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F.A. Díaz et alii, Frattura ed Integrità Strutturale, 25 (2013) 109-116; DOI: 10.3221/IGF-ESIS.25.16 114 C ALCULATION OF THE OPENING LOAD FROM THE ANALYSIS OF THE COMPLIANCE TRACES he hardware employed consisted of a PC (Viglen-Pentium 200 MHz Intel processor) provided with an enhanced multifunction I/O PCI board (National Instruments model PCI 6052E) connected to a BNC accessory (National Instrument model BNC-2090). The system was controlled using Lab View software version 6.0 and made it possible to store load and strain data corresponding to a loading-unloading cycle as ASCII files. Since data were affected by random noise, it was necessary to employ a noise reduction technique. The technique adopted was an incremental polynomial method similar to the recommended ASTM method for data reduction in fatigue tests [11]. Subsequently, for the calculation of the opening/closing loads the strain offset technique was adopted [12]. Initially, data was divided into two sets according to the loading and the unloading branches and plotted as load versus strain plots. The unloading branch was first employed to determine the specimen compliance in the absence of closure by fitting a first order polynomial to its upper part. After that, the fitted first order polynomial was employed to perform the strain-offset calculation, which consisted of subtracting the actual strain data (affected by closure) from the fitted equation (not affected by closure). Subsequently, the load against strain-offset was plotted to estimate the closing load. The closing load was then calculated as the points where data in the plot last crossed the zero strain offset (Figure 6.A). Figure 6 : Illustration showing load vs. strain and load vs. strain-offset plots for a 42 mm crack in an aluminium CT specimen loaded between 0.94 and 0.04 kN (R-ratio 0.04). a) Unloading branch. b) Loading branch. For the loading branch an identical methodology was followed. However, in this case the strain-offset calculation was performed using the same fitting first order polynomial obtained for the unloading branch. Finally, the opening load was calculated following the same criteria as previously adopted for the unloading branch (Figure 6.B) . Results corresponding to the opening and closing loads estimated from data at the different locations (horizontal, 45˚ and back face) are presented in figures 7.B, 7.B and 7.C. Figure 7 : A) Illustration showing the variation of the opening and closing loads with the R-ratio for a 42 mm crack using a horizontal strain gauge, B) a 45˚ strain gauge, C) a back face strain gauge. T a) b)