This work was supported by the DOE/NNSA Grant No. DE-NA0002007

Nathan Briggs

University of Utah

Your Title

Advised by Dr. J. L. Ding

School of Mechanical and Materials Engineering

Microstructure Analysis To gain some insights on the deformation and fracture mechanisms,

microstructure analyses using Scanning Electron Microscope or SEM

were also conducted for both untested and tested samples. Samples

of the images are shown in Figures 7 and 8.

Figure 7. Reinforced HDPE tensile test specimens:(a) .066/s (b) 1000/s

(c) 2000/s (d) 4000/s

Figure 8. Reinforced HDPE compression test specimens: (a) .066/s

(b) 1000/s (c) 2000/s (d) 4000/s

Higher strain rate samples have smoother fracture surfaces, this

supports the experimental result that higher strain rates result in lower

total strain.

Conclusions • The material response is strongly rate dependent.

• Materials exhibit different properties and behavior under

compression and tension loadings.

• The strength and ductility are higher under compression than

tension.

Numerical Modeling and Future Work To gain further insights on the material behavior particularly on the

interaction between the reinforcements and matrix, finite element

simulations will also be conducted. A nonlinear viscoelastic model [4]

is used for the matrix material. This work is ongoing.

Acknowledgements I would like to thank the Institute for Shock Physics and Dr. Ding for

setting up and advising the project. In addition I was helped greatly

and advised by Yueqi Hu, Yuanyuan Liu, and Nandita Biswas.

References [1] Hu, Y., Liu, T., Ding, J. L., and Zhong, W. H., 2013, “Deformation Behavior of

High Density Polyethylene and Its Nanocomposites under Static and Dynamic

Compression Loadings,” Polymer Composites, 34(3), 417-425.

[2] Tian Liu, Yu Want, Allen Eyler, Wei-hong Zhong, 2014, “Synergistic effects of

hybrid graphitic nanofillers on simultaneously enhanced wear and mechanical

properties of polymer nanocomposites”, European Polymer Journal, Vol. 55, pp

210-221.

[3] Ramesh, K.T. 2008, “High Strain Rate and Impact Experiments”, Springer

Handbook of Experimental Solid Mechanics, Part D|33.

[4] Pedro Areias, Karel Matous, 2007, “Finite Element Formulation for Modeling

Nonlinear Viscoelastic Elastomers”, Computational Methods in Applied Mechanics

and Engineering, 197(2008) 4702-4717.

Background and Introduction High Density Polyethylene or HDPE and other polymers have been widely

used in many industrial purposes due to its high strength to weight ratio.

Carbon nanofibers (CNF’s) and graphene platelet (GNP’s) and their

combination presents a potentially cost effective way to improve the

strength of the material without significant impact on the weight.

The mechanical behavior of HDPE composites under compressive loading

has been studied previously [1]. The aim of this study is to characterize

mechanical behavior of HDPE nanocomposites under dynamic tension

loading, and compare the results to the compression case. In particular this

study focuses on the materials response under dynamic loading with strain

rates of 1000/s, 2000/s, and 4000/s. Characterization of the material

response under these types of loading conditions allows us to better

understand the material response under impact loading such as that

encountered in a car crash.

Experimental Setup and the Operation

Principle of SHPB (Split Hopkinson

Pressure Bar)

Figure 1:Split Hopkinson Pressure Bar (SHPB) setup

Figure 2: SHPB schematic. Top: compression; Bottom: tension.

Compressed gas is used to propel a striker. When the striker impacts the

incident bar, a stress pulse is generated. The pulse travels into the sample

where some of the stress wave is reflected back into the incident bar and

the rest is transmitted through the sample into the transmitter bar. Strain

gauges on the incident and transmitter bars record the strains generated by

the stress pulse as they travel down the bars. These strain gauge data can

then be used to calculate the stress and strain characteristics of the sample.

A solid cylinder is used as a striker in the compression test, and a tube is

used in the tension test.

Materials and Test Samples HDPEs reinforced with 3% wt CNF’s and GNP’s were used in in this study.

Processing of materials was done in Dr. Zhong’s lab at WSU [2]. The

dimensions of the samples are shown in Figure 3.

Figure 3: Schematics of test specimens

Experimental Characterization and Numerical

Modeling of the Carbon Nanofiber Reinforced High

Density Polyethylene under Dynamic Compression

and Tension Loadings

a

g

Strain Gauge Data and Analysis Samples of strain gauge data from compression and tension test are shown in

Figure 4.

Figure 4: Strain gauge data are shown for compression (left) and tension (right).

Tension data has more noise than the compression data. The noise may be

attributed to more complicated geometry for the tension experimental setup

including the specimen fixture.

This data can be used to extract stress strain relations through the following

relations [3]:

𝜀 𝑒 = −2𝑐𝑏𝑙0

𝜀𝑅 , 𝜀𝑒 = 𝜀 𝑒 𝜏

𝑡

0

𝑑𝜏, 𝜎𝑒 =𝐸𝑏𝐴𝑏𝐴𝑠

𝜀𝑇𝑟

𝜀𝑡 = −𝑙𝑛 1 − 𝜀𝑠 , 𝜀 𝑡 =𝜀 𝑒

1 − 𝜀𝑒, 𝜎𝑡 = 𝜎𝑒 1 − 𝜀𝑒

where 𝜀𝑒 , 𝜎𝑒 are the engineering stress and strain, 𝜀𝑡 , 𝜎𝑡 are the true stress and

strain, 𝜀𝑇𝑟 , 𝜀𝑅 are the reflected and transmitted strains, 𝐴𝑠, 𝑙0 are the initial length

and cross sectional area of the specimen, and 𝐸𝑏 , 𝐴𝑏 are the young's modulus

and cross sectional area of the transmitted and incident bars.

Experimental Results on Macroscopic

Behavior The stress-strain curves for 3% wt CNF/GNP samples under tension and

compression loadings at different strain rates are shown in Figure 5. Pictures of

the tested samples are shown in Figure 6.

Figure 5: Comparison of 3% wt CNF/GNP samples at different strain rates under

tension (left), and compression (right).

Figure 6: Left, Tension samples arranged from .66/s(top) to 4000/s(bottom).

Right, compression samples arranged from .66/s(left) to 4000/s(right).

Some major observations are listed below.

• Material response depends on the imposed strain rate.

• Higher stiffness is seen at higher strain rates

• Lower total strain to fracture at higher strain rates indicates a decrease in

ductility.

• Material exhibits lower strength and toughness under tension compared to

compression loading.

• Material necking during tension is more pronounced for quasi-static loading,

but less visible for high strain rate loadings.