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Experimental investigation of static and fatigue behaviour of composites honeycomb materials using four point bending tests
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Composite Structures 87 (2009) 265–273 www.elsevier.com/locate/compstruct
Experimental investigation of static and fatigue behaviour of composites honeycomb materials using four point bending tests S. Belouettar a,*, A. Abbadi a,b, Z. Azari b, R. Belouettar c, P. Freres d a
Centre de Recherche Public Henri Tudor, 29, Avenue John F. Kennedy, L-1855 Luxembourg, G.D of Luxembourg, Luxembourg b Laboratoire de Fiabilite´ Me´canique, Ecole des Inge´nieurs de Metz, Ile du Saulcy, F-57045 Metz, France c De´partement de Ge´nie Civil, Universite´ Badji Mokhtar de Annaba, BP 12 Sidi Amar, DZ 23000 Annaba, Algeria d EURO-COMPOSITESÒ, S.A. Zone Industrielle, L-6401 Echternach, G.D of Luxembourg, Luxembourg Available online 12 February 2008
Abstract In this study static and fatigue behaviours of honeycomb sandwich composites, made of aramide fibres and aluminium cores, are investigated through four-point bending tests. Damage and failure modes are reported and discussed. Global and local parameters were considered to evaluate the fatigue life of the analysed sandwich composites. Effects of core densities and the cell orientation (L or W) on the maximum load and on the damage processes (initiation and evolution) are also investigated. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Sandwich beam; Fatigue; Aramide fibres; Aluminium honeycomb; Damage
1. Introduction The use of sandwich structure continues to increase rapidly due to the wide fields of their application, for instance: satellites, aircraft, ships, automobiles, rail cars, wind energy systems, and bridge construction to mention only a few. The sandwich composites are multi-layered materials made by bonding stiff, high strength skins facings to lowdensity core material (see Fig. 1). The main benefits of using the sandwich concept in structural components are the high stiffness and low weight ratios. These structures can carry both in-plane and out-of-plane loads and exhibit good stability under compression, keeping excellent strength to weight and stiffness to weight characteristics. The many advantages of sandwich constructions, the development of new materials and the need for high performance and low-weight structures insure that sandwich construction will continue to be in demand. Sandwich constructions are being considered for application to aircraft *
Corresponding author. Tel.: +352 54 55 80 500; fax: +352 42 59 91 333. E-mail address: [email protected] (S. Belouettar). 0263-8223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2008.01.015
primary structures, where durability and damage tolerance is a first rank consideration, therefore, understanding the adverse effect of in-service events. In fact, expansion of composite structure to sensitive fields, where high reliability is demanded, such as civil aviation, was so far restricted by the poor knowledge of their behaviour under complex dynamic loads. For widespread application and in order to introduce sandwich in primary structures several challenges must be meet. Therefore, the structure needs to be evaluated in order to prove that damage occurring during the service life will not lead to failure or excessive structural deformation until the damage is detected. In order to use these materials in different applications, the knowledge of their static and fatigue behaviours are required and a better understanding of the various failure mechanisms under static and fatigue loadings conditions is necessary and highly desirable. The strength of the sandwich is a result of a combination of properties from the skin, core and interface. Any damage accumulated in one, or more, of these base materials will have an overall effect on the properties of the sandwich. It is imperative to understand how potential damage occurs in service will affect structural performance. The
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Aluminium skin
Nomex/Aluminium honeycomb core Aluminium skin
Glue
Fig. 1. Schematic detailed description of the honeycomb sandwich structure.
ability of the overall structure to behave fail-safe means the ability of load redistribution after partial, obvious damage/ single failure without loss of overall structural load bearing function is another key issue. The fundamentals of sandwich constructions and reviews of experimental and analytical methods are described in early work by Allen [1] and recent works by Zenkert [2]. Gibson and Ashby studied the in-plane stiffness of honeycomb cores according to the bending model of cell edges [3]. Masters and Evans developed a theoretical model for predicting the in-plane elastic stiffness of honeycomb cores based on the deformation of honeycomb cells [4]. Becker studied the effective in-plane stiffness of honeycomb cores and the thickness effect using the closed-form description [5]. Meraghni et al. presented a new analytical method to analyze the out-of-plane stiffness of honeycomb cores based on the modified laminate theory [6]. Regarding to sandwich composites modelling, a complete review of the various kinematics and theories could be found in the work of Hu et al. [7]. From phenomenological point of view, fatigue behaviour of sandwich honeycomb composites can be evaluated, in the global sense by stiffness, residual strength or other mechanical properties. Hwang and Han [8] proposed a fatigue modulus concept for fatigue life prediction of composite materials. It is suggested that changes in stiffness might be an appropriate measure of fatigue damage. Many investigators [9–12] have examined the effectiveness of the stiffness degradation in composite materials as a measure of accumulated damage. As residual strength, stiffness and life are affected by fatigue damage, only residual stiffness can be monitored non-destructively [13]. While residual strength demonstrates minimum decrease with the increase in the number of cycles until a stage close to the end of life of the specimen, when it begins to change then abruptly and is destructive in real sense. On contrary, stiffness exhibits greater changes during fatigue specifically at the early stage of fatigue life of specimen [14]. Much important work on residual stiffness has been done by Reifsnider et al. [15]. Wen-Shyong et al. [16] selected the residual stiffness as a parameter to describe the degradation behaviour and to predict the fatigue life. There is interesting feature in stiffness degradation approach that only limited amount of
data is needed for obtaining reasonable results [17]. Kim [18] reported in his studies that the reduction of bending strength of foam cored sandwich specimen is caused by the stiffness reduction of foam due to ageing of polyurethane foam during fatigue cycles. Shenoi et al. [19] investigated the static and flexural fatigue characteristics of foam core polymer composite sandwich beams. Failure modes relate to both core shear and skin failure. Burman and Zenkert [20] also tested the fatigue characteristic of two cellular foam core materials as used in load carrying sandwich structures. Judawisastra et al. [21] studied the bending fatigue behaviour of pure epoxy and 3D woven sandwich composites. Panels with different core properties were selected. The results of the stiffness degradation correlated well with the mechanical properties of the sandwich structure. Focusing on sandwich materials and structures, an investigation on how material properties and geometrical configuration can affect damage and fatigue damage has not extensively investigated. Of utmost importance, which is the very basis for the use of a sandwich, is the high bending stiffness and strength to weight ratios, which is achieved when the face and core interact in an optimal way. It is hence most important to have a comprehensive knowledge of the sandwich static and fatigue performance. This paper hopes to address some deficiency in the analysis of static and fatigue NomexÒ and aluminium honeycomb sandwich composites and also provides new findings on the analysis of the fatigue of honeycomb structures. The first and foremost step in this paper is to develop an experimental investigation describing the pure bending behaviour of sandwich composite panels under static and cyclic fatigue behaviours. As composite structures, flexible structures have, for pure bending loading, significant displacements and rotations. The design and the dimensioning of these structures require good material characterization beyond the small deformations domain. An additional outcome of this study is the analysis of the core density and the cell orientation (L or W) effects on the maximum load and on the process of damage. 