Page 420 - Biaxial Multiaxial Fatigue and Fracture

404 Y ACBIN. I? QCAEGEBEUR AND S. DEGALLAIX

Eeq = ,/q (3)
Equivalent total strain ceq was defined by analogy as:

All the tests were performed under total strain control at room temperature. A preliminary
tension test with progressive stepped strain rates in [6.6 s-I; 6.6 IO” s-l] showed that
viscosity at room temperature is significant. On this ground, all the further tests were carried
out with a constant equivalent strain rate equal to 6.6 IO4 s-I. A program was developed using
Labview software in order to control the biaxial machine. About 500 points were recorded for
each cycle during the cyclic tests.
A given loading is completely defined by its path, its amplitude and its mean strain in the
plane (c,y/&). The loading path amplitude is defined as the radius of the smallest circle
circumscribed around the path and the mean strain is the position vector of the centre of this
circle. We studied separately the influence of the history of each of these three parameters on
the cyclic stress response. The seven selected loading path shapes are shown in Fig. 3. A
proportional loading path is a path whose control parameters remain proportional to each other
during the time.

..
Tension-commession Torsion Proportional 45O

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2 Square Clover Hourglass Circle

Fig. 3. Loading paths in (E, Y/&) Plane.

This work aims at studying the influence of the loading type on the cyclic stress response,
therefore the cyclic tests were carried out up to stress stabilization, and not until fracture.