Altair Students Guides - Instructors Manual - A Designer's Guide to FEA

Student Project Summaries A Designer’s Guide To Finite Element Analysis

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Contents

Introduction.............................................................................................2

Installation Instructions: .......................................................................2 FRF Analysis of a Cross Member................................................................3

Description of the Problem ....................................................................3 The Analysis Model ...............................................................................4 Results.................................................................................................5 Further Work........................................................................................5 Summary .............................................................................................6

Thermal Expansion Of Screw Shaft............................................................7 Description of the Problem ....................................................................7 The Analysis Model ...............................................................................8 Results.................................................................................................9 Further Work........................................................................................9 Summary ........................................................................................... 10

Transient Analysis Of Gun Barrel ............................................................. 11 Description of the Problem .................................................................. 11 The Analysis Model ............................................................................. 12 Results............................................................................................... 13 Further Work...................................................................................... 13 Summary ........................................................................................... 14

Force & Stress Analysis of Engine Mechanism .......................................... 15 Description of the Problem .................................................................. 15 The Analysis Model ............................................................................. 16 Results............................................................................................... 17 Further Work...................................................................................... 17 Summary ........................................................................................... 18


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Introduction This material is best used after reading the book A Designer’s Guide To Finite Element Analysis. Access to HyperWorks software is not essential for you, the instructor. Of

course, if you choose to solve the problem yourself before working with your students, you will need HyperMesh and OptiStruct.

This book describes 4 assignment problems that highlight different applications of HyperWorks. Each problem is independent, and is complete

in itself. Students may choose to do more than one, depending on their

interest.

To make best use of this material you will need a computer with a sound-

card and speakers. Your computer should have a media-player programme

(such as Windows Media Player) and an Internet Browser that supports JavaScript. The material can be copied to a server and accessed by clients.

You can customize the HTML files to suit your

requirements. After opening the file, double-

click on any text to edit it. Use the save changes link on the left of your Browser window when

you are finished.

Installation Instructions: 1. Copy the folders to your computer or to your server. If you are

working on a server, it is a good idea to set the folders to “read only” to prevent inadvertent modifications.

2. The videos are best played in full-screen at a resolution of 1024 x

768. You may need to install the CamStudio Codec to view video on your computer. To do this, right-click on the file camcodec.inf and

choose Install from the popup menu. You may need administrator privileges to do this.

3. Ensure that JavaScript is enabled on your browser.

4. Each folder contains one HTML file. Double-click on it to open the instructions.

5. Data files are provided as relevant – IGES files, HM files, etc.

6. In case you need support, contact your distributor or email

[email protected]


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FRF Analysis of a Cross Member Areas covered:

• Geometry abstraction for FE modeling • Automatic mesh generation • Application of FRF loads • Design interpretations of FE results

Software used: • HyperMesh • OptiStruct/Analysis • HyperView

Description of the Problem Vibration response plays an important part in

the design of vehicle frames. The range of excitation is usually specified by the vehicle

designer for each sub-system.

In this project, a cross-member has been

proposed for a vehicle chassis. The purpose of the analysis is to evaluate the response of

the component as the frequency is "swept" through the given range - from 0 to 1000 Hz.

The starting point of the problem is the IGES file of the CAD assembly. A frequency-domain

load is applied to simulate a frequency-sweep: from 0 to 1000 Hz.

The student should be encouraged to understand the modal-testing approach so

that the importance of the various data is understood from a designer's perspective. The

vibration characteristics of the assembled frame will be very different from that of the

single component, of course, but the

approach of setting requirements on individual components is essential at the

preliminary design stage.


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The Analysis Model The model is characteristic of many sheet-

metal parts: small radii, flanges, bends, etc.

The element choice is easy - shell elements.

A shell element represents the "neutral"

surface of a thin shell. Except for a flat plate,

the neutral surface is not at the center. The assignment discusses the relative benefits of

generating elements on the inner surface, the outer surface or the mid-surface. The

discussion highlights the fact that if thin-shell

elements are justified, the difference is not important from a mechanics point of view. It

is more important for convenience of

modeling. When working with assemblies,

considerable effort goes into extending or trimming surfaces to ensure that they meet!

HyperMesh does a good job of mid-surface extraction. We will use it simply because it

makes our meshing job easier, not because it is more accurate than meshing the inner or

outer surfaces.

Since the model is small, we will not need to

worry much about the element size, though the student must, of course, ensure that the

results are adequately accurate.