2. Material specimens The Honeycomb sandwich panels are provided by EuroComposites (Luxembourg) and intended for the aircraft industry. The geometrical dimensions of the specimen are shown in Table 1. The faces of a thickness equal to 0.60 mm are made of aluminium (AlMg3), the core structure is made either from aluminium (ECM) sheets or from aramide fibres (ECA) folded and glued together (Fig. 1) forming a hexagonal cell structure. As in the standard layTable 1 Specimen dimensions L (mm)
b (mm)
h (mm)
hc (mm)
tf (mm)
L2 (mm)
L1 (mm)
d = hc + tf (mm)
500
250
10
8.80
0.60
420
210
9.40
S. Belouettar et al. / Composite Structures 87 (2009) 265–273 Table 2 Mechanical properties of faces made of aluminium Young’s modulus (MPa)
Strength to failure (MPa)
Maximum elongation (%)
70,000
367
13
Table 3 Mechanical properties of the cores [22] Core
Cell size Density (kg/m3) Shear resistance L (MPa) Shear modulus L (MPa) Shear resistance W (MPa) Shear moudulus W (MPa) Compression resistance (MPa)
Aluminium core
Fibre aramide core
ECM
ECA
6.4 82 2.40 430 1.40 220 1.50
9.6 55 1.48 253 0.88 170 2.75
3.2 4.8 1.32 51 0.56 49 2.10
3.2 144 3.50 128 2.20 94 15.20
out for commercial honeycombs, the assembly of the structure produces some cell walls with double thickness. In the tested configuration these double thickness walls were parallel to the specimen longitudinal axis. The honeycomb core is an opened cell with various densities of 55 kg/m3 and 82 kg/m3 of aluminium core and 48 kg/m3 and 144 kg/m3 of aramide fibre core, respectively. The cell size is 6.4 and 9.6 mm for aluminium core and 3.2 mm for aramide fibres core, respectively. The geometrical and mechanical properties of the panels are depicted in tables (Tables 2 and 3). 3. Experimental method Both static and fatigue tests were carried out through a four-point bending testing fixture device schematically shown in Fig. 2. Such device, designed and built expressly for these tests, was connected to a servo-hydraulic universal testing machine INSTRON model 4302 controlled by an INSTRON electronic unit. The electronic unit performs the test control and the data acquisition. Another PC equipped with a NI acquisition device was used to acquire the load and stroke signals. Load was measured with a
tf
d
Ec, Gc H
tc
Ef
b
Fig. 2. Sketch of the four point bending test and specimen dimensions.
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10 kN strain-gage load cell directly mounted on the testing machine cross head, while stroke was measured by means of a LVDT transducer directly connected between the frame of the testing machine and the head of the hydraulic actuator. The design of the fixture device allows the inner supports to rotate around the neutral axis of the specimen. When a damaged specimen is tested, the specimen deformation is asymmetric due to the different bending stiffness of the two portions (with and without defect) of the specimen. The rotation of the inner supports allows the testing device to adapt the testing conditions, following the asymmetric displacement of the specimen. In this way it is possible to obtain that, during the whole loading process, the same loads are applied on both the inner supports and consequently to keep the four point bending condition. To apply the loading signal, a Labview program has been set which enables to introduce all points of the cycle. The portions of specimen between the inner and the outer supports are loaded with a constant shear force. The bending moment raises linearly from the outer support to the inner one. The portion of specimen between the two inner supports is loaded with a constant bending moment while the shear force is zero. The maximum value of the bending moment is in the portion between the two inner supports. Two kinds of tests, static and fatigue, were performed on undamaged specimens. 4. Static tests results Static tests were carried out on all configurations at room temperature in stroke control mode at a constant displacement rate of 2 mm mn 1 in order to archive a quasistatic loading condition according to the military standards: MIL-STD-401 DIN 53291. Four replicate sandwich specimens of different densities and in two different configurations were tested. Static tests were performed before fatigue ones with the aim of evaluating both the stiffness and the ultimate strength of the specimens in order to set properly the load cycling amplitude for the fatigue tests. During the tests the displacement of the inner supports of the four-point bending rig and the total applied load were acquired. From these data the specimen bending stiffness was calculated as slope of the initial portion of the bending moment-displacement curve by means of a linear regression. A typical loading maximum displacement curves are shown in Figs. 3–6. The analysis of the experimental results of the static four point binding tests permit to make the following statements: the sandwich composite stiffness increases when increasing the core density and the load to failure increases with increasing cores densities; the maximum loads are higher in the L-direction than in the Wdirection for low densities and almost of the same order of higher core densities values and the maximum deflection is higher in L configuration than in the W for the same sandwich core. Considering together aramide fibres and aluminium cores, the sandwich panels with aramide fibres
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7
6
6
5
5
Loads (kN)
Loads (kN)
density 82kg/m3 (Alu-Alu)
4
3
4
144kg/m3 W
3
48kg/m3 W
2
2
1
55kg/m3
1
(Alu-Alu) 0
0 0
10
20
30
40
0
50
20
40
60
80
100
Max. deflection (mm)
Max. deflection (mm)
Fig. 3. Evolution and comparison of the deflection according to the applied vertical load for the L- and W-directions. The cores are made of aluminium (55 kg/m3 and 82 kg/m3).
Fig. 5. Evolution and comparison of the deflection according to the applied vertical load. The cores are made of aramide fibres (48 kg/m3 and 144 kg/m3) and W-oriented.
5.5 5.0 5 4.5
(L)
L
4
4.0
Loads (kN)
Loads (kN)
3.5
3
3.0 2.5 2.0
2
(W)
w 1.5 1
1.0 0.5
0
0.0 0
5
10
15
20
25
Max. deflection (mm)
Fig. 4. Comparison of the deflection according to the applied vertical load. The cores are made of aluminium (55 kg/m3) and L-oriented.
are almost more ductile than those made of aluminium cores. In all cases the trend was as expected up the failure and therefore no change in the specimen stiffness was detected, while the failure appeared abruptly. The visual and optical observations made on the damaged honeycomb sandwich panels point out that all the specimens failed due to face wrinkling: a local buckling of the compressed face; an indentation and plastic deformations of faces at the loads application area and to cell walls wrinkling in the zone between the load application zone and the support zone. It appears from these observations that the failure modes depend essentially on the nature of core itself: material, density and cells orientation.
