s10_using flexible bodies

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ADM740, Section 10, May 2012

Copyright 2012 MSC.Software Corporation

SECTION 10

USING FLEXIBLE BODIES



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● In this section, you will learn how to create flexible bodies in

your models, as well as how to swap a rigid body for a flexible

body.

Using Flexible Bodies



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● What’s in this section:

● Flexible Body Overview

● Limitations of Flexible Bodies

● Getting Flexible Bodies

● Modal Superposition

● Visualization Attributes

● About Joints and Motions

Using Flexible Bodies



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● Adams/Flex uses an assumed-modes method of modeling

flexible bodies, called modal flexibility. Modal flexibility

assigns a set of mode shapes to a flexible body. This modal

method of modeling flexibility can be very useful in problems

that are characterized by high elasticity and moderate

deflections. That is, deflections less than 10% of acharacteristic length of the body.

Flexible Body Overview



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● By integrating flexible bodies into your model, you can:● Capture inertial and compliance properties during handling and

comfort simulations.

● Predict loads with greater accuracy by allowing MD Adams to

account for flexibility during simulations.

● Study deformation (stress/strain can be calculated using Adams/Durability).

● Examine the linear system modes of a flexible model when you

use Adams/Flex with Adams/Linear.

● You should use flexible bodies wherever you expectcomponent flexibility to affect the dynamic behavior of your

model or when you require accurate information about the

deformations of a component in your model.

Flexible Body Overview



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● When you use flexible bodies, remember that flexible body

deformations are a linear combination of deformation shapes.

Consequently, take special precautions when modeling

higher order deformations, such as those that occur when

deformations are large, or when attempting to correctly model

centrifugal stiffening of rotating systems. You can overcomethese limitations by dividing a flexible body into multiple

flexible bodies and assembling them in Adams/Car.

● Also, note that flexible bodies are not parametric. If you want

to substitute a new flexible body in your system, you mustcreate a new flexible body.

Limitations of Flexible Bodies



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Getting Flexible Bodies

● Two ways to incorporate flexible bodies● Importing modal neutral files (.mnf) - To create a new flexible body,

go to Build Part Flexible Body in the Template Builder.

Adams/Car imports the .mnf file and creates the flexible body.

You need to create interface part at the interface nodes to attach

other parts to the flexible body.● Replacing a rigid part with a flexible part at the subsystem level

using the Flex Swap Dialog box.

For more information on Adams/Flex, see the Adams/Flex

online help.



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● Adams/Flex treats flexible body deformations as small, linear

deformations relative to a local reference frame undergoing largemotion

● Represent deformation as a linear combination of mode shapes

● Simple example

● Craig-Bampton modes distinguish boundary nodes from interior nodes

Modal Superposition





● Fixed boundary normal modes

● Constraint modes

For more Adams/Flex theory, see the Adams/Flex online help (specifically,

see the index entry Craig-Bampton modes - considerations whentranslating FE models).

Modal Superposition





● You can use the Plot Type option

to display contour or vector plots.

Visualization Attributes




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2012 MSC.Software Corporation

● Color contours

● Indicate the deformation magnitude of the flexible component

● Are continuous attributes, not discrete nodal values

● Show relative deformation, not stress

● Contours turned on in Adams/PostProcessor with the Contour Plots

tab





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● Deformation scale factor

● Used to exaggerate deformation

● Can scale up or down

● Constraints can appear violated when the scale is >1. This is merely

visual: the analysis will, of course, maintain the constraints you’ve

defined.● When the scale is equal to 0, the shape will not be deformed but the

colors will be displayed.





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● Datum node

● Deformation is a relative term and should be expressed with respect to

a node:

● You select which node Adams/Flex considers as the undeformed (datum)

node and then all other nodes deform relative to it

● Nodal deformations are colorized relative to the datum node

● LBRF (local body reference frame, the default, also known as the body

coordinate system or BCS) is in the same location as the reference frame

used in FEA





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Node 1000

Node 1001

Node 1000

Node 1001

● Example of using different datum nodes

● Here, node 1000 is the datum node.

● In this case, node 1001 is the datum node.

● The color red denotes maximum deformation relative to the datum

node.





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About Joints and Motions

● Using joints

● After you bring a flexible body into Adams/Car, you can connect it to your rigidmodel using the Adams/Car library of constraints.

● You can avoid nodal mismatch by:● Using consistent numbering

● Paying attention to alignment issues

● Joints are commonly used to attach flexible bodies

● Fixed● Revolute

● Spherical

● Universal (or Hooke)

● If you build a template with flexible body then you need to create interfaceparts to attach joints to the flex body.

● On the other hand if you use the flex swap tool at the subsystem level theadditional step is not required.

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