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Honeycomb compositelightweight structuresmade of aluminium oraramid fibres are used inairplanes, railway

carriages and automobiles. Thesestructures are subjected to dynamicloading but hardly any fatigueproperties of the honeycomb coreexist in current literature (Asummary of the state of the art: [1]).

The lightweight panels which wereinvestigated are made of ahoneycomb core of aluminium,which is connected by an adhesivelayer with two outer sheets ofaluminium (Figure 1).

During this project, fatigue testswith failures of the core structurewere conducted in parallel withFinite Element calculations. Ananalytical model was created, whichexplains the experimental results.

Since the behaviour of the panels isorthotropic, the panels reactdifferently depending on the

direction of the loading. For thisreason, it is necessary to distinguishbetween the three directions ofsymmetry, which are called L, Wand T direction (Figure 2).

The walls of the honeycomb cellshave different wall thicknesses. This

is due to the manufacturingprocess, where the foils are partlyglued together. The glued wallswith double thickness are calledribbons (Figure 2). The dimensionsof the examined panels are shownin Table 1.

Figure 1: Sandwich structure with honeycomb core [2]

Table 1: Material and dimensions of examined Panels

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Failure Modes of Honeycomb coreSandwich PanelsIn a 3-point bending test, sandwichstructures are mainly subjected tothree types of stress:• Tension / Compression in the

cover sheets due to bending• Shear stress in the core• Compressive stress in the core in

proximity of the load application

Each stress type must be examined inorder to figure out which is thecritical one.

The bending stress leads to cracks inthe face sheets, which was examinedin a former project [3].

The core of the sandwich panelsusually fails due to shear orcompressive stress (Figure 3). The typeof stress which prevails, depending ofthe geometry and the loadapplication, is responsible for the corefailure.

The distribution of the stresses inFigure 3 was simulated in ANSYS bymoving the load horizontally. Theshear stress is maximal somewherebetween the two points of the forceapplication. The compression stress inthe core has a maximum just belowthe middle load. Core indentation isoccurring, when the compressionstress surpasses the buckling strengthof the honeycomb core. In this case,

the structure fails locally due tobuckling of the core (Figure 3).

MaterialsThe sandwich structure consists ofthree different materials:• Glue• Aluminium alloy AlMg3 H44 (AW

5754) for the face sheets• Aluminium alloy AlMn1Cu H19

(AW3003) for the honeycombstructure

Test MethodologyDynamic 3-point bending tests wereperformed in order to provoke corefailure. The test setup is powered bya hydraulic cylinder from InstronStructural Testing Systems (IST). Thehydraulic cylinder can be exciteddisplacement or force controlled.

In order to provoke the two failuretypes of Figure 3, the load wasapplied in two different ways:• Steel roll with a small diameter

(25mm), which implies a highcontact pressure and thecomponent fails by coreindentation (Figure 5a)

• Elastomeric roll (Vulkollan 80Shore A) with a big diameter(76mm), which implies a low

Figure 2: L, W and T directions [2] (Ribbon has double thickness, due to the manufacturing process)

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contact pressure so that the corefails due to the shear forces(Figure 5b)

Fatigue Test ResultsDynamic tests were carried out tostudy the fatigue properties of thestructure. The samples were loaded ina three-point bending test with asinusoidal load with constantamplitude at a power ratio of R=10.The excitation was force controlled.

The soft load application (Figure 5b)leads to shear failure in thehoneycomb core. Cracks are initiatedin the interior of the honeycombcore, which grow predominantly inthe diagonal direction of the cells(Figure 6). These cracks are notexactly under the load, but some cellsaway from it. Here the shear stress ismaximal, as shown in Figure 3.

If a hard load application is chosen(Figure 5a), the specimens fail due tothe pressure load induced by the load(core indentation). In the damagepattern of Figure 7, it can be seen,that the cracks are exactly under theload application. The W-specimenshows horizontal and diagonal cracksin the cell walls. The L specimenshows only horizontal cracks.

The tests showed that first cracksoccurred after less than 10% of thetotal life period of the specimen. Thebuckling process creates locally highstresses and cracks, which are notimperatively leading to the total directfailure of the structure.

In Figure 8, the fatigue diagrams of Land W-samples with identicaldimensions are shown. In theordinate of the fatigue diagram theforce amplitude is displayed and notthe stress amplitude at the location ofthe crack initiation. These two valuesare related, but the relationship is notnecessarily linear. The number ofcycles on the abscissa corresponds tothe number of cycles to completefailure of the part and not until thefirst crack. These boundary conditionsimply that the diagrams are notconventional SN-diagrams. Theexperimental results, however, lie wellalong a straight line.

The curves of the specimens whichfail due to buckling (coreindentation), are flat, compared tothe shear failure curves. This flatcurve is due to the high nonlinearstress increase during buckling.

SimulationsA model of the sandwich structurewas created using ANSYS. Thestructure is modelled with shell281elements, which have 8 nodes with 6degrees of freedom each. Shell281elements are also suitable for largedeformations and plastic behaviour.The roll for the load application ismodelled with solid95 elements,volume elements with 20 nodes with3 degrees of freedom each. Thecontact condition between the rolland the sample is modelled with theelements conta174 and targe170.These elements have 8 nodes and areplaced on the surface of the shellelements. Contact occurs when thesurface of a conta174 elementpenetrates one of the targe170elements.

