- Email: [email protected]
Experimental and numerical evaluation of sandwich composite structures
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 64 (2004) 1709–1715 www.elsevier.com/locate/compscitech
Experimental and numerical evaluation of sandwich composite structures C. Borsellino a b
a,1
, L. Calabrese
a,1
, A. Valenza
b,*
Department of Industrial Chemistry and Material Engineering, University of Messina, Italy Process and Materials, Department of Chemical Engineering, University of Palermo, Italy
Received 25 March 2003; received in revised form 9 January 2004; accepted 12 January 2004 Available online 21 February 2004
Abstract The main problem working with sandwich composite structures is their intrinsic anisotropy and non-homogeneity that does not allow their correct modelling. Nowadays the available data on mechanical properties of complex structures, necessary to allow a correct and reliable design, are not sufficient. The aim of the present work is to extend the knowledge of mechanical properties both on single components and on complete structures, focusing on the effects induced by different kind of skin arrangements (Kevlar, glass and carbon fibres). Compressive, shear and flexural tests were performed for a complete static mechanical characterisation of the sandwich structure both on each single component and on the complex structures in order to acquire important comparison parameters. The mechanical results of each component were used as input data in order to implement the FEM analysis by the commercial ANSYS code. A simplified model is proposed to simulate the compressive and flexural tests of a glass fibre sandwich structure. In addition their mechanical behaviour has been compared with experimental data by the aforesaid static tests of complex sandwich structures. 2004 Elsevier Ltd. All rights reserved. Keywords: Sandwich; B. Mechanical properties; C. Finite element analysis
1. Introduction The increasing efforts aimed to find out structures characterised by reduced weight and better mechanical performances, in this last years, has led to the development and the employment of sandwich structures mainly in land and sea transportation [1]. A structural sandwich is a particular type of a laminated composite characterised by a combination of different materials bonded together to cooperate with their single properties to the global structure performance. Usually the sandwich structure is divided in three main parts; two external thin and stiff faces (skin) and a
*
Corresponding author. Fax: +39-0916567280. E-mail addresses: [email protected] (C. Borsellino), [email protected] (L. Calabrese), [email protected]. it (A. Valenza). 1 Fax: +39-090391518. 0266-3538/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2004.01.003
central thick and soft core. The skins are bonded to the core to permit the load transfer between the components. The main problem in designing and verifying such structures is their intrinsic anisotropy and non-homogeneity that does not allow their correct modelling. Nowadays the available data on mechanical properties of complex structures, necessary to allow a correct and reliable design, are not sufficient [2,3]. The aim of the present work is to extend the knowledge of mechanical properties both on single components and on complete structures, focusing on the effects induced by different kind of skin arrangements (Kevlar, glass and carbon fibres). Therefore, using the vacuum bag technology, three different sandwich structures, used in windsurf boards construction, were realised. The sandwiches are characterised by a middle light core of expanded polystyrene externally covered by high density PVC foam. The employed technology is suitable for small batch production of complex shapes and structures that
1710
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715
cannot be obtained by press. The procedure is performed as in the following: the whole prepreg structure is encapsulated between a rigid, airtight mould and a flexible vacuum bag. This last is sealed around the periphery of the tool. The cavity is then evacuated via peripheral channel. In this way the structure is compacted using the vacuum as compression force [4]. Compressive and flexural tests were performed for a complete static mechanical characterisation of the sandwich structure both on each single component and on the complex structures in order to acquire important comparison parameters. Moreover shear tests were performed on polystyrene foam to enhance the effect of the density on its mechanical performance. The mechanical results of each component were used as input data in order to implement a simplified model, developed by a commercial FEA code. The proposed model has been verified by simulating compressive and flexural tests of a reference sandwich structure composed of fibreglass skins. In addition their mechanical behaviour has been compared with experimental data by the aforesaid static tests of complex sandwich structure. The so obtained results show that the numerical model has a good forecasting capability. Due to its versatility it can also be employed for other structures differing from those above described both in the single element (material) and in the layer stacking sequence.
2. Experimental procedure 2.1. Materials The sandwich beam facing were realized with different woven fibres (detailed characteristics of the fibres used are reported in Table 1). A PX-420 epoxy resin combined with the hardener 128, both supplied by Prochima, was used as matrix. Polystyrene and PVC foams are used together as core. The realized sandwich structure is reported in Fig. 1. The sandwich plate was fabricated by vacuum bagging.
Table 1 Woven properties
Thickness (mm) Elastic modulus (MPa) Shear modulus (MPa) PoissonÕs ratio, m Density, q (kg/m3 ) Woven specification (g/m2 )
Glass fibre
Carbon fibre
Kevlar fibre
0.12 72,000 30000 0.2 2,550 165
0.25 297,000 114231 0.3 1,750 200
0.25 124,000 45926 0.35 1,450 168
Fig. 1. Scheme of the sandwich structure.
