International Journal of Applied Physics and Mathematics, Vol. 2, No.

1, January 2012

Comparison with Low-Velocity Impact and Quasi-static

Indentation Testing of Foam Core Sandwich Composites

Yan Li, An Xuefeng, and Yi Xiaosu

quasi-static indentation testing than from low-velocity
Abstract—The need for quasi-static indentation test method impact testing. Several studies [5-10] show a similarity
for modeling low-velocity foreign object impact events would between quasi-static indentation and drop weight impact
prove to be very beneficial to researchers. In order to examine testing. While other studies[11-13] have shown a limit to the
whether it is feasible, series of quasi-static indentation and
low-velocity impact tests were carried out and compared. An
applicability of using quasi-static indentation to represent
analysis of the relationships between impact energy (or low velocity impact. It must be noted that there are many
quasi-static indentation force) and damage area, dent depth variables involved in these tests such as boundary conditions,
indicates clearly that dent depth was selected as the damage specimen size, specimen thickness, stacking sequence,
parameter to set up damage relationship between the two tests. impactor size, impactor shape and type of fiber/resin system.
The knee points of dent depth appearing in the two tests curves Such studies on the comparison of quasi-static indentation to
were very close. The variation tendency of the dent depth, the
process curves and the cross sectional damage views of the two
low velocity impact testing are with respect to composite
tests were in similarity. Results show that no distinct differences laminates. But the relevant researches about sandwich
could be seen between low-velocity impact and quasi-static composite panels are few relatively. An experimental
indentation testing, indicating that quasi-static indentation investigation by Ferri and Sankar [14] has revealed that the
testing can be used to represent low-velocity impact testing. contact force-indentation relations for a quasi-static test and a
low-velocity impact event about sandwich composites are
Index Term—Sandwich composites; low-velocity impact;
virtually equivalent. Lindholm C-J studied impact and
quasi-static indentation; dent depth.
indentation behavior of sandwich panels [15]. Such studies
indicate that it is limited to reflect the relationship between
I. INTRODUCTION low-velocity impact testing and quasi-static indentation
testing.
Composite sandwich panels are widely used in aerospace,
The method of assessing damage resistance using
marine, automotive and recreational industries, due to the
low-velocity impact tests is time-consuming and expensive,
high specific strength and stiffness, corrosion-resistance,
and the test results have large dispersibility. By simulating an
tailorability and stability [1]. Sandwich composites structures
impact event using a quasi-static indentation test, damage
consist of two thin fiber reinforced composite face sheets,
initiation and propagation can be more easily detected, and
which are bonded to a lightweight foam core. The function of
deflection can be directly measured with great accuracy and
the face sheets is to carry bending and inplane forces, while
maximum transverse force can be better controlled.
the role of the core is to keep the face sheets together and
Low-velocity impact test of composites accepted ASTM
carry transverse shear loads. One of the main concerns in the
D7136-05 standard [16], and quasi-static test of composites
application of sandwich composite structures is that their
adopted ASTM D6264-98 standard [17]. In this study, it is
load carrying ability may be significantly reduced by the
the focus of the work to examine if drop weight impact tests
presence of a local damage (delamination) between the core
and quasi-static indentation tests give the same tests
and the face. A frequent cause of such damage is in-service
conditions (size, shape, and location of damage for a given
incidents (runway debris, careless handling, etc.) or
maximum transverse load) according to the same ASTM
interactions with attached structures (pillars, bends, etc) [2-4].
standard. It is of significance for learning and simulating the
It should be noted that such damages of sandwich composites
damage of foam core sandwich composite panels.
usually cannot be observed outside. However, they may
cause serious reduction in load-bearing capacity. Thus, it is
of great practical importance to develop numerical models
II. EXPERIMENTAL STUDY
capable of characterizing the response of sandwich structures
subjected to local loading. The core of sandwich panels was constituted of Rohacell
The need for quasi-static indentation test method for 71WF-HT closed cell polymethacrylimide (PMI) foam made
modeling low-velocity foreign object impact events to by Röhm foam GmbH. The nominal density of the foam was
sandwich composites would prove to be very beneficial to 75 kg/cm3. One of the advantages of the Rohacell foam was
researchers since much more data can be obtained from its relatively high temperature resistance. The investigated
specimens consisted of relatively thin (4 mm) foam core. The
Manuscript received September 10, 2011; revised December 20, 2011. core was sandwiched between two face sheet carbon fiber
This work was supported in part by the Weapon Equipment Pre-Research laminates with thickness of 1.52 mm. The face sheets were
Foundation of China under grant No. 9140C4405041003.
Y. Li, A. Xuefeng, and Y. Xiaosu are with Science and
produced from T700 unidirectional textile with an areal
Technology on Advanced Composites Laboratory, Beijing Institute density of 192 g/m2 and bismaleimide (BMI) resin.
of Aeronautical Materials (e-mail: yanli0827@ 163.com). Quasi-isotropic and symmetric lay-up [45/0/-45/90]s was

