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element with an offsetshear center formodeling channels,
saysKensinger. But if yourFEA program doesnt
have such an element,youd have to model thechannel geometry
usingplate or solid elements toget the correct answer.
An FEA model canalso be used to determineat what condition
astructure deviates fromthe linear behaviordefined by most
simpleequations. In lateranalyses, the equationcan be used until
nonlin-
earity affects the per-formance of similarstructures. This
reducesthe time needed to runFEA. And while youredoing this, you
should belearning, he adds. Bythe time you get a finalmodel, you
should haveidentified known andunknown variables, amax and min
range for
variables, whether theyare linear or not, and
what type of nonlineari-ties exist. Each ring ofthe target
clarifies theobjective of the analysisand leads closer to thebulls
eye.
Another perspectivecomes from an analyst ata Midwest
aerospacemanufacturer. Forexample, to find thefrequency of a plate
withirregular or complexgeometry, he suggests
first get a feel for theanswer by finding thefrequency of a
plate withregular geometry onethats rectangular orsquare with
dimensionssimilar to the irregularplate. The value can bechecked
with a handbookto get a ballpark figure.
FFT analysis isanother tool for
frequency analysis. Itsessentially an oscillo-scope working with
acomputer. In a nutshell,the user hits a sample
part with a hammer andthe machine finds theparts natural
frequen-cies. Analysts are usuallyinterested in the funda-mental
mode. So if thematerial properties of aprototype are
completelyunknown, the analyzercan sense the
stiffnesscharacteristics, then givethe frequency along withclues
for the materialsother properties.
For instance, expertssuggest workingbackward with an
appro-priate equation. Youinsert values forstiffness, modulus
ofelasticity, or density, andsolve for the unknown.Stiffness is a
function ofmodulus of elasticity andmoment of inertia. Whenthe
geometry is known,the missing variable isthe modulus of
elasticity.
This is a simple yetpowerful method, sayexperts, because as
moreplastic comes into use,
vendor-provided infor-mation has such a widerange, it is not as
usefulas it should be. Findingproperties becomes theresponsibility
of theanalyst.
Analysts also suggestkeeping summaries ofpast analyses to
check
for similar information toprovide approximateanswers.
For thermal analysis,classical thermalequations fromhandbooks
become oneplace to start. MathCADsoftware, for example,can
incorporateequations for free convec-tion and radiation in the
58 MACHINE DESIGN SEPTEMBER 13, 2001 www.machinedesign.com
FE update
The Web page for Maple software from Waterloo
Maple(www.waterloomaple.com) provides a range of user-written
programs for a few specialized engineeringproblems. For instance,
under the Fluid section, a
program describes equations and mechanics forskydiving. Users
can download the equations and runthem in Maple software. Circle
623
clearly define design ob-jectives and product re-quirements to
engineer-ing. Without them, ananalysis program cannotrun
efficiently.
Another necessity isaccess to all technologybehind the design
re-
quirements referencebooks through sophisti-cated analysis
tools.Everyone on the designteam must understandthe physics of
theproduct as well as thephysics behind the tools.Finally, the
design teammust have access topersonnel trained usingeach tool.
These peoplecan be in-house or at aconsultancy.
Analysis aids are in-formation tools, saysKensinger. They
dontgive answers, theyprovide information.They characterizephysical
concepts, notthe real world. Its up tothe engineer to decidewhat
the answer is.
geometry. Experiencecan tweak the equationwhen the box
orenclosure is not a con-
venient rectangular con-figuration. Heat is notnecessarily
uniformlydistributed, even thoughone assumes a uniform
distribution for deter-mining preliminary
values. That means atemperature from anequation is the low end
ofthe answer range. FEAshould give slightlyhigher answers.
Andexpect isolated hot spots.Then define a range forthe final
temperature.Based on experience anda few assumptions, youcan always
check what
you did with MathCADor other software. Youshould find it is
within afew degrees, say experts.
Successful and costeffective analysisdepends on severalelements,
saysKensinger. And not allare related to theanalysis tool. First of
all,management should
