Create FEA Model

8/7/2019 Create FEA Model

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y Meshing is the process used to fill the solid modelwith nodes and elements, i.e, to create the FEA model.

y Remember, you need nodes and elements for the finiteelement solution, not just the solid model. The solid

model does NOT participate in the finite elementsolution.

Solid model FEA model

mes i g


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y There are three ste s t meshi g:

y Defi e eleme t attri tes

y S ecifymesh c tr ls

y Ge erate the mesh

y

Eleme t attributes are characteristics f the fi iteeleme t model that youmust esta lish rior tomeshi g. They ca i clude:

y Eleme t ty es

y

eal constantsy Material roperties

y Sectionproperties


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Element Type

y The element type is an important choice that determinesthe following element characteristics:

y Degree of reedom (D ) set. thermal element type,for example, has one dof: TEMP, whereas a structuralelement type mayhave up to six dof: UX, UY, UZ, TX,

TY, TZ.y Element shape -- rick, tetrahedron, quadrilateral,

triangle, etc.

y Dimensionality-- 2-D (X-Yplane only), or 3-D.

y ssumeddisplacement shape -- linear vs. quadratic.y SYS has a li rary ofover 170 element types from

which you can choose.


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Element rder

y Element order refers to the polynomial order of theelements s ape fu ctio s.

yWhat is a shape function?y It is a mathematical function that gives the shape of

the results within the element. Since EAsolves forD values only at nodes, we need the shape function tomap the nodal D values topoints within the element.

y The shape function represents assumed behavior for agiven element.

y Howwell each assumed element shape functionmatches the true behavior directly affects the accuracyof the solution, as shownon the next slide.


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Quadratic distributionofD values

Actual quadraticcurve

Linear approximation(Poor Results)

Quadratic approximation(Best Results)

Linear approximationwithmultiple elements

(Better Results)


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yWhen you choose an element type, you are implicitlychoosing and accepting the element shape functionassumed for that element type. Therefore, check the

shape function informationbefore you choose anelement type.

y Typically, a linear element has only corner nodes,whereas a quadratic element alsohas midside nodes.


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Linear elementsy Can support only a linear variation of

displacement and therefore (mostly) only aconstant state of stress within a sin leelement.

y i hly sensitive to element distortion.

y Acceptable if you are only interested in

nominal stress results.y Need to use a lar e number of elements to

resolve hi h stress radients.

Quadratic elementsy Can support a uadratic variation of

displacement and therefore a linearvariation of stress within a sin le element.

y Can represent curved ed es and surfacesmore accurately than linear elements. Notas sensitive to element distortion.

y Recommended if you are interested inhi hly accurate stresses.

y ive better results than linear elements, inmany cases with fewer number of elementsand total OF.


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Real onstants and Section Properties:y Used for geometric properties that cannot be

completelydefinedby the elements geometry. orexample:y Abeam element is definedby a line joining twonodes.

This defines only the lengthof the beam. To specify thebeams cross-sectional properties, such as the area,moment of inertia or dimensions, youneed touse realconstants or sectionproperties.

y Ashell element is definedby a quadrilateral or triangulararea. This defines only the surface area of the shell. To

specify the shell thickness or layers, youneed touse realconstants or sectionproperties.y Most 3-D solid elements donot require a real constant

since the element geometry is fullydefinedby its nodes.


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Material Properties

y Every analysis requires some material property input:Youngs modulus EX for structural elements, thermal

conductivity KXX for thermal elements, etc.


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Ge erati t e mesis the final step inmeshing.

y First save the database.

y Thenpress [Mesh] in the MeshTool.

y This brings up a picker. Press[Pick All]in the picker to indicate all entities.


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SOLUTION


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y There are five categories of loads:DOFCo strai ts Specified D F values, such as

displacements in a stress analysis or

temperatures in a thermal analysis.Co ce trated Loads Point loads, such as forces or heat flow

rates.

Surface Loads Loads distributedover a surface, such aspressures or convections.

Body Loads Volumetric or field loads, such astemperatures (causing thermalexpansion) or internal heat generation.

I ertia Loads Loads due to structural mass or inertia,such as gravity and rotational velocity.

A. Define Loads


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L ADING :y You can apply loads either on the solidmodel or directlyon

the FEAmodel (nodes and elements).y Solidmodel loads are easier to applybecause there are fewer

entities topick.

y

Moreover, solidmodel loads are independent of the mesh. Youdont need to reapply the loads if you change the mesh.

C

onstraintsat nodes

FEA model

Pressures on element faces

Force at node

C

onstraint on line

Solid model

Pressure on line

Force at keypoint

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Create FEA Model