The choice of appropriate units is also

discussed. Analysis results and data are specified in cycles-per-unit-time. The use of

SI units means we can work with Hz (cycles / second).

The specifications call for an excitation between 0 and 1000 Hz. We use the Lanczos

method to obtain mode shapes upto 3000 Hz.

We use the modal-analysis method, where

we approximate the transient behavior as a weighted sum of the mode shapes. The


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source of error lies in the choice of mode shapes, so to be safe we use

mode shapes upto 3000 Hz - thrice the excitation range.

Results Interpreting the results of the optimization requires a good understanding of the FRF approach.

Since the frequency-domain and time-domain

are entirely equivalent, the results can be

viewed either as phase-magnitude plots (i.e. the frequency domain) or as animations or

time-histories (i.e. the time domain).

Depending on the output-control options,

either or both can be generated by OptiStruct / Analysis.

Further Work The assignment brings home the advantages of OptiStruct / Analysis:

• Excellent data import capabilities

• Quick, easy and convenient geometry abstraction

• Powerful mesh generation

• Easy control over the solution process

• convenient reporting options with easy viewing

Depending on their proficiency, students may want to research

• the use of optimization to improve response

• a comparison between the direct-

analysis method and the modal-

analysis-method • the sensitivity of the results to mesh

size

• the sensitivity of results to the

solution parameters: FREQ1 options, number of frequencies, etc.


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• the response for multiple loads with a phase lag

Summary By the end of this assignment, the student will know how to

• import IGES files • use the Model Browser

• zoom, pan and rotate

• change colors of entities

• control visibility of geometry

• create collectors for materials, elements, forces and restraints

• measure distances and the diameter of circles • generate mid-surfaces automatically • use QI meshing • find the centers of circles • use consistent units

• check for different types of element-edges - free, shared, etc. • fill and stitch surfaces

• use temp nodes

• apply frequency-domain loads

• use the modal-method for FRF analysis

• plot stress contours • view deformed shapes

• check for warnings using the text output files • view results in HyperView


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Thermal Expansion Of Screw Shaft Areas covered:

• Geometry abstraction for FE modeling • Hex mesh generation • Application of temperature loads • Use of symmetry boundary conditions • Use of contact in FE analysis • Design interpretations of FE results


Description of the Problem Aircraft components go through stringent

testing before they are accepted. The testing conditions are often more demanding than the

anticipated deployment conditions.

A design proposal has been received for a

subassembly, and the task is to simulate its

performance. The screw-shaft subassembly,

which fits into a housing, is manufactured to very close tolerances. The dimensional

accuracy is measured in 10s of microns.

In the test chamber, the complete assembly is

cooled to -40 degrees Kelvin and tested. It is subsequently raised to 135 degrees Kelvin and

tested again.

Will the sub-assembly function correctly? How

will the gap in the groove behave?


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Since the entire assembly is "soaked" in the

testing-chamber to reach the uniform

temperature, we do not need to perform a thermal analysis. Instead, we need to apply

the temperature field and calculate the deformation of the components. In case there

is interference, we can then use contact

analysis to investigate stresses.

The Analysis Model The model is characteristic of turned parts,

displaying axial symmetry.

The element choice is easy - solid elements

While tetrahedral elements are easy to create,

we will want to use hexahedral elements since the clearances are very fine. We will use three

different facilities HyperMesh gives us to create the hexahedral mesh on the shaft and

the plugs.

We must, of course, ensure that the results

are adequately accurate. A convergence study is very important, particularly since the curved

faces of the grooves should be captured

adequately accurately.

We will want to use quite a fine mesh in our final analysis, so we should take advantage of

symmetry. Since there are 4 plugs, we use quarter symmetry to reduce the model size.

Since the differential expansion of the shaft

and the plugs is the likely source of design-

problems, the student should appreciate the importance of the reference temperature and

the coefficient of thermal expansion.

Automeshing can be used if we are willing to

settle for tetras. HyperMesh makes it very easy to do this. Since we want hexahedral

elements, however, we use a different

approach. First, since the shaft has an axis of


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symmetry, we mesh one face with

quadrilaterals and "spin" these to get hexas.

Next, we focus on one of the plugs. We create a 2D mesh on one face, and use HyperMorph

to map these to another face. Once we have this, the Linear Solid option generates hexas

between the element sets. Finally, we "drag"

elements along a line to generate the stub-handle on the plug.

