Aluminium foldcores for sandwich structure application: Mechanical properties and FE-simulation

Thin-Walled Structures 90 (2015) 31–41

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Aluminium foldcores for sandwich structure application: Mechanical properties and FE-simulation Sebastian Fischer n IFB—Institut für Flugzeugbau, Universität Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany

art ic l e i nf o

a b s t r a c t

Article history: Received 17 December 2014 Received in revised form 5 January 2015 Accepted 5 January 2015

Foldcore is an origami-like structural sandwich core which is manufactured by folding a planar base material into a three dimensional structure. The manufacturing technology is open to a variety of base materials and also a range of unit cell geometries is feasible. This results in a wide spectrum of homogenized mechanical properties which can be achieved by foldcores. Aluminium was found to be a suitable base material for building high performance foldcores with high stiffness, strength and energy absorption capabilities. In this study aluminium foldcores are built and tested in compression, shear, bending and impact. Simulation methods are developed and validated along these test series. Simulation shows good agreement to experiments and is therefore a good method for further foldcore developments. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Foldcore, thin walled structures Finite element analysis (FEA) Mechanical testing

1. Introduction Sandwich construction is a lightweight design principle which is used in many industrial sectors like automotive industry or aerospace. Especially in structures which are subjected to bending loads or are sensitive to buckling, a lightweight design can be achieved by using sandwich construction. A sandwich is a three-layered composite material. It consists of the outer layers, the face sheets, which carry the in-plane loads and bending moments. The inner layer, the core, keeps the face sheets on their given distance and carries transverse shear loads. A variety of established materials for core and face sheets are available. As face sheets, any material with high strength and stiffness can be used. These are for example metals, fibre reinforced plastics or plywood. The core is a spacer between the face sheets and does not carry in-plane loads. So materials with low density are used. For example balsa wood, foam core or honeycomb are typical for application as a sandwich core [1–3]. Besides those established core materials, novel core materials are under development. Foldcore is such a novel sandwich core material. It is manufactured by folding a planar base material into a three dimensional structure [4–8]. This manufacturing technique is open to various base materials and also different unit cells are feasible [9]. Typical base materials are plastics, fibre reinforced materials or metal

n

Corresponding author. Tel.: þ 49 711 685 60378; fax: þ 49 711 685 62449. E-mail addresses: sfi[email protected], sebastian–fi[email protected]

http://dx.doi.org/10.1016/j.tws.2015.01.003 0263-8231/& 2015 Elsevier Ltd. All rights reserved.

foils. Also cheap materials like paper or cardboard can be used for applications with low requirements regarding mechanical properties. Due to its high elastic modulus, aluminium is a good candidate for construction of foldcores with high stiffness [10]. Moreover energy absorption capabilities are given through plastic material behaviour. Consequently aluminium is used in this study as base material for the foldcores and the face sheets. Static and dynamic tests are performed on base materials, foldcores and sandwich structures in order to characterize the foldcores mechanical performance. A comparison to other cellular metals like aluminium foam and aluminium honeycomb is given and modelling methods for FE-simulation of foldcores are developed.

2. General properties and comparison with other core materials The principle of the manufacturing method for foldcores is shown in Fig. 1. The folding edges are embossed onto the base material and then it is folded along these edges. The samples for this study are manufactured in a semi-automatic process. The embossing of the base material is done automatically using a 2D CNC machine; the folding step is performed manual. No further cutting or machining steps are necessary. That is why the manufacturing concept for foldcores is also called origami-like manufacturing. A variety of base materials is applicable for foldcore production. Typical base materials are thin metal foils, plastic foils or resin impregnated paper-like materials compounded from synthetic or natural fibres [11–13].

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S. Fischer / Thin-Walled Structures 90 (2015) 31–41

honeycomb properties. Aluminium foam properties are inferior to those of foldcore and honeycomb.

