AN EXPERIMENTAL STUDY ON THE IMPACT COLLAPSE
CHARACTERISTICS OF CF/EPOXY CIRCULAR TUBES
Y. N. Kirn1, K.H. Im 2, J. W. Park 3 and I. Y. Yang3
1
Department of Mechanical Design Eng., Chosun University, 375 Seosuk-dong, Dong-gu,
Kwangju, 501-759, Korea
2
3
Dept. of Automotive Engineering, Woosuk University, Wanju-gun, Chonbuk, 565-701, Korea
Factory Automation Research Center for Parts of Vehicles, Chosun University, 375 Seosuk-
dong, Dong-gu, Kwangju, 501-759, Korea
ABSTRACT. This study is to investigate the energy absorption characteristics of CF/Epoxy (CarbonFiber/Epoxy Resin) circular tubes in static and impact tests. The experimental results varied
significantly as a function of interlaminar number, orientation angle of outer and trigger. When a CFRP
composite tube is crushed, static/impact energy is consumed by friction between the loading plate and the
splayed fronds of the tube, by fracture of the fibers, matrix and their interface, and the response is
complex and depends on the interaction among the different mechanisms, such as transverse shearing,
laminar bending and local buckling. The collapse mode depended upon orientation angle of outer of CFRP
tubes and loading status(static/impact). Typical collapse modes of CFRP tubes are wedge collapse mode,
splaying collapse mode and fragmentation collapse mode.
INTRODUCTION
Because of their high strength, stiffness and low density, composites are currently
being considered for many structural (aerospace vehicles, automobiles, trains and ships)
applications due to their potential for reducing structural weight. Although composite
materials exhibit collapse modes that are significantly different from the crushing modes of
metallic materials, numerous studies have shown that composite materials can be efficient
energy absorbing materials. [1-3]
Many studies have been conducted to study the effects of different reinforcements,
crush rates, environment temperature and specimen architecture on the energy absorption
capabilities of composite tubes. Thomton [3], Parley [4-6] and Hull [7] have carried out a lot
of experimental work to study the crushing behavior of the composite tubes and the influence
of various parameters on their crushing characteristics. Parley and Jones also identified three
unique crushing modes, transverse shearing, laminar bending and local buckling.
Previous studies on the collapse characteristics of composite tubes have involved only
collapse testing and the determination of the effects of shape, t/D ratio and fiber orientation.
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/$20.00
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Therefore, the objective of this study was to determine, in CFRP (carbon fiber reinforced
plastics) composite tubes, how changing the interlaminar number and ply orientation angles
of outer affect the crushing characteristics and energy absorption capabilities and to further
fundamental understanding of the collapse mechanisms and modes of composite tubes under
impact loadings.
EXPERIMENTAL PROCEDURE
Specimens
The CFRP composite tubes were manufactured from uni-directional prepreg sheets of
carbon fibers (CU125NS) by the HANKUK Fiber Co. in Korea. The CFRP tubes are made of
8 layers of these sheets stacked at different angles. They are cured by heating to the
appropriate hardening temperature (130°C) as means of a heater at the vacuum bag of the
autoclave. The tubes, which are 30mm inside diameter, are cut into 100mm lengths by a
diamond-cutting machine. One end of each table was then chamfered at 45 degrees so that
crushing could be initiated without causing catastrophic failure. Table 1 shows details the
specimens. From Table 1, A indicates interlaminar number 2, B does 3,C does 6 and D does 7.
The outer ply orientation angle was 90° in the case of layer-ups for the under line stacking
method, and 0° in tile case of layer-ups for the other method. Their marks presented to 00
and 90. Also, specimens with a trigger are labeled T and those without trigger are labeled N.
TABLE 1. Specimen definitions.
S(I)
A(B,C,D)
00(90)
- S : Static
I : Impact
• A:[02/902]sor[902/02]s
B:[902/02]2or[02/902]2
C:[0/90]2sor[90/0]2s
D: [90/01, or rO/901,
The laminated angle at outer ply
00:0°
90 :90°
T: Trigger
N:Non-trigger(without trigger)
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Axial Collapse Experiment
The static axial collapse test was performed to examine changed interlaminar
number, outer ply orientation angle and trigger, using a universal testing machine (UTM)
with a 5 ton capacity. Loading plates were set parallel to each other prior to the initiation of
the tests. The load-displacement curves were recorded using an automatic data acquisition
system. In case of the static compression test, all tubes were compressed at a rate of
approximately lOmm/min, static speed, until a limited displacement of 60mm was reached.
