Impact Testing of Advanced Composites
by Joshua M. Duell
The advantages of composite materials are numerous and well documented.
Composite materials are often used in environments in which they will
suffer from impact damage. For example, damage can occur from a
hammer being dropped on a composite pipe or from a bullet striking
composite armor. Since impact damage resistance is such an important
property for composite materials, this chapter will be devoted to the theory
behind impact testing, and the procedures used to perform impact testing.
The chapter will describe in detail the best way to perform impact tests.
Different ways to evaluate impact data will be examined, as well as ways to
characterize the impact induced damage.
Impact testing fits into two main categories: (a.) low velocity impact, and
(b.) high velocity impact [1]. These two main categories lead to three main
types of impact testing. Charpy impact testing and drop weight impact
testing fall into the category of low velocity impact testing (here it should be
noted that an impact test machine can be used for high velocity impact also;
for reference see ASTM D 3763 [2]). Ballistics impact testing falls into the
category of high velocity impact testing. Technology has increased to the
point that there are now sophisticated measuring devices for instrumented
impact testing. For all low velocity instrumented impact test devices there
are three major components: the dynamic load cell (or tup), the data display
system, and the signal conditioning unit [3]. The tup is placed on the
impactor used to strike the specimen. Within the tup is a strain gage that
measures the change in strain vs. time as the impactor strikes the specimen.
The signal conditioning unit removes the noise associated with the signal,
and the data display system plots the measured data. An in-depth
exploration on the benefits and methods of each type of impact test will be
explored more in the following sections.
6.1 Charpy Impact Testing
Charpy impact testing has been used for many years to test the impact
toughness of various metals. The advent of modern composites brought
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about materials with properties that depend on their orientation.
Consequently, new test methods had to be found to accurately test the
directionally dependent impact resistance of composite materials. Since
Charpy impact testing is both cheap and fast, its use was extended to
composites. A Charpy impact test machine is shown in the following
picture.
Figure 6.1
Charpy impact tester.
6.1.2 Specimen Preparation
The specimen that fits into the Charpy impact tester is rectangular with a
notch cut in one side. The notch allows for a predetermined crack initiation
location. Many composite Charpy impact tests are performed without the
notch cut into the specimen. In these cases it should be noted in the
experimental procedure if the notch is not present. A typical Charpy impact
specimen is shown in the following figure.
Figure 6.2
Charpy impact specimen
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For a typical fiber reinforced polymer Charpy specimen, L = 126 ± 1 mm,
D = 12.7 ± 0.15 mm, and 3.00 mm < w < 12.7 mm [4]. A typical composite
Charpy specimen is first made by creating a large panel of laminae oriented
in the direction that is dictated by the problem definition. When creating a
panel using a wet lay-up or prepreg technique, the wet-out reinforcement
plies are stacked in the desired configuration to get a panel that is several
layers thick. The specimen is then placed in a vacuum to remove excess
resin, and allowed to cure (with or without external pressure and heat). A
figure of the final panel lay-up is displayed in the following figure.
Figure 6.3
Plate lay-up
Once the composite specimen has finished curing, the composite
specimen is removed from the vacuum bag setup. (For more information on
composite lay-up preparation, refer to the work by Adams, Carlsson, and
Pipes [5].) The resultant plate can then be cut into small rectangles which
will be used as Charpy impact specimens. The final step is to cut the notch
into the specimen.
6.2.2 Test Setup and Procedure
The Charpy impact test method works by placing a notched specimen (with
the notch facing away from the point of contact) into a large machine with a
pendulum of a known weight. The pendulum is raised to a known height
and allowed to fall. As the pendulum swings, it impacts and breaks the
specimen, rising to a measured height. A figure displaying the process is
shown below.
