Strain Rate Effects on the Mechanical Properties of

Polymer Composite Materials

George C. Jacob,1 J. Michael Starbuck,2 John F. Fellers,1 Srdan Simunovic,3

Raymond G. Boeman2
1
Materials Science and Engineering Department, University of Tennessee, Knoxville, 434 Dougherty Engineering,
Knoxville, TN 37996
2
Polymer Matrix Composites Group, Metals and Ceramics Division, Oak Ridge National Laboratory, Post Office Box
2009, Oak Ridge, TN 37831-8048
Computational Material Science, Computer Science and Mathematics Division, Oak Ridge National Laboratory, Post
Office Box 2008, Bldg. 6025, MS-6359, Oak Ridge, TN 37831-6359

Received 4 November 2003; accepted 16 March 2004

DOI 10.1002/app.20901
Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: This paper is a detailed review of the strain to better understand the strain rate effects on these mechan-
rate dependence of some mechanical properties of polymer ical properties of fiber-reinforced polymer composite mate-
composite materials. An attempt is made to present and rials. © 2004 Wiley Periodicals, Inc.* J Appl Polym Sci 94: 296 –301,
summarize much of the published work relating to the effect 2004
of strain rate studies done in the past on the tensile, shear,
compressive, and flexural properties of composite materials Key words: composites; fibers; resins; mechanical properties

INTRODUCTION of these composites would change with strain rate is

warranted to be able to design structures that would
Composites in the past were mainly used for savings
not fail prematurely and unexpectedly at high loading
in secondary structures. With several advances made
rates. Determination of dynamic mechanical proper-
in understanding the behavior of composite materials,
ties of these composites would also ensure the design
many fiber-reinforced polymer composite materials
of composite structures that are weight efficient and
are finding increasing use as primary load-bearing
structurally sound when they are subjected to higher
structures and also in a wide range of high technology
dynamic loads. The above argument reinforces the
engineering applications. The ability to tailor compos-
ites, in addition to their attributes of high stiffness-to- need for dynamic characterization of fiber-reinforced
weight and strength-to-weight ratios, fatigue resis- polymer composite materials to understand the strain
tance, corrosion resistance, and lower manufacturing rate effects on their mechanical properties. In this
costs, makes them very attractive when compared paper an attempt is made to review much of the work
with conventional metals for use in many naval, aero- published in the literature that investigates the strain
space, and automotive structural components. rate effects on the tensile, shear, compressive, and
High strain rate loading is probable in many of the flexural properties of fiber-reinforced polymer com-
applications where fiber-reinforced polymer compos- posite materials (Table I).
ites find use as candidate materials. It has always been Inertial effects prevalent at elevated rates of strain
a cause for concern that the mechanical properties of are an experimental difficulty encountered by scien-
composite materials may be poor at high rates of tists investigating the effects of strain rate on perfor-
strain. Hence, study of how the mechanical properties mance properties of a composite material. For exam-
ple, test fixtures can be subject to inertial disturbances
at medium to high rates of strain. These disturbances
Correspondence to: G. C. Jacob (gjacob@utk.edu). are due to the phenomenon of mechanical resonance
DISCLAIMER: The submitted manuscript has been authored that the test equipment acquires at higher speeds.
by a contractor of the U.S. Government under Contract
DE-AC05– 00OR22725. Accordingly, the U.S. Government re-
Inertial responses of test systems increase with test
tains a nonexclusive, royalty-free license to publish or reproduce speed and obscure test data, causing the analysis of
the published form of this contribution, or allow others to do so, for the test data to be difficult and inaccurate. Therefore,
U.S. Government purposes. it is important for investigators to overcome the iner-
tial problems while studying strain rate effects on
Journal of Applied Polymer Science, Vol. 94, 296 –301 (2004)
© 2004 Wiley Periodicals, Inc. * This article is a US Govern- composites.
ment work and, as such, is in the public domain in the Various test methods have different advantages and
United States of America. limitations and must be chosen appropriately to pro-
STRAIN RATE EFFECTS ON THE MECHANICAL PROPERTIES OF POLYMER COMPOSITE MATERIALS 297

