Issue 41

J.A.O. González et alii, Frattura ed Integrità Strutturale, 41 (2017) 227-235; DOI: 10.3221/IGF-ESIS.41.31 232 concluded that  K eff  K max  K op was not the FCG controlling driving force in this case. Indeed, since  K eff significantly increased while the FCG rate da/dN essentially remained fixed while the cracks grew,  K eff could not possibly be their driving force. Figure 8 : Cracked surfaces of one of the thick specimens, showing their successive homologous or quasi-parallel crack fronts, clear evidence that all of their points across the specimen thickness grew under iso-driving force conditions. In fact, since CAL conditions {  K , K max } were maintained during those tests, there is no doubt that  K eff steadily increased as the cracks grew, because the decrease in the K op /K max ratio is beyond the (small) uncertainty of the measured data. Moreover, despite their slightly lower R-ratios, notice that the opening loads were a little bit higher along the crack path in the thicker than in the thinner test specimens, or under plane strain instead of plane stress conditions, contrary to what could be expected beforehand using PICC models. Notice that such tests used the very same technique proposed by Elber to identify crack closure [3], and Paris and Herman's linearity-subtractor ideas to enhance the K op identification [8, 10-11]. If these techniques can be used to support Elber’s arguments, they can also be equally used to question them. Moreover, they used independent and redundant compliance and DIC techniques to measure the fatigue crack opening loads, to avoid questions about their sensitivity. Therefore, according to Kemp’s [6] advices, these measurements can indeed be used to evaluate the actual  K eff role in those FCG tests, since they are based on direct crack closure measurements, not on indirect evidence of any sort. Figure 9 : FCG rates da/dN and crack opening ratios K op /K max measured under quasi-constant {  K  20MPa  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) specimens ( t  2mm ), supposedly under plane stress conditions.