336 Prakt. Met. Sonderband 46 (2014) 
resolution and high displacement measurement accuracy. The combination of SEM and 
DIC can satisfy these requirements. 
DIC was tested on two standard problems. The static strain field that shows the influence 
of a crack on the local strain distribution was investigated first because higher 
displacements were expected. Then the displacement shifts between the same crack 
under loading and unloaded are examined in order to show the cyclic strain field during 
fatigue and to identify the elastic dominated amount of the static strain field. 
2. EXPERIMENTAL METHODOLOGY 
The measurement surface of a tensile specimen of the f.c.c. nickel-based super alloy 
CMSX-4 was prepared stepwise using diamond polishing suspensions down to 0.25 um. 
CMSX-4 has a strong tendency to planar slip resulting in a stage-I-crack growth of several 
hundred micrometres. The y/v’-microstructure with coherent cubic v’-precipitates of the 
intermetallic type Nis(Al, Ti) in a y-matrix of a nickel based solid solution cause a very 
strong material contrast in SEM imaging (Fig. 1). This high contrast is favourable for good 
image correlation results. Fig 
An EBSD analysis was performed firstly to identify the orientation of the {111} slip planes magn 
and the Schmid factors of all <110> slip directions of the 12 slip systems. The slip system 
with the highest Schmid factor was chosen and a 100 ym semicircular notch was 
positioned on the slip plane by focused ion beam technique. More detailed information can 
be found in [4]. The regions of interest for the investigation were marked bv a cross about 2.2Ci 
75 um in the extension of the notch (Fig. 1). 
The specimen was repolished to sharpen the borders of the notch to support crack The s 
initiation. MPa. 
detect 
ASTM 
2.1 REFERENCE IMAGES 
The analysed images were gathered using a Kammrath&Weiss in-situ tensile module. The 
stress was 585 MPa during the in situ measurements. This load is higher than the 
maximum load during fatigue to ensure that the dislocations in the plastic zone rearrange 
in a new equilibrium position. This new arrangement differs from the fatigue dislocation 
distribution. So this overload allows to study the static crack strain field although the crack 
was initiated and grew as a fatigue crack. 
The SEM parameters for the reference images were kept constant to achieve exactly the 
same detector and electron beam settings like the gun position and the lens alignment 
parameters for the measurement images. Images were taken with the highest achievable 
pixel resolution and a maximum grey value contrast in backscatter mode in a Zeiss Sigma 
VP SEM. The detector contrast and brightness settings were optimized to get a maximum 
image contrast without white and black pixel (Fig. 3a/b). 
A worst case estimation (assumption of a crack instead of a notch) showed that the stress 
field in the image area is nearly unaffected by the notch (less than 6% stress gradient 2.3 MI 
along the image centreline). So the reference images are assumed to show the ROI with a 
constant stress of 585 MPa. The rr 
The accuracy of DIC was verified by the comparison of images of the same region of an with e 
uncracked specimen in the loaded and unloaded case. The measured strain was in good Speci: 
agreement with the expected value. tensile 
correl: