338 Prakt. Met. Sonderband 46 (2014) 
The image correlation was performed with the commercial software VEDDAC 6. The dx/d 
images were corrected by a displacement and rotation offset filter and the resulting x- and 
y-shifts are filtered with a smooth Gaussian filter. An example of the high correlation factor 
[0;1] as a quality indicator for the shift measurements between reference and 
measurement images is given in figure 3c. 
ow mm 
Fig. 3: a: reference image c: correlation factor for cyclic 
b: measurement image strain investigation 
2.4 CYCLIC STRAIN FIELD EXPERIMENT 
Images of the same loaded and unloaded crack were compared by DIC at the step of 
78000 cycles. Noticing the elastic dominated nature of the cyclic strain field of the crack, 
this smaller strain field compared to the static strain field was also investigated to find 
significant differences related with the presence of a plastic zone in the static case. 
3. RESULTS 
The strain tensor component exy and the Tresca stress field were calculated from the x- 
and y-shift measurements using MATLAB 2013b. ex and gyy follows directly from dx/dx 
and dy/dy. €zz is not achievable by DIC due to the 2D character of the images. The elastic 
strain components ex: and gy. vanish because of the plane stress state at the surface. , 
Results for the static strain fields are shown in fig. 4 and for the cyclic strain field in figure 
5. The strain component &xy calculated from the shear components for the static and cyclic 
case are shown in fig. 6. A comparison of both crack sides for both components of &xy 
dy/dx and dx/dy is shown in fig. 7 and shows an opposite character of shear as expected. 
4. DISCUSSION 
Fig. 8 shows the estimated Tresca stress field, based on the measured strains for the 
static and cyclic case. The Tresca stress field was chosen because it is not affected by €z 
in the plane stress case, in contrast to the v.-Mises stress field. 
It can be seen that the measured Tresca stress field is in very good agreement with the 
expected Tresca stress field. The Tresca stress field for the static case shows the plastic 
zone as an additional line emanating from the crack tip. The shear caused by the 
dislocations in the plastic zone can also be seen at dx/y in fig. 7. 
DIC especially the dx/dy component was used to measure the size of the plastic zone. The 
results for the different analysis steps were compared with those measured by other 
techniques (Fig. 9). Fig. 7 
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