary and a collector electrode. The collector electrode can have either zero voltage or the negative voltage of the capillary (see Fig. 3 LHS). D E