0
5
10
15
20
25
30
Max deflection (mm)
Fig. 6. Comparison of the deflection according to the applied vertical load for the L- and W-directions. The cores are made of aramide fibres of 48 kg/m3 density.
Indeed, for honeycomb cores made of aramide fibre of a density of 48 kg/m3 and W-oriented the failure is almost characterized by cell walls buckling (Fig. 7), small indentation of the faces in the vicinity of the loading application and a plastic deformation of the sandwich skins. On the other hand, for L-oriented configuration the failure is essentially due to a significant faces wrinkling in the vicinity of the load application zones (Fig. 13). Fig. 8 illustrates the failure modes of L-oriented and W-oriented aluminium core of a density of 55 kg/m3. These damages are essentially characterized by cell walls buckling in the zone between the load application and the fixed support and
S. Belouettar et al. / Composite Structures 87 (2009) 265–273
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Fig. 7. Failure modes of sandwich with aramide fibre honeycomb core of 48 kg/m3 density.
Fig. 8. Failure modes of sandwich with aluminium honeycomb core of 55 kg/m3.
to a significant skins wrinkling. On the contrary, the failure modes (Fig. 9) in the case of aluminium cores with a density 82 kg/m3 and W-oriented are essentially due to the skin wrinkling and a significant buckling of the cell walls. Also, it is important to mention that in the case of sandwich made of aramide fibres cores and of high density (144 kg/m3), the failure is essentially due the important
Fig. 9. Failure modes of sandwich with aluminium honeycomb core of 82 kg/m3 and W-oriented.
wrinkling of the faces (see Fig. 10). Table 4 summarizes the various failures observed modes for the different tested configurations. 5. Fatigue test results The objectives of the investigated tests were to obtain basic knowledge on the fatigue behaviour of honeycomb sandwich composites as the standard stress-life, SN diagrams and fatigue damage modes. Tests were performed at a room temperature under direct load control, while the load cycling amplitudes were chosen on the basis of the static test results. The test load was sinusoidal with a frequency f = 2 Hz and a load ratio R = 0.1 and a constant amplitude loading. With such load ratio the face in contact with the outer supports was submitted to time varying inplane tensile stresses, while the other face that is in contact with the inner supports was submitted to time varying
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Fig. 10. Static failure modes of sandwich with aramide fibre honeycomb core of 144 kg/m3. The L-specimens failed in local buckling of the face while the W specimens showed an anticipated core shear failure.
Table 4 Principal failure modes under of honeycomb structure under static four point bending test Core material
Density (kg/m3)
Cell orientation
Observed failure mode
Reference
Aramide fibres
48
W
Small buckling of the cell walls and face wrinkling Important buckling of the cell walls and significant face wrinkling Small buckling of the cell walls et face wrinkling Face wrinkling and cell walls buckling Cell walls buckling, face wrinkling Important face wrinkling Local cell walls buckling Important Local wrinkling of the skins
Fig. 7
L
Aluminium
55
W
L Aluminium
82
W L
Aramide fibres
144
W L
Fig. 7
Fig. 8
Fig. 8
Fig. 8
applicable load vary with the different configurations and with the different load ratios the levels were slightly adjusted accordingly. The fatigue life of the specimens is characterized as the number of cycles to ultimate failure. The number of cycles from crack initiation to final fracture was in all cases short when compared to the fatigue life. The variation of the stiffness during the major part of the lifetime was insignificant. The stiffness did not decrease below 90% of the original stiffness until just before final failure. During this last part of the fatigue life the degradation was more pronounced due to the crack formation further discussed below. The fatigue curves in terms of load level versus the number of cycles, shown in Figs. 11–13, illustrate a qualitative comparison between the fatigue lifetime of sandwich composites made of aramide fibres cores and aluminium cores. It comes out that for honeycomb sandwich composites, made of aramide fibres cores, the lifetime of
Fig. 10 Fig. 10
1.1
Load level, F app/F sta,max
1.0
in-plane compressive stresses. In this way it has been possible to evaluate separately the effect of a tensile and a compressive stress field on the fatigue behaviour of the tested specimen. The value of the specimen bending stiffness was monitored during all the tests in order to gather information about the possible reduction of the sandwich structural properties with fatigue cycling. Fatigue data were generated at load levels of 100%, 90%, 80%, 70%, 65% and 60% of the static ultimate load. Two core densities are used: 48 kg/m3 (aramide fibres) and 82 kg/m3 (aluminium) and a minimum of three specimens within each configuration. The data acquisition performed monitored the stiffness variation during the tests using the output signals of the load cell and the deflection of the hydraulic piston. However, since the fatigue threshold and maximum
sens L 0.9
NR =854331 0.8
r=F app/F sta,max =0.5
sens W
0.7
0.6
10000
Alu/Alu, 82 kg/m3
100000
1000000
Cycles to failure, NR
Fig. 11. Comparison of the fatigue lifetime of the L and W configurations in terms of load level vs cycles to failure.
S. Belouettar et al. / Composite Structures 87 (2009) 265–273 1.2
Load level, F app/F sta,max
1.1
1.0
L
0.9
-0.08378 0.8
W
0.7
0.6
NR =1.07E07 r=F app/F sta,max =0.5
0.5 1000
10000
100000
1000000
1E7
Cycles to failure, NR
Fig. 12. Fatigue curves in terms of load level vs cycles to failure. The cores are made of aramide fibres and of density of 48 kg/m3, direction L and W.