The "Position" option quickly lets us move an

entire mesh from one position to another,

completing our mesh.

Results Interpreting the results of the optimization

requires some care. In the initial analysis, we are trying to decide whether or not a more

expensive contact analysis is required or not. So the focus is on the deformation pattern of

the shaft and the plugs.

If contact is necessary, a subsequent analysis

with contact yields the actual deformation pattern.

Further Work The assignment brings home the advantages

of OptiStruct / Analysis:






You may choose to assign further investigations to your students based on their

level of proficiency on CAD, the time available,

etc.


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Some of the areas that could include

• the use of contact to assess stresses

• the use of spring supports on the exterior, to simulate the flexibility of the housing

• an investigation into the sensitivity to mesh size

• a study of possible alternate steel-alloys

for the plugs


• import IGES files • use the Model Browser




• create material collectors • create component collectors

• create load collectors

• create loadcases

• measure distances • use consistent units

• work with different types of edges

- free, shared, etc. • fill and stitch surfaces

• measure the diameter of circles

• find the centers of circles • use temp nodes

• use autocleanup to do all of the

above

• create restraints or SPCs • apply symmetry BCs • use HyperMorph to map elements

to a geometry • use the Linear Solid option to

generate hex elements • check for warnings using the text

output files • view deformation plots and stress

contours


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Transient Analysis Of Gun Barrel Areas covered:

• Hexahedral Mesh Generation • Direct-integration for short-time response • Generation of plots for parametric studies • Design interpretations of FE results


Description of the Problem An artillery system is being designed to allow

for four barrels, and the goal is to understand the impact of the firing order on the

deflection of the barrel tips. When the shell

leaves a barrel, the tip of the barrel

experiences a disturbance. As a result, all 4

barrel tips vibrate. If the amplitude of the vibration has not died down before the next

shell is fired, accuracy will suffer.

Our design problem is to estimate the

amplitudes of vibration of the barrel tips under different firing orders. if we number the

barrels 1-2-3-4, several choices are available to the artillery designer: the order could be

1/2/3/4, or 1/3/2/4, etc.

Since the gun is still at the design stage, the

load characteristics are not frozen. The simulation should also cast light on the

sensitivity of the proposed configuration and firing order to the amplitude-versus-time

variation of the load.

A Finite Element model is used, with the

direct-integration approach, to carry out the simulation. A series of analyses will be carried

out to generate graphs of the displacement

characteristics of the barrel-tips under


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different firing orders, and under different

amplitude-time variations of the impulse load.

The Analysis Model We will want to carry out the analysis for a few second after the excitation has ceased.

The graphs we obtain of amplitude vs. time can be used by the designer to estimate when

it is safe to fire the next round.

Beam elements would yield a very quick

model, but the problem lies in connecting the baffle plates to the barrels. Shell elements are

a possible choice for the baffles, but shells

and solids are incompatible elements - shells have 6 dof / node while solids have 3.

In a geometry like this, hexahedral elements

are almost as easy to create as tetrahedra. Accordingly there is no justification to use the

less-accurate tetras. Therefore we use

hexahedral elements for all components. We

take care to ensure that nodes match up at

the junctions of the baffles and the barrels, since as per the assembly instructions the

baffles are press-fitted. We assume perfect

transmission of forces from the barrels to the baffles.

We use the different facilities HyperMesh

gives us - linear solid, ruled-surface meshing, etc.- to create the hexahedral mesh.

First, for the barrels, we use the "ruled"

option to generate elements between curves.

We then use the linear-solid option to create hexas. Next, we copy elements from one

barrel to another, filling all 4 with elements.

When meshing the baffles, the elements on

the barrels must match those on the holes in the baffles. This ensures continuity of forces.


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Clamps at the specified locations are best

represented by SPCs. The loads are a little

more complicated. For the first analysis, we assume the load-vs-time curve is triangular:

going from 0 to 200N in 1 second and falling back to 0 in the next second. We specify the

curve using the TABLED1 card. The DAREA

card specifies the amplitude of the excitation, and the TLOAD1 card puts the curve and the

amplitude together.

We then specify the time-integration method,

by choosing delta-t, which is the time-step size and the number of time steps.

Results Animated plots can be viewed, but graphs of amplitude vs. time are more useful in this

case.

The assignment shows how to evaluate results

as time-history plots, and how to export these

to files for use in subsequent reports.






• convenient reporting options with easy

viewing

You may choose to assign further investigations to your students based on their

level of proficiency on CAD, the time available, etc.