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hf
hi
Figure 6.4
Charpy impact procedure
The difference in the initial and final heights is directly proportional to
the amount of energy lost due to fracturing the specimen. The total energy
of fracture is determined by
Γtotal = mg (ho − h f )
(6.1)
where total is the total energy, m is the mass, g is gravitational acceleration,
ho is the original height, and hf is the final height. The failure types for
composite Charpy impact tests depend on the specimen orientation. Often,
specimens exhibit fiber fracture and fiber pull-out, while other times
delamination failure is the primary failure mode. Examples of post fracture
Charpy impact specimens with different failure modes are shown below.
Figure 6.5
Picture of failed composite Charpy impact specimens [6].
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101
Specimens were tested with lay-up angles of 0, 10, 22.5, 30, 45, 67.5, and
90 degrees (from left to right in Figure 6.5). It is possible to see that
specimen 1 failed from fiber breakage and pull-out. Specimen 2 failed from
a combination of fiber pull-out and fiber-matrix separation, and specimens
3-7 failed at the fiber-matrix interface. Composites therefore may need to
be tested in different fiber directions due to the anisotropy of the material.
The failure type is important when characterizing composites.
6.2.3 Instrumented Charpy Impact Machines
Advances in technology have led to instrumented Charpy impact testers. A
Charpy impact testing machine like the one shown in Figure 6.1 can be used
with high speed photography to yield data that shows the progression of the
crack growth in accordance with time [7]. Other types of instrumented
Charpy impact testers use an electronic tup on the pendulum to measure
strain rate as the pendulum passes through the specimen. The strain data
can be converted to force and recorded as a function of time. Data can be
gathered to show the change in the toughness with respect to the velocity of
the pendulum. A graph comparing three different types of composite
materials is shown in the following figure where Cv is called the Charpy
energy:
Figure 6.6
A toughness comparison of three different composite materials [8].
From Figure 6.6 it can be seen that the S-glass composite performs much
better than that of the other two composites. Charpy impact testing can also
be used to compare different types of composite layups, including woven
and unidirectional laminates. In composite materials, unidirectional prepreg materials are the most tested of any configuration of composite
materials. In the following table, a comparison is shown for the test results
of both woven and unidirectional carbon fiber composites.
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Table 6.1 Impact Energy of Carbon Fiber Composites [9]
Unidirectional
450 kJ/m2
4.48 J
Impact Strength
Absorbed Energy
Woven
229 kJ/m2
2.36 J
Charpy impact testing has been expanded to devices that are capable of
measuring impact strength, impact force, and impact deflection [9].
6.2 Drop Weight Impact Testing
Drop weight impact testing is another type of low velocity testing, and it is
the most common test for composite materials. Drop weight impact tests are
done to test the impact behavior on composite plates, which most closely
resemble impact damage in the field. When using a drop weight impact
tester, two categories of damage can occur. The first is clearly visible
impact damage (CVID), which can easily be seen by the naked eye. The
second type of damage is barely visible impact damage (BVID), which can
seldom be seen by the naked eye. Evaluation of both types of damage can
be enhanced through the use of post-impact testing (discussed in Section
6.5).
In drop weight impact testing, a mass is raised to a known height and
released, impacting the specimen. The choice can be made between either
an instrumented or non-instrumented test machine. A figure displaying how
an instrumented impact machine works is shown below.
W
Instrumented
Tup
Flag
Velocity
gate
H
v0
x(t)
Support
Specimen
Figure 6.7
Example of the operation of a drop weight impact test machine
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In Figure 6.7, x(t) is the coordinate system, H is the initial drop height, W
is the impact weight, and vo is the impact velocity. The tup is a hemispherical impactor that measures the strain during impact [10]. An example
of an instrumented impact test fixture is shown in the following figure.
Figure 6.8
Drop weight test device (This photo has been provided by Instron® Corporation)
6.2.1 Specimen Preparation and Test Setup
The specimens used for drop weight testing are flat panel composite
specimens. The lay-up and material used is dependent on the desired results
of the testing. The flat panel specimen preparation is similar to that of the
panels used in the Charpy impact testing shown in Figure 6.3. Following
the creation of the panel, specimens are cut to the desired size. This flat
specimen is then inserted into the test machine and clamped along its edges.