TABLE I
Summary of Published Data on the Effects of Loading Rate on Tensile, Compressive, Shear, and Flexural Properties
Reference Materials studied Range of rates investigated Observations

Davies and Magee1,2 Glass/polyester 10⫺3–103 s Increase in ultimate tensile strength with
increasing loading rate
Rotem and Lifshitz3 Glass/epoxy 10⫺6–30 s Tensile strength and modulus increased with
increasing loading rate for unidirectional
glass/epoxy composites
Lifshitz4 Glass/epoxy Static–4.2 m/s Tensile modulus and failure stress were strain
rate independent for the angle ply glass/
epoxy laminate
Melin and Asp5 Carbon/epoxy 10⫺3–103 s Transverse tensile properties only exhibited a
weak dependence on strain rate
Okoli and Smith6,8,9 Glass/epoxy 0.008, mm/s–4 m/s Tensile strength, tensile modulus, shear
strength, and shear modulus increased with
increasing loading rate; increase in tensile,
shear, and flexural energy with increase in
loading rate
Armenakas and Glass/epoxy 0.0265 min⫺1–30,000 min⫺1 Tensile modulus increased with increasing
Sciamarella10 loading rate while ultimate tensile strain
and stress decreased with increasing
loading rate
Vashchenko et al.11 Glass/polyamide 3.3⫻10⫺5–12 m/sec Tensile strength increased with increasing
loading rate
Staab and Gilat12,13 Glass/epoxy 10⫺5–103 s Maximum tensile stress and strain increased
with increasing loading rate
Harding and Welsh14,15 Graphite/epoxy, 10⫺4–103 s⫺1 Tensile modulus and failure stress for
glass/epoxy, graphite/epoxy were strain rate insensitive;
glass/polyester, tensile modulus for glass/epoxy,
graphite/polyester, glass/polyester, graphite/polyester, and
Kevlar/polyester Kevlar/polyester increased with increasing
loading rate
Roberts and Harding16 Glass/phenolic resin 1–20,000 mm/sec Increase in tensile strength, stiffness, and
displacement with increasing loading rate
Bai et al.18 Glass bead/HDPE 3⫻10⫺5–8⫻10⫺3 s⫺1 Tensile modulus and strength increased with
increasing loading rate
Daniel et al.19 Carbon/epoxy 1⫻10⫺4–500 s⫺1 Longitudinal tensile and compression
modulus increased with increasing loading
rate; longitudinal tensile and compression
strength and strain were loading rate
insensitive; transverse tensile and
compression modulus and strength
increased with increasing loading rate
while the tensile strain was loading rate
insensitive
Hayes and Adams20 Glass/epoxy and 1.7–4.9 m/s Tensile modulus and strength of glass/epoxy
graphite/epoxy was strain rate insensitive while that of
graphite/epoxy decreased with increasing
loading rate
Daniel and Liber21,22 Boron/epoxy, glass/ 1.4⫻10⫺4–27 s⫺1 Increase in tensile modulus and failure
epoxy, Kevlar/ strength of Kevlar/epoxy with increasing
epoxy, graphite/ loading rate while that of boron/epoxy,
epoxy glass/epoxy, and graphite/epoxy remained
strain rate insensitive
Chamis and Smith23 Graphite/epoxy Static–381 s⫺1 Longitudinal tensile strength for graphite/
epoxy was loading rate insensitive;
transverse tensile and shear properties
increased with increasing loading rate
Daniel et al.24 Graphite/epoxy 100 s–500 s⫺1 Longitudinal tensile strength for graphite/
epoxy was loading rate insensitive;
transverse tensile and shear properties
increased with increasing loading rate
Kawata et al.25,26 Glass/polyester, 0.001–2000 s⫺1 Tensile strength for graphite/epoxy and
glass/epoxy, graphite/nylon 6,6 increased while that of
graphite/epoxy, glass/epoxy and glass/polyester decreased
short graphite with increasing loading rate
fiber/nylon 6,6
298 JACOB ET AL.