the L configuration (Fig. 12) is larger than in the W-direction at constant load level. After extrapolation of the two (L and W configurations) fatigue curves, one notices the intersection of these curves for a load level of 0.5 (Fig. 12) and a lifetime of 1.07 107 cycles. Notice also from Fig. 13 that for a given load level, the lifetime of honeycomb structure made of aluminium cores are significantly larger than the lifetime of material made of aramide fibres cores in all analysed situations. Also, it is not worthy to mention that the endurance limits for both configurations are roughly similar for two analysed materials and corresponds to 60% (see Figs. 11 and 12) of the maximum loading. The following discussions regarding the fatigue failure processes are only based on visual inspection of the free sides of the beams. For sandwich structures made of aramide fibres, both W and L configurations failed in shear with a crack propagating through the thickness of the core (details are shown on Fig. 15). The failure propagation is always in the diagonal direction in the case of the L configuration and horizontal for the W one. In both cases, cracks
271
or micro defects appear before any macro size crack is formed. Crack propagation in one direction, subjected to an opening load during half of load cycle, is unaffected by the cracks growing in the other diagonal direction since these cracks are closed. Hence, cracks initiate from the crack tip of the horizontal macro crack: one growing upwards during half the load cycle and the other one growing downwards during the other half of the load cycle. Notice here that the final crack length is independent of the maximum load and loading ratio. We also noticed a subsequent shear buckling (Fig. 15) of the vertical cell walls in the centre region between the inner and outer support from the first load cycles as well as the formation of several clusters of small horizontal cracks in the cell walls formed within separate cell as shown in Fig. 15. The fatigue cracks formation in the L configuration is similar to those in the W configuration. However, the number of observed micro cracks was significantly less important and the failure was more abrupt. In both cases, the fracture pattern of a diagonal crack is the same (Fig. 15) and not affected by the number of cycles to failure. When applying high loads i.e. 70–80% of the static failure load, some specimens failed in the region close to the supports rather than the expected central region. In that case, the crack did not initiate directly at the support but near the support corner. Two factors can explain this behaviour; the small transverse (or compressive) stresses close to the supports and the thermal influence from the warm loading piston and support. Regarding the fatigue failure process of sandwich composites made of aluminium core, the failure is constantly caused by cracking (Fig. 16) in the lower face of sandwich. This observation is applicable for the two considered configurations. We also observed a subsequent shear buckling of the vertical cell walls in the centre region between the inner and outer support from the first load cycles. This buckling did however not induce any significant change of stiffness. Fig. 14 shows the evolution crack length according to the applied load. One can see from Fig. 14 that in the W configuration case, the crack propagation
6.0 5.5
5
Alu/Alu, 82 kg/m3
L
Alu/Alu, 82
kg/m3 Load level [kN]
Load level [kN]
5.0 4.5 4.0 3.5
4
W-configuration Al3u/A. fibres, 48 kg/m3 3
L 3.0
Alu/aramide 48 kg/m3 2
2.5 100000
1000000
Cycles to failure, NR
1000
10000
100000
1000000
Cycles to failure, NR
Fig. 13. Comparison of the fatigue lifetime of two sandwich structures. The cores are made of aramide fibres of a density of 48 kg/m3 and aluminium of a density of 82 kg/m3.
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S. Belouettar et al. / Composite Structures 87 (2009) 265–273 140
140
Alu/Alu, density 82 kg/m3 Alu/Alu, core density 82 kg/m3
L
120
L
Crack length, a [mm]
Crack length, a [mm]
120
100
Zone I 80
Zone II High load level
Low load level
60
100
80
W 60
W
40 3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
40 10000
100000
Applied load [kN]
1000000
Cycles to failure, NR
Fig. 14. Evolution of the crack length vs the applied load and vs the number of cycles to failure for the sandwich structure made of aluminium core of a density of 82 kg/m3.
W-configuration
L-configuration
Top view y y x x
Side view z x
Fig. 15. Failure modes of the aramide fibres cores in the W and L-directions.
Fig. 16. Failure modes of the aluminium core in the W and L-directions.
is almost stationary in the zone I for low loads level. While in zone II (high level loads), the crack propagation is almost stationary for the L configuration. From Fig. 14,
we observe that L and W configurations are roughly similar in terms of crack length in the zone I. On the contrary, the variation is more significant in the zone II. We can
S. Belouettar et al. / Composite Structures 87 (2009) 265–273
conclude that when fatigue is a concern, the L configuration appears more suitable. 6. Summary and conclusion Fatigue tests in four-point bending were performed on two different sandwich configurations; one with an aluminium cores and one with aramide fibres cores. The fatigue test results were presented in standard S/N diagrams. It was also found from this experimental program that the stiffness might not be a good monitoring measure for the ‘‘health” of a specimen. When the stiffness starts to decrease during the last part of the fatigue life tests there was already considerable damage present in the core material. The damage formation process in the test specimens could be described as follows: damage initiated in the zone of high shear stresses over the entire length of the zone and in the middle of the specimen. An investigation of the influence of support distances was made concluding that the size of the failure process zone depended on the lengths between the load supports. For structural fatigue, when considering sandwich with aluminium core, the L configuration appears more suitable. References [1] Allen G. Analysis and design of structural sandwich panel. Oxford (UK): Pergamon Press; 1969. [2] Zenkert D. An introduction to sandwich construction. Solihull (UK): EMAS; 1995. [3] Gibson LJ, Ashby MF. Cellular solids: structure and properties. 2nd ed. Cambridge (UK): Cambridge University Press; 1997. [4] Masters IG, Evans KE. Models for the elastic deformation of honeycombs. Compos Struct 1996;35:403–22. [5] Becker W. Closed-form analysis of the thickness effect of regular honeycomb core material. Compos Struct 2000;48:67–70.
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[6] Meraghni F, Desrumaux F, Benzeggagh ML. Mechanical behaviour of cellular core for structural sandwich panels. Composites: Part A 1999;30:767–79. [7] Hu H, Belouettar S, Daya EM, Potier-Ferry M. Evaluation of kinematic formulations for viscoelastically damped sandwich beam modeling. J Sandwich Struct Mater 2006;8:477–95. [8] Hwang W, S Han K. Fatigue of composites fatigue modulus concept and life prediction. J Compos Mater 1986;20:155–65. [9] Yang JN, Jones DL, Yang SH, Meskini A. A stiffness degradation model for graphite/epoxy laminate. J Compos Mater 1990;24:753–69. [10] Miner M. Cumulative damage in fatigue. J Appl Mech Part A 1945:159–64. [11] Belingardi G, Martella P, Peroni L. Fatigue analysis of honeycombcomposite sandwich beams. Compos Part A: Appl Sci Manuf 2007;38:1183–91. [12] Demelio G, Genovese K, Pappalettere C. An experimental investigation of static and fatigue behaviour of sandwich composite panels joined by fasteners. Compos Part B: Eng 2001;32:299–308. [13] Burman M, Zenkert D. Fatigue of foam core sandwich beams, Part I: Undamaged specimens. Int J Fatigue 1997;19:551–61. [14] Wang D. Strength failure and fatigue analysis of laminates. Composites Eng Mater Handbook. ASTM International; 1985. p. 236–48. [15] Reifsnider KL. Fatigue of composite materials. London (UK): Elsevier Science Publishers; 1991. [16] Wen-Shyong K, Jiunn F, Horng-Wen L. Failure behavior of 3D woven composites under transverse shear. Compos Part A: Appl Sci Manuf 2003;34:561–75. [17] Burman M, Zenkert D. Fatigue of foam core sandwich beams, Part II: Effect of initial damages. Int J Fatigue 1997;19:563–78. [18] Kim RY. Fatigue strength. Composites Eng Mater Handbook. ASTM International; 1985. p. 436–43. [19] Shenoi RA, Clark SD, Allen HG. Fatigue behaviour of polymer composite sandwich beams. J Compos Mater 1995;29:2423–45. [20] Burman M, Zenkert D. Fatigue of undamaged and damaged honeycomb sandwich beams. J Sandwich Struct Mater 2000;2:50–74. [21] Judawisastra H, Ivens J, Verpoest I. The fatigue behaviour and damage development of 3D woven sandwich composites. Compos Struct 1998;43:35–45. [22] ECA-Honeycomb Data Sheet. EuroComposites S.A.
