Fatigue Crack Paths 2003

TwoBounds of Accumulated Damageunder Sequential Loading Blocks

Tension and torsion (as well as bending and torsion) are considered as non similar

loading since they do not show the same generalised fatigue limit [2-4] (fatigue limit

expressed in terms of a shear stress) and hence the same mesoplasticity behavior (Fig.

1). Moreover, the critical plane where damage accumulates is different for the two load

conditions. By reasoning with individual critical planes, it appears that the critical plane

of a tension loading is not affected by a torsion loading and vice versa [8].

Consequently, with the discussed model, the predicted number of cycles for the block

test is deduced from the only tension loading or the only torsion loading. Indeed, the

first applied block being in tension and the corresponding stress level exceeding the

fatigue limit, some damage is accumulated on a 45° oriented plane. In the second block

under torsion, shear stress above the fatigue limit leads to some damage on either

longitudinal or transversal planes. For this stress state, it is easy to prove that there is no

shear stress on the 45° planes (i.e. planes of the first tension block).

1,00E+07

Bending, Torsion Tension, Torsion

N ointer. NoINIintetrIentrer.

(105 cycles)

(105 cycles)

(5104 cycles)

Tension, Torsion

(5104 cycles)

cycle s

Bending, Bending+Torsion,

N ointer.

Inter.

e r o f

Torsion

d n u m b

1,00E+06

C36

P r e d i c t e

C18

(104 cycles)

1,00E+10,500E+05

1,00E+06

1,00E+07

Observednumberof cycles

Figure 3. Predicted and observed number of cycles for the sequential tension, torsion

tests (block length: 105 and 5104) applied to C36 and for the sequential bending,

bending+torsion, torsion tests (block length: 104) applied to C18 [9].