Issue 34

R. Brighenti et alii, Frattura ed Integrità Strutturale, 34 (2015) 59-68; DOI: 10.3221/IGF-ESIS.34.05

0.6

1.4

2L f

/  f

l / L f

=0.1 =0.7

num. data

2L f

/  f

num. data

l

/ L f

=0.1 =0.7

20 40 80

20 40 80

/ L f

l

1.2

l / L f

0.5

0.2 Dimensionless SIFs, K* IIz 0.4 0.6 0.8 1.0

0.1 Dimensionless SIFs, K* IIr 0.2 0.3 0.4

(b)

(a)

(b)

(a)

0.0

0.0

1

40

5 10 15 20 25 30 35

1

40

5 10 15 20 25 30 35

Young modulus ratio , E f

/E

Young modulus ratio , E f

/E m

m

Figure 4 : Dimensionless radial and longitudinal Mode II SIF, (a) , II r K

K

f m E E , for different

and (b)

, vs Young modulus ratio,

, II z



f 

L

values of fibre aspect ratio, 2

, and relative debonded length equal to 0.1 and 0.7.

f

0.4

0.4

interpol. l

/L f 0.05 0.20 0.50 0.90

interpol. E f

/E m 1

num. data

num. data

0.0 Dimensionless SIFs, K* Ir (b) (b) 0.1 0.2 0.3

0.1 Dimensionless SIFs, K* Ir (a) (a) 0.2 0.3

10 25 40

/ E m

=10

E f

= 80

L f

/ 

f

0.0

0

20

40

60

80 100

0

0.2 0.4 0.6 0.8 Relative crack length, l/L f

1

Fiber aspect ratio , 2 L f

/  f

Figure 5 : Dimensionless radial and longitudinal Mode II SIF, (a) , II r K

K

f m E E , for different

and (b)

, vs Young modulus ratio,

, II z



f 

L

values of fibre aspect ratio, 2

, and relative debonded length equal to 0.1 and 0.7.

f

m

/ v dl dN C K    

   

K K K

, i

(9)

cg

i

i

th

i

IC

In Eq. (9), K  is the equivalent stress intensity factor range produced by the cyclic remote stresses. The above crack growth rate takes place in a thin weak layer placed between two different materials. Such a layer can be considered as a third material whose properties are themselves affected by the fatigue process. For the above reason, the interface crack propagates in such a degraded material, and a suitable evaluation of the damage can be done by using Eqs. (4) to (6). In the present study, the above biaxial damage evaluation is applied to the Paris constant i C : 0 , ( ) / (1 ) i i f i C N C D   , where 0 i C is the undamaged initial constant value. The critical detached length, c l , and the corresponding number of loading cycles, c N , leading to such an ultimate condition can be evaluated as follows: , i C m are the Paris constants of the interface, l is the debonded length (Fig. 1b), and i i

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