Tensile Strength of Composites
Santa Rosa Junior College
Engineering 45
Fall 2011
Tim Robbins, Nick Howard, Nick Johnson, Saul Estrella
An Introduction to Composites
Composites are defined as
any material containing
two or more substances
which significantly displays
the properties of all
constituent parts in order
to create a whole with a
better combination of
properties.
Picture of ski composite
(Courtesy: Materials Science and Engineering:
An Introduction, 7th Edition)
An Introduction to Composites
•
Much of the time, composites are made of two materials:
•
•
•
The matrix, which is the main and continuous material.
The dispersed phase, which is suspending within the matrix and
does not have to be continuous.
There are three main categories of composite materials:
•
•
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Particle-reinforced
Fiber-reinforced
Structural
Particle-reinforced Composites
• Particle-reinforced
composites use small
particulates as the
dispersed phase
Example: concrete, where
the cement is the matrix
and the sand/gravel
forms the dispersed
phase
(Courtesy: Materials Science and Engineering:
An Introduction, 7th Edition)
Fiber-reinforced Composites
• Fiber-reinforced
composites use fibers
with large strength to
weight ratios as the
dispersed phase
Example: our samples
containing carbon fiber
fabric and fiberglass
(dispersed phase) in an
epoxy matrix
Different possible fiber alignments
(Courtesy: Materials Science and Engineering:
An Introduction, 7th Edition)
Structure Composites
• Structure composites
are made of layers of
materials that have
direction specific
strength
Example: skis, where
layers of fiberglass are
laminated together in
order to give strength in
more than one direction
(Courtesy: Materials Science and Engineering:
An Introduction, 7th Edition)
Applications of Fiber-reinforced
Composites
•
•
•
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Aerospace components
(wings, propellers,
fuselages, etc.)
Bicycles
Racecars
Helmets
Surf Boards
Boeing 787 composite fuselage.
(Courtesy: Leo Watson)
Our Experiment
Objective: The purpose of our experiment was
to find the stress-strain profiles for
composites made of combinations of epoxy,
carbon fiber fabric, and fiberglass.
Our intent was to compare the results from two
composite samples with that of a third purely
epoxy control sample.
Mold Creation



We used the TAP Silicone
RTV Mold Making System.
This system is composed of
liquid silicone which is
activated by a tin catalyst.
After mixing the two parts of
the molding material
together, we slowly and
carefully poured it over the
sample.
After about a week, the mold
was completely cured.
Resin
•
•
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Resin choice:
• Polyester
• Epoxy
Tap Plastics: 1 to 1 General
Purpose Epoxy Resin
• Ideal for high strength
applications
• Minimal Shrinkage (<1%)
Published Properties:
(Courtesy: TAP Plastics)
• Tensile Strength: 7,500
psi
• Flexural Strength:
11,500psi
• Hardness: 95 Rockwell R
Samples
We created three
samples:



Pure epoxy control
Epoxy with S-2 woven
fiberglass
Epoxy with unidirectional
carbon fiber fabric
Testing


We were unable to successfully test our
samples in the tensile strength machine, so
we instead did a three-point test.
However, since we did not design our
experiment around the three-point test, our
results are largely qualitative.
Three-Point Test


The three-point test is done
by placing the specimen
across two supports and
applying a compressive load
in the middle.
Due to our sample shape,
the flexural strength is
calculated according to the
equation:
(Courtesy: Materials Science and Engineering:
An Introduction, 7th Edition)
Ff = load at fracture point
L = distance between
supports
R = radius of the specimen
Control Sample
Measured: σfs= 3853
psi
(the number for L is taken
as ~3 in.)
 σfs= 11,500 psi
(from TAP technical
specifications)

Epoxy with Fiberglass
Measured: σfs= 33015
psi
(the number for L is taken
as ~3 in.)

Epoxy with Carbon Fiber Fabric
Measured: σfs= 41130
psi
(the number for L is taken
as ~3 in.)

Control Sample
Image shows
difference in thread
sizes between
control and carbon
fiber sample after
stretching control.
Epoxy and Fiberglass
Epoxy and Carbon Fiber Fabric
Issues
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•
Our biggest issue was that our samples slipped out
of the tensile machine due to not being completely
cured.
If we were to repeat this experiment, there are
several things we could do to fix this problem:
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Apply heat
Not test samples prematurely
Do a three-point test from the start
Acknowledgments
Dr. Younes Ataiiyan for all his guidance and
advice, especially when things did not go
according to plan.
TAP Plastics for giving us student discounts
and advice concerning material selections.
Works Cited
Callister, William. Materials Science and
Engineering: An Introduction. 7th ed. York, PA:
John Wiley & Sons, Inc., 2007.
TAP Plastics. TAP Plastics, Inc., 2011. Web. 13
Dec. 2011. <http://www.tapplastics.com/>.
Watson, Leo. Aviation Spectator. Aviation
Spectator, 2011. Web. 13 Dec. 2011.