2.2. Mechanical testing Static-mechanical tests were preliminarily conduced on constituents to acquire important parameter for the mechanical characterization of the sandwich structure and to implement the numerical simulation. By performing the Flatwise Compression Test (in the following FC test), according to ASTM C365, the compressive strength and modulus of sandwich cores, under load direction normal to the facing plane, have been determined. The samples cross-section area for this test is 25 25 mm. The Edgewise–Compressive test (in the following EC test), according to ASTM C364, was carried out to obtain the flat compressive properties of structural sandwich under load direction parallel to the facing plane. The sample dimensions are 20 50 120 mm. Moreover the Three Point Bending Test (in the following 3PF test), according to ASTM C393, was realized to determine the properties of flat sandwich panel subjected to flatwise flexure. In this case the sample dimensions are 20 40 130 mm, with span length of 80 mm. 2.3. Finite element analysis Numerical simulations were conducted using the ANSYS 5.6 finite element software. A 2-D model with 8-node quadrilateral element (Plane 82) has been realised, according to other authors [5]. The simulation is performed for FC, EC and 3PF tests employing a reference sandwich structure composed of fibreglass skins. A schematic description of the finite element modelling parameters is reported in Table 2. We focused our interest on a composite modelling which is based on the knowledge of the material data obtained via experimental test. These tests are performed using appropriate value of the mechanical and physical properties, obtained by the experimental characterization of the constituent. The knowledge of the material data is obtained by experimental tests; the woven skin properties are
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715 Table 2 Simulation set-up
FCT ECT 3PF
Nodes
Elements
Sample dimension (mm)
Steps
6332 21,153 20,144
2050 6930 6600
25 20 120 20 130 20
11 8 6
detected by a homogenization model [6], using the constituent properties as input produces the lamina properties as output. The post-elastic analysis is intentionally neglected, not only to obtain a numerical simulation simple and versatile, always required conditions in an effective design methodology, but rather because the aim of the work is to characterise the composite structure by a numerical procedure performed in the elastic regime. In fact in plastic regime the foam cell collapse heavily influences the mechanical stability of the sandwich structure. The numerical analyses, carried out to compare the theoretical model with the experimental results, are able to reproduce the experimental determination of the initial stiffness achieved in the experimental tests described before.
3. Results and discussion
1711
In the beads foam the compression yield stress is often lower than the tensile failure stress. Differences between the two types of foam can be explained considering that in compression the collapse is due to the buckling of the cells walls while in the tensile condition the critical cracks initiate locally in a lowdensity core region at a bead boundary. Consequently the tensile mechanical tests are particularly influenced by un-homogeneity into the cellular structure [7]. Such consideration can explain why in the 15 kg/m3 polystyrene foam, the compression yield stress is slightly higher than the tensile failure stress. The mechanical properties of PVC foam are higher than polystyrene foam about one size order, due principally to the high density value (40 kg/m3 ). The PVC foam acts as junction between the low-density foam and the high-performance fibres, offering a soft structural contribute on the mechanical stability of the composite sandwiches. 3.1.2. Flatwise compression tests Fig. 2 shows the general trend of composites in the FC tests for all test samples. The trends for all samples are very similar. The first part of the compressive stress–strain curve is linear elastic until it reaches a local maximum at very low deformation value, where the cell structure of the
3.1. Mechanical results 8 Glass 6
Stress [Mpa]
3.1.1. Constituents In Table 3 the static mechanical results for all constituents are reported. Moreover in the same table the mechanical properties two different densities polystyrene are compared. On polystyrene foam core an increase in modulus and failure stress is evidenced, associated to a small increase of density. About the obtained results it is possible to notice that on polystyrene foam, at a density increase of 20%, a considerable increasing in modulus and ultimate stress is achieved (respectively about 51% and 108% on flexural test). In the polystyrene foam with density 18 kg/m3 there is a significant difference between tensile and compression properties while a similar trend is not evidenced in the polystyrene with 15 kg/m3 density.
Carbon 4
Kevlar
2
0 0
20
40
60
80
Strain [%]
Fig. 2. Stress–strain curve in FC test for all sandwich structures.
Table 3 Static mechanical properties Tensile test (MPa)
Flexural test (MPa)
Compression test (MPa)
Shear test (MPa)
Epoxy resin
– rr ¼ 0:05 E ¼ 1:21 rr ¼ 0:12 E ¼ 5:09 rr ¼ 1:82 E ¼ 54:0
rr ¼ 99:8 E ¼ 3; 220 rs ¼ 0:06 E ¼ 0:8 rs ¼ 0:08 E ¼ 1:25 rs ¼ 0:93 E ¼ 13:1
–
Polystyrene foam 15 kg/m3
rr ¼ 85:7 E ¼ 4; 378 rr ¼ 0:12 E ¼ 3:78 rr ¼ 0:25 E ¼ 5:71 rr ¼ 2:25 E ¼ 58:2
Polystyrene foam 18 kg/m3 PVC foam
100
rs ¼ 0:57 E ¼ 1:005 rs ¼ 1:11 E ¼ 1:25 –
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715
polystyrene foam starts to collapse. At intermediate deformations (at about e ¼ 50%) a second modification of the trend of the curve is evidenced, associated at the beginning collapse of the PVC foam core. In the last part of the curve the stress increase exponentially, due to the interaction of the cellular structures of the compressed foam. The last part of the curve is called the regime of densification. On this experimental test, only the foam core used influences the flatwise-compressive properties of sandwich panel. In Fig. 3 comparative analysis between a FC test on glass-fibre sandwich and a theoretical trace obtained adding the strain contribute of each single component is reported. It can be noticed that the curve have similar shapes, confirming the collapse sequence polystyrenePVC; on the other hand their small difference at intermediate stresses can be attributed to a viscous liquid resin which permeate from the woven into the voids of the foams, inducing locally, at the interface, a density increase. 3.1.3. Edgewise compression tests Fig. 4 shows the load versus displacement for EC test. Excluding the first stabilization trace of the test, all specimens show a linear behaviour until the local buckling is reached. The deformation to break decreases with increasing elastic modulus of the fibres. The fracture load in Kevlar sandwich is lower than other fibre skins sandwich, due to the reduced compressive properties of aramid fibres. Generally, because the core shear modulus is lower than the skins stiffness, the initial failure mode of the samples is crimping (Fig. 5(a) and (b)). Sometimes, due to poor adhesion between skin and core associated to thin skins, a wrinkling rupture initially occurs (Fig. 5(c)). After that the failure has started, also shearing and delamination take place in the sandwich samples [8].
