Fatigue Crack Paths 2003

110

C r a c kD e t e c t i oin P I N

15678900

P I N

Failure

C r a c k

D e t e c t i o nBiOn X

S t r e s s ( M P a )

B O X

Failure

Box failure

Pin failure

40

1,E+05

1,E+06

1,E+07

1,E+08

Cycles

Figure 4. S-N curve for D CNC50.

- Alternating tension compression fatigue tests

- Fatigue tests with non zero mean stress (tensile and compressive)

- Fatigue tests on notched specimens (for notch sensitivity analysis)

- Faigue tests on specimens with different surface finish.

The results of these tests were employed, together with finite element (FE) analysis to

predict actual cycles for crack initiation in TJ.

E X P E R I M E N TOABLS E R V A T I OONNSF A T I G UCE R A CGKR O W H T

In few tests a beach marking technique was employed to determine subsequent crack

front. Figure 6 shows the crack fronts of specimen 55VM4that was subjected to beach

marking at 50000 cycles intervals.

The crack front, during propagation, undergoes significant shape variations, passing

from the initial semi-elliptical shape to a through crack shape. In the first phase the

crack was observed to growth with a constant aspect ratio between depth a and semi

amplitude c (Fig. 7).

5 5 V M 4

8

5

6

3 4

2

1

4

2

Crack fronts

0

6 , E + 0 5 7 , E + 0 5 8 , E + 0 5 9 , E + 0 5 1,E+06 1,E+06 1,E+06

C y c l e s

Figure 5. Dynamic Amplification Coefficient. Figure 6. Crack fronts in PIN component.