Crack Paths 2012

Specimens

All the fatigue tests were carried onto micronotched hourglass specimens. After

machining the specimens were hand polished and then electro-polished (surface

removal 30-40 Pm)in order to reduce the residual stresses.

After surface finish, artificial micronotches were then introduced onto the surface of

the specimens by E D Mmachining: three different defects, characterized by a size

(expressed in terms of Murakami’s ¥area parameter) of 220, 315 and 630 Pm. Defect

sizes are shown in Fig. 1.

Figure 1. Micronotches adopted for fatigue tests.

2

2

2

1

1

1

0

0

-1

0

-1

-2

-2

-21

0.0

0.2

0.4

0.6

0.8

1.0

W W

W W

Time

W W

-3

-3

m a x

m a x

m a x

V W

V W

V W

m a x

m a x

m a x

-4

-4

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Time

Time

(a)

(b)

(c)

Figure 2. Load patterns. (a) Load Path 1 (LP1) – Bearing steel, (b) Load Path 2 (LP2) –

Bearing steel, (c) Load Path 3 (LP3) – Gear steel and R7Tsteel.

Precracking & fatigue test details

Specimens made of the gear and bearing steel were firstly tested under tension at R=-1

in order to determine Mode I thresholds. Then, for the sake of torsional and out-of

phase tests, the specimens were precracked at R=-2 at stress levels very close to fatigue

limits at R=-1 for 107 cycles, for inducing the formation of small non-propagating

cracks at the bottom of the notches. All the specimens were observed under S E Mto

verify the success of pre-cracking procedure (if not successfull the ModeI loading was

repeated).

After the pre-cracking procedure, the specimens were subjected to torsional and out

of-phase tests at different

'KIII

levels. The O O Ptests were carried out according to

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