This work was supported by the National Science Foundation’s REU program under grant number EEC 1157094

Objectives

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.

Results

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.

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 fiber-matrix 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.

Experimental Procedure

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 Split-Hopkinson Pressure Bar system, with strain rates ranging from 1000 to 7000/sec.

Crashworthiness reflects a materials ability to absorb dynamic loading

and protect its occupants during these impacts.

Split-Hopkinson Pressure Bar setup

Striker Transmitted Bar Incident Bar

Specimen Velocity (m/s) Strain Gage Strain Gage

Future Study

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.

Typical/Ideal stress wave generation.

(Found for HDPE at 1000/sec)

Specimen Specifications

The specimen was specifically

designed to a size so the stress

wave would reach equilibrium faster.

7mm 3mm

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

Striker Specifications

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.

Nanocomp

D

PCNF

TR 3

TR 1

GNP

HDPE

.

Scanning Electron Microscope (SEM)

Pictured above is the true stress-strain of all the materials at different

strain rates. Below is the resulting yield stresses.

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 ox-

CNF.

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).

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.

Acknowledgements

Thanks to Dr. Zhong ‘s lab for providing the material and SEM analysis.