Objectives
Experimental Procedure
As the world advances toward more efficient machines, scientists
and engineers undergo a process to produce lighter weight
materials that are easy to manufacture and also cost effective.
However these materials must still withstand different loads such as
high speed impact. Applications in which the material must
perform under dynamic loading conditions include crashworthiness
in automobiles and airplanes as well as armor development. The
main purpose of this study is to investigate the dynamic response
of some polymeric materials and their composites. In addition,
the static responses were also studied and used as a reference for
comparing with the dynamic behavior.
For the static loading, a strain rate of 0.01/sec was applied to a
small specimen using a standard material test machine. High
strain rates or dynamic tests were carried out with a SplitHopkinson Pressure Bar system, with strain rates ranging from
1000 to 7000/sec.
Materials
The materials investigated in this study included both
commercial and lab-processed materials. The former included
low density polyethylene (LDPE), high density polyethylene
(HDPE), ultra high molecular weight polyethylene (UHMWPE).
The latter included HDPE, HDPE composites reinforced with
pristine carbon nanofibers (PCNF-Comp A), CNF with chemical
coating of thickness 3nm (TR3-Comp B) and 46nm (TR1-Comp C),
and pristine graphite nanoplatelet (GNP-Comp D).
Treatment of CNF occurs by
boiling
Octadecyltrimethoxysilane
(ODMS)-ethanol solution. This
causes ODMS to react with
water and form hydroxyl groups
on ends of silane coupling
agent. Thus a thin silane layer
is formed around surface of oxCNF.
Crashworthiness reflects a materials ability to absorb dynamic loading
and protect its occupants during these impacts.
Split-Hopkinson Pressure Bar
The experiment was carried out in that a striker bar was fired
toward the incident bar. The strain rate of the experiment was
controlled by the velocity of the striker which in turn was controlled
by a set pressure. Upon impacting the striker bar to the free end of
the incident, a stress wave was generated which travelled through
the incident and transmitted bars. The wave profile was detected by
the strain gages mounted on these bars and recorded using a
LabView program. These wave profiles, in the form of square sine
waves, could then be used to extract the stress-strain relation of the
material. Each material was tested 3-5 times to reduce experimental
error. Due to the compliance of the polymer materials, aluminum
rods were used for these bars.
Striker Specifications
Striker Length (m)
0.9144
0.6096
0.1524
Striker Velocity (m/sec)
3.96
14.63
24.38
Strain Rate Generated (1/sec)
1000
4000
7000
The striker as well as the incident, transmitted and momentum
bars all have matching diameters of 0.01905 meters. The striker
bar varied in length depending on the desired strain rate while both
the incident and transmitted bar are 2.13 meters.
Using the program Origin, all the different materials tested were
set side by side on the same graph to see if any variances in the
stress-strain curves were evident. The final results for the five
different materials created in the lab show that there is slight
improvement with the reinforced material, particularly with the
PCNF. All the lab created materials performed better than the
commercial HDPE. At higher strain rates, the stress wave became
very noisy thus 1000-3000/sec strain rate was the optimum
dynamic loading.
Scanning Electron Microscope (SEM)
HDPE
PCNF
3mm
Split-Hopkinson Pressure Bar setup
Velocity (m/s)
Striker
Strain Gage
Incident Bar
Specimen
Strain Gage
Conclusions
The commercial and lab created material showed strong strain
rate sensitivity. Except Comp D (GNP) at 4000/sec strain rate, the
lab created composites showed overall improvement of yield
stress over the HDPE. In general the carbon nanofibers increased
the materials strength better than the graphite nanoplatelet.
Theoretically, the coated CNF should have been the strongest and
performed the best. The SEM images revealed that the fibermatrix interface was greatest with the coated CNF. However, the
analytical results showed more accurate results than the SEM
image based on the number of times the material was tested
where as the SEM image was only done on one spot of each
specimen instead of doing a complete statistical analysis, thus no
surface trend was discovered. In conclusion, the PCNF revealed
the best improvement on the material’s strength.
Future Study
TR 3
The specimen was specifically
designed to a size so the stress
wave would reach equilibrium faster.
Pictured above is the true stress-strain of all the materials at different
strain rates. Below is the resulting yield stresses.
Results
Specimen Specifications
7mm
Typical/Ideal stress wave generation.
(Found for HDPE at 1000/sec)
The current study focuses on the compression behavior . Since
tension loading is an important loading mode particularly for
leading to material failure, the tension behavior will be
investigated in the future study.
TR 1
Acknowledgements
Nanocomp
D
GNP
Transmitted Bar
.
Thanks to Dr. Zhong ‘s lab for providing the material and SEM
analysis.
This work was supported by the National Science Foundation’s
REU program under grant number EEC 1157094