Issue 43

F. Berto et alii, Frattura ed Integrità Strutturale, 43 (2018) 1-32; DOI: 10.3221/IGF-ESIS.43.01

     2 

  

   (1 )(5 8 ) 4  

K

th



R

(9)

0



0

2



  K

 (1/ )(

 / ) th 0

a

When ν =0.3, Eq. (4) gives R c

= 0.845 a 0

, where

is the El Haddad-Smith-Topper parameter [106].

0

Under plane stress conditions, simple algebraic considerations give:

      2 

  

 (5 3 ) 4 

K

th



R

(10)

0



0

so that we have now R 0 when the Poisson’s ratio ν is 0.3. Fig. 11 exactly fits the Kitagawa diagram plotting fatigue strength of a material in the presence and absence of cracks [103]. The plateau on the left hand side of Fig. 2 is due to the fact that the small cracks are fully embedded in the elementary structural volume, so that the value of 1 W coincides with that of the plain specimens (under plane strain conditions). The transition crack size a 0 has been employed as an empirical parameter to account for the differences between long and short fatigue cracks. In particular El Haddad et al. [106] suggested to add in the SIF range definition the fictitious crack length a 0 to the crack amplitude a. Doing so, the fatigue limit Δ σ th of cracked components was correlated to plain specimen fatigue limit by means of the expression: =1.025 a 0





 1 1 /

th



(11)





a a

0

0

By observing Fig. 11, when a >> R 0

, it is possible to write:

f

f

Re

Re

W

W

1

1

1

1







(12)

f

Re

 1 / 1 / (1 / ) aI R a a  1 0 1 0

1 W a

( )

 W aI R

0

1

  Re 2 0 1 f

W

/ 2 E is the reference value. The analogy with Eq.(11) is evident.

where

Figure 12: Fatigue behavior of a material weakened by notches or cracks (log-log scale).

11