TABLE I Continued
Reference Materials studied Range of rates investigated Observations

Barre et al.27 Glass/polyester and 0.1–10 s⫺1 Tensile modulus and strength increased with
glass/phenolic increasing loading rate
resin
Paterson et al.28 Chopped glass fiber 1.67⫻10⫺3–6 s⫺1 Tensile modulus and strength increased with
in styrene/maleic increasing loading rate
anhydride resin
Groves et al.29 Carbon/epoxy 0–3000 s⫺1 Compressive and tensile properties (strength
and modulus) increased with increasing
loading rate
Powers et al.30,31 Graphite/epoxy and 49–1430 s⫺1 Compression yield stress and elastic strain
graphite/polyimide energy increased with increasing loading
rate for the graphite/epoxy composite
while the ultimate strength and modulus of
elasticity were strain rate insensitive for
both composites
Li et al.32 Short glass fiber/ 10⫺4–350 s⫺1 Compression modulus and strength increased
liquid crystalline with increase in loading rate
polymer
Takeda and Wan33 Glass/polyester 10⫺3–750 s⫺1 Compression strength increased with
increasing loading rate
Tzeng and Abrahamian34–36 Graphite/epoxy 10–100 in/s Compression strength and strain increased
with increasing loading rate
Amijima and Fuji37 Glass/polyester 10⫺3–103 s⫺1 Compression strength increased with
increasing loading rate
Cazeneuve and Maile38 Graphite/epoxy 10⫺3–600 s⫺1 Longitudinal and transverse compression
strength increased with increasing loading
rate
Sims et al.39 Glass mat/polyester 10⫺6–10⫺1 m/s Increase in flexural strength with increasing
loading rate

duce good and comparable results. The drop weight stress was only moderately higher than the static
impact test allows easy variation of strain rate and is value (20 –30% higher). The dependence of the trans-
inexpensive. However, it is difficult to increase the verse tensile properties on strain rate of a high perfor-
maximum limit of strain rate since the speed is di- mance carbon/epoxy composite loaded in transverse
rectly related to drop height. The use of hydraulic tension was investigated by Melin and Asp.5 Dog-
machines is convenient and accurate but they are ex- bone-shape specimens were tested under quasi-static
pensive and the strain rate is limited. Hopkinson bars and dynamic loading conditions (10⫺3–103 s⫺1). The
are used for dynamic characterization above 1000 s⫺1. average transverse modulus was observed to be inde-
However, the system is very sensitive to contact sur- pendent of strain rate while the initial transverse mod-
face conditions. The use of thin ring specimens under ulus was found to decrease slightly with increased
internal or external pressure can also be used for high strain rate. The strain to and stress at failure was
rate dynamic testing but it is expensive and complex. found to increase slightly with increased strain rate.
Thus, when loaded in the transverse direction it was
concluded that the carbon/epoxy composite exhibited
LITERATURE SURVEY
a weak dependence on strain rate.
Davies and Magee1,2 studied the effect of strain rate on Tensile tests were performed on a glass epoxy lam-
the ultimate tensile strength of glass/polyester com- inate at different rates (1.7 ⫻ 10⫺2-2000 mm/s) by
posites. They reported the glass/polyester composites Okoli and Smith6,7 to determine the effects of strain
to be rate sensitive with the magnitude of the ultimate rate on Poisson’s ratio (ratio of transverse strain to the
tensile strength increasing by 55% over the strain rate corresponding axial strain below the proportional
change. Rotem and Lifshitz3 investigated the effect of limit) of the material. Poisson’s ratio was found to be
strain rate on the tensile properties of unidirectional rate insensitive. It was suggested that the rate insen-
glass fiber/epoxy composites and found that the dy- sitivity in Poisson’s ratio of the laminates tested is due
namic strength is three times the static value and the to the presence of fibers in the composites. The effect
dynamic modulus is 50% higher than the static value. of strain rate on the tensile properties of a glass/epoxy
However, while investigating angle ply glass/epoxy composite was investigated by Okoli and Smith.8 The
laminates Lifshitz4 found that the elastic modulus was tensile strength of the composite was found to increase
independent of strain rate and the dynamic failure with strain rate. This increase in tensile strength with
STRAIN RATE EFFECTS ON THE MECHANICAL PROPERTIES OF POLYMER COMPOSITE MATERIALS 299