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• the use of spring supports at the base, to simulate the flexibility of the support system

• an investigation into the sensitivity to mesh size

• a study of possible alternate steel-alloys for the baffles

• an investigation into the response if perforated baffle plates are

used, which is likely to be the case when the design evolves


• import IGES files of assemblies into HyperMesh • use the Model Browser



• delete unwanted imported data


• create material collectors

• create and edit component collectors


• create restraints or SPCs

• measure distances

• use consistent units

• apply time-variant loads as (time,amplitude)

• perform an analysis and obtain baseline results

• check for warnings using the text output files


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Force & Stress Analysis of Engine Mechanism Areas covered:

• Geometry Abstraction for FE modeling • Automatic Mesh Generation • Modeling for MBD • Use of Component Mode Synthesis

Software used: • HyperMesh • OptiStruct • HyperView

Description of the Problem An assembly model for the engine of a

model aircraft has been proposed. The

designers want to know at what speed the engine can be run without the stress in the

connecting rod exceeding the permissible

stress for the material.

This problem involves more than just an "FEA for stress analysis". Forces and

restraints are not supplied. The designer has to estimate these and then perform the

stress-analysis. The components move as a

mechanism so the first task is to calculate the forces in the components as the engine

reciprocates. These forces should then be used to calculate the dynamic stresses in

the component of interest - the connecting

rod.

The first task involves a rigid-body analysis, while the second involves a flexible-body

analysis. The approach taken in this assignment makes use of the capability of

OptiStruct / Analysis to mix both forms of

analysis using component-mode synthesis (CMS).

The assignment explores the use of


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"bodies" and "joints" together with

"elements".

The Analysis Model The assignment involves continuum mechanics for stress analysis, and rigid

body dynamics for calculation of forces.

The latter does not require 3D geometry.

The mass and inertial properties are adequate. However the availability of the

3D geometry makes it easier to understand the results of the analysis. Also, the use of

CMS means that the "flexibility" as

calculated by the finite element method can be used to calculate dynamic stresses

easily.

A "traditional" analysis would require that • the properties be calculated using a 3D

model, typically a CAD model • these properties be transferred to a

motion-simulation tool that can perform "dynamic" analysis, not simple

"kinematic" analysis • the dynamic simulation be carried out

and forces obtained as output

• forces be transferred to a Finite Element model

• the stress analysis be performed using the Finite Element model

With the integrated approach, the various "transfers" of data are eliminated, making

the process easier and faster and reducing

scope for error.

The modeling focus is on creation of joints and bodies. OptiStruct / Analysis requires

that the two grids be used to define revolute and translational joints - one grid

on each of the bodies. The grids that define the joints need to be on the axis of

rotation, for a revolute joint. Since there

may not be elements at the desired


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location, we make liberal use of "rigid"

elements. This approach allows us to define

the grids for the joints at locations that are correct from the point of view of the multi-

body solver.

The use of "ground" bodies means we do

not need any restraints on the FEA model. We provide an initial velocity to the piston,

to simulate the effect of a "kick-start". Unlike an FEA solution, the time-integration

scheme used by the MBD solver embedded

inOptiStruct / Analysis calculates the step-size for time-integration internally.

Running OptiStruct/Analysis is easy, but

errors can occur and can be diagnosed using the text otput file and the "fem" file

Results The motion of the bodies is best interpreted

using animations. These are different from

the animated plots of a more "traditional"

FEA. In this case, the deformations are large. HyperView allows us to view the

animation, to superimpose stresses on the

animated displays, and to plot graphs of forces at points of interest.







You may choose to assign further investigations to your students based on


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their level of proficiency on CAD, the time

available, etc.


• the use of a higher-quality mesh

• application of a constant angular velocity motion to the crank

• use of the Flexible Body definition option (PFBODY) for the connecting rod

• an investigation of the variation of the stress in the connecting rod with

changes in the rpm of the engine

Summary By the end of this assignment, the student

will know how to

• import IGES files of assemblies

into HyperMesh • use the Model Browser




• create material collectors

• create component collectors


• define units for the MBD solver

• mix flexible and rigid bodies in

the same model • create flexible bodies, rigid

bodies and grounded bodies

• apply initial motion to bodies

• define revolute and

translational joints • check for warnings using the

text output files

• view animated plots of stresses

• view animated plots of the

mechanism’s motion

Documents

Altair Students Guides - Instructors Manual - A Designer's Guide to FEA