Composite Structures 87 (2009) 265–273 www.elsevier.com/locate/compstruct
Experimental investigation of static and fatigue behaviour of composites honeycomb materials using four point bending tests S. Belouettar a,*, A. Abbadi a,b, Z. Azari b, R. Belouettar c, P. Freres d a
Centre de Recherche Public Henri Tudor, 29, Avenue John F. Kennedy, L-1855 Luxembourg, G.D of Luxembourg, Luxembourg b Laboratoire de Fiabilite´ Me´canique, Ecole des Inge´nieurs de Metz, Ile du Saulcy, F-57045 Metz, France c De´partement de Ge´nie Civil, Universite´ Badji Mokhtar de Annaba, BP 12 Sidi Amar, DZ 23000 Annaba, Algeria d EURO-COMPOSITESÒ, S.A. Zone Industrielle, L-6401 Echternach, G.D of Luxembourg, Luxembourg Available online 12 February 2008
Abstract In this study static and fatigue behaviours of honeycomb sandwich composites, made of aramide fibres and aluminium cores, are investigated through four-point bending tests. Damage and failure modes are reported and discussed. Global and local parameters were considered to evaluate the fatigue life of the analysed sandwich composites. Effects of core densities and the cell orientation (L or W) on the maximum load and on the damage processes (initiation and evolution) are also investigated. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Sandwich beam; Fatigue; Aramide fibres; Aluminium honeycomb; Damage
1. Introduction The use of sandwich structure continues to increase rapidly due to the wide fields of their application, for instance: satellites, aircraft, ships, automobiles, rail cars, wind energy systems, and bridge construction to mention only a few. The sandwich composites are multi-layered materials made by bonding stiff, high strength skins facings to lowdensity core material (see Fig. 1). The main benefits of using the sandwich concept in structural components are the high stiffness and low weight ratios. These structures can carry both in-plane and out-of-plane loads and exhibit good stability under compression, keeping excellent strength to weight and stiffness to weight characteristics. The many advantages of sandwich constructions, the development of new materials and the need for high performance and low-weight structures insure that sandwich construction will continue to be in demand. Sandwich constructions are being considered for application to aircraft *
Corresponding author. Tel.: +352 54 55 80 500; fax: +352 42 59 91 333. E-mail address: [email protected] (S. Belouettar). 0263-8223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2008.01.015
primary structures, where durability and damage tolerance is a first rank consideration, therefore, understanding the adverse effect of in-service events. In fact, expansion of composite structure to sensitive fields, where high reliability is demanded, such as civil aviation, was so far restricted by the poor knowledge of their behaviour under complex dynamic loads. For widespread application and in order to introduce sandwich in primary structures several challenges must be meet. Therefore, the structure needs to be evaluated in order to prove that damage occurring during the service life will not lead to failure or excessive structural deformation until the damage is detected. In order to use these materials in different applications, the knowledge of their static and fatigue behaviours are required and a better understanding of the various failure mechanisms under static and fatigue loadings conditions is necessary and highly desirable. The strength of the sandwich is a result of a combination of properties from the skin, core and interface. Any damage accumulated in one, or more, of these base materials will have an overall effect on the properties of the sandwich. It is imperative to understand how potential damage occurs in service will affect structural performance. The
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Aluminium skin
Nomex/Aluminium honeycomb core Aluminium skin
Glue
Fig. 1. Schematic detailed description of the honeycomb sandwich structure.
ability of the overall structure to behave fail-safe means the ability of load redistribution after partial, obvious damage/ single failure without loss of overall structural load bearing function is another key issue. The fundamentals of sandwich constructions and reviews of experimental and analytical methods are described in early work by Allen [1] and recent works by Zenkert [2]. Gibson and Ashby studied the in-plane stiffness of honeycomb cores according to the bending model of cell edges [3]. Masters and Evans developed a theoretical model for predicting the in-plane elastic stiffness of honeycomb cores based on the deformation of honeycomb cells [4]. Becker studied the effective in-plane stiffness of honeycomb cores and the thickness effect using the closed-form description [5]. Meraghni et al. presented a new analytical method to analyze the out-of-plane stiffness of honeycomb cores based on the modified laminate theory [6]. Regarding to sandwich composites modelling, a complete review of the various kinematics and theories could be found in the work of Hu et al. [7]. From phenomenological point of view, fatigue behaviour of sandwich honeycomb composites can be evaluated, in the global sense by stiffness, residual strength or other mechanical properties. Hwang and Han [8] proposed a fatigue modulus concept for fatigue life prediction of composite materials. It is suggested that changes in stiffness might be an appropriate measure of fatigue damage. Many investigators [9–12] have examined the effectiveness of the stiffness degradation in composite materials as a measure of accumulated damage. As residual strength, stiffness and life are affected by fatigue damage, only residual stiffness can be monitored non-destructively [13]. While residual strength demonstrates minimum decrease with the increase in the number of cycles until a stage close to the end of life of the specimen, when it begins to change then abruptly and is destructive in real sense. On contrary, stiffness exhibits greater changes during fatigue specifically at the early stage of fatigue life of specimen [14]. Much important work on residual stiffness has been done by Reifsnider et al. [15]. Wen-Shyong et al. [16] selected the residual stiffness as a parameter to describe the degradation behaviour and to predict the fatigue life. There is interesting feature in stiffness degradation approach that only limited amount of
data is needed for obtaining reasonable results [17]. Kim [18] reported in his studies that the reduction of bending strength of foam cored sandwich specimen is caused by the stiffness reduction of foam due to ageing of polyurethane foam during fatigue cycles. Shenoi et al. [19] investigated the static and flexural fatigue characteristics of foam core polymer composite sandwich beams. Failure modes relate to both core shear and skin failure. Burman and Zenkert [20] also tested the fatigue characteristic of two cellular foam core materials as used in load carrying sandwich structures. Judawisastra et al. [21] studied the bending fatigue behaviour of pure epoxy and 3D woven sandwich composites. Panels with different core properties were selected. The results of the stiffness degradation correlated well with the mechanical properties of the sandwich structure. Focusing on sandwich materials and structures, an investigation on how material properties and geometrical configuration can affect damage and fatigue damage has not extensively investigated. Of utmost importance, which is the very basis for the use of a sandwich, is the high bending stiffness and strength to weight ratios, which is achieved when the face and core interact in an optimal way. It is hence most important to have a comprehensive knowledge of the sandwich static and fatigue performance. This paper hopes to address some deficiency in the analysis of static and fatigue NomexÒ and aluminium honeycomb sandwich composites and also provides new findings on the analysis of the fatigue of honeycomb structures. The first and foremost step in this paper is to develop an experimental investigation describing the pure bending behaviour of sandwich composite panels under static and cyclic fatigue behaviours. As composite structures, flexible structures have, for pure bending loading, significant displacements and rotations. The design and the dimensioning of these structures require good material characterization beyond the small deformations domain. An additional outcome of this study is the analysis of the core density and the cell orientation (L or W) effects on the maximum load and on the process of damage. 