CYCLIC BUCKLING TESTS OF CFRP BOXES UNDER
COMPRESSION AND TORSION
P. Cordisco and C. Bisagni
Department of Aerospace Engineering
Politecnico di Milano
Milan, IT
ABSTRACT
The paper presents the results of the first tests performed at the Department of Aerospace Engineering of Politecnico di Milano
inside the European project COCOMAT (Improved MATerial Exploitation at Safe Design of COmposite Airframe Structures by
Accurate Simulation of Collapse). The tests were performed on CFRP stringer stiffened composite panels applying axial
compression and shear loading, separately and in combination. First, static buckling tests were performed with different load
combinations, then cyclic buckling tests were performed applying repeated shear loading beyond the buckling load with a
constant applied axial compression. To perform the tests under shear loading, the panels, manufactured by Agusta/Westland,
were assembled in a closed box. In this way, it was possible to test the panels under shear, applying a torque to the closed
box. The results obtained statically, and the results obtained after the repetition of the load are compared in terms of loaddisplacement curves, strains and out of plane deformation. From the tests results it is possible to note the capability of CFRP
stringer stiffened panels to work in the post-buckling regime without changes in the global behavior, even if the buckling load is
reached several times.
Introduction
Fiber reinforced composites are an important class of material for aerospace applications because of their high specific
mechanical properties. Anyway, unlike isotropic materials, they have a complex response to loading, and this is the main limit
to their large use. But aeronautical industries hope to reach a cost reduction of 20% in the short period and of 50% in the long
one [1].
From the structural point of view, the only way to achieve such an objective seems, nowadays, from one hand, the extension
of the use of composite materials and, from the other hand, the adoption, during the design phase, of strength criteria less
conservative than the actual ones. In order to give next future composite designers the same level of knowledge actual
metallic designers have [2], it is very important to carry out accurate experiments for investigating the behavior of structures
made of composite materials facing different operating conditions. One of this operating conditions is the buckling behavior,
and the post-buckling range, especially when it is reached several times.
In literature, few results are nowadays available on stringer stiffened CFRP panels subjected to combined axial compression
and shear loading [3-5], and even less on panels subjected to cyclic load repetitions [6]. Indeed, loading of a single curved
panel, during buckling tests raises a difficult problem, especially for what concerns shear loading. At Politecnico di Milano, the
problem has been come through by assembling four panels in a closed box [5-6]. This allows to test each panels under shear
by applying a torque on the box.
The results of the first tests performed inside the European project COCOMAT (Improved MATerial Exploitation at Safe Design
of COmposite Airframe Structures by Accurate Simulation of Collapse) [1] are here presented. First, static buckling tests were
performed, applying axial compression and shear loading, separately and in combination. Then cyclic tests were performed
applying repeated shear loading beyond the buckling load, with a constant applied axial compression.
Specimens
Four panels, manufactured by Agusta-Westland, were assembled and tested in a closed box (Figure 1a). In particular, the box
consists of two large panels and two small panels. Each panel is made of CFRP with both fabric and unidirectional plies. The
long panels are 700 mm high, 693 mm wide and have a radius of curvature of 1500 mm. They present four L-shaped stringer
stiffeners (28x28 mm), which are bonded, as well as riveted, to the skin. The two smaller panels have the same height and the
same radius of curvature, but they are 241 mm wide and present only one stiffener (identical to the stiffeners of the two large
panels).
At the top and at the bottom of the closed box, two aluminum ending tabs are bonded using a mixture of epoxy resin and
aluminum powder, so to allow the fixing of the box into the loading machine. The bonding to the aluminum ending tabs is
realized using an ad-hoc designed and manufactured facility (Figure 1b), which assures the planarity of the upper on the lower
basis and their correct alignment in order to guarantee an uniform distribution of the load during the tests.
The box is instrumented with 54 strain gauges. In particular, strain gauges are placed on three sides of the box, both internally
and externally (back-to-back configuration). On the fourth side, the strain gauges were bonded only internally, so to leave the
external surface clean for a laser-based displacement measurements.
(a)
(b)
Figure 1. (a) CFRP box. (b) CFRP box within the facility for fixing the aluminum ending tabs.
Experimental equipments
The testing machine at the Department of Aerospace Engineering of Politecnico di Milano [5-8] was used to perform the
buckling tests (Figure 2). The equipment is able to apply axial compression and torque, separately or in combination. Besides
both static buckling tests and cyclic buckling tests can be performed.
During static tests, the equipment is controlled using a position mode. The axial compression is applied using four stepping
motors, while the torque is applied using a stepping motor and a torsion lever.
