From: parfrk@rs6106.ecs.rpi.edu (Karl Andrew Parfrey)
Newsgroups: rec.autos.sport
Subject: Porsche @ Indy, carbon fiber
Date: 30 Sep 92 04:27:27 GMT
Reply-To: parfrk@rpi.edu
Organization: Rensselaer Polytechnic Institute, Troy, NY.

(This is my first post to r.a.s, so allow me a moment to put my Nomex
 on...)

If you're not interested in The Great IndyCar vs. F1 Safety Debate then
please hit 'n'.   This is a rather lengthy post.  The recent post 
inquiring about Porsche's Indy effort was relevant to this debate:

I quote Advanced Composites magazine, March/April 1991:

	"In 1989, the Porsche Motorsport Indy team and March Engineering
created the first all-carbon-fiber/epoxy Indy car chassis, slated to run on
the 1990 circuit.  However, CART's board of directors disallowed its use.
The positive experience of F1 cars notwithstanding, CART questioned 
crashworthiness and driver safety of composite chassis and bodywork for
the Indy circuit, where the crash scenarios are radically different...
...often resulting in a bounce and a second hit against the barrier.
CART was unsure how well an all-carbon-fiber/epoxy chassis would perform
relative to this secondary impact.
	"Though unable to race, the Porsche/March chassis proved a test bed
for primary structure in Indy cars.  With assistance from many suppliers,
including ICI/Fiberite [supplier to Williams, much IMSA GTP, and my lab] in
establishing materials specifications and Hitco in determining impact test
methods (based on aerospace testing), CART subjected the chassis to 
thourough testing and analysis.  Those results satisfied CART, which then
dropped a green flag for use of composites in primary chassis structure
for all 1991 Indy cars."

	I'm not sure if the March connection is accurate.  If memory 
serves me the original Porsche Indycar was manufactured in Germany by
Messerschmidt (sp?).  Perhaps the dismal results led them to seek
professional help!  I believe the first all-composite Indycar was the
TrueSports 91C.  

Composites are extraordinarily complex materials to design with (if you
beg to differ you can write my thesis for me! :) ).  Contrary to some of 
the opinions expressed recently, carbon fiber is a 
very poor absorber of energy and the crashworthiness of carbon fiber 
structures is a major issue.  Doubt about the post-impact strength has
kept composite primary structures out of the aerospace industry (especially
the civilian sector) until only recently.  In contrast to the race car
engineering community, the aerospace community has an extremely cautious
and deliberate attitude when dealing with new technology.

The particular material property in question is frequently termed "Compression
after Impact" strength; a sample is subjected to a standard impact and 
then its strength tested.  Carbon fiber composites fare very poorly in this 
test (compared to aluminum, for instance) and this is the basis for CART's
beliefs concerning safety at oval tracks.  CART's beliefs are very justifiable
with extensive research data, not just anectdotal evidence of the sort that 
tries to compare racing accidents which occur in vastly different circumstances.

For reasons involving the crystalline structure of graphite, typically carbon
fibers can be either strong or stiff but not both.  Traditionally racecar
structures were manufactured from standard modulus graphite (high strength,
average stiffness) but the trend among some teams is to use high modulus
graphite (low strength, high stiffness).  This aids the designer in his goal
of removing chassis flexing from the suspension movement.  These high modulus
materials were developed for use in satellite structures where stiffness 
is paramount but strength is unneccesary.  In Automotive Materials, 
Autumn/Fall 1991 Williams engineer Brian P O'Rourke (senior engineer for composite materials at WGPE) writes:

	"There are still those who believe that the more stiffness that is
available, the better the result will be and have changed to an HM [high
modulus, low strength] material in spite, or unaware, of the fact that 
their safety margins may be depleted.  An even worse situation occurs when
a designer, having achieved the right level of stiffness performance,
decides to replace his standard material with a smaller quantity of a HM
fibre to save weight.  Here the strength margins are degraded both by
the use of a less capable material and by there being less of it.
	"There have been certain accidents where the manner in which 
structures have failed arouse the suspicion that the thinking behind
their design may have been along those lines...
	"...There seems to be an attitude among some engineers in Formula 1
(where beating the regulations is as much a part of the sport as beating
the opposition) that as long as the car complies with the letter of 
the impact regulations then that is as much as is necessary.  The argument
is that if the car were to fail catastrophically at 11.1 m/s impact
speed, but passed the test at 11.0 m/s, then it is someone elses fault..."

It seems apparant to me that he was talking about Martin Donelly's accident.
There is a very dramatic photo of what (isn't) left of Martin's Lotus
included in the article.  The article (part of two O'Rourke wrote) is very
interesting.  It includes engineering drawings of the FW14 monocoque as
well as photos of an unpainted FW14 undergoing the FISA crash test.  If you
want a copy email me (I think I may regret saying this!...)  Williams is 
to my knowledge generally the only F1 team which makes significant 
contributions to the technical literature of the academic world.  I commend FW
for allowing his boys to share their wealth...

