Issue 41

J.M. Vasco-Olmo et alii, Frattura ed Integrità Strutturale, 41 (2017) 166-174; DOI: 10.3221/IGF-ESIS.41.23 170 The two faces of the specimen were prepared for experimental observations in two different ways. The surface to be used for digital image correlation (DIC) was treated by spraying a black speckle with an airbrush over a white background, while the other face was polished to assist in tracking the crack tip with a zoom lens. Fatigue test was conducted on an ElectroPuls E3000 electric machine (Fig. 1b) at a frequency of 10 Hz. A CCD camera fitted with a macro-zoom lens (MLH-10X EO) to increase the spatial resolution at the region around the crack tip, was placed perpendicular to each face of the specimen. During fatigue testing, the cycling was periodically paused to allow acquisition of a sequence of images at uniform increments through a complete loading and unloading cycle. The CCD camera viewing the speckled face of the specimen was set up so that the field of view was 17.3 x 13 mm (resolution of 13.5 μm/pix) with the crack path located at the centre of the image. Illumination of the surface was provided by a fibre optic ring placed around the zoom lens (shown in Fig. 1b). E XPERIMENTAL METHODOLOGY n this section, the two methodologies developed to evaluate the plastic zone size and shape are described. The first methodology is a direct method in which the plastic zone is estimated from the displacement fields determined by experiment, while the second one is an indirect method in which data from experiments is employed in the models to determine the plastic zone size and shape. Direct method for estimating the plastic zone This method is based on the application of a yield criterion and consists in identifying the plastic stress field by differentiating the experimentally measured displacement fields. In this work, 2D-DIC has been employed to make experimental displacement measurements. Then, the procedure for implementing this method is described. The first step in the methodology consists in obtaining the horizontal and vertical displacement fields around the crack tip. In Fig. 2 typical examples of displacement maps are shown for a crack length of 9.40 mm and a load level of 750 N. The developed process will be illustrated using these displacement maps. Figure 2 : Horizontal (a) and vertical (b) displacement fields measured with DIC for a crack length of 9.40 mm and a load level of 750 N, and data point collection employed for the calculation of stress intensity factors. The next step in the process involves determining the strain fields at the crack tip by differentiation of the displacement fields. For this purpose, the Green-Lagrange strain tensor [13] is employed because it considers the second order terms and is more accurate than those using only first order terms. Thus, this strain tensor is given by the following expressions: (10) I                                                                                                        y v x v y u x u x v y v x u y u y v y u y v x u x v y u y v x u xy yy xx 0 0 0 0 2 1   