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⎧ M1U<br />

1<br />

= F1+<br />

R1<br />

⎪<br />

⎨M U<br />

= F + R<br />

⎪<br />

CU<br />

⎩ + CU<br />

=<br />

2 2 2 2<br />

1 1 2 2<br />

0<br />

Coupling at the<br />

Interfaces (3)<br />

• It may be useful to split the problem into a free and a link<br />

problem by an additive decomposition. Define:<br />

1<br />

U<br />

−<br />

i,free<br />

Mi Fi<br />

so that: U −1<br />

i<br />

= U i,free<br />

+ U<br />

i,link<br />

U<br />

M R<br />

i,link<br />

i i<br />

• The problem becomes:<br />

Free problem<br />

(involves all dofs)<br />

⎧ M1U<br />

⎪ 1,free<br />

= F1<br />

⎨<br />

M U<br />

⎪⎩ = F<br />

2 2,free 2<br />

⎧ MU<br />

1 1,link<br />

= R1<br />

⎪<br />

⎨MU<br />

= R<br />

⎪<br />

⎪⎩ CU<br />

<br />

2 2,link 2<br />

1 1+ CU<br />

2 2<br />

= 0<br />

Link problem<br />

(involves only<br />

dofs on interface)<br />

111<br />

⎧ M1U<br />

⎪ 1,free<br />

= F1<br />

⎨<br />

M<br />

2U<br />

<br />

⎪⎩ 2,free<br />

= F2<br />

Coupling at the<br />

Interfaces (4)<br />

⎧ MU<br />

1 1,link<br />

= R1<br />

⎪<br />

⎨MU<br />

<br />

2 2,link<br />

= R2<br />

⎪<br />

CU<br />

⎪⎩ + CU<br />

=<br />

1 1 2 2<br />

0<br />

• The free problem may be solved directly to obtain U i,free<br />

• To solve the link problem, we introduce as usual Lagrange<br />

multipliers and associated reaction (interaction) forces:<br />

so that:<br />

R<br />

i<br />

= C Λ<br />

1 1 2 2<br />

T<br />

i<br />

T<br />

MU<br />

1 1,link<br />

−C1<br />

Λ= 0<br />

T<br />

MU<br />

2 2,link<br />

−C2Λ=<br />

0<br />

CU<br />

+ CU<br />

= 0<br />

Unknowns:<br />

U Λ an d U<br />

, i,link i<br />

112<br />

56

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