2.1. Unit cell design The unit cell of a zigzag-foldcore is defined by four independent geometric parameters which are shown in Fig. 2. The dimensions of these geometric parameters are typically above 5 mm, folding gets impossible if the unit cells get too small. Typical values for the height H for example are from 5 to 50 mm. Dependent on the base material, typical core densities are between 20 and 200 kg/m³. The zigzag unit cell type was proven in preliminary studies being a good candidate for structural applications and will be used in this study. Usually a foldcore sample consists of a repetitive arrangement of identical unit cells like shown in Fig. 1. This gives a sample with constant height and constant core density. Above that, it is also possible to build samples with varying unit cells like shown in Fig. 3. So it is possible to adjust the core to surrounding geometry or to increase the density in parts with load introductions [14]. 2.2. Multifunctional aspects Thermal insulation is a multifunctional aspect featured by all types of cellular sandwich cores. Trapped air inside the cells is a good insulator and the thin cell walls allow only little heat flux. The foldcores open cellular design allows another multifunctional aspect: ventability like shown in Fig. 4. The open channels allow fluid transport and could be used for heat exchange for example. 2.3. Benchmark Foldcores feature advanced design capabilities and multifunctional aspects as discussed above. But for a structural sandwich core, good mechanical properties are a key issue. Table 1 gives homogenized mechanical properties of commercial available aluminium sandwich cores and compares with analogue foldcore data. Two types of cellular metals are available, aluminium foam and aluminium honeycomb. For this comparison, data for ALPORASs aluminium foam [15] and an aluminium honeycomb from EURO-COMPOSITESs [16] are given. As the aluminium foam has a higher density than foldcore and honeycomb, specific values are given in Table 2. The foldcores shear properties are lower but in the same order of magnitude than

Fig. 1. Manufacturing-principle for foldcores.

3. Foldcores investigated The foldcores investigated in this study all feature the same base material and the same unit cell geometry. The unit cell is a zigzag type and will be referenced as geometry 182. Geometry data is given in Table 3. The parameters H, L, S and V are shown in Fig. 2. The homogenized core density ρ is defined as usual for sandwich core materials and the specific density ρspecific is a geometric density and is calculated by dividing the core density through the area density of the cell wall material. The base material for all foldcores in this study is aluminium EN AW-1050A. It is pure aluminium with very little alloying constituents. The foil thickness is 0.1 mm. Preliminary manufacturing studies with this aluminium foil were successful, making it a good candidate as cell wall material for metallic foldcores [10]. For face sheets, aluminium 2024 with a thickness of 0.8 mm is used. Face sheets and core are bonded with Reduxs 319 film adhesive. Fig. 5 shows a foldcore sample and a sandwich structure with aluminium foldcore and aluminium face sheets.

4. Test program The test program follows a bottom-up approach. First the base materials aluminium EN AW-1050A and aluminium 2024 are tested in tensile tests. This is necessary in order to get Young’s modulus, yield stress, strength and breaking strain for later simulations. The next steps are static tests with foldcores. Flatwise compression tests and transverse shear tests are performed. These are typical tests for characterizing sandwich core materials. After that tests with complete sandwich structures are performed. Tests chosen in this study are bending tests and impact tests. Test methods and results are discussed briefly in this chapter. Results are presented in more detail in the next chapter in comparison with simulation results. 4.1. Base material tests Tensile tests are performed with samples from aluminium EN AW-1050A and aluminium 2024. The samples from EN AW-1050A have little axial rigidity, so optical strain measurement is used in order not to disturb the measurement with a strain gauge. Material data is given in Table 4. When building the foldcore samples, aluminium EN AW-1050A passes through a heat treatment of 250 1C before and after the folding process to decrease residual stresses. The tensile test samples go through the same heat treatment. The heat treatment decreases the yield stress σY and the ultimate stress σU. Contrary the failure strain εF is increased. The face sheets pass temperatures of 175 1C when bonding them to cores. This temperature doesn’t affect the material, so the samples out of aluminium 2024 don’t pass any heat treatment before testing.

