Issue 41

J.A.O. González et alii, Frattura ed Integrità Strutturale, 41 (2017) 227-235; DOI: 10.3221/IGF-ESIS.41.31 230 the near and the far-field gages, as well as by the DIC analyses, showed no significant discrepancy, meaning that practically the same value was obtained from all of them, see Figs. 6 and 7. Fig. 8 shows the crack faces of a thick specimen with homologous or quasi-parallel crack fronts, which indicate a constant driving force along them. Figure 3 : Gage strips and back-face strain gages bonded on a thin and on a thick DC(T) specimens. Figure 4 : Experimental setup used to measure the strain fields on the specimen surface with the DIC system, to redundantly measure the crack opening loads K op . The thin t  2mm < pz max  (1/  )(K max /S Y ) 2  (1/  )[20/(0.9  262)] 2  2.3mm DC(T) specimens were loaded under quasi- constant {  K  20MPa  m , R  0.1 } conditions, to grow fatigue cracks under nominally pl-  conditions (using Irwin’s pz estimate, assuming it can define pl-  states in FCG as well). The cracks grew in the thicker specimens in a nominally pl-  state under identical {  K  20MPa  m , R  0.1 } loads (their t  30mm > 2.5  (K max /S Y ) 2  2.5  [20/(0.9  262)] 2  17.9 mm ). FCG rates da/dN and crack opening ratios K op /K max measured along the crack path are shown in Figs. 9 and 10, where the crack size is quantified by a/w , the ratio between the crack length a and the original ligament size w , measured from the load line. Three thin and three thick specimens were tested, two of each (called old tests in Figs. 9 and 10) using only strain gages to measure K op . The differential in such simple tests was the careful K op measurements, made using a piece of software written in LabVIEW to apply the linearity subtractor procedures on the redundantly measured signals [11]. The data acquisition was performed using National Instruments NI 9215, NI 9235, and cDAQ-9172 instruments [12]. To apply the DIC technique, the specimen surfaces opposed to the strain gage strip were first covered with a coat of white paint, over which small black dots were uniformly sprayed, see Fig. 4. The commercial DIC system from Correlated Solutions used for these measurements include two 5-MP Point Grey GRAS-50S5M CCD cameras with two Tamron SP AF180mm F/3.5 lenses, an adjustable double fiber-optic light source, calibration grids, a suitable data acquisition system, and the software package VIC-3D [13]. The digital cameras were mounted on an adjustable tripod in front of the specimen. Before starting the DIC tests, an accurate stereo calibration of the system is needed, and it was performed using standard precision calibration grids. About 25 image pairs of a grid with 9×9 dots and dot spacing of 0.89 mm were acquired during this calibration procedure.