The clamps can be placed in a circumferential configuration or in a
rectangular configuration, based on the design and test specifications (a
common standard is ASTM D 5628 [10]). Once the test specimen is
clamped down, the mass is raised to the desired height, and the mass is
locked into place.
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6.2.2 Test Procedure and Data Evaluation
Once the machine is set to its correct configuration and all data acquisition
software is running, the mass is released and allowed to impact the plate. A
picture of an impacted test specimen is shown in the following figure.
Figure 6.9
Post impact drop weight test specimen [11].
The strains measured by the tup are loaded into a software program, and the
data obtained from the test can be examined to see the impact resistance of
the plate. The data is plotted as force, energy, or displacement vs. time.
The data, however, is not always conclusive and often post-impact analysis
is required [11]. A graph typical of an instrumented impact test is shown
below.
Energy(N-M)
Load(Nt)
Specimen FF Damage Initiation
1200
2
1000
1.5
1
600
400
0.5
Energy (J)
Load (N)
800
200
0
0
-200
-0.5
45
50
55
60
65
70
75
Time (msec)
Figure 6.10
Example plot of force vs. time curve for drop weight impact test.
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105
The graph shown in Figure 6.10 gives information about how the
material behaves during the impact process. The load drops upon fracture
initiation. In order to try to gain more information, the impacted sample is
often analyzed using post-impact analysis techniques. Post-impact testing
can determine the amount of strength left before ultimate failure, or examine
the failure mode of the specimen after it has been impacted.
6.2.3 Plate Impact Theory
In order to characterize composites during impact, theory has been
developed by Jang, Huang, Hsieh, Kowbel, and Jang [12]. This theory
consists of treating the specimen like a beam in bending. In this treatment,
the plate is clamped along two sides only. This theory is different from a
more classic approach in which the plate is constrained on all sides. In order
to begin, Newton’s second law is used and the solution for acceleration, a(t),
is given as
a (t ) = g −
P (t )
M
(6.2)
where g is the gravitational acceleration constant, P(t) is the load with
respect to time, and M is the mass of the impactor. t = 0 at the time when
impact has just begun, and, knowing the initial conditions,
v(t ) = V , at t = 0
x(t ) = 0, at t = 0
(6.3)
where V is the velocity just prior to impact. Equation 6.2 can be integrated
to obtain an expression for v(t), and this can be integrated to obtain a
solution for x(t).
t
v(t ) = V + g −
0
P(t '
)
dt '
M
(6.4)
t
x(t ) = 0 + v(t '
)dt '
0
It is important to remember that equations 6.4 work as long as the composite
laminate is not punctured. Once the previous quantities are known, it is now
possible to solve for the energy absorbed as a function of time.
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t
Γ(t ) = P (t '
)v(t '
)dt '
(6.5)
0
Integrating equation 6.5 from 0 until the time t that the impactor is no longer
touching the specimen will yield the total energy total. This is a simplified
method used to determine the energy absorbed by the specimen.
6.3 Ballistics Testing
Ballistics testing is a form of high speed testing that is used to test the
ultimate impact strength of composites.
High velocity testing is
characterized by an impactor traveling in the range of 400-2000 m/s [13].
For high velocity impact conditions, structural response is less important
than in a low velocity case, and the damage area is more localized; therefore
the geometrical considerations are less important [14]. Ballistics testing
consists of firing a high speed projectile at an object and determining after
the impact how localized the damage is. This is a good method for testing
impact resistance of composites, and has been used for testing products such
as composite armor.
6.3.1 Test Setup and Procedure
Ballistics testing is complicated, and care has to be taken during the setup.
A typical setup of a conventional ballistics test apparatus is shown in the
following figure.
