Austenitic-ferritic duplex stainless steel has a high strength and an excellent corrosion resistance. Therefore it is
used, for example, in offshore systems, where the material experiences cyclic loading of high stress amplitudes
which may cause failure after long periodic operation. Laboratory experiment in very high cycle fatigue (VHCF)
regime (up to 109
cycles) may help to evaluate the defect evolution as function of load cycles and to predict possible
origins of crack formation [1]. Besides macroscopic probes TEM and X-ray techniques are important because with them it is possible to study the defect structure on the micro and nanoscale. Moreover, X-ray diffraction is a suitable
technique to probe the defect formation during the fatigue treatment. Recently we have demonstrated that by use of
micron sized beam and high energy photons X-ray diffraction allows single grain analysis of ferritic duplex steel
samples after certain fatigue cycles. It has been indicated that X-ray Bragg peaks of austenite grains exhibit a certain
probability of peak splitting which increases as a function of local stress load [2]. At the same time the shape of
ferrite grains kept nearly not affected. Considering that this peak splitting can be used as indication of local stress
impact, this technique has been applied for in-situ probing of the defect formation. It has been found that the
probability of peak splitting increases with the number of fatigue cycles [3]. This paper will show the results of insitu
investigation of duplex steel samples with increasing stress load in reflection geometry. Because former fatigue
experiments have shown, that the crack initiation predominately takes place at the surface of the investigated duplex
steels [4], surface sensitive in-situ measurements were carried out in reflection mode using flat dog bone shaped
sample. Here the probed information depth is in the order of 30 microns and allows for a closer comparison with
other surface probe techniques.