Each test was repeated at least three times and in some cases four or five times. Absorbed
energy is obtained by integrating the load and displacement as shown by Equation (1), and
the average collapse load is calculated by dividing the absorbed energy by the deformed
length. The average collapse stress is then calculated by dividing the average collapse load
by the sectional area of the specimen, as shown by Equation (2).
(2)
— lav-**-
where Ea is the absorbed energy in the specimen, P is the collapse load during the crushing
process, o av is the average collapse stress and P av is the average collapse load. In this
study, axial impact collapse tests were performed to use a vertical impact testing machine and
a manufactured load cell. The vertical impact testing machine consists of an air chamber, a
cross head, a load cell, an anti-vibration rubber, guide bars and frames. During the test,
impact loads were obtained by converting electrical resistance variations on the
semiconductor strain gauges into loads. The resistance variations of the semiconductor strain
gauge going through the shield line and the bridge-circuit, are fed into a dynamic strain
amplifier, which converts them into voltage variations. Deformation of a specimen is
measured using a non-contacting optical deformation system, which catches the movement
of the target on the cross head. A load-deformation curve showing the collapse history is
obtained by eliminating the time axis from each measured time-load and time-deformation
curve. Based on the load-deformation curve, absorbed energy (Ea), average collapse load
(Pav), average collapse stress (a av) and maximum collapse load (Pmax) are derived. The
energy absorption characteristics and collapse modes of each specimen may then be studied.
40
-ICOOT
-SCOON
-SC90N
--IC90T
30
£,20
0
10
20
30
40
50
10
60
20
30
40
Displacement [mm]
Displacement [mm]
(a) SC_N
(b) IC_T
FIGURE 1. Relationship between load and displacement.
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Figure l(a) is the load-displacement curve of the static test on the CFRP composite
tubes with an interlaminar number of 6. Fig. l(b) represents the load-displacement diagram
of a specimen with a 6 interlaminar number and a trigger after impact testing. The solid line
is of the 0°ply orientation angle of the outer and the broken line is of the 90°ply orientation
angle of the outer.
RESULTS AND DISCUSSION
Figures 2-3 compare the averages of average collapse stresses of the static and
impact collapse tests upon changing interlaminar number, ply orientation angle of the outer
and trigger/non-trigger. On considering the values in Figs. 2~3 with respect to the changing
interlaminar number and ply orientation angle of the outer, it is apparent that the higher
average collapse stress is presented in the case of C-type composite tubes with an
interlaminar number of 6 and a 90°outer ply orientation angle under static and impact loads.
Also it is apparent that the average collapse of the C-type composite tubes with a trigger is
higher than that of the others. The average collapse stress under impact tests is presented in
Fig. 3, which shows that the average collapse stress increases to interlaminar number 6, as
the interlaminar number increase. However, a decreasing trend is evident on moving to an
interlaminar number of 7. Because a key element of energy absorption is crack growing and
extension, and cracks may be classified as interlaminar cracks, intralaminar cracks and
central cracks. It is apparent that on increasing interlaminar number that interlaminar
cracking increases, but the growth of. intralaminar cracks and central cracks decrease. The
average collapse stress under static tests show the same trend as the static tests. The collapse
mode depended upon the ply orientation angle of outer and upon the loading
status(static/impact). The collapse mode of the CFRP composite tube in of three types; i.e.,
the wedge collapse mode, the splaying collapse mode and the fragmentation collapse mode.
The wedge collapse mode was shown in case of the CFRP composite tubes with a 0° ply
orientation angle of outer under static and impact loading. The splaying collapse mode was
W
3D
2
4
6
8
Interiarrinsr ruTter [k|
10
2
4
6
8
10
Interlaninar nurrter [ k|
(b) tubes with trigger
(a) tubes without trigger
FIGURE 2. Relationship between the average collapse stress and the interlaminar number for various CFRP
thin-wall structures under static load.