Diaphragm
Sabot
Pellet
Sabot
Velocity
Strain gages
Gas
Magnet
Sabot pin
Specimen
Figure 6.11
Apparatus for ballistic impact testing
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The test apparatus shown in Figure 6.11 is typically known as a gas-gun
impact test machine. The gas-gun apparatus works by forcing gas into the
back of the diaphragm which expands and applies pressure to the sabot. The
sabot pin is released, and the sabot is forced along the barrel. The velocity
pins measure the time it takes for the front of the sabot to travel the distance
between the two pins, and outputs a velocity. The sabot is then blocked by
the sabot stops, and the pellet is allowed to proceed to impact the test
specimen. The strains can then be measured using strain gages applied to
the specimen. The pellet used for testing is usually one with a high
hardness, and will be made of a hard steel, or zirconia (for the setup shown
in Figure 6.11 the pellet would have to be magnetic, as it is held in place by
a magnet). An inert gas is used to fill the chamber to cut down on possible
accidents.
6.3.2 Data Evaluation
When performing ballistics testing, data evaluation often has to be
accompanied by a different method for defining the damage done. There are
different ways to characterize that damage, and those pertaining to postimpact damage analysis will be discussed in the following section. A
specimen with ballistics impact damage is shown,
Figure 6.12
Example of Ballistics Impact Damage [15]
where Ra is a radial crack propagating outwards from the impact center, and
Ri is a ring crack propagating around the impact center. When doing
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calculations for ballistics testing, some assumptions must be made. One
assumption is that the velocity of the pellet remains constant from the time
that it is measured with the velocity pins until the time that it contacts the
specimen. The second is that the energy lost from the pellet is proportional
to the energy absorbed by the specimen. The remaining energy of the pellet
can be measured using sensors to detect its rebound velocity (or in the case
of a through laminar failure its remaining velocity). When calculating the
necessary unknowns that come with ballistics testing, the equations can
become rather complex. For those higher order calculations, refer to the
results by Fujii, Yasuda, and Tanabe [13].
6.4 Comparison of Test Methods
A comparison of the different types of test methods can help in making a
decision regarding the correct test to use. Charpy impact testing is easy and
fast, which allows the researcher to generate large amounts of data. Charpy
impact testing is simple in its scope, and therefore its results are not indepth, nor do they reveal a great amount about the material. The Charpy
impact test itself is easily set up, and therefore a great number of specimens
and a large amount of data can be collected in a very small amount of time.
With the advent of high speed photography, Charpy impact tests are now
able to produce results that help to show the propagation of the crack, and
instrumentation allows the user to measure the force more accurately. The
data, however, may not be suitable for some composite materials due to the
anisotropy of the material [16]. For these composites, little value can be
gained from the results as to the overall behavior of the material. Due to the
simplistic model presented by Charpy impact testing, very few types of post
impact test methods are effective.
Drop weight impact testing is a more common “real world” scenario than
that of the Charpy impact test, and consequently greater amounts of
information can be gathered as to the behavior of the laminate. Drop weight
impact testing also allows for different configurations to be used. Therefore,
drop weight testing tends to be the preferred method when using low
velocity impact testing. Drop weight testing results may also be enhanced
from post impact testing.
Ballistics testing cannot be directly compared to the previous two testing
techniques, because it is a high velocity test. Ballistics testing can be more
complicated. However, often high velocity impact testing is needed, and
cannot be avoided. Ballistics testing is highly effective at replicating the
behavior of the composite at high speed impact and allows the user to
characterize the material by analyzing measured data. It should be noted
Impact Testing of Advanced Composites
109
that it is best to perform a low velocity test when possible, due to simplicity.
As is the case with drop weight machine testing, ballistics test results can be
enhanced greatly by post-impact testing.
6.5 Test Methods for Post Impact Damage Testing
Impact testing of composites may not be all that is needed for
characterization. In order to see the damage mechanism or the failure type,
sometimes a post-impact analysis is needed for the damaged specimen.
There are several ways in which to test a damaged composite specimen.