Fig. 2. Zigzag unit cell and geometric parameters.

S. Fischer / Thin-Walled Structures 90 (2015) 31–41

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Fig. 3. Sample with constant zigzag unit cell (left), sample with varying density (middle) and sample with varying height (right).

Fig. 5. Aluminium foldcore and foldcore-sandwich.

Fig. 4. Venting the foldcore through open channels.

Table 4 Base material data.

Table 1 Comparison with commercial core materials, absolute values. ρ [kg/ m3]

Core

250 ALPORASs E-C ECM 6.4– 60 60 Foldcore 182 54

Material

EZ [MPa]

σZ [MPa]

GL [MPa]

τL [MPa]

GW [MPa]

τW [MPa]

700 n.a.

1.50 3.55

300 370

1.20 1.72

300 166

1.20 0.95

34

0.63

160

0.64

202

0.66

E [MPa]

t ν [  ] σY [mm] [MPa]

Aluminium EN AW- 68,628 0.1 1050A Aluminium 2024 70,055 0.8

ρ [kg/m3]

σU [MPa]

εF [  ]

0.306 108.3

148.1

0.0969 2583

0.368 337.8

527.9

0.2553 2750

4.2. Static core tests Table 2 Comparison with commercial core materials, specific values. Core

σZspec GLspec τLspec GWspec EZspec ρ [kg/ [MPa/ [MPa/ [MPa/ [MPa/ [MPa/ 3 m ] (kg/m³)] (kg/m³)] (kg/m³)] (kg/m³)] (kg/m³)]

ALPORASs 250 2.80 E-C ECM 60 n.a. 6.4-60 Foldcore 54 0.63 182

τWspec [MPa/ (kg/m³)]

0.0060 0.0592

1.20 6.17

0.0048 0.0287

1.20 2.77

0.0048 0.0158

0.0117

2.96

0.0119

3.74

0.0122

Table 3 Dimensions of unit cell.

Flatwise compression tests and transverse shear tests are performed for basic characterization and for validation of simulation [17,18]. The test setup is shown in Fig. 6. For all load cases, a stabilized foldcore is used. This means that the core is bonded to face sheets forming a sandwich. In the compression test, the sample is loaded between two parallel platens. Reaction force is measured with the machines load cell; displacement is measured with an external displacement transducer. Homogenized stress and strain is calculated from force and displacement. The setup for the shear tests is similar and also shown in Fig. 6. The shear test has to be performed in L- and W-direction of the core due to orthotropy imposed by the unit cell geometry. Stiffness and strength are evaluated from stress–strain curves and listed in Table 1. 4.3. Bending test

Geometry

H [mm]

L [mm]

S [mm]

V [mm]

ρspecific [1/mm]

ρ [kg/m3]

182

12

6.28

8.84

8.84

0.193

54

Face sheets are bonded to the foldcore to build a sandwich structure. Here, aluminium 2024 is used as face sheet material. The bonding is performed with a Reduxs 319 film adhesive.

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S. Fischer / Thin-Walled Structures 90 (2015) 31–41

Fig. 6. Test setup for foldcores, compression, shear L and shear W (from left to right).

Fig. 7. Bending test.

Fig. 8. Setup for LVI-test (left), samples tested with 20 J and 30 J (right).

The test setup is shown in Fig. 7. Using a conventional sample like shown in Fig. 7 left leads to local failure at the load introduction. Therefore a modified sample is built which is reinforced by solid aluminium blocks at the side. As shown in Fig. 7 right and middle, a

bending deflection of the sample is achieved and failure occurs by local buckling of the upper face sheet. The dimensions of the sample are 300 mm length and 70 mm width. The height of the sample is 14.2 mm, where the face sheets

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Fig. 9. Setup for HVI-test (left) and impactor inside sabot (right).

Fig. 10. FE-model for flatwise compression test.