O'Rourke was also indirectly criticizing McLaren.  Composite materials 
manufacturer Hercules has been known to use the MP4/* chassis as a materials
testbed similar to how Honda uses the engines to develop new technologies.
Hercules supplies McLaren with a material they term UHM (ultra high modulus).
Prost had a structural failure in Montreal (two seasons ago?) where a 
suspension mount pulled out of the monocoque.  This was directly caused by
a switch to the ultra-high modulus material and McLaren's lack of understanding
of the structural implications therein.  Details in the relevant Autocourse.
In contrast to most F1 manufacturers, McLaren does not include Kevlar
(material used in bulletproof vests) in the monocoque laminates.  (It
is frequently used under the assumption that it improves crashworthiness.)

Speaking of which, the FISA crash test specifies a crash energy of 
47 KiloJoules to be applied to the complete monocoque/nosebox assembly in a
single frontal impact.  The CART crash test involves an impact of 58.5 kJ
followed by a _second_ impact of 29.25 kJ.  Both stipulate a maximum 
deceleration of 25 g.  Any comments, F1 safety fans? (I'm one too).

It is easy to look at carbon fiber and say things such as "on an equal
weight basis, a carbon fiber laminate can be  5-25 times stronger
and 8 times stiffer than aluminum."  This is true.  However, the relevant
property is the energy input required to fail the material.  For linear
materials this is inversely proportional to stiffness, thus aluminum
absorbs more energy.  In addition, aluminum plastically deforms during
failure which absorbs even more energy.  Finally, the extremely complex
behavior of composite materials means that many times a seemingly 
safer structure is actually unsafe.  Just ask Martin and Alain.

The reason racecar structures in general have become safer has much more
to do with the sandwich construction employed rather than the material from
which the sandwich faces are made.  A sandwich structure is one in which 
two thin facings are separated by a lightweight spacer such as foam or
honeycomb.  It functions identically to an I-beam: separate the flanges of
the I-beam and it becomes much stiffer in bending due to geometric reasons.
A positive side effect of sandwich construction is that the energy 
absorbtion capability is vastly improved.  Honeycomb will crush progressively
absorbing energy as it does so.  As work = force x distance the amount of
energy absorbed (work done to fail the material) is proportional to
its failure stress (force/unit area) and its failure strain (elongation/original
length).  Failing a carbon fiber laminate may involve applying billions of
Newtons per square meter but with an elongation of only 0.5% : thus very little
work is required to cause failure.  Crushing a honeycomb core may involve
applying only millions of Newtons per square meter but with a deformation of
virtually 100% : thus an enormous amount of energy is absorbed.

Consider the racing helmet.  Most helmets are constructed of a shell (typically
fiberglass or Kevlar) covering ~20 mm of styrofoam which contacts the head.
You guys can (and probably will) argue till you're blue in the face over
the relative merits of aluminum vs. carbon fiber helmet shells.  The point is
moot: let me inform you that I am about to smack you upside the head with a
ball peen hammer.  You have at your disposal to place between your skull and
the hammer a) a 3 mm thick plate of Hercules' finest ultrahigh modulus material
procured at a cost of ~US$250/lb, or b) a 25 mm thick slab of styrofoam you 
found in the dumpster behind K-Mart. 

Which would you choose?  Think about it.  The point of the helmet shell is
simply to protect the styrofoam from incedental damage and wear.  I have
never owned a helmet but I'm told it is recommended to replace the styrofoam
after _any_ head impact.  It is the styrofoam that absorbs energy, saving 
your life.  Likewise it is sandwich construction which makes racecars safe.
Both Indycars and F1 use sandwich construction; they merely cover the 
honeycomb with a different material.  The use of carbon fiber in F1 (and now
in Indy cars) is done simply to make the chassis stiffer in torsion; insofar
as safety is concerned it may actually be slightly detrimental.  It is an
_extremely_ complex issue involving not only the materials but the methodology
of manufacture and the understanding (or more accurately, lack thereof) of
the materials' response.  The more I learn the more I realize I don't know.
Most F1 and Indy engineers would probably say that as well.

IMHO if you want to argue about safety at Indy argue instead about chicanes
and sand traps.  Put an F1 car at Indy and things won't change much.

Now where'd I put that balaclava...

Hope I've cleared up some misunderstandings.

		-Kap 
-- 
Kap Parfrey   (parfrk@rpi.edu)			The opinions expressed
Center for Composite Materials  		herein are not my own
 and Structures 				but rather those of my
Dept of Mechanical Engineering 			employer. I am merely a
Rensselaer Polytechnic Institute		puppet of the regime.
Troy Ny USA 						- Pinochio	
Newsgroups: rec.autos.sport