6.5.1 Ultrasound
Ultrasonic methods are commonly used to analyze composites. The
ultrasonic method works by sending continuous waves of sound into the
object and capturing the reflected sounds. The reflected sounds paint an
image of the object, and in this manner determine the size and shape of the
defects. One main disadvantage of the conventional ultrasound test is that in
order to gain a good picture of the material layers, the measurement device
has to be in contact with the material in order to minimize impedance.
Newer technology can enable non-contact ultrasonic testing techniques to be
used [17]. Different types of ultrasonic testers currently exist. There are
simple devices that give only a digital readout of the thickness to complex
systems that show the size and shape of the defects.
6.5.2 Microwave Testing
Microwave testing is another way to analyze impact damage in composite
materials. Microwave testing works by sending microwaves through a
material, and the defect causes the waves to change. These waves are then
absorbed into a thin film, which can be enhanced to display defects in the
specimen. The microwave testing techniques are beneficial in that the air
surrounding the composite gives very little impedance to the microwaves.
Microwave testing can be hazardous, as microwaves can cause health
concerns. As with ultrasonic testing, there are different types of microwave
testing. One technique of interest is the microwave NDI technique [18].
6.5.3 Compression Testing
Compression testing after impact is a way to determine how much strength
the impacted specimen still contains after initial testing. Compression
testing is different from the previous two techniques, in that it is a
destructive test. Compression after impact can be used to determine the
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ultimate stress and strain of a specimen in compression after impact. The
data is useful to see if the damaged specimen is still strong enough to
perform its designated function.
There are many different types of compression testing, and therefore
many different types of compression fixtures. There are several different
ASTM standards that deal with the different types of compression testing
(however, they do not mention compression after impact, and care needs to
be taken if using these standards). A good outline of the mathematical
relationships, and test procedures can be found in SACMA SRM 2R [19].
The SACMA standard is one of two standards that exist on post impact
compression testing of composite materials, the other being a standard
created at Boeing.
6.5.4 Other Types of Post Impact Testing
There are also several other types of post-impact damage testing, most of
which are non-destructive (ND) test methods. Some of the other types of
testing are three and four-point bending, x-ray, optical, scanning electron
microscopy (SEM), and thermographic testing. Three and four-point
bending works by performing the bending test on the impacted specimen.
However, there are no standards to govern performing this test on postimpact specimens. X-ray testing works by blasting the sample with x-rays
and measuring the difference in the reflected x-rays. Optical testing works
by placing the impacted specimen under a microscope to analyze the
fracture surface. The SEM technique works by sending electrons to the
specimen and measuring the returning electrons to gain a visual image of the
fracture surface. Both optical and SEM allow the user to see only the
surface of the specimen, instead of being able to see the inner layers of the
lamina. Thermographic testing works by flash-heating the material and
measuring the heat as it is dissipated from the sample using an infrared
camera. Thermographic testing is gaining popularity due to its accuracy and
effectiveness.
6.5.5 Deciding on a test technique
The costs and benefits of the various methods presented here are diverse,
and in order to decide on what type of post-impact test to do, time, cost,
availability of a test method, and the information that is needed has to be
considered. For Charpy impact testing, only the fracture surface would need
to be viewed, and optical techniques would be advantageous (both
inexpensive and fast). However, for ballistics and impact testing, the layers
beneath the surface would need to be seen to determine if any residual
damage has been caused. In the case of a ballistics and drop weight impact
Impact Testing of Advanced Composites
111
test specimen, one may want to characterize the strength left in the
composite. Therefore, a compression test may need to be done to the
specimen. The post-impact test technique for a laminated plate may be be
either qualitative or quantitative. Non-destructive testing is qualitative and
shows the damage caused by impact testing. Destructive testing is
quantitative and will give the material properties (such as modulus, stress,
strain, etc.) of the composite after impact. If necessary, both types of testing
can be done, provided the ND technique is done first.
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