The characteristics of an interactive graphics systems to facilitate the finite element method of structural analysis are described. The finite element model analysis consists of three phases: (1) preprocessing (model generation), (2) problem solution, and (3) postprocessing (interpretation of results). The advantages of interactive graphics to finite element structural analysis are defined

Burk, R. C.

Held, F. H.

English

NASA Technical Reports Server

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AN INTERACTIVE GRAPHICS SYSTEM !!
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TO FACILITATE
FINITE ELEMENT STRUCTURAL ANALYSIS !
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a_. t
BY ROBERT C. BURK AND FRED H. HELD
MCDONNELL DOUGLAS ASTRONAUTICS COMPANY - EAST
SUMMARY
Industry's growing use of finite element structural analysis
requires that an increasing portion of the engineer's time be spent
in building, checking, executing, and interpreting results of
finite element models. The following discussion explains the use
of a rapid, inexpensive, graphically oriented system for performing
this job. Wi_% much of the bookkeeping drudgery removed and the
i visibility of results enhanced, the inspiration/perspiration ratio }I '_, of the engineer is significantly improved, i
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I INTRODUCTION !
I The effectiveness of a finite element analysis depends on the
, accuracy with which the model represents the actual structure and
on the time and money spent to build, solve, and interpret the
analysis results. Usually, finite element analysis involves the
generation and manipulation of large quantities of data by hand
using up valuable engineering time and introducing many i
opportunities for human errors. A typical analysis would normally _ j
require tne following steps_
I. Idealize the actual structure int discrete elements.
2. Make sketches of _,e idealized _gdel and label with node
and element numbers for use An referencing element _:_
properties, applied loads, reactions, output results, etc.
3. Fill out data sheets. _
., 4. Have data sheets keypunched. _5. Obtain listing of deck and check for any errors (keypunch _
or coding).
6. Make appropriate corrections.
7, Obtain batch plot of structure to check for incorrect
element connectivity or node location.
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8. Submit for batch solution, i
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9. Evaluate results for any errors. Often, several computer i_
runs (with their corresponding costs and turnaround
delays) are required before error-free output is obtained, i
10. Obtain batch plots of deformed shape. Ii
11. Make freebody sketches of components showing reactions and i
internal loads. These are used for detailed stress t
analysis and formal reports of the analysis.
Each of these steps takes time and allows considerable chance for
error.
In order to minimize the time spent on each of the steps, to reduce
the chance for error, and to enhance the understanding by the
, engineer, interactive graphics is 5eing harnessed. Reference I
I presents an excellent review of theuses of interactive computer
i graphics. In the area of structur&l analysis computer graphic
applications, a system developed for two-dimensional modeling and
display of results is discussed. This has been implemented on both
the UNIVAC 418 - DEC 340 and the IBM 360/50-IBM 2250. Reference 2
_,. presents a system developed by the Jet Propulsion Laboratory usinginteractive graphics for three-dimensional model checking.!
I This paper presents a rapid, low cost system which uses interactive
graphics in bo_, _e preprocessing and postprocessing of finite
element data. It can be used wi_, the NASTPAN, ICES STRUDL, and
CASD (Computer Aided Structural Design -- an in-house developed and
used program) finite element analysis programs to minimize modeling
errors, reduce the time required for the design/analysis cycle, and
maximize the visibility of results.
A PROMISING SOLUTION i
The system being effectively used by the McDonnell Douglas
Astronautics Company, Eastern Division, places the engineer at an
interactive g_aphics terminal. He can build, check, edit, solve, _
and interpret the results of a finite element model static analysis
without leaving his chair. Many useful alternatives are controlled :_
by the user at his remote terminal, i
The basic hardware employed is an inexpensive, semiportable,
Computek interactive graphics terminal wi_, attached hard copier
and digitizing tablet (see Figure I). The terminal is a teletype
compatible device with a cathode ray tube (CRT) that can transmit
and display both alphanumerics and graphics. A hand held stylus is
used to identify point coordinates on the digitizing tablet. These
_, coordinates along with the status of three push buttons and two
_ switches are transmitted to _le computer for processing. The
_ stylus position on the tablet is tracked on the CRT, allowing the
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user to coordinate k,is input with the displayed image. The hardcopier makes 8 I/2 by 11 inch copies of the current image on the
screen (in approximately 10 seconds for about 6 cents each) for I_documentation. The terminal communicates with our XDS Sigma 7 }
conversational, direct access computer via a standard telephone.
Thus, the user may locate his terminal anywhere electrical outlets i
_ and a telephone exist. !
A finite element model analysis can be broken into three phases:
preprocessing (model generation), problem solution, and i
_ postprocessing (interpretation of results). The use of our system i
_ will be explained in each of these phases and demonstrated with an
l example problem. The problem involves determination of internal
_ loads and deflections for a swept, multi-cell wing structure
, _ _ubjected to a simplified landing condition.
PREPROCESSING
Preprocessing includes building, checking, and editing a finite
element model to prepare it for solution. The preprocessing phase