4.5. High velocity impact test

Table 5 Static core tests, comparison of test and simulation. Modulus/strength

Minimum

Experiment Maximum

Mean

FEM

EZ [MPa] σZ [MPa] GL [MPa] τL [MPa] GW [MPa] τW [MPa]

16 0.49 148 0.63 159 0.63

58 0.77 183 0.65 276 0.67

34 0.63 160 0.64 202 0.66

35 0.67 166 0.66 224 0.74

have a thickness of 0.8 mm each. The blocks have a length of 75 mm leaving 150 mm space for the foldcore.

4.4. Low velocity impact test Next step in the test programme are dynamic tests. Low velocity impact tests (LVI) have the advantage, that an instrumented impactor is used. Force–time data is captured during the impact. This data is valuable for calculation of absorbed energy and for validation of numerical models. The test setup is shown in Fig. 8. The size of the sample is 150 mm  100 mm, the impactor has a diameter of 16 mm. Impact energies between 2 J and 40 J are realized. The disadvantage of LVI is a limitation of impact energy due to the limitation of impactor speed. So no rupture of face sheets could be investigated during LVI.

Additional HVI tests were performed in order to determine the energy needed for penetration of the face sheets. The test setup is shown in Fig. 9. The sample is mounted into a massive steel frame. The size of the sample is 300 mm  300 mm. The impactor is a steel ball with diameter 30 mm and a mass of 110 g. It is accelerated by its sabot in a gas gun to the given velocity. The impact energy is varied between 159 J and 345 J. A penetration of both face sheets could be achieved with energy of 263 J and higher.

5. Simulation The commercial FE-program ABAQUS/Explicit is used in this study. Isotropic elastic-plastic material models are used for aluminium EN AW-1050A and for aluminium 2024. The simulations include geometric and physical nonlinearities like large deformations, buckling, contact and material failure. After defining the material models for the base materials, a modelling approach for a foldcore micro-model is developed. This micro-model is validated with data from static foldcore tests. The models for bending tests and impact tests use the same foldcore micro-model. 5.1. Base material tests The stress–strain data is evaluated and elastic modulus, strength, yield stress and breaking strain are determined. Material data is

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Fig. 11. Flatwise compression, stress–strain curve and model.

Fig. 12. Shear L-direction and W-direction, stress–strain curves.

Fig. 13. FE-model of the bending test.

Fig. 14. Force–displacement curve and FE-model.

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listed in Table 4. An isotropic elastic-plastic bilinear material behaviour is chosen and enhanced with the ductile failure criterion available in ABAQUS. 5.2. Static core tests A modelling method for foldcore micromodels was developed in preliminary studies [19–22]. The unit cell is meshed with S4R shell elements. The element size is below 1 mm so that cell wall buckling can be captured by the model. The micromodel also captures irregularities in the unit cell geometry which occur during manufacturing. First irregularity is a weakening of the folding edges due to the embossing and folding processes. Local plasticization occurs here leading to residual stresses and thinning of the cell wall material. This effect is captured by using a modified material model and modified section properties inside the folding edge. The second irregularity is geometric imperfections. The foldcores faces are affected by the folding process and do not stay perfectly plane. Curvatures occur which are captured by scanning a foldcore sample. This geometry is then used as basis for meshing instead of the ideal geometry with perfectly plane cell walls [22]. The model for the flatwise compression test is shown in Fig. 10, models for the transverse shear tests are set up accordingly. Load is introduced by the face sheets which are modelled with only one layer of C3D8R continuum elements. The face sheets don’t undergo any relevant deformation during the static tests so the choice of element type for the face sheets doesn’t influence the simulation result, C3D8R elements are used here because they are computationally efficient having only one integration point. Core and face sheets are connected by contact conditions. During the tests, no adhesive failure was observed; therefore the contact condition was modelled without adhesive failure. Foldcore properties in test and simulation are shown in Table 5. The moduli are evaluated in the region before peak stress, where the stress–strain curve is fairly linear. Strength is taken as the peak stress of the stress–strain curve. The stress–strain curve for flatwise compression is shown in Fig. 11. Peak stress is captured well by the model and also plateau stress and compaction at high strains is predicted with good accordance to tests. The first row of cells is also shown in Fig. 11. The upper picture is just after reaching peak stress, intensive buckling occurs at this stage. The lower picture shows the model at full compaction. Table 6 Stiffness and strength in bending test. Stiffness/strength

