numero25

P. Lopez-Crespo et alii, Frattura ed Integrità Strutturale, 25 (2013) 153-160; DOI: 10.3221/IGF-ESIS.25.22 156 Figure 3 : The variation in the crack opening elastic strain measured mid-thickness (z=0) along the crack plane (y=0) as the samples was unloaded from K max to K min at OL-1. Fig. 4 shows the evolution in the strain field in the crack opening direction along the crack plane at mid-thickness both at maximum and minimum loading as the sample undergoes the overload event and then as the crack grows through the plastic zone created by the overload event. It is clear that the crack growth is very small after 20 and 1000 cycles. After 10,000 additional cycles the crack has progressed around 300  m while after an additional 20,000 cycles the crack has progressed 1070  m beyond the overload event. It should be noted that during the loading event the CT sample moves slightly in response to the applied load, however we have tried to correct for movement in both the crack growth direction (x) and perpendicular to it (y). It is interesting to note that the compressive regions near the crack tip are not so sharp, or deep for the baseline fatigue response (OL-1) or immediately after overload (OL) as compared to those after 20 or 1000 additional fatigue cycles, although this could be due to the fact that the linescan has missed the peak value in the 2D (x,y) strain field. Closer comparison of the shapes and magnitudes of the strains local to the crack-tip can be obtained by examination of Figs. 5a) and b). These show the strain evolution for the maximum and minimum loads respectively shifted so that each curve has the current position of the crack-tip (defined as the point of zero strain at K max ) coincident with the others. It is clear from these curves that the peak tensile strain at K max falls immediately after overload and this rises towards the baseline fatigue level as the crack advances. This is in agreement with our previous paper on austenitic steel [12]. There it was shown that this is due to the compressive residual stress in the near crack tip zone after overload through which the crack must grow. That this is due to the effect of climbing out of the compressive residually stressed zone is confirmed by the fact that when the difference in strain between K max and K min is plotted in Fig. 6 the elastic stress changes each cycle are almost identical for OL-1, OL+20 and OL+1000. It is clear from Fig 5a that considerable plastic deformation has been induced in the crack-tip region by the overload event since the tensile peak at overload (OL) is only slightly higher than prior to overload (OL-1) but the tensile region is broader. Fig. 5b) shows the curves at minimum load relative to the position of the crack tip at that instant. The fact that for most of the points behind the crack the strains lie slightly below zero (~-400x10 -6 ) might be suggestive of a slight error in the strain free lattice spacing as one would certainly expect there to be no contact at K max (Fig. 5a) across the crack faces yet the strains are similarly compressive. However, it should be noted that towards the back face the strains do appear to approach zero suggesting that we do indeed have a representative value. Alternatively it could be that the stress is zero across the crack, but the strain is not. Crack face closure forces would be evident as compressive strains behind the crack- tip, while compressive residual stresses arising from the plastic zone would lie ahead of the current crack-tip position.