is schematically represented in Figure 2. There are several
methods we use to build a finite element model. The most common
method is to model in the interactive graphics mode using the
_ finite element modeling (FEM) program. Alternately, the data can
be entered in card format directly into an on-line file using a
conversational terminal and the computer's editor system. The
model data may also be entered onto data sheets, keypunched, then
loaded into an on-line file on the computer. In many cases, a
small special purpose computer program is used to generate sections
of the desired model where extreme accuracy is required or where
structural geometry is very repetitious. For all methods, the F_4
program is used to display the model for checking and to make any
additions or necessary corrections.
8
When using the FEM program, node geometry is normally digitized
directly to the computer utilizing actual scale drawings or layouts
of structural cross sections as shown in Figure 3. Optionally,
points can be keyed in (to obtain a more accurate location) or
_. specified as vertical or horizontal from previously defined nodes.
Bar elements may be indicated between these nodes while in this
two-dimensional -mode. The wing problem was entered using the
parallel rib stations as the entry planes. A typical CRT display
of one of these sections, as copied by the attached hard copier, is
shown in Figure 4. The type of operation performed is controlled
by the menu items on the CRT and the position of the buttons and
": switches on _le digitizing tablet. For example, after the first of
t,' the two similar root sections was idealized, the menu item "DUP
li ALL", along with the appropriate Y station value of the second
station, was indicated. This is done by moving the hand-held
stylus across the tablet until the cursor on the screen is in the
' . target "O". The stylus is then depressed on the tablet which sends
_ _%e tablet coordinates of" _,is point to the comput_z. The computer
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decodes this into a selection of the a_propriate menu item. For ..}
_e "DUP ALL" item, the computer responds with a prompt on the I'
screen to enter _le new plane station. After these data are typed !'
in from _le keyboard, all elements in the current plane are i
duplicated at the newly defined plane. The other rib stations are t
entered by using the "NEW CUT" option on uhe menu and arc digitized !
as was the first rib. }!!
After building all cross sections, a three di_ensional display is
obtained by indicating the "EXIT" item of the "2-D Node Building i
Mode" menu. Control is then transferred to the "3-D Element i
Building Mode". The first activity in this mode is the display of
the three-dimensional projection of the current model in the last
defined orientation with a new menu along the right hand edge of
. the screen. This display is shown in Figure 5 for the example
problem. All remaining connection elements are nov added. This is
accomplished by indicating the appropriate element type from the
menu, and then indicating the corresponding nodes on the model.
i Representations of _,e elements (bars, bending bars, shear panels,
triangular plates, and quadrilateral plates) are shown immediately
I on the CRT as they are generated, thus providing graphic assurance
of model correctness. _[any options are available in _,is mode and
i are displayed as a menu along the right hand side of the screen.
One such option, "WINDOW", allows magnification of the specified
_ portion of the display for eas in viewing or inputing data in
complex areas. This display is obtained by indicating the menu
I item, then the lower left and upper right corner of the desired
• view. The screen is erased, _e indicated portion of the model is
rescaled to fill the screen, and the display redrawn. A companion
' mehu item, "PAN", allows the operator to move the center of the
window to a new point on the structure (at the same scale factor)
as indicated by the location of the cursor. Thus, he can
effectively pan across the structure in discrete steps at a
magnified scale. Additional menu items allow the input of reaction
and applied load vectors, load magnitudes (for up to six
conditions), and symmetry plane (if any). The example problem,
with reaction and force vectors at this stage of completion, is
_ii shown in Figureby6. program.
The problem is then stored in a data file in the appropriate format _"
for one of the following three computer codes: NASTPAN, ICES
_,., STRUDL, or CASD. A set of standard program control "cards" and
default value element properties are automatically inserted into
the data file ti_e
On some models, we have saved 80 percent of the time over the old
method of submitting tabulated data for keypunching. The example
problem took 46 minutes of "clock on the wall time" and expended
approximately 7 dollars of computer costs to bring it to this stage
of completion_
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_'_ At this point in the idealization, a listing of the data file is
_ typically obtained on either a time-sl_are alphanumeric terminal or
a high-speed batch printer (See Figure 2) and appropriate I
solution commands and element properties edited from the default p
values I these same data can be edited for dynamic analysis (e.g.,
add inertia matrix) and solved for mode shapes and frequencies i
using the computer code consistent with the data format. I
Additional preprocessing programs can be run for such functions as I
adding the appropriate job control language heafler cards and ,I
performing additional data checks. For large, complex models, the
geometry can be plotted on a remote plotter to obtain larger copies
of the structural idealization. At the end of _ese operations, Ii
the completed model is ready for sol-tion. Portions of the NASTPAN
r
card image data file generated for the example problem are shown in
Figure 7.
}laving formulated the input data using preprocessing programs, such
errors as "key punch errors", data in wrong card columns, incorrect
