Create FEA Model
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Transcript of 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