This document summarizes an experiment that tested carbon fiber composite materials at high strain rates using a split Hopkinson pressure bar (SHPB). Several carbon fiber composite specimens of different shapes were tested on the SHPB to analyze how properties like Young's modulus and Poisson's ratio are affected at high strain rates. The results found that carbon fiber properties change significantly at higher strain rates. The experiment also revealed issues with the flexibility of carbon fiber specimens and stress concentrations that caused premature failure. Future work will aim to better design specimens for SHPB testing of carbon fiber composites and further analyze the strain rate dependent properties revealed by this experiment.

1. Introduction:
Polymer Composite Testing by split Hopkinson pressure bar
Kennon Owens1,2, Shruti Nair1, and Shreyas Joglekar1, Dr. Mark Pankow1
1Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695
2Department of Mechanical Engineering, North Carolina A&T State University, Greensboro, NC 27401
kmowens@aggies.ncat.edu
Objective:
Background:
Results:
Conclusion:
Discussion:Methods:
Future Work:
Acknowledgements:
This material is based upon work supported by the National Science Foundation under Grant No. 1460935.
• Carbon fiber composites typically consist of fibers or nanoparticles (“fillers”) that have
been cured in a resin (“matrix”).
• These components incorporate properties that mesh well and complement each other to
create a material with many benefits and few weaknesses.
• To obtain a stronger understanding of how relevant
properties of carbon fiber, such as: Young’s modulus
(force needed to manipulate a material’s size and
shape) and Poisson’s ratio (negative ratio of
transverse strain to axial strain) are affected by a high
strain and loading rate.
• The experiment shown utilizes a split Hopkinson pressure bar (SHPB) and the material
testing techniques that it encompasses.
• Carbon fiber is a fiber-reinforced plastic, developed and utilized for its material property
combination of extremely high-level strength and light weight.
• The first step would be calculating the specimen
geometry and fabricating them.
• The specimen would be made to fit into the SHPB,
while the connections with all measuring equipment
are established.
• This setup leads into the performance of the test and
gathering of data.
• This data would be analyzed and simplified through
the use of MATLAB software.
• The applications of the composite form of these fibers can be found in the automotive
and aerospace industries, being used as a competitively alternative material for the
vehicle chassis.
• An SHPB, like the one utilized in this experiment, is a tool used to characterize the
mechanical response of materials being made to deform at high strain rates.
• The strain rate range of an SHPB is 102 s-1 to 104 s-1, which is the same rate commonly
faced in collision-related loading situations, such as a car collision.
A diagram depicting the layout of an SHPB is displayed below.
• Testing samples of carbon fiber on an SHPB
proved to be troublesome due to their
flexibility.
• We were forced to use specialized tools to
assist with fabrication and loading of
specimens.
• Failure to handle the specimens properly will
result in premature failure and/or
experimental failure along areas of stress
concentration.
Above is a picture of the SHPB
setup, including the camera
used, lighting, and specimen.
Displayed above is a carbon fiber sheet
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Test Specimen 1
Test Specimen 2
Test Specimen 3
Test Specimen 4
Some of the equations used
to determine specimen
design size are displayed at
the right.
Shown above is a picture of the
SHPB used to retrieve the data
for this experiment.
• The next step for this lab would be to use the data found here to
extrapolate the definite material response of these fibers, in
terms of how there properties changed with respect to strain rate
• Future experiments should have a means of mitigating the easily
damaged nature of the carbon fiber specimen.
• It is also recommended that any future experiments work to find
a means of mitigating the threat of stress concentrations in the
specimens.
Found below is another example
of carbon fiber utilized in an
automotive vehicle.
At the left is an example of
carbon fiber utilized in an
automotive vehicle.
At the left is an airplane
utilizing carbon fiber in the
skin of its wingspan.
Above is a picture of a few sheets, from
which carbon fiber specimens were cut.
• An SHPB works by using a gas pressure
gun to fire a striking rod into an impact
with the incident rod.
• This impact sends a stress pulse
through the rod, which is then
sent through the specimen,
causing failure.
• While in the specimen, the pulse is both reflected back into the incident rod and
transmitted into the transmission rod, where it can be measured by strain gauges.
• The first specimen utilized a dog-bone shape.
• As can be seen in the above picture, failure
occurred in the middle section of the specimen.
• Further inspection showed signs of premature
failure during the experiment, thus leading to a
small transmitted signal.
• The second specimen used a straight
rectangular shape (no dog-bone).
• The experiment showed signs of failure
initiation, however failure did not occur due to
it being loaded into the SHPB incorrectly.
• A strong signal was transmitted despite not
having failure.
• The third specimen was another straight rectangular
one.
• As can be seen above, the type of failure that took
place with this specimen was very irregular.
• This type of failure has not been seen in this lab
before, however the signals were strong.
• The fourth specimen was that of a dog-bone.
• The failure shown took place at the gripped
edge of the specimen, due to stress
concentrations along the edges.
• This type of failure typically results in weak
signals, as was the case here.
Shown at the left is the rear-view
mirror of an automotive vehicle,
which incorporates carbon fiber.
Shown at the right is the front area
and front left wheel of another vehicle
with carbon fiber.
Shown below is a
carbon fiber sheet.
• The data gathered appears to favor the straight
rectangular specimen shape, as opposed to the
dog-bone shape.
• Both dog-bone specimens gave weak signal
readings, however previous experiments have
seen these types of readings in straight
specimens also.
• The reading distribution may be a coincidence.
• Based on these results, it would appear as though the strength of the carbon fiber tested,
along with several other of it’s properties, experience significant change at higher strain
rates.
• The difficulty in the experimentation itself leads us to believe that the SHPB was not
designed with this type of specimen shape and flexibility in mind.
• In terms of the signals received, the experiment showed much greater strain pulses in the
reflected signal than that of the transmitted signal, leading us to believe that the carbon
fiber is well suited to block strain inducing force.
• The way in which the specimens failed leads us to believe that this type of carbon fiber
doesn’t have a well-structured and even strength distribution.
• The tests conducted in this
lab will go onto be used as
a means to classify the
properties of carbon fiber
as they apply to conditions
involving a high strain rate.