During cyclic tests, the torque is applied cyclically. In this case, the stepping motor is substitutes by an hydraulic cylinder
driven by a servo-valve. The torque is so transmitted to the box by simply controlling the length of the cylinder’s shaft.
Moreover, this configuration allows to perform torsion tests at any desiderate velocity. This aspect is very important in the
cyclic torsion tests, in which the load must be applied for thousands of times.
During the tests, the load and the displacements applied on the specimen are recorded using a load cell (able to read both
axial compression and torque) and four LVDTs (two for the axial shortening and two for the rotation), respectively.
The out of plane deformation of the external surface of the front panel of the box are measured using a scanning equipment
(Figure 3), based on a laser sensor, whose position is computer-controlled. The laser has the possibility to translate in three
directions and to rotate around the vertical axis by means of four stepping motors. Combining the four movements, the laser is
able to follow the curvature of the panels in order to maintain a constant distance from it, remaining at the same time
perpendicular to the scanned surface. Anyway, for low curvature specimens like the ones here presented, the use of the
rotational and of the normal motors are not required and the scanning is performed just translating the laser along the vertical
and the horizontal axes.
Figure 2. Two views sketch of the buckling test facility of Politecnico di Milano.
Figure 3. Two views sketch of the laser scanning system of Politecnico di Milano.
Test methodology
The test sequence was decided in agreement with Agusta/Westland. At the beginning six static tests were performed in order
to record the static response of the panels subjected to pure axial compression, pure torque and to four combinations of axial
compression and torque. In particular, combined tests were carried out at four constant compression values (one for each test)
and increasing the torque until the post-buckling range. Defining PCRIT the buckling load of the pure axial compression test,
the values of the axial load were fixed to: 95% PCRIT, 75% PCRIT, 50% PCRIT and 25% PCRIT. In each test, the maximum
reached torque was equal to 125% of the buckling torque.
Then, the box was loaded under cyclic buckling. The box was tested for 2000 cycles fixing the axial load to 50% PCRIT and
repeating the torque from 0 to 125% of the buckling torque measured in the static test. The loading frequency was equal to 0.2
Hz.
During the static tests, the axial load vs. shortening curve, the torque vs. rotation curve and the strain gauge readings were
measured. Moreover, the external surface of the front panel of the box was scanned by the laser sensor. During the cyclic
tests, only the applied loads and the corresponding displacements were recorded. Anyway, every 500 cycles, a static test was
performed recording measurements from the strain gauges and the laser sensor.
Static tests results
Table 1 reports the buckling load measured during the tests with different load combinations, while Figure 4 presents the
obtained interaction curve.
Buckling torque
(TCrit) [kNm]
Maximum torque
(125% TCrit) [kNm]
Test
name
Axial compression
[kN]
% of the pure axial
compression test (PCrit)
Test1
90.0
100
0
0
Test2
85.5
95
0.84
1.04
Test3
67.5
75
4.2
5.25
Test4
45.0
50
5.8
7.25
Test5
22.5
25
7.0
8.75
Test6
0.00
0
8.5
10.6
Table 1. Buckling loads measured during the static combined tests.
100
Axial load [kN]
80
60
40
20
0
0
1.5
3
4.5
6
7.5
9
Torque [kNm]
Figure 4. Interaction curve measured in the static combined tests.
Figure 5 reports a picture of the box taken during the pure axial compression test, while Figure 6 shows the external surfaces
of the box as it was scanned in each static test by the laser system equipment at the maximum reached torque.
Figure 5. Photo of the box during the pure axial compression test.
(a)
(b)
(c)
(d)
(e)
(f)
Figure 6. Out of plane deformation of the box front panel obtained by the laser system at the maximum reached torque.
(a) Test1 - pure axial compression; (b) Test2; (c) Test3; (d) Test4; (e) Test5; (f) Test6 - pure torque.
Cyclic test results
Figure 7 presents the comparison of the four torque vs. rotation curves recorded in the initial static tests and after 500, 1000,
1500 and 2000 cycles, respectively.
Figure 8 presents a comparison of the micro-strain vs. torque curves recorded by two couples of back-to-back strain gauges
(±45°, in the middle of the back panel of the box) at 0, 500, 1000, 1500 and 200 cycles.
Figure 9 shows a comparison of the external surface of the front panel of the box as measured by the laser system at the
maximum reached torque after 0, 500, 1000, 1500 and 2000 cycles.
As it can be seen in the comparisons, only a slight change in the strain gauge readings was observed, while the torque vs.
rotation curves and the deformed surfaces were practically the same. This seems to confirm the capability of this kind of
structure to work in the post-buckling field, reaching the buckling load thousands of times without changes in the global
behavior.
