Abstract-Sandwich structures comprising para-aramid paper honeycomb core and carbon fiber epoxy matrix composite facesheets were fabricated for ...
2013 International Conference on Aerospace Science & Engineering (ICASE)
Optimization of the Manufacturing Parameters of Honeycomb Composite Sandwich Structures for Aerospace Application 1
U. Farooq, 2A. A. Khurram, 1M.S.Ahmad ,2S. A. Rakha, 2N. Ali, 2A. Munir, 1T. Subhani
1
Materials Science and Engineering Department, Institute of Space Technology, Islamabad, Pakistan 2
Experimental Physics Facility, NCP, Quaid-i-Azam University, Islamabad, Pakistan
Abstract-Sandwich structures comprising para-aramid paper honeycomb core and carbon fiber epoxy matrix composite facesheets were fabricated for aerospace applications. For the adhesion of honeycomb core and composite facesheets, two different types of adhesive films were used. The curing parameters for adhesive films, including temperature and time, were optimized for maximum bonding strength.
Keywords: Sandwich structure; honeycomb; Mechanical testing.
Para-aramid
I. INTRODUCTION Sandwich composite structures offer a very high stiffness-to-weight ratio. A sandwich composite panel is composed of high strength composite skins separated by and bonded to a light weight honeycomb or foam cores. A sandwich panel enhances the flexural rigidity of the structure without adding substantial weight. Also an increase in the thickness of honeycomb and/or foam cores provides higher stiffness and strength of the panel at the minimum addition of weight. Sandwich structures in several applications have shown to have higher fatigue strength and better acoustical and thermal insulation [1]. Honeycomb cores have numerous advantages over the foam or wood based sandwich structures due to their high crushing strength and stiffness, fatigue and moisture resistance [2]. Sandwich panels have a variety of applications in aerospace, automobile, sports and marine industry. In aerospace industry the parts of the aircrafts like ailerons, flaps and rudders, have been manufactured from honeycomb sandwich panels. Generally, in sandwich composites two composite facesheets and a core material are bonded together using an adhesive material. The properties of the 978-1-4799-0993-3/13/$31.00 © 2013 IEEE
adhesive material play a key role in sandwich panel technology. The adhesive material must attach the facesheets to the core material firmly in order to ensure the load transfer from the face sheet to the core, which results in a high bending stiffness. Since two different materials are bonded together in a sandwich panel, shear stress is generated at the interface when the panel is under a bending load. As a result a facesheet usually detaches from the core material. The load bearing capacity of the structural sandwich is therefore dependent on the quality of adhesion between the core and the face sheets [3-4]. The adhesive materials must have the filleting property between the facesheet and the honeycomb core, since a reliable bond is only possible if the flow of adhesive material create a symmetric fillet on the walls of the honeycomb core as shown in the Fig.1.
Fig.1 The fillets formed by adhesive material between a honeycomb wall and facesheet.
Adhesive bond between the face sheets and the core material critically affects the mechanical properties of the sandwich composite structures. In the present
study two types of the adhesive materrials, i.e. films have been used to join the composite ffacesheets with a honeycomb core. The processingg and curing parameters of the adhesive materials w were optimized to achieve the maximum bonding strength of a honeycomb sandwich structure.
sandwich panels as a function n of the processing (curing) parameters of the adhesiv ve films.
II. MATERIALS Para-aramid paper honeycomb core used in the present study was purchased froom Armicore Company, China. The thickness of thhe honeycomb core was 20mm while the cell sizee was 5.5mm. Carbon fiber epoxy matrix compositte face sheets having the thickness of 1mm were pprepared using Vacuum Assisted Resin Transfer Moldding (VARTM) technique. Carbon fibers were embeddded as a carbon fabric of 3K plain weave with 200g/m m2 mass density while an aerospace grade areldite epooxy 5052 from tetraglycidyle methylene dianiline classs of resins was used as matrix. Two different types of adhesive films (69B and 272D) were used for the bbonding of the face sheets and the honeycomb cores.
Fig.2. A typical hexagonal honeycomb co ore.
III. EXPERIMENTAL The honeycomb core, as shown in Figg.2 was cut in the desired dimensions (76.2mm × 203.2mm) following the ASTM standard C393 for 3-point The composite bending test as shown in Fig.3a. T facesheets were also cut in the same dimensions in order to attach with the honeycomb coore. Composite facesheets and the honeycomb core were bonded together by two different adhesivve films, as mentioned above. The curing of thesee two adhesive films (69B and 272D) was performeed at different temperatures and time durations (Tablee.1) in order to optimize their bonding strength.
Fig.3a 3-point bending test of a compositee panel.
Table.1 Processing parameters of 69B and 272D adhesive films. Sr. No.
