Issue 41

J.A.O. González et alii, Frattura ed Integrità Strutturale, 41 (2017) 227-235; DOI: 10.3221/IGF-ESIS.41.31 234 that FCG is promoted by local driving forces along the crack front, so that partially closed crack tips do not necessarily pin the entire crack fronts, as assumed by many  K eff models based on simplified 2D PICC arguments. Then, fatigue crack growth rates da/dN and crack opening loads P op were redundantly measured on FCG tests under quasi-constant ∆K and R conditions, in thin and thick DC(T) specimens, to simulate plane-stress and plane-strain FCG conditions. The opening loads were measured by Elber’s compliance techniques, using Paris and Hermann’s linearity subtractor technique to enhance the P op identification. Initially, two strain-gages, one bonded on the back face and the other bonded ahead of the crack along the residual ligament of the specimens, were used to identify P op by far and by near-field measurements. However, to avoid any doubts about the P op measurements’ quality, a third independent DIC-based technique was used in a new set of tests. The P op values obtained by these three redundant methods showed no discrepancy, confirming the reliability and repeatability of the data obtained in the measurements. Since the ∆K eff measured along those tests augmented significantly with the crack size, whereas the measured FCG rates da/dN remained practically constant, it can be concluded that Elber’s effective stress intensity factor range was not the controlling driving force for the analyzed tests. Figure 11 : FCG rates da/dN and crack opening ratios K op /K max measured under quasi-constant {  K  15MPa  m , R  0.1 } loading conditions by the four redundant techniques (near and far-field strain gages and DIC-based COD and strain fields) along the crack path in the thin DC(T) specimen with t  2mm , supposedly under plane stress conditions. Figure 12 : FCG rates da/dN and crack opening ratios K op /K max measured under quasi-constant {  K  15MPa  m , R  0.1 } loading conditions by the four redundant techniques (near and far-field strain gages and DIC-based COD and strain fields) along the crack path in the thick DC(T) specimen with t  12mm , supposedly under plane strain conditions.