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 International Journal of Applied Physics and Mathematics, Vol. 2, No. 1, January 2012

chosen. Sandwich composite panels were manufactured by As shown in Fig.2 (a), with increasing of the impact energy,
resin transfer molding (RTM) process. Rectangular the dent depth increased gradually. The dent depth was
specimens cut from the panels with in-plane size of 150 increased from 0.18 mm to 0.27 mm only when the impact
mm×100 mm were used. energy rose from 5 J to 15 J. When the impact energy reached
The two categories of tests are depended on the test 20 J, knee point appeared. After that, the impact energy has a
conditions. The sample size, supported mode and large influence on the dent depth. As the impact energy
impactor/indenter size of the two test standards are increasing from 15 J to 40 J, the dent depth also grew from
inconsistent. In order to compare with the two tests, drop 0.30 mm to 0.55 mm. As shown in Fig.2 (b), it is apparent
weight test standard was used as the basement. So the sample that the curve of quasi-static indentation force and dent depth
size, supported mode and impactor/indenter size were has similar tendency to Fig.2 (a). During the stage of lower
according to ASTM D 7136-05. The sample size was load, the dent depth has risen up to 0.27 mm, when the
150mm×100mm, and the specimens were clamped on all four quasi-static indentation force reached 4000 N. After the knee
edges of 125mm×75mm rectangular opening size on center. point, the dent depth has increased promptly. The knee points
The steel impactor/indenter used for the tests has a of the dent depth of the two tests were very close and the
hemispherical strike tip with a 16 mm diameter. variation tendency is identical, which indicated that
The impact testing was performed using a drop-weight quasi-static indentation tests may replace low-velocity
impact tester (INSTRON 9250HV). The drop-weight testing impact tests. As indicated in Fig.2 (c), damage area after
machine consists of a drop tower equipped with an impactor, low-velocity impact and quasi-static indentation testing was
which has high bandwidth digital signal processing (DSP) equal approximately when the impact energy was less than
electronics, Impulse TM control and data acquisition 12 J, while dent depth had much more difference at higher
software. And the quasi-static indentation testing was impact energy. So when the impact energy was lower (about
performed using a electronic tensile tester (INSTRON 5569) less than 12 J), quasi-static indentation force was seen as
with loading speed 1.25 mm/s. approximately equal to the maximum force caused by impact
In this test, damage area and dent depth will be measured.. process of corresponding impact energy.
After the specimens were impacted or subjected to
quasi-static indentation testing, evaluation of the impact 12000

damage inflicted to the specimens is performed by means of 11000

ultrasonic C-scan. The samples were set aside for at least 24 10000

hours so that the resulting dent depth would have enough

2
Damge area/mm

9000
time to relax to its equilibrium state. The dent depth was
measured by vernier calipers.
8000

7000

6000

III. RESULTS AND DISCUSSION 5000

0 5 10 15 20 25 30 35 40 45
Impact energy/J

A. Analysis of Damage Parameters (a) Low-velocity impact testing

Once the impact testing was completed, the maximum 12000

impact force obtained for each of the different subgroups was 10000

used as the independent variable for the quasi-static

2
Damge area/mm

8000

indentation testing. Fig.1 shows the damage area obtained

from C-scan as a function of impact energy (quasi-static 6000

indentation force) for the sandwich composite panels. As 4000

shown in Fig.1 (a), the data dispersibility of damage area

versus impact energy and is large. Also, regularity cannot be 2000
2000 2500 3000 3500 400 0 450 0 500 0 550 0 60 00 65 00
Quasi-static indentation force/N
found, and did not reflect the damage of sandwich
composites directly. As indicated in Fig.1 (b), with (b) Quasi-static indentation testing
Fig. 1. Relationship between impact energy (or quasi-static indentation
increasing of the quasi-static indentation force, damage area force) and damage area.
increased gradually. Due to dispersibility of the data, it could
not be found whether there was little resemblance between 0.60

the change trend of the impact energy (quasi-static 0.55

indentation force) versus damage area curves. So damage 0.50

area is not suitable as damage parameter to establish the 0.45

Dent depth/mm

equivalence of low-velocity impact testing and quasi-static 0.40

indentation testing. 0.35

The relationship between impact energy (quasi-static 0.30

indentation force) and dent depth was presented in Fig.2. The 0.25

0.20
curves of impact energy versus dent depth, quasi-static
0.15
indentation force versus dent depth had good regularity and 0 5 10 15 20
Impact energy/J
25 30 35 40 45

corresponding relationship. Dent depth could reflect the

changes of the damage resistance of sandwich composites. (a) Low-velocity impact testing

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 International Journal of Applied Physics and Mathematics, Vol. 2, No. 1, January 2012

0.65
6000
0.60
5J
0.55 5000 8J
10J
0.50
12J
4000 15J
Dent depth/mm

20J

F/N
0.45
25J
3000 30J
0.40
35J
40J
0.35 2000

0.30
1000
0.25

0
0.20 0 5 10 15

2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 t/ms
Quasi-static indentation force/N
Fig. 4. Force-Time curves during impact.
(b) Quasi-static indentation testing
6000
40J
0.6 Quasi-static indentation testing F5
Low-veloocity impact testing F8
5000
F10
0.5 30J 35J F12
4000 F15
25J F20
0.4
Dent depth/mm