i node number reference, etc., are largely eliminated and the .
probability of a successful solution on the first try is greatly
, increased.
PROBLEM SOLUTION
_ Solution can be initiated by a command from the user's remote
terminal, as shown in Figure 8. Small problems (less than [50
degrees of freedom), such as the example problem, are usually
solved using an abbreviated version of CASD which can be executed
in real time on the XDS Sigma 7 computer. Larger problems, and all
NASTRAN and ICES STRUDL analyses, require a batch solution on our :
IBM 360/195-195 computer. In Gither case, results are normally
routed back to an on-line disk file for suisequent batch listing
and evaluation using postprocessing programs.
5;
POSTPROCESSING _._
Postprocessing includes all processes that operate on the solution "_
results. The postprocessing phase of our system is schematically _:
represented in Figure 9. For example, the first postprocessing '_
operation normally executed is the editin_ of the solution file to •_,_
find any error statements. This can be performed at any terminal ._
connected to the direct access computer. ,_/_
The primary postprocessing tool is a program called VUOUT. This _,
program allows results to be graphically displeased at the user's _:_
terminal. It permits isometric viewing for static loads of the :q
undeflected shape with, optionally, the deflected shape (with any
magnification), buckle shape (NASTPAN only), internal loads for i_
axial ba-s and shear panels, applied joint loads, and reactions for
static p_obl_ms. These may be displayed with superimposed bar or
node numbers, and may be of the entire model, or of only a
specified small section. In addition, load sheet (free body)
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1974006473-671
Ldisplays of a single bar or panel element with its applied loads |
may be requested. In this case, aFpropriate element properties I
(length, area, thickness, modulus of elasticity, etc. ) can be !
displayed witA the load._. I
The program first processes the solution results line by line and I
writes the data into a compacted on-line file for ready access by i!
the display subroutines. This file is only generated during the
first viewing of tile results. Subsequent reviewing of the data _
bypasses this step. The display tecnnique requires only a minimal I
amount of data in core at any one time and hence allows the results
of an essentially unlimited size model to be displayed. The type of
display is controlled by the menu selections shown in Figure 10.
Actual hard copies of the results of the example problem are shown
in Figures 11 through 16. llard copies provide a permanent record
for futher study, stress analysis, and use in reports. The
graphical displays highlight any errors and enhance the engineer's
understanding of the problem, allowing a more creative use of the
finite element technique.
CONCLUSION
The application of interactive graphics to finite element i
.. structural analysis is one of the most exciting developments since j
the introduction of high speed computers. Significant engineering {
time can be saved and tile usability of results enhanced. It allows i
t/_e engineer to concentrate on t/le creative aspects of his job,
freeing him from tedious mechanical tasks of input coding and
output data reduction. It is no substitute for good engineering,
but rather provides another tool for unlocking the complexities of
intricate st-uctures. Interactive graphic displays of the problem
and its solution enhance understanding of structural interactions.
'£he coupling of our preprocessing and postprocessing _;rograms to
work with analysis programs, such as NASTRAN, has given us an
inexpensive tool to quickly analyze complex structures. This
system ha_ played an important part in our static and dynamic
structural analyses of several projects including our NASA Space _
Shuttle effort, the avy Harpoon missile, and the NASA Skylab, as I
well as several advanced design efforts. It has proven to be an
i[_ effectiv_ tool to eliminate many errors a0sociated with finite
< element model generation and interpretation of the analysis
. i results.
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ACKNOWLEDGEMENTS
1, Tl,e authors wish to acknowledge the many contributors to tile
: structural analysis system presented herein. The basic system |
requirements were defined by the Strength and Structural Dynamics I
Departments of iicDonnell Douglas Astronautics Cc,1_any - East i
(MDAC-E). Actual software development _.as berformed primarily by i
the authors, Dr. b .C. "£sai, and P. A. Giarritano. The CASD I
analysis program was developed Ly tile Advanced Procedures Group Ii
within Structural Engineering at tile Douglas Aircraft Company and
incorporated into our XDS Sigma 7 computer by _LDAC-E.
REFERENCES
I. Prince, II. Uavid, Lockileed-Geor .oia Company, "Interac rive
', Graphics for Computer-Aided Design," Addison-Wesley PuLlisi_ing
' Co., 1971[
i 2. Katow, 14. Smoot, Cooper, Barry Ii., "N,_STP/_ Data Gene 'ion and
. l!anagement Using Interactive Graphics," paper cont..ined in
"NASTRAN User's Experiences," NASA TMX-2637, 1972.
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1974006473-673

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_-===" _ 77 INTERACTIVE
_ ALPRANUMERIC J GRAPHICS
TERMINAL %'_.l T.E,RMIRAL
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DIREC1ACCFSSCOMPUTER :
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LARGE PROBLEMS IN BATCH
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1974006473-680
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FIGURE 10-'VUOUT" MENU OPTIONS l
FOR POSTPROCESSING DISPLAY. [
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2FIGURE I1.- "VUOUT" DISPLAY OF DETAIL SECTION WITH
NODE NUMBERS AND PANELS INDICATED. _
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An interactive graphics system to facilitate finite element structural analysis

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