10000 Carbon Glass
8000 Kevlar Load [N]
1712
6000 4000 2000 0 0,0
0,5
1,0
1,5
Displacement [mm]
Fig. 4. Load vs Displacement in the EC test for all sandwich composites.
Fig. 5. Typical Edgewise Compressive fracture modes of sandwich laminates.
3.1.4. Three point bending tests The stress/displacement curves, obtained from a 3PB tests for all sandwich structures, are reported in Fig. 6. The mechanical behaviour is well matched with the results reported in the literature [9,10]. Observing the traces of the tests, we can identify three regions: A. Initially the load increases proportionally with deflection; the material shows a linear elastic behaviour, until about 20–30 MPa stress value. Beyond this point the load/deflection curve becomes non linear and it follows a plateau in which the cellular structure collapse
5000 Additive model
40
B
Experimental data
4000
A
3000
Stress [Mpa]
Load [N]
32
2000
C 24 16
1000
Carbon Kevlar Glass
8 0 0
5
10
15
20
Deflection [mm]
0 0
5
10
15
20
25
Displacement [mm]
Fig. 3. Comparative analysis of experimental FC test and additive compression model.
Fig. 6. Stress versus displacement from a 3PB test.
30
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715
Fig. 8. Specimen with edge failure in 3PB test.
the ends of the top skins, especially for the short beam samples. In Fig. 8 two pictures of a carbon sandwich are reported, they evidence the fracture on the edge of the sample, which occurs through the beads or along the beads boundaries [12], propagated at the polystyrenePVC foam interface. 3.2. Finite element analysis results 3.2.1. Flatwise compression tests The Figs. 9 and 10 show respectively the comparative results of ANSYS simulation of FC test with experimental data and the strain distributions in the load application direction (y direction), of the reference sandwich structure composed of fibreglass skins. The model does not present an appreciable compatibility with the experimental data at high deformations. This is due at a bad interaction at the interface between two different materials, especially at the foam cores interface. High deformation values are localised in the foam cores. Particularly the densification regime starts in the polystyrene foam, which have an average strain of about 80% against the PVC foam where we have an average strain of about 35%. Confirming the experimental results the skin deformations are negligible compared with the other ones. 6000 Experimental data Ansys model 4500 Load [N]
initiate by buckling of the cell walls and edges. In fact, from a visual examination of the running test, we can observe the crushing of the top part of the polystyrene foam, associated to the distortion of the cellular structure of the foam. This wrinkling effect is evident in sandwich structure with low-density foam core [11]. The wrinkle locally reduces the distance between the two faces, altering the behaviour of the panel. B. Further loading lead to a top skin failure by bending under the load application point and this phenomenon is followed by a large decrease in load. The failure of the top skin starts by local wrinkling on the compressed side of the panel. For carbon fibre sandwich the fracture stress is higher than the glass or Kevlar one. The addiction of higher modulus carbon fibre induce evident effects on flexural strength or modulus. The deformation to break of the Kevlar fibre sandwich (5 mm) is lower than other structures one (8–9 mm). On the other hand the wrinkling fracture of the top skin is not abrupt as other structures. This behaviour of the Kevlar sandwiches is explained considering that this fibre has a low compression stress to break but high absorbed deformation energy. C. The last zone of the load/deflection curve is particularly interesting studying the formation and evolution of failure modes. On the carbon sandwich a further loading leads to a gradual stress increase, because the carbon top layer is not fully break down, so a residual strength of the skin is showed. In fact sometimes a second cracking noise was heard accompanied by a second load drop on the load/deflection curve. On the other sandwich structures this phenomenon is not present. Subsequently at the complete fracture of the top skin, the crack begins to propagate at the interface skin-PVC foam-Polystyrene foam, and at the same time the crack propagates itself towards the centre of the sample (Fig. 7(a)). At higher deformation value the densification of the foam cores take place. Foams have almost completely collapsed and the wall cells begin to interact, inducing a further load increase, until we have the fracture of the lower skin associated to the delamination and debonding at the interface where material discontinuities are present (Fig. 7(b)). Sometimes a quite relevant load drop was observed, due to the tensile fracture of the core, owing to lifting of
1713
3000
1500
0 0
5
10
15
20
Displacement [mm]
Fig. 7. Specimens with flexural failure, 6.5.
Fig. 9. Numerical/experimental comparison for load/displacement plots in FC test.
1714
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715
Fig. 10. Strain distribution in y direction in FC test simulation (load ¼ 950 N).