F/N
F25
20J 3000 F30
0.3
15J F35
12J
8J 10J F40
2000
5J
0.2

1000
0.1

0
0 1 2 3 4 5 6 7
0.0
s/mm

Fig. 2. Relationship between impact energy (or quasi-static indentation Fig. 5. Force-Displacement curves during quasi-static indentation.
force) and dent depth.
As presented in Fig. 3-Fig. 5, the curves of damage process
As presented in Fig. 1 and Fig. 2, the testing data of impact of low-velocity impact and quasi-static indentation testing
energy versus damage area, quasi-static indentation force had the similar variation tendency. Initially the force
versus damage area has large dispersibility. However, the increased gradually in linearity with the growth of
curves of impact energy versus dent depth, quasi-static displacement. When the displacement was up to 0.5 mm and
indentation force versus dent depth had good corresponding the force reached near 650N, the knee point appeared in
relationship. Compared with damage area, dent depth was low-velocity impact testing. While the displacement was 0.6
proper to be damage parameter to reflect the changes of mm and the force was near 550 N, the knee point occured in
damage resistance of sandwich composites. Z. Shen, et al. quasi-static indentation testing. The knee point appeared in
[18]-[21] have found dent depth is most sensitive to the the two kinds of tests was very close. Then the force grew
capability of resistance to impact of composites, and dent rapidly relatively in index similarly until the maximum. The
depth could reflect the changes of the damage resistance knee point onset load would increase and the time when the
(toughness) of composites obviously. At the meantime, knee point appearing was in advance as the impact energy
compared with damage area and other damage parameters, (quasi-static indentation force) increased. Different impact
the testing data of dent depth has smallest dispersibility and energy was selected randomly to compare with the two kinds
dent depth is a visible impact damage parameter easiest to be of tests, as in Fig.6. The curves of low-velocity impact and
measured. In other words, dent depth could be as the damage quasi-static indentation testing were in good similarity. In
parameter to establish the equivalence of impact energy and general, the resemblance of the process curves of the two
quasi-static indentation testing. kinds of tests indicated the damage equivalence of sandwich
B. Damage Process of Low-Velocity Impact and composite panels subjected to low-velocity impact and
Quasi-Static Indentation Testing quasi-static indentation tests.

6500 3500

6000 Impact(8J)
5J 3000 Quasi-static indentation(F8)
5500
8J
5000
10J 2500
4500 12J
4000 15J
20J 2000
3500
F/N

25J
F/N

3000 30J 1500

2500 35J
40J
2000 1000
1500

1000 500
500

0 0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 0 1 2 3 4 5 6

Deflection/mm s/mm
(a) 8 J
Fig. 3. Force-Deflection curves during impact.

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 International Journal of Applied Physics and Mathematics, Vol. 2, No. 1, January 2012

up to the maximum force. From the cross-sections, the

4000 impact(15J) damage behavior of the two kinds of tests at low impact
quasi-static indentation(F15)
energy was similar, a little different at high impact energy, so
3000 quasi-static indentation force was seen as approximately
equal to the maximum force caused by impact process of
corresponding impact energy during the stage of low impact
F/N

2000
energy.
8J 15J
1000

0
0 1 2 3 4 5 6 7
s/mm
(b) 15 J 25J 40J
6000

impact(25J)
5000 Quasi-static indentation(F25)

4000

3000
25J
F/N

2000

1000

0
0 2 4 6 8
s/mm

(c) 25 J 40J

6000 impact(40J)
Quasi-static indentation(F40)
5000

4000
F/N

3000

2000
(a) Low-velocity impact testing

1000

0
0 2 4 6 8 10
s/mm

(d) 40 J
Fig. 6. Comparison of low-velocity impact and quasi-static indentation
testing process.

C. Inspection of Specimens
The specimens after low-velocity impact and quasi-static
indentation testing were cut through the impact (or static
indentation) location for visual and microscopic investigation
of the sustained damage. The cross-sections reveal that
delamination and fiber breakage only occur in the upper
(impacted) facing. In addition, local crushing of foam core
could clearly be observed. The bottom facing is left damaged.
Fig.7 displays the cross sectional views of both halves of the
sectioned specimens subjected to low-velocity impact and
quasi-static indentation testing. The damage process of the
two kinds of tests was similar as shown in Fig.7 (a) and Fig.7
(b). Initially collapse of foam appeared, while the knee point
appeared as shown in Fig. 3-Fig. 5. With increasing of the
impact energy (or quasi-static indentation force), the collapse
height of foam had grown gradually and the damage area was (b) Quasi-static indentation testing (the maximum force of impact testing)
also increasing. And then foam generated crack which
extended in transverse direction. Finally fiber breakage and Fig. 7. Cross sectional views of sandwiches subjected to low-velocity
delamination appeared on the upper facing when the load was impact and quasi-static indentation testing.

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 International Journal of Applied Physics and Mathematics, Vol. 2, No. 1, January 2012

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