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Rock Mechanics.pdf - Mining and Blasting

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THE FINITE ELEMENT METHOD<br />

The components of [N], i.e. the terms Ni, are prescribed functions of position, <strong>and</strong><br />

[u e ] is a column vector listing the nodal displacements uxi, u yi, uxj . . . etc.<br />

The interpolation functions which constitute the elements of [N] must be chosen<br />

to return the nodal displacements at each of the nodes. This requires that<br />

[Ni]xi,yi = [I]<br />

[Ni]xi,yj = [0], etc.<br />

where [I] <strong>and</strong> [0] are the identity <strong>and</strong> null matrices respectively. Also, since both<br />

components of displacement at a point are to be interpolated in the same way, it is<br />

clear that<br />

[Ni] = Ni[I]<br />

where Ni is a scalar function of position within the element.<br />

A simple development of a linear interpolation function is demonstrated by representing<br />

the displacements in terms of linear functions of position, i.e.<br />

ux = 1 + 2x + 3y<br />

u y = 4 + 5x + 6y<br />

(6.37)<br />

The six interpolation constants are determined by ensuring that the displacements<br />

ux, u y assume the nodal values when nodal co-ordinates are inserted in equation<br />

6.37. Thus 1, 2, 3 are determined by solving the simultaneous equations<br />

uxi = 1 + 2xi + 3yi<br />

uxj = 1 + 2x j + 3yj<br />

uxk = 1 + 2xk + 3yk<br />

Solution for 1, 2, 3 <strong>and</strong> some rearrangement, produces<br />

ux = 1<br />

2 [(ai + bi x + ci y)uxi + (a j + b j x + c j y)uxj + (ak + bkx + ck y)uxk]<br />

where<br />

ai = x j yk − xk y j<br />

bi = y j − yk<br />

ci = xk − x j<br />

with cyclic permutation of i, j, k to obtain a j, etc., <strong>and</strong><br />

185<br />

2 = 2 × area of the triangular element<br />

⎡ ⎤<br />

1 xi yi<br />

= 2 ⎣1<br />

x j y j⎦<br />

1 xk yk<br />

(6.38)

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