Page 129 - Biaxial Multiaxial Fatigue and Fracture

114 G.B. MARQUIS AND E! KARALAINEN-ROIKONEN

RESULTS AND DISCUSSION
Fatigue life

Fatigue test data are shown in Fig. 6. At comparable fatigue lives, the specimens subject to
torsion loading failed at noticeably lower stress levels than did the uniaxial tension specimens.
The fatigue limit was approximately 78% of the tensile fatigue limit. Under equi-biaxial
loading the nodular iron had nearly the same fatigue limit as under uniaxial loading. It can be
noted that the uniaxial test data in this figure was based on constant mean stress testing at a
mean stress level of 182 MPa as compared to the constant R = 0.1 testing used for the other
two stress states. For this reason the uniaxial curve required some adjustment. At the fatigue
limit, however, the mean stress resulted in a stress ratio very similar to that used for the other
stress states. The fatigue limit for uniaxial tension shown is based on Eq. (7) and the finite life
curve is based of an extensive test program using this material [15]. Specific R values for each
set of tension fatigue data are shown in Fig. 6.

250

200
h
ai
a
E
a,
-0
3 150
i?
6
In
v)
p1
tj
100

IO IO 10 IO
Number of Cycles to Failure

Fig. 6. Experimental results for tension, equi-biaxial, and torsion fatigue.