Crack Paths 2006

From the crack length versus number of cycles curves reported in Fig. 5, it appears that by changing the angle M from 0o to 60o then fatigue life increases under both force and

displacement control. Sometimes two phases were observed in cracks initiation and

propagation; the first was characterized by initiation and propagation under a mixed

moderegime whilst in the second phase the crack growth took place according to mode

I. These two phases could not always be clearly observed and distinguished during the

tests. Fromthe test results obtained under force and displacement control it also appears

that the specimen life is higher under force control (Fig. 5). This behavior could be due

to the compressive part of the loading cycle present under the displacement-controlled

cycle at R = 0, which could influence, in particular, the initial, mixed-mode crack

initiation and short crack propagation.

(a)

M q

P

max = 16 kN

2 am( m )

R V = 0

16 M q

M q

12

a c k l e n g t h

8

4

C r

0

0

20000

40000

60000

80000

Numberof Cycles, N

(b)

40

Gmax= 0.21 m m

M q

(m m )

M q

RG=0

30

M q

r a c k l e n gt h 2 a

20

10

C

10000 20000 30000 40000 50000

0 0

Numberof Cycles, N

Figure 5. Comparison of experimental test results for M = 0q, 45q and 60q: (a) force

control and (b) displacement control