3.2.2. Edgewise compression tests In Figs. 11 and 12 are reported the results respectively of the experimental and numerical comparison of loaddisplacement curve and the stress distribution in the load direction for the reference sample, when a load P ¼ 8000 N is applied. The numerical results agreed very well with the experimental ones. The Young modulus of the skins (Eskin ¼ 21,750 MPa) is greater than foam cores ones (EPVC ¼ 54 MPa; EEPS ¼ 5.09 MPa), consequently the axial load is carried out mainly by the skins. Consequently the elastic and geometrical parameters of the skins influence compres-
sive performances of the foam core sandwich structure. However a bad skin/core adhesion can induce a premature failure of the sample, due to crack formation at the interface between these two components [13]. 3.2.3. Three point bending tests Fig. 13 compares 3PF experimental load/displacement results with those from the numerical analysis, evidencing a close agreement in elastic regime between them. Fig. 14 shows strain distributions in the applied load direction for the 3PF test, just before the non linear behaviour in the load/deflection curve.
500
10000 Experimental data
400
Ansys model Load [N]
Load [N]
7500
5000
2500
300
200
100 Experimental data Ansys model
0
0
0,0
0,3
0,5
0,8
1,0
1,3
1,5
Displacement [mm]
Fig. 11. Experimental/numerical comparison for load/displacement plots in EC test.
0
2
4
6
8
10
12
Displacement [mm]
Fig. 13. Experimental/numerical comparison for load/displacement plots in 3PB test.
Fig. 12. Stress distribution in y direction in EC test – P ¼ 8000 N.
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715
1715
Fig. 14. Strain distribution in y direction–3PBT simulation – P ¼ 365 N.
The higher strain values are reached in the polystyrene foam, while skins are less deformed [10]. Moreover in the y strain distribution figure the load effect on the supports is evidenced, but the strain values are lower than in the polystyrene centre core. This confirms that the plateau evidenced in the experimental curves is due to the wall collapse of the polystyrene foam cells.
4. Conclusions • On the basis of the experimental compressive, shear and flexural tests performed the mechanical characteristics of the single components of the sandwich structure are obtained, these last show the influence of the density in the polystyrene foam and the difference between polystyrene and PVC foam. • Carrying out the compression and flexural tests has allowed a better comprehension of the fracture mechanisms of the sandwich composite. • The presence of different fibre skins does not induce relevant effects on the stiffness of the sandwich structure but it heavily influences the fracture mechanism. In particular in FC tests: only the foam core used influences the flatwise-compressive properties of sandwich panel. • In flexural test the failure of the top skin starts by local wrinkling on the compressed side of the panel. For carbon fibre sandwich the fracture stress is higher than the glass or Kevlar one. The addiction of higher modulus carbon fibre does not induce evident effects on flexural strength or modulus. The deformation to break of the Kevlar fibre sandwich is lower than other structures one. On the other hand the wrinkling fracture of the top skin is not abrupt as other structures. • In edgewise compression test, because the core shear modulus is lower than the skins stiffness, the initial failure mode of the samples is crimping. Sometimes, due to poor adhesion between skin and core associated to thin skins, a wrinkling rupture initially occurs. After that the failure has started, also shearing and delamination take place in the sandwich samples. • The FEM analysis, performed by employing the commercial ANSYS code, was carried out to simulate the
static mechanical tests. The static-mechanical behaviour of the composite structure is well approximated by numerical simulations in elastic zone. As it was expected the proposed model does not show a good compatibility with the experimental data at high deformation (plastic regime). • The proposed model is suitable to perform a proper engineering design of sandwich composite structures. This last is helpful to choose the correct geometry as well as the correct layer arrangement at varying loading conditions that are not often experimentally available. References [1] Atckinson R. Innovative uses for sandwich constructions. Reinf Plast 1997;41(2):30–3. [2] Manning JA, Crosky AG, Bandyopadhay S. Flexural and impact properties of sandwich panels used in surfboard construction. In: Proceedings of the International Conference on Advanced Composites. Wollongong, Australia; 1993. p. 123–7. [3] Bannister MK, Braemar R, Crothers IP. The mechanical performance of 3D woven sandwich composites. Compos Struct 1999;47:687–90. [4] Coniff Darren. Closed mould processes why use them? Reinf Plast 1999;43(2):32–4. [5] Manet V. The use of Ansys to calculate the behaviour of sandwich structures. Compos Sci Technol 1998;58:1899–905. [6] Tanov R. A contribution to the finite element formulation for the analysis of composite sandwich shells. PhD, Aerospace Engineering, University of Cincinnati Engineering; 2000. [7] Moosa ASI, Mills NJ. Analysis of bend test on polystyrene bead foams. Polym Test 1998;17:357–78. [8] Kwon YW, Murphy MC, Castelli V. Buckling of unbalanced sandwich panels with titanium and GRP skins. J Press Vess Technol 1995;117:40–4. [9] Corigliano A, Rizzi E, Papa E. Experimental characterization and numerical simulations of a syntactic-foam/glass-fibre composite sandwich. Compos Sci Technol 2000;60:2169–80. [10] Mines RAW, Alias A. Numerical simulation of the progressive collapse of polymer composite sandwich beams under static loading. Compos: Part A 2002;33:11–26. [11] Weissman-Berman D. Marine sandwich structures – Part II. SAMPE J 1992;28(5):9–17. [12] Mills NJ, Kang P. The effect of water immersion on the fracture toughness of polystyrene foam used in soft shell cycle helmets. J Cell Plast 1994;30:196–222. [13] Mouritz AP, Thomson RS. Compression, flexure and shear properties of a sandwich composite containing defects. Compos Struct 1999;44:263–78.