strain rate was attributed to the increased strength of crease in the tensile strength, stiffness, and displace-
the glass fibers with strain rate. In other studies the ment at failure was observed at higher displacement
effects of strain rate on the tensile, shear, and flexural rates. This was attributed to the rate dependence of the
properties of glass/epoxy laminate was investigated resistance of the resin matrix to fiber straightening and
by Okoli and Smith.6,9 Tensile modulus increased by of the fracture strength of the glass fibers.
1.82%, tensile strength increased by 9.3%, shear The tensile mechanical behavior of a short carbon
strength increased by 7.06%, and shear modulus in- fiber-filled liquid crystalline polymer composite, Vec-
creased by 11.06% per decade increase in log of strain tra A320, was examined under static loading (10⫺2
rate.6 The above observation was in agreement with s⫺1) and dynamic loading (400 s⫺1) by Shim et al.17 A
the results of the investigation conducted by Armena- pendulum-type tensile split Hopkinson bar device
kas and Sciamarella10 that suggested a linear variation was used to apply dynamic tension. The fracture
of the tensile modulus of elasticity of unidirectional strain and Young’s modulus of the composite were
glass/epoxy composites with the log of strain rate. found to be noticeably influenced by changes in the
However, the ultimate tensile strain and stress of the strain rate. Experimental studies on the effects of
composite decreased with the increase in strain rate. strain rate on the tensile properties of glass bead/
An increase in tensile, shear, and flexural energy of 17, HDPE composites were conducted by Bai et al.18 Both
5.9, and 8.5%, respectively, per decade of increase in Young’s modulus and the tensile strength of the glass
the log of strain rate was observed.9 The study indi- bead/HDPE composite were found to increase with
cated that it is a change in failure modes as strain rate strain rate. Daniel et al.19 investigated the dynamic
is increased, which brought about the increase in en- response of carbon/epoxy composites at high strain
ergy observed. rates using three different test methods. In the first test
Work done by Vashchenko et al.11 on glass/poly- method used for dynamic testing of thin laminates in
amide composites also suggested a linear relationship tension, a carbon/epoxy laminate was characterized
between the tensile strength characteristics of the com- under longitudinal, transverse, and in-plane shear
posite and the log of strain rate. A systematic study of loading at strain rates up to 500 s⫺1. In the longitudi-
the strain rate effects on the mechanical behavior of nal direction the modulus increased moderately with
glass/epoxy angle ply laminates was done by Staab strain rate (up to 20% over the static value) but the
and Gilat12,13 using a direction tension split Hopkin- strength and ultimate strain did not vary significantly.
son bar apparatus for the high strain rate tests and a The modulus and strength increased sharply over
servo hydraulic testing machine for the quasi-static static values in the transverse (to the fiber) direction
tests. The tensile tests at higher strain rates (in the but the ultimate strain only increased slightly. There
order of 1000 s⫺1) showed a marked increase in the was a 30% increase in the in-plane shear modulus and
maximum normal stress and strain when compared to strength. In the second test method used for dynamic
the values obtained in the quasi-static tests. Although testing of thin laminates in compression, longitudinal
both fibers and matrix are strain rate sensitive, the properties were obtained up to a strain rate of 90 s⫺1.
fibers were thought to influence laminate rate sensi- The longitudinal modulus increased with strain rate
tivity more than the matrix. Harding and Welsh vali- (up to 30% over the static value) but the strength and
dated a dynamic tensile technique by performing tests ultimate strain were equal to or a little lower than
(over the range 10⫺4 to 1000 s⫺1) on graphite/epoxy, static values. The dynamic modulus and strength at
glass/epoxy, glass/polyester, graphite/polyester, and 210 s⫺1 increased sharply over static values in the
Kevlar/polyester composites.14,15 The modulus, fail- transverse (to the fiber) direction while the ultimate
ure stress, and failure mode of the graphite/epoxy strain was lower than the static one. There was a 30%
composite were found to be strain rate insensitive. The increase in the in-plane shear modulus and strength.
dynamic modulus and strength for the glass/epoxy In the third test method used for dynamic testing of
composite were about twice the static value. This in- thick laminates in compression, transverse properties
crease in failure strength was explained on the basis of were obtained up to a strain rate of 80 s⫺1. The trans-
the observed change in failure mode. Similarly, the verse modulus moderately increased with strain rate
elastic tensile modulus of the glass/polyester, graph- (up to 18% over the static value) but the strength and
ite/polyester, and Kevlar/polyester composites in- ultimate strain increased by 50 and 30% over corre-
creased with strain rate and the strain rate dependence sponding static values.
of the elastic modulus was suggested to be derived Hayes and Adams constructed a specialized pendu-
from the elastic interaction between the reinforcement lum impactor to investigate the strain rate effects on
and the matrix and was determined by the strain rate the tensile properties of unidirectional glass/epoxy
dependence of the matrix strength. Tensile tests were and graphite/epoxy composites.20 The modulus and
performed at up to five displacement rates, from about strength of the glass/epoxy composites were con-
1 to 30,000 mm/s, by Roberts and Harding16 to deter- cluded to be rate insensitive at impact speeds in the
mine the effect of strain rate on the tensile properties range of 2.7 to 4.9 m/s. However, the modulus and
of a glass/phenolic resin composite. A significant in- strength of the graphite/epoxy composites decreased
300 JACOB ET AL.

