Issue 57

R. Fincato et alii, Frattura ed Integrità Strutturale, 57 (2021) 114-126; DOI: 10.3221/IGF-ESIS.57.10

            

            

k

  k

 n



f

Y

3 2

σ α





n

1

1

k

k n

k

 n



   (

F

B R

3 2

)

σ β







n

1

1

1

   

   

k



2 3

 *, 1 n

k n

k

k n

k



 





Ca

B

 *, 1 n α α

N

N

(10)



1



 *, 1 n

n

*,

1

a





k n

k n

k n

 







2 3 m b

β β

N

β







n

1

1

1

 H H 1 k n

 

2 3

n



1

     k k x x A B x 1 k



k n

  k

k

1



x

(11)

   1 n











n

n

n

n

1

1

1

1

1

where A is the matrix of the partial derivatives of the equations in B against the unknowns. The idea is to perform a multi- equation Newton-Raphson scheme that guarantees a quadratic rate of convergence, speeding up the computation time compared to an explicit integration scheme. Moreover, the algorithm is stable and accurate for large  D inputs. The sub- iterations k stop whenever the following condition is fulfilled:

   1 1

k n

tol

1

(12)

B

where tol1 is a threshold imposed by the user, usually 10 -8 , as in the present work. Another benefit of the current approach consists in the possibility of computing the consistent tangent operator  TO directly after the local equilibrium is fulfilled (i.e., Eqn. (12) is satisfied) as:

1

  1 12 :  

TO

( 1) k

  



n A

E

(13)



where k n A is a matrix with the same dimensions as elastic constant matrix E and the first term is located in the first row and second column of        1 1 1 k n A . A detailed explanation of the algorithm, together with iso-error maps for the evaluation of the finite step accuracy, can be found in [24,25]. Ghaei and Green algorithm for the definition of the workhardening stagnation In large finite steps integration, the conditions in Eqn. (4) can be rewritten as the condition in Eqn. (14), which implies that the update of the workhardening stagnation has to be performed whenever the back stress   1 1 k n β lies outside the non-IH estimated at the n step. Ghaei and Green proposed a single scalar equation for the update of  g . Assuming to know the back stress   1 1 k n β of the bounding surface, the radius of the non-IH surface at the n+1 step is approximated with a Taylor expansion as:              2 2 1 1 1 1 , , 3 2 0 k k n n n n n n g r r β q β q (14)        1 1 1 12

    2 1 n r



2



k n

2

1

    

 h χ β  1 : n

r

r

r

3





n

n

n

1

1









k n

k n

1

1

  

β β β

(15)





n

1

1







β

q



 

χ

n

n



k n

1

n

   q



χ β











n

n

1

1





 

 

1

1

119