Issue 28

D. Gentile et alii, Frattura ed Integrità Strutturale, 28 (2014) 42-50; DOI: 10.3221/IGF-ESIS.28.05 44 profile. Results showed that the COD profile is not affected by the shape of the initial blunting since the initial crack opening, due to the cut, scales the overall opening displacement [10]. Each pipe was loaded under four point bending. Tests were performed with Instron 300kN electromechanical testing machine, and theCODwasmeasured for two applied load values: 500N and 1500N. A pre-loadof 40Nwas used in order to compensate loading fixture compliance, Fig. 1. Themeasure of the COD distribution along the crack lengthwas performed bymeans of digital image correlation (DIC). The system used is ARAMIS byGOM, which ensures an accuracy on strainmeasurements of 0.01%. In order to proceed withDIC, the pipe surface is initially sprayed with white coating and successively sprayed with black coating to produce the reference speckle surface. Two calibrated cameras were used in order to account for the 3D nature of the measurement. InFig. 2 an example of the acquired and processed images fromDIC is shown. For each flawed pipe configuration, the COD profile was measured initially for the in-axis (symmetric) position with respect to the bending moment. Successively, the measure was repeated, at the same prescribed load, rotating the pipe around its longitudinal axis by anoff-axis angle. Figure 1 : Four point bend test set-up. a) b) c) Figure 2 : Example of processedDIC images: a) in green the analysis area red dots are the virtual clip gauge ends; b) in blu the crack and green squares shows the grid for strainmeasurements; c) the resulting strainmap. The crack opening displacement was measured defining a series of virtual clip gauges applied on the computational grid generate by Aramis software. Since the COD profile is a 3D curve in the coordinate space, two cameras were used to determine the coordinates of the physical points, which are mapped onto the image pixels, to obtain deformation and strain comparing digital images at different times.