with increasing impact speeds. Daniel and Liber21,22 properties of graphite/epoxy composites and graph-
attempted to characterize the effect of strain rate on ite/polyimide composites. For both composites, in all
the mechanical properties of unidirectional boron/ three directions, the modulus of elasticity, strain to
epoxy, glass/epoxy, graphite/epoxy, and Kevlar/ep- failure, and mean ultimate strength did not change
oxy composites. While the Kevlar/epoxy composite with strain rate.
showed a 20% increase in tensile modulus and failure Li et al.32 investigated the effect of strain rate on the
strength in the fiber direction with increasing strain compression stress strain characteristics of a short
rate from 10⫺4 to 27 s⫺1, the tensile modulus and glass fiber-reinforced thermotropic liquid crystalline
failure strength of the boron/epoxy, glass/epoxy, and polymer (an aromatic copolyester consisting of p-hy-
graphite/epoxy composites were found be rate insen- droxybenzoic acid and 2,6-hydroxy-naphthoic acid)
sitive. The increase in modulus and failure strength of over a wide range of strain rates (10⫺4 to 350 s⫺1). The
the Kevlar/epoxy composite was 40 and 60%, respec- low strain rate compression tests were conducted us-
tively, during transverse and shear (off-axis) loading. ing a Instron universal tester while the high strain rate
Work done by Chamis and Smith23 and further tests were carried out using a split Hopkinson pres-
investigations by Daniel et al.24 on unidirectional sure bar technique. The compression modulus was
graphite/epoxy laminates yielded similar results found to be insensitive to strain rate in the low strain
wherein the tensile strength in the fiber direction did rate regime (10⫺4 to 10⫺2 s⫺1) but it increased more
not change with strain rate. However, there was an rapidly with strain rate at higher strain rates. The
increase in the transverse tensile properties and shear compression strength changed linearly with log(strain
properties with increasing loading rate. rate) over the entire strain rate range. Macroscopic
The effect of strain rate (10⫺3 to 2000 s⫺1) on the inspection of the compression failed specimens indi-
tensile properties of glass/polyester, glass/epoxy, cated that the strain rate had a strong influence on the
graphite/epoxy, and graphite short fiber-reinforced failure mode. Takeda and Wan33 studied the effects of
nylon 6,6 composites was investigated by Kawata et strain rate on the compression strength of unidirec-
al.25,26 The strength of the graphite/epoxy and graph-
tional glass fiber-reinforced polyester resin composites
ite/nylon 6,6 composites increased with strain rate
using the compression-type improved split Hopkin-
while that of the glass/epoxy and glass/polyester
son pressure bar apparatus, where the impact loading
composites decreased. The influence of strain rate on
can be stopped at any moment in the impact process
the tensile properties of glass/phenolic resin and
so that the specimen can be recovered at various levels
glass/polyester resin composites was studied by Barre
of loading. The compressive strength was found to