Adhesive film
1
272D
2
69B
Curing temperature (oC) 100 110 120 100 120 130
Curing time (hrs) 2,3 2,3 2,3 3,4 2,3,4 2,3,4
The cured specimens were tested for 3-point bending Model WDWtest using a tensile testing machine (M 30). Load was applied along L directtion (Fig.2) by loading span while performing 3-poinnt bending test as shown in Fig.3a. Load-displacemennt curves were obtained to evaluate the bending sttrength of the
Fig. 3b A typical localized core crushing after a 3-point bend test
IV. RESULTS AND DISSCUSSION A typical load-deflection curve reesulting from 3-point bending test of a composite sandwich panel is shown y evident: region I is in Fig.4. Four regions are clearly the linear elastic region where the applied load is mainly born by the upper facesheet and the honeycomb core. In region III, the applied load dropped suddenly because of the t crushing of the honeycomb core, which started along a with the severe bending of the upper facesheet. During crushing of
Fig.4 Load-displacement curves of sandwich panel bonded with 69B adhesive film as function of curing temperature.
There can be many modes of failure but shear stress is generally the overwhelming mode of failure in honeycomb composite structures, in which a core material fails when shear stresses approach to their critical values. However, situations arise when normal stresses in the sandwich core dominate the shear stresses. Such a combination of different stresses is called multi-axial state of stress.
The load-displacement curves of honeycomb sandwich panels with 69B adhesive film are shown in Fig.4. The curing time for three composites was same, i.e. 4hrs; however, curing temperature varied, i.e. 1000C, 1200C and 1300C. It can be seen from this figure that the sample cured at 1200C has a higher failure load 1.094kN as compared to the samples cured at 1000C and 1300C. However, these two samples have shown higher deflection before the fracture of the top face sheet. In Fig.5 the loaddisplacement behavior of sandwich panels is shown, which were cured at a temperature of 1200C. However, curing time was varied from 2hrs to 4hrs. The sample cured at 1200C for 2hrs has shown higher load, i.e. 1.157kN before failure as compared to those cured for 3hrs and 4hrs. Fig.4 and Fig.5 show optimum curing temperature and time for 69B adhesive film, i.e. 1200C and 2hrs respectively. Curing conditions for the 272D adhesive film for optimum bonding strength of honeycomb sandwich panels were optimized in the same manner as for 69B adhesive film. The results of 3-point bending test of honeycomb sandwich core bonded with 272D adhesive film are shown in Fig.6 and Fig.7. 272D-100C-2hrs Fmax= 0.797KN
1.0
272D-110C-2hrs Fmax= 0.939KN 272D-120C-2hrs Fmax= 0.759KN
0.8 Load (KN)
the honeycomb core, the force applied on the specimen was almost uniform until the upper facesheet fractured. Region III is showing the sudden drop of applied load when the upper facesheet fractured. In this region detachment of upper facesheet from the honeycomb core has also been observed. In region IV, the load again started increasing, which can be related to the densification of the crushed honeycomb core; also in this region the load was now born by lower facesheet of the honeycomb sandwich panel.
0.6 0.4
69B-120C-4hrs Fmax=1.094kN
1.2
69B-120C-3hrs Fmax=0.954kN 69B-120C-2hrs Fmax=1.157kN
1.0
0.2 0.0
0
5
10
15
20
25
Load (KN)
Displacement (mm)
0.8
Fig.6 Load-displacement curves of a sandwich panel bonded with 272D as function of curing temperature.
0.6 0.4 0.2 0.0
0
5
10
15
20
25
Displacement (mm)
Fig.5 Load-displacement curves of sandwich panel bonded with 69B as function of curing time.
The results in Fig.6 and Fig.7 show that curing at 1100C for 2hrs has resulted in higher peak load 0.939kN before failure. However, after curing at 1000C and 1200C, a gradual decrease in the applied load during region II, which is also a desirable effect in sandwich beams. 3-point bending tests were also conducted as a function of curing time. The curing time for three composites (Fig.6) was same i.e. 2hrs.
However, curing temperature for the three composites was different, i.e. 1000C, 1100C, 1200C. Moreover, 2hrs curing has also resulted in larger displacement (Fig.6 and Fig.7) before the complete failure of the top face sheet. These results show that curing of 272D adhesive film at 1100C for 2hrs has optimum mechanical properties as compared to the samples cured at other temperatures for different time durations as shown in Fig. 7.
272D-2hrs-110C F max = 0.939KN
Load (KN)
1.0
272D-3hrs-110C F max = 0.899KN
V. CONCLUSIONS It is concluded from the 3-point bending tests of honeycomb sandwich panels that the bonding strength depends upon curing conditions of the adhesive films, which join the facesheets to the honeycomb core. The curing of 69B film at 1200C for 2hrs has shown the optimum results, whereas optimized curing temperature and time for 272D adhesive film were found to be 1100C and 2hrs, respectively. These honeycomb sandwich composites are ideal lightweight materials for aerospace applications.
0.8
REFERENCES
0.6
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[2] www.hexcel.com
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Displacement (m m )
Fig.7 Load-displacement curve of sandwich panel bonded with 272D as function of curing time.
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