Minimum

Experiment Maximum

Mean

FEM

K [kN/mm] Fmax [kN]

4.59 13.5

4.71 15.5

4.65 14.7

4.59 14.2

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The stress–strain curves in shear are shown in Fig. 12. Again stiffness, peak stress and compaction are well predicted by the model. The model is able to capture the effects which occur during loading of the core, like cell wall buckling, material failure and compaction.

5.3. Bending test The model for simulation of the bending test uses the same foldcore micro-model as the models in chapter 5.2. For bending, it is important to use an adequate element type for the face sheets in order to get correct bending stiffness. Only one layer of continuum elements, like used in the compression test, would be too stiff and lead to false simulation results. So either a number of layers of continuum elements could be used to model the face sheets or a single layer of shell elements. Here SC8R continuum shells are used because they are able to model the bending behaviour of the face sheets in a computationally efficient way. They are connected to the core using a tied contact. The aluminium blocks and fins are meshed with C3D8R continuum elements. Blocks and face sheets are meshed coincident and connected by merging opposing nodes. The fins are connected to the face sheets by contact conditions. The model in detail is shown in Fig. 13. Force–displacement curves and the model at failure are shown in Fig. 14. The model fails by local buckling of the upper face sheet. The same failure mode is observed in the experiments, see Fig. 7. This failure mode depends on face sheet and on core properties. So it is important to use a realistic model of the core in order to get reliable results for the whole sandwich structure. Also the force–displacement curves of test and simulation show good accordance. Stiffness and strength of the model is within the range of test data as shown in Table 6.

5.4. Low velocity impact test Basically the same modelling is used for the sample as in the bending simulation. The impactor and the bearing are modelled with C3D8R continuum elements. Contact conditions are used between impactor, sample and bearing. The model is shown in Fig. 15. A geometric and physical nonlinear explicit dynamic analysis is carried out. The analysis is performed in two steps. First the impactor is accelerated to its impact velocity. In the second step, the impactor is freed from its boundary condition in direction of motion and impacts the sample. Force–time data from test and simulation is compared. Maximum contact force is used as comparative value. A test series with constant impactor mass of 2.49 kg and impact energy from 2 J up to 40 J is carried out first. Force–time data is shown in Fig. 16. After that, another series is carried out with

Fig. 15. FE-model of low-velocity-impact.

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S. Fischer / Thin-Walled Structures 90 (2015) 31–41

Fig. 16. Force–time data from LVI with 2.49 kg impactor mass.

Fig. 17. Force–time data from LVI with 40 J impact energy.

constant impact energy of 40 J, but varying mass from 2.49 kg up to 7.52 kg. Force–time data is shown in Figs. 16 and 17. A summary of results is shown in Table 7. Impact energy, mass m, impact velocity v, maximum contact forces Fmax and contact

time tcontact are given. Generally speaking, simulation is able to predict the failure modes which occur during the impact with different energy. This is core failure and plasticization of the upper face sheets which occurs for small impact energies like 2 J or 5 J.