Experimental and numerical evaluation of sandwich composite structures C. Borsellino a b
a,1
, L. Calabrese
a,1
, A. Valenza
b,*
Department of Industrial Chemistry and Material Engineering, University of Messina, Italy Process and Materials, Department of Chemical Engineering, University of Palermo, Italy
Received 25 March 2003; received in revised form 9 January 2004; accepted 12 January 2004 Available online 21 February 2004
Abstract The main problem working with sandwich composite structures is their intrinsic anisotropy and non-homogeneity that does not allow their correct modelling. Nowadays the available data on mechanical properties of complex structures, necessary to allow a correct and reliable design, are not sufficient. The aim of the present work is to extend the knowledge of mechanical properties both on single components and on complete structures, focusing on the effects induced by different kind of skin arrangements (Kevlar, glass and carbon fibres). Compressive, shear and flexural tests were performed for a complete static mechanical characterisation of the sandwich structure both on each single component and on the complex structures in order to acquire important comparison parameters. The mechanical results of each component were used as input data in order to implement the FEM analysis by the commercial ANSYS code. A simplified model is proposed to simulate the compressive and flexural tests of a glass fibre sandwich structure. In addition their mechanical behaviour has been compared with experimental data by the aforesaid static tests of complex sandwich structures. 2004 Elsevier Ltd. All rights reserved. Keywords: Sandwich; B. Mechanical properties; C. Finite element analysis
1. Introduction The increasing efforts aimed to find out structures characterised by reduced weight and better mechanical performances, in this last years, has led to the development and the employment of sandwich structures mainly in land and sea transportation [1]. A structural sandwich is a particular type of a laminated composite characterised by a combination of different materials bonded together to cooperate with their single properties to the global structure performance. Usually the sandwich structure is divided in three main parts; two external thin and stiff faces (skin) and a
*
Corresponding author. Fax: +39-0916567280. E-mail addresses: [email protected] (C. Borsellino), [email protected] (L. Calabrese), [email protected]. it (A. Valenza). 1 Fax: +39-090391518. 0266-3538/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2004.01.003
central thick and soft core. The skins are bonded to the core to permit the load transfer between the components. The main problem in designing and verifying such structures is their intrinsic anisotropy and non-homogeneity that does not allow their correct modelling. Nowadays the available data on mechanical properties of complex structures, necessary to allow a correct and reliable design, are not sufficient [2,3]. The aim of the present work is to extend the knowledge of mechanical properties both on single components and on complete structures, focusing on the effects induced by different kind of skin arrangements (Kevlar, glass and carbon fibres). Therefore, using the vacuum bag technology, three different sandwich structures, used in windsurf boards construction, were realised. The sandwiches are characterised by a middle light core of expanded polystyrene externally covered by high density PVC foam. The employed technology is suitable for small batch production of complex shapes and structures that
1710
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715
cannot be obtained by press. The procedure is performed as in the following: the whole prepreg structure is encapsulated between a rigid, airtight mould and a flexible vacuum bag. This last is sealed around the periphery of the tool. The cavity is then evacuated via peripheral channel. In this way the structure is compacted using the vacuum as compression force [4]. Compressive and flexural tests were performed for a complete static mechanical characterisation of the sandwich structure both on each single component and on the complex structures in order to acquire important comparison parameters. Moreover shear tests were performed on polystyrene foam to enhance the effect of the density on its mechanical performance. The mechanical results of each component were used as input data in order to implement a simplified model, developed by a commercial FEA code. The proposed model has been verified by simulating compressive and flexural tests of a reference sandwich structure composed of fibreglass skins. In addition their mechanical behaviour has been compared with experimental data by the aforesaid static tests of complex sandwich structure. The so obtained results show that the numerical model has a good forecasting capability. Due to its versatility it can also be employed for other structures differing from those above described both in the single element (material) and in the layer stacking sequence.
2. Experimental procedure 2.1. Materials The sandwich beam facing were realized with different woven fibres (detailed characteristics of the fibres used are reported in Table 1). A PX-420 epoxy resin combined with the hardener 128, both supplied by Prochima, was used as matrix. Polystyrene and PVC foams are used together as core. The realized sandwich structure is reported in Fig. 1. The sandwich plate was fabricated by vacuum bagging.
Table 1 Woven properties
Thickness (mm) Elastic modulus (MPa) Shear modulus (MPa) PoissonÕs ratio, m Density, q (kg/m3 ) Woven specification (g/m2 )
Glass fibre
Carbon fibre
Kevlar fibre
0.12 72,000 30000 0.2 2,550 165
0.25 297,000 114231 0.3 1,750 200
0.25 124,000 45926 0.35 1,450 168
Fig. 1. Scheme of the sandwich structure.
