numero25

P. Lopez-Crespo et alii, Frattura ed Integrità Strutturale, 25 (2013) 153-160 ; DOI: 10.3221/IGF-ESIS.25.22 158 Careful examination of Fig. 5b would appear to show a compressive trough centred on the crack position for OL-1. Similarly the overload event is accompanied by a compressive strain dip on either side of the crack-tip location. By contrast for a low numbers of cycles (OL+20 and OL+1000) the compressive stress at K min appears to be sharper and associated only with plasticity ahead of the crack tip. This may be because of in the absence of crack face contact the crack is no longer wedged open allowing a larger compressive zone in the plastic zone to develop. While great care was taken to ensure that the crack tip was lying exactly on the line of the scan, 2D maps are currently being acquired [13] to find out whether this effect is instead merely due to scanning slightly off the line of the compressive maximum with some of the linescans. With further cycling and crack growth (OL+11000 & OL+21000) Fig. 5b appears to show a distinct compressive contact stress in the region of the overload cycle. This compressive stress in the region of the overload at K min is even more clearly evident in Fig. 6 as positive peaks in the K max -K min curves for OL+10000 and OL+21000 at the location of the plastic zone generated by the overload. (a) (b) Figure 5 : Strain evolution mid-thickness along the crack plane at a) K max and b) K min for all the stages of fatigue crack growth analysed. The coordinate along the crack plane (horizontal axis) has been corrected so that the crack-tip position coincides for all cases. Figure 6 : Plots showing the change in elastic strain between K max and K min as a function of number of cycles. The arrow shows the location of the extensive plastic deformation from the overload event. It is also noteworthy that for all the crack locations the tensile peak ahead of the crack at K min is approximately constant (~500x10 -6 ) and located around 400-500  m ahead of the crack (Fig. 5b). 