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Derive the shape function for a linear quadrilateral element and show its variation over the element.

shape function

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Shape function for Quadratic Element

quadratic element

The element has 3 nodes.

Pilot function will be ϕ=Aˉx[ˉxhe2][ˉxhe]   ....(1)

For node 1, ˉx vanishes.

ϕ1=A[ˉxhe2][ˉxhe]

At node 1, ϕ1=1 & ˉx=0, above equation becomes

1=A[he2](he)

A=2h2e

ϕ1=2h2e(ˉxhe2)(ˉxhe)

Rearranging the term

ϕ1=(12ˉxhe)(1ˉxhe)

diagram

For node 2, term (ˉxhe2) vanishes.

Equation (1) becomes ϕ2=Aˉx(ˉxhe)

At node 2, ˉx=he2    ϕ2=1

1=A.he2(he2he)

   =A(he2)(he2)

A=4h2e

ϕ2=4h2eˉx(ˉxhe)

Rearranging the term we get

ϕ2=4ˉxhe(1ˉxhe)

diagram

For node 3, (ˉxhe) vanishes.

Equation (1) becomes ϕ3=Aˉx(ˉxhe2)

At node 3, ϕ3=1   ˉx=he

1=A.he(hehe2)

   =A(he)(he2)

A=2h2e

ϕ3=2h2e.ˉx(ˉxhe)

Rearranging the term we get

ϕ3=ˉxhe(12ˉxhe)

diagram

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