2.2. Mechanical testing Static-mechanical tests were preliminarily conduced on constituents to acquire important parameter for the mechanical characterization of the sandwich structure and to implement the numerical simulation. By performing the Flatwise Compression Test (in the following FC test), according to ASTM C365, the compressive strength and modulus of sandwich cores, under load direction normal to the facing plane, have been determined. The samples cross-section area for this test is 25 25 mm. The Edgewise–Compressive test (in the following EC test), according to ASTM C364, was carried out to obtain the flat compressive properties of structural sandwich under load direction parallel to the facing plane. The sample dimensions are 20 50 120 mm. Moreover the Three Point Bending Test (in the following 3PF test), according to ASTM C393, was realized to determine the properties of flat sandwich panel subjected to flatwise flexure. In this case the sample dimensions are 20 40 130 mm, with span length of 80 mm. 2.3. Finite element analysis Numerical simulations were conducted using the ANSYS 5.6 finite element software. A 2-D model with 8-node quadrilateral element (Plane 82) has been realised, according to other authors [5]. The simulation is performed for FC, EC and 3PF tests employing a reference sandwich structure composed of fibreglass skins. A schematic description of the finite element modelling parameters is reported in Table 2. We focused our interest on a composite modelling which is based on the knowledge of the material data obtained via experimental test. These tests are performed using appropriate value of the mechanical and physical properties, obtained by the experimental characterization of the constituent. The knowledge of the material data is obtained by experimental tests; the woven skin properties are
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715 Table 2 Simulation set-up
FCT ECT 3PF
Nodes
Elements
Sample dimension (mm)
Steps
6332 21,153 20,144
2050 6930 6600
25 20 120 20 130 20
11 8 6
detected by a homogenization model [6], using the constituent properties as input produces the lamina properties as output. The post-elastic analysis is intentionally neglected, not only to obtain a numerical simulation simple and versatile, always required conditions in an effective design methodology, but rather because the aim of the work is to characterise the composite structure by a numerical procedure performed in the elastic regime. In fact in plastic regime the foam cell collapse heavily influences the mechanical stability of the sandwich structure. The numerical analyses, carried out to compare the theoretical model with the experimental results, are able to reproduce the experimental determination of the initial stiffness achieved in the experimental tests described before.
3. Results and discussion
1711
In the beads foam the compression yield stress is often lower than the tensile failure stress. Differences between the two types of foam can be explained considering that in compression the collapse is due to the buckling of the cells walls while in the tensile condition the critical cracks initiate locally in a lowdensity core region at a bead boundary. Consequently the tensile mechanical tests are particularly influenced by un-homogeneity into the cellular structure [7]. Such consideration can explain why in the 15 kg/m3 polystyrene foam, the compression yield stress is slightly higher than the tensile failure stress. The mechanical properties of PVC foam are higher than polystyrene foam about one size order, due principally to the high density value (40 kg/m3 ). The PVC foam acts as junction between the low-density foam and the high-performance fibres, offering a soft structural contribute on the mechanical stability of the composite sandwiches. 3.1.2. Flatwise compression tests Fig. 2 shows the general trend of composites in the FC tests for all test samples. The trends for all samples are very similar. The first part of the compressive stress–strain curve is linear elastic until it reaches a local maximum at very low deformation value, where the cell structure of the
3.1. Mechanical results 8 Glass 6
Stress [Mpa]
3.1.1. Constituents In Table 3 the static mechanical results for all constituents are reported. Moreover in the same table the mechanical properties two different densities polystyrene are compared. On polystyrene foam core an increase in modulus and failure stress is evidenced, associated to a small increase of density. About the obtained results it is possible to notice that on polystyrene foam, at a density increase of 20%, a considerable increasing in modulus and ultimate stress is achieved (respectively about 51% and 108% on flexural test). In the polystyrene foam with density 18 kg/m3 there is a significant difference between tensile and compression properties while a similar trend is not evidenced in the polystyrene with 15 kg/m3 density.
Carbon 4
Kevlar
2
0 0
20
40
60
80
Strain [%]
Fig. 2. Stress–strain curve in FC test for all sandwich structures.
Table 3 Static mechanical properties Tensile test (MPa)
Flexural test (MPa)
Compression test (MPa)
Shear test (MPa)
Epoxy resin
– rr ¼ 0:05 E ¼ 1:21 rr ¼ 0:12 E ¼ 5:09 rr ¼ 1:82 E ¼ 54:0
rr ¼ 99:8 E ¼ 3; 220 rs ¼ 0:06 E ¼ 0:8 rs ¼ 0:08 E ¼ 1:25 rs ¼ 0:93 E ¼ 13:1
–
Polystyrene foam 15 kg/m3
rr ¼ 85:7 E ¼ 4; 378 rr ¼ 0:12 E ¼ 3:78 rr ¼ 0:25 E ¼ 5:71 rr ¼ 2:25 E ¼ 58:2
Polystyrene foam 18 kg/m3 PVC foam
100
rs ¼ 0:57 E ¼ 1:005 rs ¼ 1:11 E ¼ 1:25 –
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715
polystyrene foam starts to collapse. At intermediate deformations (at about e ¼ 50%) a second modification of the trend of the curve is evidenced, associated at the beginning collapse of the PVC foam core. In the last part of the curve the stress increase exponentially, due to the interaction of the cellular structures of the compressed foam. The last part of the curve is called the regime of densification. On this experimental test, only the foam core used influences the flatwise-compressive properties of sandwich panel. In Fig. 3 comparative analysis between a FC test on glass-fibre sandwich and a theoretical trace obtained adding the strain contribute of each single component is reported. It can be noticed that the curve have similar shapes, confirming the collapse sequence polystyrenePVC; on the other hand their small difference at intermediate stresses can be attributed to a viscous liquid resin which permeate from the woven into the voids of the foams, inducing locally, at the interface, a density increase. 3.1.3. Edgewise compression tests Fig. 4 shows the load versus displacement for EC test. Excluding the first stabilization trace of the test, all specimens show a linear behaviour until the local buckling is reached. The deformation to break decreases with increasing elastic modulus of the fibres. The fracture load in Kevlar sandwich is lower than other fibre skins sandwich, due to the reduced compressive properties of aramid fibres. Generally, because the core shear modulus is lower than the skins stiffness, the initial failure mode of the samples is crimping (Fig. 5(a) and (b)). Sometimes, due to poor adhesion between skin and core associated to thin skins, a wrinkling rupture initially occurs (Fig. 5(c)). After that the failure has started, also shearing and delamination take place in the sandwich samples [8].