2. Material specimens The Honeycomb sandwich panels are provided by EuroComposites (Luxembourg) and intended for the aircraft industry. The geometrical dimensions of the specimen are shown in Table 1. The faces of a thickness equal to 0.60 mm are made of aluminium (AlMg3), the core structure is made either from aluminium (ECM) sheets or from aramide fibres (ECA) folded and glued together (Fig. 1) forming a hexagonal cell structure. As in the standard layTable 1 Specimen dimensions L (mm)
b (mm)
h (mm)
hc (mm)
tf (mm)
L2 (mm)
L1 (mm)
d = hc + tf (mm)
500
250
10
8.80
0.60
420
210
9.40
S. Belouettar et al. / Composite Structures 87 (2009) 265–273 Table 2 Mechanical properties of faces made of aluminium Young’s modulus (MPa)
Strength to failure (MPa)
Maximum elongation (%)
70,000
367
13
Table 3 Mechanical properties of the cores [22] Core
Cell size Density (kg/m3) Shear resistance L (MPa) Shear modulus L (MPa) Shear resistance W (MPa) Shear moudulus W (MPa) Compression resistance (MPa)
Aluminium core
Fibre aramide core
ECM
ECA
6.4 82 2.40 430 1.40 220 1.50
9.6 55 1.48 253 0.88 170 2.75
3.2 4.8 1.32 51 0.56 49 2.10
3.2 144 3.50 128 2.20 94 15.20
out for commercial honeycombs, the assembly of the structure produces some cell walls with double thickness. In the tested configuration these double thickness walls were parallel to the specimen longitudinal axis. The honeycomb core is an opened cell with various densities of 55 kg/m3 and 82 kg/m3 of aluminium core and 48 kg/m3 and 144 kg/m3 of aramide fibre core, respectively. The cell size is 6.4 and 9.6 mm for aluminium core and 3.2 mm for aramide fibres core, respectively. The geometrical and mechanical properties of the panels are depicted in tables (Tables 2 and 3). 3. Experimental method Both static and fatigue tests were carried out through a four-point bending testing fixture device schematically shown in Fig. 2. Such device, designed and built expressly for these tests, was connected to a servo-hydraulic universal testing machine INSTRON model 4302 controlled by an INSTRON electronic unit. The electronic unit performs the test control and the data acquisition. Another PC equipped with a NI acquisition device was used to acquire the load and stroke signals. Load was measured with a
tf
d
Ec, Gc H
tc
Ef
b
Fig. 2. Sketch of the four point bending test and specimen dimensions.
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10 kN strain-gage load cell directly mounted on the testing machine cross head, while stroke was measured by means of a LVDT transducer directly connected between the frame of the testing machine and the head of the hydraulic actuator. The design of the fixture device allows the inner supports to rotate around the neutral axis of the specimen. When a damaged specimen is tested, the specimen deformation is asymmetric due to the different bending stiffness of the two portions (with and without defect) of the specimen. The rotation of the inner supports allows the testing device to adapt the testing conditions, following the asymmetric displacement of the specimen. In this way it is possible to obtain that, during the whole loading process, the same loads are applied on both the inner supports and consequently to keep the four point bending condition. To apply the loading signal, a Labview program has been set which enables to introduce all points of the cycle. The portions of specimen between the inner and the outer supports are loaded with a constant shear force. The bending moment raises linearly from the outer support to the inner one. The portion of specimen between the two inner supports is loaded with a constant bending moment while the shear force is zero. The maximum value of the bending moment is in the portion between the two inner supports. Two kinds of tests, static and fatigue, were performed on undamaged specimens. 4. Static tests results Static tests were carried out on all configurations at room temperature in stroke control mode at a constant displacement rate of 2 mm mn 1 in order to archive a quasistatic loading condition according to the military standards: MIL-STD-401 DIN 53291. Four replicate sandwich specimens of different densities and in two different configurations were tested. Static tests were performed before fatigue ones with the aim of evaluating both the stiffness and the ultimate strength of the specimens in order to set properly the load cycling amplitude for the fatigue tests. During the tests the displacement of the inner supports of the four-point bending rig and the total applied load were acquired. From these data the specimen bending stiffness was calculated as slope of the initial portion of the bending moment-displacement curve by means of a linear regression. A typical loading maximum displacement curves are shown in Figs. 3–6. The analysis of the experimental results of the static four point binding tests permit to make the following statements: the sandwich composite stiffness increases when increasing the core density and the load to failure increases with increasing cores densities; the maximum loads are higher in the L-direction than in the Wdirection for low densities and almost of the same order of higher core densities values and the maximum deflection is higher in L configuration than in the W for the same sandwich core. Considering together aramide fibres and aluminium cores, the sandwich panels with aramide fibres
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7
6
6
5
5
Loads (kN)
Loads (kN)
density 82kg/m3 (Alu-Alu)
4
3
4
144kg/m3 W
3
48kg/m3 W
2
2
1
55kg/m3
1
(Alu-Alu) 0
0 0
10
20
30
40
0
50
20
40
60
80
100
Max. deflection (mm)
Max. deflection (mm)
Fig. 3. Evolution and comparison of the deflection according to the applied vertical load for the L- and W-directions. The cores are made of aluminium (55 kg/m3 and 82 kg/m3).
Fig. 5. Evolution and comparison of the deflection according to the applied vertical load. The cores are made of aramide fibres (48 kg/m3 and 144 kg/m3) and W-oriented.
5.5 5.0 5 4.5
(L)
L
4
4.0
Loads (kN)
Loads (kN)
3.5
3
3.0 2.5 2.0
2
(W)
w 1.5 1
1.0 0.5
0
0.0 0
5
10
15
20
25
Max. deflection (mm)
Fig. 4. Comparison of the deflection according to the applied vertical load. The cores are made of aluminium (55 kg/m3) and L-oriented.
are almost more ductile than those made of aluminium cores. In all cases the trend was as expected up the failure and therefore no change in the specimen stiffness was detected, while the failure appeared abruptly. The visual and optical observations made on the damaged honeycomb sandwich panels point out that all the specimens failed due to face wrinkling: a local buckling of the compressed face; an indentation and plastic deformations of faces at the loads application area and to cell walls wrinkling in the zone between the load application zone and the support zone. It appears from these observations that the failure modes depend essentially on the nature of core itself: material, density and cells orientation.
