Page 210 - Biaxial Multiaxial Fatigue and Fracture

194 T LRGODA ET AL.

At stage 5 the critical plane is determined. In this paper the authors used the damage accu-
mulation method, as in [4 - 7, 221. According to this method, the critical plane is the plane
where the calculated damage degree according to the assumed fatigue parameter is the maxi-
mum, and the fatigue life is the minimum. Thus, the fatigue lives were determined in many
planes according to the successive stages.
After determination of energy parameter histories in a given plane (stage 6), the rain flow
algorithm was used for cycle and half-cycle counting and next damage was accumulated (stage
7) according to the Palmgren-Miner hypothesis [25,26]

where:
S(T0) - degree of the material damage at time TO,
j -number of the class intervals of the amplitude histogram,
Waf- fatigue limit expressed in the strain energy density,
NO - number of cycles corresponding to the fatigue limit oaf,
n,- number of cycles with amplitude W, ,
m - exponent of the fatigue characteristic (0, - Nf ),
m' = m12 - exponent of the fatigue characteristic (W, - N, ),
a - coefficient (e 1) allowing to consider amplitudes below the fatigue limit in the
damage accumulation.

Damage was also accumulated according the Serensen - Kogayev hypothesis [25]:

where:

Serensen - Kogayev coefficient,

"i
-
ti = - frequency of occurrence of particular levels W,i at time TO.
hni
i=l
W,, - maximum amplitude of the energy parameter.