10000 Carbon Glass
8000 Kevlar Load [N]
1712
6000 4000 2000 0 0,0
0,5
1,0
1,5
Displacement [mm]
Fig. 4. Load vs Displacement in the EC test for all sandwich composites.
Fig. 5. Typical Edgewise Compressive fracture modes of sandwich laminates.
3.1.4. Three point bending tests The stress/displacement curves, obtained from a 3PB tests for all sandwich structures, are reported in Fig. 6. The mechanical behaviour is well matched with the results reported in the literature [9,10]. Observing the traces of the tests, we can identify three regions: A. Initially the load increases proportionally with deflection; the material shows a linear elastic behaviour, until about 20–30 MPa stress value. Beyond this point the load/deflection curve becomes non linear and it follows a plateau in which the cellular structure collapse
5000 Additive model
40
B
Experimental data
4000
A
3000
Stress [Mpa]
Load [N]
32
2000
C 24 16
1000
Carbon Kevlar Glass
8 0 0
5
10
15
20
Deflection [mm]
0 0
5
10
15
20
25
Displacement [mm]
Fig. 3. Comparative analysis of experimental FC test and additive compression model.
Fig. 6. Stress versus displacement from a 3PB test.
30
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715
Fig. 8. Specimen with edge failure in 3PB test.
the ends of the top skins, especially for the short beam samples. In Fig. 8 two pictures of a carbon sandwich are reported, they evidence the fracture on the edge of the sample, which occurs through the beads or along the beads boundaries [12], propagated at the polystyrenePVC foam interface. 3.2. Finite element analysis results 3.2.1. Flatwise compression tests The Figs. 9 and 10 show respectively the comparative results of ANSYS simulation of FC test with experimental data and the strain distributions in the load application direction (y direction), of the reference sandwich structure composed of fibreglass skins. The model does not present an appreciable compatibility with the experimental data at high deformations. This is due at a bad interaction at the interface between two different materials, especially at the foam cores interface. High deformation values are localised in the foam cores. Particularly the densification regime starts in the polystyrene foam, which have an average strain of about 80% against the PVC foam where we have an average strain of about 35%. Confirming the experimental results the skin deformations are negligible compared with the other ones. 6000 Experimental data Ansys model 4500 Load [N]
initiate by buckling of the cell walls and edges. In fact, from a visual examination of the running test, we can observe the crushing of the top part of the polystyrene foam, associated to the distortion of the cellular structure of the foam. This wrinkling effect is evident in sandwich structure with low-density foam core [11]. The wrinkle locally reduces the distance between the two faces, altering the behaviour of the panel. B. Further loading lead to a top skin failure by bending under the load application point and this phenomenon is followed by a large decrease in load. The failure of the top skin starts by local wrinkling on the compressed side of the panel. For carbon fibre sandwich the fracture stress is higher than the glass or Kevlar one. The addiction of higher modulus carbon fibre induce evident effects on flexural strength or modulus. The deformation to break of the Kevlar fibre sandwich (5 mm) is lower than other structures one (8–9 mm). On the other hand the wrinkling fracture of the top skin is not abrupt as other structures. This behaviour of the Kevlar sandwiches is explained considering that this fibre has a low compression stress to break but high absorbed deformation energy. C. The last zone of the load/deflection curve is particularly interesting studying the formation and evolution of failure modes. On the carbon sandwich a further loading leads to a gradual stress increase, because the carbon top layer is not fully break down, so a residual strength of the skin is showed. In fact sometimes a second cracking noise was heard accompanied by a second load drop on the load/deflection curve. On the other sandwich structures this phenomenon is not present. Subsequently at the complete fracture of the top skin, the crack begins to propagate at the interface skin-PVC foam-Polystyrene foam, and at the same time the crack propagates itself towards the centre of the sample (Fig. 7(a)). At higher deformation value the densification of the foam cores take place. Foams have almost completely collapsed and the wall cells begin to interact, inducing a further load increase, until we have the fracture of the lower skin associated to the delamination and debonding at the interface where material discontinuities are present (Fig. 7(b)). Sometimes a quite relevant load drop was observed, due to the tensile fracture of the core, owing to lifting of
1713
3000
1500
0 0
5
10
15
20
Displacement [mm]
Fig. 7. Specimens with flexural failure, 6.5.
Fig. 9. Numerical/experimental comparison for load/displacement plots in FC test.
1714
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715
Fig. 10. Strain distribution in y direction in FC test simulation (load ¼ 950 N).