0
5
10
15
20
25
30
Max deflection (mm)
Fig. 6. Comparison of the deflection according to the applied vertical load for the L- and W-directions. The cores are made of aramide fibres of 48 kg/m3 density.
Indeed, for honeycomb cores made of aramide fibre of a density of 48 kg/m3 and W-oriented the failure is almost characterized by cell walls buckling (Fig. 7), small indentation of the faces in the vicinity of the loading application and a plastic deformation of the sandwich skins. On the other hand, for L-oriented configuration the failure is essentially due to a significant faces wrinkling in the vicinity of the load application zones (Fig. 13). Fig. 8 illustrates the failure modes of L-oriented and W-oriented aluminium core of a density of 55 kg/m3. These damages are essentially characterized by cell walls buckling in the zone between the load application and the fixed support and
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Fig. 7. Failure modes of sandwich with aramide fibre honeycomb core of 48 kg/m3 density.
Fig. 8. Failure modes of sandwich with aluminium honeycomb core of 55 kg/m3.
to a significant skins wrinkling. On the contrary, the failure modes (Fig. 9) in the case of aluminium cores with a density 82 kg/m3 and W-oriented are essentially due to the skin wrinkling and a significant buckling of the cell walls. Also, it is important to mention that in the case of sandwich made of aramide fibres cores and of high density (144 kg/m3), the failure is essentially due the important
Fig. 9. Failure modes of sandwich with aluminium honeycomb core of 82 kg/m3 and W-oriented.
wrinkling of the faces (see Fig. 10). Table 4 summarizes the various failures observed modes for the different tested configurations. 5. Fatigue test results The objectives of the investigated tests were to obtain basic knowledge on the fatigue behaviour of honeycomb sandwich composites as the standard stress-life, SN diagrams and fatigue damage modes. Tests were performed at a room temperature under direct load control, while the load cycling amplitudes were chosen on the basis of the static test results. The test load was sinusoidal with a frequency f = 2 Hz and a load ratio R = 0.1 and a constant amplitude loading. With such load ratio the face in contact with the outer supports was submitted to time varying inplane tensile stresses, while the other face that is in contact with the inner supports was submitted to time varying
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Fig. 10. Static failure modes of sandwich with aramide fibre honeycomb core of 144 kg/m3. The L-specimens failed in local buckling of the face while the W specimens showed an anticipated core shear failure.
Table 4 Principal failure modes under of honeycomb structure under static four point bending test Core material
Density (kg/m3)
Cell orientation
Observed failure mode
Reference
Aramide fibres
48
W
Small buckling of the cell walls and face wrinkling Important buckling of the cell walls and significant face wrinkling Small buckling of the cell walls et face wrinkling Face wrinkling and cell walls buckling Cell walls buckling, face wrinkling Important face wrinkling Local cell walls buckling Important Local wrinkling of the skins
Fig. 7
L
Aluminium
55
W
L Aluminium
82
W L
Aramide fibres
144
W L
Fig. 7
Fig. 8
Fig. 8
Fig. 8
applicable load vary with the different configurations and with the different load ratios the levels were slightly adjusted accordingly. The fatigue life of the specimens is characterized as the number of cycles to ultimate failure. The number of cycles from crack initiation to final fracture was in all cases short when compared to the fatigue life. The variation of the stiffness during the major part of the lifetime was insignificant. The stiffness did not decrease below 90% of the original stiffness until just before final failure. During this last part of the fatigue life the degradation was more pronounced due to the crack formation further discussed below. The fatigue curves in terms of load level versus the number of cycles, shown in Figs. 11–13, illustrate a qualitative comparison between the fatigue lifetime of sandwich composites made of aramide fibres cores and aluminium cores. It comes out that for honeycomb sandwich composites, made of aramide fibres cores, the lifetime of
Fig. 10 Fig. 10
1.1
Load level, F app/F sta,max
1.0
in-plane compressive stresses. In this way it has been possible to evaluate separately the effect of a tensile and a compressive stress field on the fatigue behaviour of the tested specimen. The value of the specimen bending stiffness was monitored during all the tests in order to gather information about the possible reduction of the sandwich structural properties with fatigue cycling. Fatigue data were generated at load levels of 100%, 90%, 80%, 70%, 65% and 60% of the static ultimate load. Two core densities are used: 48 kg/m3 (aramide fibres) and 82 kg/m3 (aluminium) and a minimum of three specimens within each configuration. The data acquisition performed monitored the stiffness variation during the tests using the output signals of the load cell and the deflection of the hydraulic piston. However, since the fatigue threshold and maximum
sens L 0.9
NR =854331 0.8
r=F app/F sta,max =0.5
sens W
0.7
0.6
10000
Alu/Alu, 82 kg/m3
100000
1000000
Cycles to failure, NR
Fig. 11. Comparison of the fatigue lifetime of the L and W configurations in terms of load level vs cycles to failure.
S. Belouettar et al. / Composite Structures 87 (2009) 265–273 1.2
Load level, F app/F sta,max
1.1
1.0
L
0.9
-0.08378 0.8
W
0.7
0.6
NR =1.07E07 r=F app/F sta,max =0.5
0.5 1000
10000
100000
1000000
1E7
Cycles to failure, NR
Fig. 12. Fatigue curves in terms of load level vs cycles to failure. The cores are made of aramide fibres and of density of 48 kg/m3, direction L and W.
