Page 458 - Biaxial Multiaxial Fatigue and Fracture

442 M. WEICK AND 1 AKTAA

stresses system, slip bands are activated in all directions. Hence, preferred directions of the
orientation of cracks cannot be expected. Consequently, it is impossible in this case to
determine an explicit correlation between the phase shift and fatigue.
With our biaxial machine the multiaxial loading was generated as follows: While the load
in axial direction is raised by a conventional tension feature, the circumferential strain is
generated by a pressure difference of the surrounding media. The advantage of this method are
the fixed directions of principal stresses and strains during the non-proportional loading cycles.
Thus, the orientation of a crack can be assigned to the directions of principal stresses.
To obtain information about the microcrack nucleation and propagation, we use a sonic
emission system which records the current sonic emissions during the experiment. From these
signals, conclusions are drawn with regard to the propagation behaviour of microcracks under
multiaxial loading.
With the developed microcrack model we calculate the lifetime on the basis of the
experimental obtained AJ's. To verify this model we compare the results to the measured life-
time of the specimens.

EXPERIMENTAL DETAILS

Test facili5

As mentioned above, the existing test facility allows to perform non-proportional cyclic fatigue
tests with fixed directions of principal stresses and strains [ 1, 21. The mechanical components
of the biaxial test facility are illustrated schematically in Fig. 1.

Fig. 1. Mechanical components of the biaxial test facility.