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shown only in the case of tubes with a 90°ply orientation angle of the outer under static
loading, whereas those were collapsed in a fragmentation mode during the impact testing. Fig.
4 is average collapse stress diagram on the difference of specimen weight before/after
experimental in the static and impact collapse test. The average collapse stress increased
gradually as debris capacity increase. Fig. 5 is a section of a CFRP composite tube showing
the each collapse mode. Fig. 5(a) shows the basic collapse mode of brittle-fiber reinforced
composite tubes, which is described as the fracture collapse mode. The principal energyabsorption mechanism in the collapse of CFRP composite tubes is inter/intralaminar crack
growth. As shown in Fig. 5(b), the splaying collapse mode involves the growth of central
cracks, inter/intralaminar cracking and laminar bending. Fig. 5(c) also shows a section of a
CFRP composite tube that has suffered fragmentation collapse. In all tests composite tubes
with a 90°ply orientation angle of the outer is higher than composite tubes with a 0°ply
TO
8
1 S3
0
0
m
«
in
^
4)
23
t)
(a) tubes without trigger
m
w
Ia
|*U3J
to
^
1
CD
§^8D
I1®
———i,——,
s
4
1 33
0
^^^^^^^^^^^^^^^ji^^,,,^,^_____
2
4
Irtalaifinsr
6
.
8
(b) tubes with trigger
FIGURE 3. Relationship between the average collapse stress and the interlaminar number for various CFRP
thin-wall structures under impact load.
1000
o s^^r
Wa*
x~~i^
i 4
1
16
12
Dfferenoe[g|
Dffensnoe[g]
(b) impact test
(a) static test
FIGURE 4. Relationship between the average collapse stress and weight difference.
(b) splaying collapse
(a) wedge collapse
(c) fragmentation collapse
FIGURE 5. Section of composite tube showing the wedge collapse and the fragmentation collapse.
orientation angle of the outer because the fibers in the outer laminate tend to extend and
break.
CONCLUSIONS
In this study, CF/Epoxy composite tubes with a variety of interlaminar numbers, outer
ply orientation angles and trigger are presented.
CF/Epoxy composite tubes with a 90°outer ply orientation angle exhibited higher crush
loads throughout the whole crush increases, and the higher energy absorption capabilities of
the composite tubes with a 90°outer ply orientation angle is higher than those of tubes with a
0°outer ply orientation angle because in this case the outer-fiber extend and break.
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In the case of the collapse of the brittle-fiber reinforced composite tube, the collapse
mode was found to depend upon the outer ply orientation angle and the loading status (i.e,
static/impact). The wedge collapse mode was shown by CF/Epoxy composite tubes with a 0°
outer 1 ply orientation angle under static and impact loadings. The splaying collapse mode
was shown only by tubes with a 90° outer ply orientation angle under static loading,
whereas these collapsed by the fragmentation mode during impact testing.
The energy absorption capability of all CF/Epoxy composite tubes was determined to be
a function of the interlaminar number, tile outer ply orientation angle and upon the presence
of a trigger. Higher energy absorption occurred composite tubes with an interlaminar number
of 6, a 90°outer ply orientation angle and a trigger.
REFERENCES
1. Gupta, N. K., Velmurugan, R., Int. J. of Solids and Structures, (1996).
2. Kirn, Y. N., Im, K. H., Park, J. W., Yang, I. Y, "Experimental Approach on the
Collapse Mechanism of CFRP Composite Tubes", Reviews of Progress in QNDE,
2000,pp. 369-376.
3. Thornton, P. H., J. Composite Materials, 24, 594-615(1990).
4. Parley, G. L., Jones, R. M., Journal of Composite Materials, 26, 1, 37-50 (1992).
5. Parley, G. L., Jones, R. M., Journal of Composite Materials, 26, 1, 78-89(1992).
6. Parley, G.L., Jones, R. M., Journal of Composite Materials, 26, 12, 1741-1751(1992).
7. Hull, D., Composites Science and Technology, 40, 3, 377-421(1991).
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