10
Torque [kNm]
8
6
Initial
At 500 cycles
At 1000 cycles
At 1500 cycles
At 2000 cycles
4
2
0
0,00
0,05
0,10
0,15
0,20
0,25
0,30
Rotation [deg]
Figure 7. Comparison of the torque vs. rotation curves measured in tests at 0, 500, 1000 ,1500 and 2000 cycles.
Strain gauges 5ext
Strain gauges 5int
1200
Initial
At 500 cycles
At 1000 cycles
At 1500 cycles
At 2000 cycles
800
200
micro-strain
1000
micro-strain
400
600
400
200
0
0
-200
-400
0
0
2000
4000
6000
8000
-600
2000
4000
Initial
At 500 cycles
At 1000 cycles
At 1500 cycles
At 2000 cycles
Torque [kNm ]
6000
8000
100
0
-100 0
2000
4000
-200
-300
-400
-500
Torque [kNm ]
8000
200
micro-strain
micro-strain
4000
6000
Strain gauges 6ext
Initial
At 500 cycles
At 1000 cycles
At 1500 cycles
At 2000 cycles
2000
8000
Torque [kNm ]
Strain gauges 6int
300
200
100
0
-100 0
-200
-300
-400
-500
-600
6000
Torque [kNm ]
Initial
At 500 cycles
At 1000 cycles
At 1500 cycles
At 2000 cycles
Figure 8. Comparison of the micro-strain vs. torque curves recorded by a couple of strain gauges (both internal and external)
at 0, 500, 1000, 1500 and 2000 cycles.
700
700
600
400
300
200
400
300
200
100
0
3.40
3.20
3.00
2.80
2.60
2.40
2.20
2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
-0.20
-0.40
-0.60
-0.80
500
Vertical axis [mm]
500
Vertical axis [mm]
600
3.40
3.20
3.00
2.80
2.60
2.40
2.20
2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
-0.20
-0.40
-0.60
-0.80
100
100
200
300
400
500
600
700
0
100
200
Horizontal axis [mm]
300
400
(a)
700
700
600
600
3.40
3.20
3.00
2.80
2.60
2.40
2.20
2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
-0.20
-0.40
-0.60
-0.80
400
300
200
3.40
3.20
3.00
2.80
2.60
2.40
2.20
2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
-0.20
-0.40
-0.60
-0.80
500
Vertical axis [mm]
500
Vertical axis [mm]
600
(b)
700
400
300
200
100
0
500
Horizontal axis [mm]
100
100
200
300
400
500
600
700
0
100
200
Horizontal axis [mm]
300
400
500
600
700
Horizontal axis [mm]
(c)
(d)
700
600
3.40
3.20
3.00
2.80
2.60
2.40
2.20
2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
-0.20
-0.40
-0.60
-0.80
Vertical axis [mm]
500
400
300
200
100
0
100
200
300
400
500
600
700
Horizontal axis [mm]
(e)
Figure 9. Comparison of the external surface of the front panel of the box as scanned by the laser scanning system at 7.2 kNm
(axial load fixed to 45kN) after (a) 0, (b) 500, (c) 1000, (d) 1500 and (e) 2000 cycles.
Conclusions
The behavior of four CFRP curved stringer stiffened panels subjected to several combination of axial load and shear was
analyzed. The panels were assembled in a closed box configuration and the box was, at first, subjected to several static load
combinations for recording the buckling loads and the post-buckling response.
After the static tests, the effect of repeated buckling was investigated performing cyclic buckling tests. The cyclic tests were
performed applying an axial compression load equal to 50% of the buckling load recorded in the pure axial compression test
and reaching the post-buckling field cycling the torque from 0 to 125% of the buckling torque obtained in the corresponding
static test, for 2000 times with a frequency of 0.2 Hz. Every 500 cycles, a static test was performed in order to evaluate if the
structure has changed the global behavior because of the repetition of the load.
Results recorded before, during and after the application of cyclic buckling loads have been compared in terms of torque vs.
rotation curves, strain gauges measurements and out of plane deformation. The obtained data are very important because
they seem to confirm the capability of this type of structure to work in the post-buckling field reaching the buckling load
thousands of times. This is one of the first steps toward the extension of the operating loads in the post-buckling field even for
the CFRP panels.
Acknowledgments
The authors want to acknowledge Prof. Vittorio Giavotto for his important suggestions during the work and for his constant
encouragements. This work is supported by the European Commission, Competitive and Sustainable Growth Programme,
Contract No. AST3-CT-2003-502723, project COCOMAT. The information in this paper is provided as is and no guarantee or
warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and
liability.
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