Crack Paths 2012

discharge machining in the center of the specimen. Furthermore, there are additional

slits of 10 m mlength at each hole in x-direction.

The fatigue crack growth experiments under biaxial loading were performed on a servo

hydraulic 250 kN INSTRONbiaxial tension compression testing machine. The biaxial

ratio

U=FY/FX

(4)

between the loading axes was varied between 0 and 1. Additionally, the load ratio was

chosen as R = 0.3 because in previous investigations on aluminium [10] this value was

found to give no crack path deviations by crack surface roughness induced friction. The

crack length was measured optically and by using crack length foils. Furthermore, in

order to study the microstructural effects during variable amplitude loading the P R M

was integrated into the biaxial testing machine, see Fig. 3.

In contrast to the SENB3experiments, there is no standard formula to calculate the

loading of the crack tip for the used cruciform specimens under biaxial loading.

Therefore, the finite element software Abaqus was used to determine the mode I and

modeII stress intensity factors KI and KII numerically.

Due to restrictions in space we focus within this paper only on results of SENB3

experiments. Nevertheless, results of biaxial experiments will be presented at the

conference.

Figure 3. Cruciform specimen used in the present study (left) and experimental setup

within servo-hydraulic INSTRObNiaxial testing machine with integrated pulsed

reflection microscope (right).

240