et al.27 The elastic modulus and strength were found
increase with increasing strain rates.
to increase with strain rate. Peterson et al.28 studied
Tzeng and Abrahamian34 –36 attempted to characterize
the tensile response of chopped glass fiber-reinforced
the dynamic responses of composite materials for ballis-
styrene/maleic anhydride materials in the range 10⫺3
to 10 s⫺1 and observed a 50 to 70% increase in the tic engineering applications. An experimental setup had
elastic modulus and strength with increase in strain been developed to investigate the dynamic effects on
rate. graphite/epoxy composite materials at strain rates typ-
Groves et al. attempted to characterize the high ically found during launching of a projectile. An air gun
strain rate response (in tension and compression) of system and a test fixture with a designed crashing mech-
continuous carbon/epoxy composites.29 Strain rates anism were used to simulate a loading condition result-
from 0 to 100 s⫺1 were generated using conventional ing from gun firing. Strain rate effects on the compres-
and high-speed hydraulic test machines, those from 10 sive strength of graphite/epoxy composites with lay up
to 1000 s⫺1 were generated using a high energy drop construction of [(0/45/-45/0)4] were determined at
tower, and those from 1000 to 3000 s⫺1 were generated strain rates of 10 –100 in/s. A 10% increase in the com-
using a split Hopkinson bar. The experimental results pressive strength was observed with increasing strain
indicated an increase in both the compression and the rate. A 1.5% strain was measured under impact failure,
tensile properties (strength and modulus) with in- which is greater than the ultimate strain of 1.1% under a
creasing strain rate. Powers et al.30 used a split Hop- static loading condition.
kinson pressure bar to obtain compressive mechanical Amijima and Fujii37 investigated the strain rate ef-
properties of a unidirectional graphite epoxy compos- fects on the compressive strength of unidirectional
ite at different strain rates varying from 49 to 1430 s⫺1. glass/polyester and woven glass/polyester compos-
For each of the three principled directions, the yield ites and found that the compressive strength of both
stress increased with strain rate and so did the elastic composites increased with strain rate, with the in-
strain energy. However, the ultimate strength, modu- crease being higher in the case of the woven compos-
lus of elasticity, and strain energy density to failure ite. Study of the effect of strain rate (over the range
were found to be strain rate insensitive. In another 10⫺3 to 600 s⫺1) on the compressive strength of unidi-
study, a split Hopkinson pressure bar was used by rectional graphite/epoxy composite specimens by Ca-
Powers et al.31 to obtain compressive mechanical zeneuve and Maile38 highlighted a 50% increase in the
STRAIN RATE EFFECTS ON THE MECHANICAL PROPERTIES OF POLYMER COMPOSITE MATERIALS 301

longitudinal strength and a 30% increase in the trans- 11. Vashchenko, A.; Spiridonova, I.; Sukhovaya, E. Metalurgija
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