Issue 49

G. Meneghetti et alii, Frattura ed Integrità Strutturale, 49 (2019) 82-96; DOI: 10.3221/IGF-ESIS.49.09

to first singular term. An advantage of the Stanley and Chan’s extrapolation method compared to other least square fitting approaches is that an accurate knowledge of the crack tip position is not required. Once K I is obtained from the maps of the thermoelastic signal  T and from Eqn. (8), then J e can be obtained from Eqs. (6). Evaluating the plastic component of J-integral The total energy included in V c can be separated in its elastic, , CC e W  , and plastic, , CC p W  , components:

, CC e V W W dV W W         CC CC

(9)

, CC p

c

where

 V W W dV    , CC e , CC e

(10a)

c

 V W W dV    , CC p , CC p

(10b)

c

In [29], a linear link was established between the plastic component of the strain energy density, , CC p W 

, and the plastic

component of the J-integral, J max,p ,:    , max, 2 , ' CC p p W k n J   



R

(11)

p c

t

k

where is a constant depending on the notch opening angle and the cyclic hardening exponent. Assuming the generalised Ramberg-Osgood law, according to which the strain is equal to the sum of its elastic and its plastic component, the plastic component of the strain energy density can be evaluated from the plastic strain component of Eqn. (12): (2 , ') p k n 

1 1 ' n

 

1 1 '   

e

W

(12)

, CC p

1

n K

  '

n

'

where e  is the Von Mises stress and K '  is the cyclic strength coefficient. For a Masing material [34], the plastic strain energy density per cycle, W , can be evaluated from the Ramberg-Osgood relation according to Halford [35]:

1

'   K      

1 ' 2 2 1 ' e n n   

' n e

 

W

(13)

Comparing Eqn. (12) and Eqn. (13), a link between W and W CC,p

is obtained, as depicted in Fig. 4:





4 1 ' W n W   

(14)

, CC p

Substituting Eqn. (14), Eqn. (10b) becomes:

86