3.2.2. Edgewise compression tests In Figs. 11 and 12 are reported the results respectively of the experimental and numerical comparison of loaddisplacement curve and the stress distribution in the load direction for the reference sample, when a load P ¼ 8000 N is applied. The numerical results agreed very well with the experimental ones. The Young modulus of the skins (Eskin ¼ 21,750 MPa) is greater than foam cores ones (EPVC ¼ 54 MPa; EEPS ¼ 5.09 MPa), consequently the axial load is carried out mainly by the skins. Consequently the elastic and geometrical parameters of the skins influence compres-
sive performances of the foam core sandwich structure. However a bad skin/core adhesion can induce a premature failure of the sample, due to crack formation at the interface between these two components [13]. 3.2.3. Three point bending tests Fig. 13 compares 3PF experimental load/displacement results with those from the numerical analysis, evidencing a close agreement in elastic regime between them. Fig. 14 shows strain distributions in the applied load direction for the 3PF test, just before the non linear behaviour in the load/deflection curve.
500
10000 Experimental data
400
Ansys model Load [N]
Load [N]
7500
5000
2500
300
200
100 Experimental data Ansys model
0
0
0,0
0,3
0,5
0,8
1,0
1,3
1,5
Displacement [mm]
Fig. 11. Experimental/numerical comparison for load/displacement plots in EC test.
0
2
4
6
8
10
12
Displacement [mm]
Fig. 13. Experimental/numerical comparison for load/displacement plots in 3PB test.
Fig. 12. Stress distribution in y direction in EC test – P ¼ 8000 N.
C. Borsellino et al. / Composites Science and Technology 64 (2004) 1709–1715
1715
Fig. 14. Strain distribution in y direction–3PBT simulation – P ¼ 365 N.
The higher strain values are reached in the polystyrene foam, while skins are less deformed [10]. Moreover in the y strain distribution figure the load effect on the supports is evidenced, but the strain values are lower than in the polystyrene centre core. This confirms that the plateau evidenced in the experimental curves is due to the wall collapse of the polystyrene foam cells.
4. Conclusions • On the basis of the experimental compressive, shear and flexural tests performed the mechanical characteristics of the single components of the sandwich structure are obtained, these last show the influence of the density in the polystyrene foam and the difference between polystyrene and PVC foam. • Carrying out the compression and flexural tests has allowed a better comprehension of the fracture mechanisms of the sandwich composite. • The presence of different fibre skins does not induce relevant effects on the stiffness of the sandwich structure but it heavily influences the fracture mechanism. In particular in FC tests: only the foam core used influences the flatwise-compressive properties of sandwich panel. • In flexural test the failure of the top skin starts by local wrinkling on the compressed side of the panel. For carbon fibre sandwich the fracture stress is higher than the glass or Kevlar one. The addiction of higher modulus carbon fibre does not induce evident effects on flexural strength or modulus. The deformation to break of the Kevlar fibre sandwich is lower than other structures one. On the other hand the wrinkling fracture of the top skin is not abrupt as other structures. • In edgewise compression test, because the core shear modulus is lower than the skins stiffness, the initial failure mode of the samples is crimping. Sometimes, due to poor adhesion between skin and core associated to thin skins, a wrinkling rupture initially occurs. After that the failure has started, also shearing and delamination take place in the sandwich samples. • The FEM analysis, performed by employing the commercial ANSYS code, was carried out to simulate the
static mechanical tests. The static-mechanical behaviour of the composite structure is well approximated by numerical simulations in elastic zone. As it was expected the proposed model does not show a good compatibility with the experimental data at high deformation (plastic regime). • The proposed model is suitable to perform a proper engineering design of sandwich composite structures. This last is helpful to choose the correct geometry as well as the correct layer arrangement at varying loading conditions that are not often experimentally available. References [1] Atckinson R. Innovative uses for sandwich constructions. Reinf Plast 1997;41(2):30–3. [2] Manning JA, Crosky AG, Bandyopadhay S. Flexural and impact properties of sandwich panels used in surfboard construction. In: Proceedings of the International Conference on Advanced Composites. Wollongong, Australia; 1993. p. 123–7. [3] Bannister MK, Braemar R, Crothers IP. The mechanical performance of 3D woven sandwich composites. Compos Struct 1999;47:687–90. [4] Coniff Darren. Closed mould processes why use them? Reinf Plast 1999;43(2):32–4. [5] Manet V. The use of Ansys to calculate the behaviour of sandwich structures. Compos Sci Technol 1998;58:1899–905. [6] Tanov R. A contribution to the finite element formulation for the analysis of composite sandwich shells. PhD, Aerospace Engineering, University of Cincinnati Engineering; 2000. [7] Moosa ASI, Mills NJ. Analysis of bend test on polystyrene bead foams. Polym Test 1998;17:357–78. [8] Kwon YW, Murphy MC, Castelli V. Buckling of unbalanced sandwich panels with titanium and GRP skins. J Press Vess Technol 1995;117:40–4. [9] Corigliano A, Rizzi E, Papa E. Experimental characterization and numerical simulations of a syntactic-foam/glass-fibre composite sandwich. Compos Sci Technol 2000;60:2169–80. [10] Mines RAW, Alias A. Numerical simulation of the progressive collapse of polymer composite sandwich beams under static loading. Compos: Part A 2002;33:11–26. [11] Weissman-Berman D. Marine sandwich structures – Part II. SAMPE J 1992;28(5):9–17. [12] Mills NJ, Kang P. The effect of water immersion on the fracture toughness of polystyrene foam used in soft shell cycle helmets. J Cell Plast 1994;30:196–222. [13] Mouritz AP, Thomson RS. Compression, flexure and shear properties of a sandwich composite containing defects. Compos Struct 1999;44:263–78.