Fatigue Crack Paths 2003

A non-linear dependency from loads of relative displacement of the crack lips during

opening can be noticed; but this will not affect the overall deflection behaviour which

always resulted quite linear.

The behaviour is obviously different when approaching the end of the crack (in

points B 31 and B39) where the relative movementof the crack lips is strongly reduced,

so that the gauges do not fail anymore or fail at very high loads.

An overview of lips opening in all measuring points is given in Fig. 9: the measured

strains displacements have been distributed symmetrically with respect to the crack.

46000

load A

load B

2000

]

load C

S t r a i n [

0

load D

-2000

load E

-4000

-6000

B.31 B 3 2 B 3 3 B 3 4 B 3 5 B 3 6 B 3 7 B 3 8 B 3 9

Strain gauge

Figure 9. The opening of crack lips as measured by the series of B gauges.

T H ESIMPLIFIEMD O D E L

The model used for calculating the breathing behaviour and the reduced stiffness of the

cracked shaft is described in detail in [1]. Here only some results are recalled. The

model of the breathing mechanism allows to evaluate with high accuracy, if compared

with 3D non linear calculations, the open and closed parts of the crack. It is then

assumed that only the closed parts together with the integer parts of the cracked cross

section contribute to the resisting area and to its second moments of area. The values of

the second moments of area are then used for evaluating the stresses due to the bending

moment, in the same manner as it would have been in a constant cross section beam,

which is obviously totally unrealistic. The highly non-linear stress distribution is not

considered.

The crack closure effect has been simulated with an external bending momentwhich

tends to hold the crack closed, generating a maximumof 140 micro-strains of

compressive strain.

These strain values have been corrected with the stress intensity factors of Fig. 4.

N o wa comparison is possible between calculated results and experimental results,

taking account of crack closure effects and stress intensity factors.