numero25

P. Lopez-Crespo et alii, Frattura ed Integrità Strutturale, 25 (2013) 153-160; DOI: 10.3221/IGF-ESIS.25.22 155 through-thickness (z) of around 1.4 mm. This allowed a 10 times greater resolution than in previous plastic zone mapping experiments [7]. Such a good resolution was possible because of the very small grain size of the bainitic steel used here. Figure 2 : Schematic of the diffraction geometry showing a CT specimen with the crack plane horizontal, and the two detectors measuring two directions of strain; note the coordinate system for ε xx and ε yy adopted after [4]. For very low  these strains can be taken as representative of those in the loading (y) and crack growth (x) directions.  F ATIGUE EXPERIMENT he specimen was fatigue pre-cracked for 43000 cycles at a frequency of 10Hz, stress intensity range Δ K = 28 MPa√m and load ratio K min /K max = 0.05. Plane strain conditions were met at the mid plane through the thickness for all loads applied during the experiment [9]. The crack length was measured perpendicularly to the loading direction from the centre of the loading holes [10]. Once the fatigue crack had grown to a length of 12.75 mm, a 100% overload (OL) was applied. Strain measurements were made at a number of fatigue stages, namely during the cycle just before the overload (OL-1), during the overload (OL), and for the cycles just after the overload (OL+1), 20 cycles after the overload (OL+20), 1000 cycles after the overload (OL+1000), 11000 cycles after the overload (OL+11000) and 21000 cycles after the overload (OL+21000). By measuring around 50 strain points along the crack plane (y=0), a profile of the strain evolution behind and ahead of the crack-tip was produced for each of the fatigue stages studied. R ESULTS AND D ISCUSSION he traditional crack closure approach defines a nominal stress intensity factor at which the crack faces touch represented by K Cl . This is often identified by a knee in the crack compliance during unloading from K max , as recorded by a back-face strain gauge, for example [11]. To investigate the manner in which the crack-tip strain field varies with unloading a series of measurements were made during an unloading cycle (K max , 0.7K max , 0.2K max , K min ) and the results are shown in Fig. 3. From these measurements it is clear that the reverse plastic zone has started to develop by the time 0.2K max has been reached. Indeed the reverse plastic zone appears to increase only marginally with further unloading to K min although the peak tensile stress ahead of the crack at the edge of the plastic zone continues to fall to a value of around 400x10 -6 at K min . T T