the L configuration (Fig. 12) is larger than in the W-direction at constant load level. After extrapolation of the two (L and W configurations) fatigue curves, one notices the intersection of these curves for a load level of 0.5 (Fig. 12) and a lifetime of 1.07 107 cycles. Notice also from Fig. 13 that for a given load level, the lifetime of honeycomb structure made of aluminium cores are significantly larger than the lifetime of material made of aramide fibres cores in all analysed situations. Also, it is not worthy to mention that the endurance limits for both configurations are roughly similar for two analysed materials and corresponds to 60% (see Figs. 11 and 12) of the maximum loading. The following discussions regarding the fatigue failure processes are only based on visual inspection of the free sides of the beams. For sandwich structures made of aramide fibres, both W and L configurations failed in shear with a crack propagating through the thickness of the core (details are shown on Fig. 15). The failure propagation is always in the diagonal direction in the case of the L configuration and horizontal for the W one. In both cases, cracks
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or micro defects appear before any macro size crack is formed. Crack propagation in one direction, subjected to an opening load during half of load cycle, is unaffected by the cracks growing in the other diagonal direction since these cracks are closed. Hence, cracks initiate from the crack tip of the horizontal macro crack: one growing upwards during half the load cycle and the other one growing downwards during the other half of the load cycle. Notice here that the final crack length is independent of the maximum load and loading ratio. We also noticed a subsequent shear buckling (Fig. 15) of the vertical cell walls in the centre region between the inner and outer support from the first load cycles as well as the formation of several clusters of small horizontal cracks in the cell walls formed within separate cell as shown in Fig. 15. The fatigue cracks formation in the L configuration is similar to those in the W configuration. However, the number of observed micro cracks was significantly less important and the failure was more abrupt. In both cases, the fracture pattern of a diagonal crack is the same (Fig. 15) and not affected by the number of cycles to failure. When applying high loads i.e. 70–80% of the static failure load, some specimens failed in the region close to the supports rather than the expected central region. In that case, the crack did not initiate directly at the support but near the support corner. Two factors can explain this behaviour; the small transverse (or compressive) stresses close to the supports and the thermal influence from the warm loading piston and support. Regarding the fatigue failure process of sandwich composites made of aluminium core, the failure is constantly caused by cracking (Fig. 16) in the lower face of sandwich. This observation is applicable for the two considered configurations. We also observed a subsequent shear buckling of the vertical cell walls in the centre region between the inner and outer support from the first load cycles. This buckling did however not induce any significant change of stiffness. Fig. 14 shows the evolution crack length according to the applied load. One can see from Fig. 14 that in the W configuration case, the crack propagation
6.0 5.5
5
Alu/Alu, 82 kg/m3
L
Alu/Alu, 82
kg/m3 Load level [kN]
Load level [kN]
5.0 4.5 4.0 3.5
4
W-configuration Al3u/A. fibres, 48 kg/m3 3
L 3.0
Alu/aramide 48 kg/m3 2
2.5 100000
1000000
Cycles to failure, NR
1000
10000
100000
1000000
Cycles to failure, NR
Fig. 13. Comparison of the fatigue lifetime of two sandwich structures. The cores are made of aramide fibres of a density of 48 kg/m3 and aluminium of a density of 82 kg/m3.
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140
Alu/Alu, density 82 kg/m3 Alu/Alu, core density 82 kg/m3
L
120
L
Crack length, a [mm]
Crack length, a [mm]
120
100
Zone I 80
Zone II High load level
Low load level
60
100
80
W 60
W
40 3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
40 10000
100000
Applied load [kN]
1000000
Cycles to failure, NR
Fig. 14. Evolution of the crack length vs the applied load and vs the number of cycles to failure for the sandwich structure made of aluminium core of a density of 82 kg/m3.
W-configuration
L-configuration
Top view y y x x
Side view z x
Fig. 15. Failure modes of the aramide fibres cores in the W and L-directions.
Fig. 16. Failure modes of the aluminium core in the W and L-directions.
is almost stationary in the zone I for low loads level. While in zone II (high level loads), the crack propagation is almost stationary for the L configuration. From Fig. 14,
we observe that L and W configurations are roughly similar in terms of crack length in the zone I. On the contrary, the variation is more significant in the zone II. We can
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conclude that when fatigue is a concern, the L configuration appears more suitable. 6. Summary and conclusion Fatigue tests in four-point bending were performed on two different sandwich configurations; one with an aluminium cores and one with aramide fibres cores. The fatigue test results were presented in standard S/N diagrams. It was also found from this experimental program that the stiffness might not be a good monitoring measure for the ‘‘health” of a specimen. When the stiffness starts to decrease during the last part of the fatigue life tests there was already considerable damage present in the core material. The damage formation process in the test specimens could be described as follows: damage initiated in the zone of high shear stresses over the entire length of the zone and in the middle of the specimen. An investigation of the influence of support distances was made concluding that the size of the failure process zone depended on the lengths between the load supports. For structural fatigue, when considering sandwich with aluminium core, the L configuration appears more suitable. References [1] Allen G. Analysis and design of structural sandwich panel. Oxford (UK): Pergamon Press; 1969. [2] Zenkert D. An introduction to sandwich construction. Solihull (UK): EMAS; 1995. [3] Gibson LJ, Ashby MF. Cellular solids: structure and properties. 2nd ed. Cambridge (UK): Cambridge University Press; 1997. [4] Masters IG, Evans KE. Models for the elastic deformation of honeycombs. Compos Struct 1996;35:403–22. [5] Becker W. Closed-form analysis of the thickness effect of regular honeycomb core material. Compos Struct 2000;48:67–70.
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[6] Meraghni F, Desrumaux F, Benzeggagh ML. Mechanical behaviour of cellular core for structural sandwich panels. Composites: Part A 1999;30:767–79. [7] Hu H, Belouettar S, Daya EM, Potier-Ferry M. Evaluation of kinematic formulations for viscoelastically damped sandwich beam modeling. J Sandwich Struct Mater 2006;8:477–95. [8] Hwang W, S Han K. Fatigue of composites fatigue modulus concept and life prediction. J Compos Mater 1986;20:155–65. [9] Yang JN, Jones DL, Yang SH, Meskini A. A stiffness degradation model for graphite/epoxy laminate. J Compos Mater 1990;24:753–69. [10] Miner M. Cumulative damage in fatigue. J Appl Mech Part A 1945:159–64. [11] Belingardi G, Martella P, Peroni L. Fatigue analysis of honeycombcomposite sandwich beams. Compos Part A: Appl Sci Manuf 2007;38:1183–91. [12] Demelio G, Genovese K, Pappalettere C. An experimental investigation of static and fatigue behaviour of sandwich composite panels joined by fasteners. Compos Part B: Eng 2001;32:299–308. [13] Burman M, Zenkert D. Fatigue of foam core sandwich beams, Part I: Undamaged specimens. Int J Fatigue 1997;19:551–61. [14] Wang D. Strength failure and fatigue analysis of laminates. Composites Eng Mater Handbook. ASTM International; 1985. p. 236–48. [15] Reifsnider KL. Fatigue of composite materials. London (UK): Elsevier Science Publishers; 1991. [16] Wen-Shyong K, Jiunn F, Horng-Wen L. Failure behavior of 3D woven composites under transverse shear. Compos Part A: Appl Sci Manuf 2003;34:561–75. [17] Burman M, Zenkert D. Fatigue of foam core sandwich beams, Part II: Effect of initial damages. Int J Fatigue 1997;19:563–78. [18] Kim RY. Fatigue strength. Composites Eng Mater Handbook. ASTM International; 1985. p. 436–43. [19] Shenoi RA, Clark SD, Allen HG. Fatigue behaviour of polymer composite sandwich beams. J Compos Mater 1995;29:2423–45. [20] Burman M, Zenkert D. Fatigue of undamaged and damaged honeycomb sandwich beams. J Sandwich Struct Mater 2000;2:50–74. [21] Judawisastra H, Ivens J, Verpoest I. The fatigue behaviour and damage development of 3D woven sandwich composites. Compos Struct 1998;43:35–45. [22] ECA-Honeycomb Data Sheet. EuroComposites S.A.