S. Fischer / Thin-Walled Structures 90 (2015) 31–41

For higher energies, compaction of the core, bending of the sample and massive plasticization of both face sheets occur. As expected, contact force and contact time increase with increasing impact energy. Test and simulation follow this trend as shown in Fig. 16. The second test series with constant impact energy show that the relation between mass and velocity of the impactor also makes a difference. Contact force and contact time increase with increasing mass. Simulation is also able to reproduce

this trend while not reaching perfect quantitative accordance in means of force–time curves, see Fig. 17. Pictures of some of the models and samples are shown in Fig. 18. In case of the 2 J impact, only a small dent is visible in test

Table 7 LVI, comparison of test and simulation. Impact energy [J]

m [kg]

v [m/ Test Fmax s] [N]

Test tcontact [ms]

FEM Fmax [N]

FEM tcontact [ms]

2 5 10 20 30 40 40 40

2.49 2.49 2.49 2.49 2.49 2.49 4.99 7.52

1.26 2.00 2.83 4.00 4.91 5.65 3.99 3.25

6.8 6.3 7.1 8.2 8.3 7.3 10.3 12.8

1229 2021 2517 2532 3710 4377 3989 4381

6.3 6.2 7.7 9.9 8.9 9.9 13.2 14.9

1077 1729 2265 2588 2889 4017 4369 4439

39

Fig. 20. FE-model of high-velocity-impact.

Fig. 18. Models and samples of LVI with 2 J, 20 J and 40 J.

Fig. 19. Model of 2 J LVI.

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S. Fischer / Thin-Walled Structures 90 (2015) 31–41

and simulation. For 20 J and 40 J, there is bending of the upper face sheet and compaction of the foldcore. Fig. 19 shows again the model in case of the 2 J impact. It can be seen that the core already failed locally while the upper face sheet shows only small plasticization.

5.5. High velocity impact test Also for the last test in this study, high-velocity-impact, a finite element model is set up. The modelling details are shown Fig. 20. For a better overview, only a quarter of the model is displayed. At

the edges, the foldcore is replaced by rigid foam to make clamping inside a frame possible without damaging the foldcore. The rigid foam and the frame are modelled with continuum elements. Top and bottom of the frame are connected with bolts, modelled with beam elements. The impactor is modelled with continuum elements. The same material models are used in HVI as in LVI and static tests, strain rate effects are neglected. In HVI, the impactor is not instrumented. So a comparison of force–time data is not possible. But it is possible to reach energy levels which are high enough for full penetration of the sample. Four tests were carried out, it was tried to identify the energy level which is needed for full penetration of the sample.

Fig. 21. FE-model and sample, 159 J and 227 J.

Fig. 22. FE-model and sample, 263 J and 345 J.

S. Fischer / Thin-Walled Structures 90 (2015) 31–41

Fig. 21 shows the samples which were impacted with 159 J (53.8 m/s) and 227 J (64.2 m/s). The impact with 159 J leads to a full compaction of the core. But no fracture of the face sheet could be observed. Increasing the impact energy up to 227 J, leads to fracture in the upper face sheet. Simulation is able to predict this behaviour very well. Impact energy was further increased. Fig. 22 shows the samples which were impacted with 263 J (69.2 m/s) and 345 J (79.2 m/s). In both cases there is penetration of both face sheets in test and simulation and plastic deformations around the point of impact.

6. Conclusions Aluminium foldcores were successfully produced and tested in flatwise compression and shear. A benchmark with aluminium foam and aluminium honeycomb is carried out showing comparable mechanical properties. The foldcores were combined with face sheets from aluminium 2024 to build up sandwich structures. These were tested in bending and in impact. Modelling methods were developed and used to recalculate the performed tests. The accordance between test and simulation is good, making it possible to simulate other foldcore sandwich configurations and different load cases. This helps reducing testing effort and is valuable in product development.

Acknowledgement Parts of this work were part of the EU project CELPACT within the Sixth Framework Programme of the European Commission (Contract AST5-CT-2006-031038, 2006-2009). The author gratefully acknowledges the funding of the research activities. Thanks also go to DLR Stuttgart and there especially to Albert Reiter, for making it possible to use DLR’s gas gun for the HVI-tests presented in this paper. References [1] Zenkert D. Handbook of sandwich construction. Engineering Materials Advisory Services Ltd; 978-0947817961. [2] Vinson J. Behavior of sandwich structures of isotropic and composite materials. Crc Pr Inc; 978-1566766999.

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