To make the simulation as realistic aspossible, several imperfections areintroduced (Figure 9):

• Roll not centred (load inserted onribbon or next to ribbon)(Δ<1mm)

• Roll rotated around the x-axis, sothat the device won’t be loadedevenly (α<0.2°)

• Roll rotated around the z-axis(β<0.5°)

Figure 3: Stress distribution and failure modes of the honeycomb core

Table 2: Mechanical properties of the materials used in the Sandwich panels

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• Cells are not regular hexagons (allthe coordinates are moved by asmall random value) (δ<0.3mm)

• Cells not planar (Small forces(Fi<0.5N) are inserted into thesimulation, which dent the walls)

The simulations showed that arotation of the roll around the x-axis(α) has a big influence. A horizontaldisplacement of the roll (Δ) can movethe force application from a ribbon toa free wall, which also has aninfluence on the results. All otherimperfections are quite insignificant.

The failure of the glue is not the mainsubject of this article, but it shouldstill be modelled in order to examinethe influence of the glue to thebuckling load. The adhesive layercovers the honeycomb core and

stabilizes it. This overlap is simulatedby expanding the shell model of thehoneycomb core with two layers ofconstant thickness, which have theproperties of the adhesive (Figure 10).The simulations showed that theinfluence of the glue to the bucklingload is less than 10%.

Core indentation (Buckling of thecore)The experiments have shown that thesamples, loaded with the hard roll,failed in the mode of coreindentation. Physically, coreindentation of honeycomb panelsmeans that the cell walls are buckling(but usually they can still carry loads).The buckling process induces bendingstresses in the cell walls, includinghigh tensile stresses. These tensilestresses influence the fatigue

behaviour of the core very negatively,so that the crack initiation phase getsmuch shorter (Figure 8). In most caseshowever, these local cracks barelyinfluence the strength of thestructure.

Some simulations showed however,that it would be too time-consumingto simulate the growth of the cracks(which is very sensitive toimperfections) within this project, inorder to see which cracks lead tofailure and which cracks not.Therefore, it was assumed that acyclic buckling of the honeycombcells is not tolerable if a part isdimensioned against fatigue. In thiscase, the stresses are distributed moreuniformly and the crack growingprocess is not so important. Thisassumption does not lead to a bigoversizing of the part, because thefatigue curve of the core indentationin Figure 8 is very flat. Therefore, itdoes not make a big difference if thepart is dimensioned for 100,000cycles or one million cycles (it isassumed, that for 1M million cycles,no buckling is occurring). In the field,core indentation is usually avoided byreinforcing the panel at the positionof the load application.

For this reason, honeycomb sandwichstructures should be dimensioned sothat no buckling occurs, because onlyin this case can good results beachieved. The buckling load can becalculated by a Finite ElementMethod in two different ways. First,by a buckling analysis, that calculatesthe theoretical buckling load for aperfect elastic system (Euler analysis).Alternatively, if nonlinearities have tobe considered, the buckling load canbe evaluated out of a nonlinearsimulation. The contact of the hardload application (Figure 5a) is not very

Figure 4: Three-point bending setup used for fatigue testing

Figure 5a: Hard load application made of steel Figure 5b: Soft load application made of Vulkollan

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load dependant, so that the linearbuckling analysis is possible. However,the soft load application (Figure 5b)causes a nonlinear contact condition,so that in this case a nonlinearsimulation is necessary.

Shear failureAfter proving that no buckling isoccurring, a normal static analysis canbe accomplished. In this case, thestress state in the core is quitehomogeneous and it will be possibleto do a fatigue prediction with thisanalysis. Just under the loadapplication, the compression stressdominates, and next to the loadapplication, the shear stressdominates like shown in Figure 3.When no buckling of the core isoccurring, the most damaging stresscomponent in the core is the shearstress. Therefore, the critical locationcan be determined from a shearstress contour plot. At this location, afatigue prediction can beaccomplished using the FKM-guideline [4].

In a 3-point bending test, away fromthe load application, the shear stresscan be checked analytically:

In this formula, it is assumed that theshear stress is distributed uniformlyover the honeycomb cells. In theelement coordinate system, the angleof the cell walls does not appear inthe formula of the shear stress.However, the number of cell wallsacross the width is important, whichimplies that the L-samples are muchmore stable against shear than W-samples (n much bigger for L-samplesthan for W-samples).

These approximate formulas are justused to understand the influence ofthe parameters and to check thesimulations. In order to have theexact stresses with all local effects,the Finite Element simulations are stillneeded.

Fatigue analysis of examinedspecimensThe procedure of the fatigue analysisfor the core structure of analuminium honeycomb sandwichshould be as follows:• Determine the buckling load of

the core. Applied load must notexceed this value

• Determine the stresses in a staticFinite Element Analysis

• Locate the critical points (e.g. in acontour plot of shear stressesbecause these stresses arepredominating)

• Calculate the lifetime of thehoneycomb core, using the FKM-guideline [4]

• Confirm the results when possibleby tests

Figure 6: Fatigue Shear failure

Figure 7: Fatigue buckling failure for W and L specimens

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Buckling loadsThe buckling loads of differentspecimens and different loadapplications are shown in Table 3.

The buckling loads of the soft loadapplication cannot be compared withthe test results, because in these

cases, the failure mode is not coreindentation, and therefore nobuckling is occurring. These bucklingloads are higher than the failure loadsin Figure 8, so the failure is not dueto buckling effects, as it is also shownin the experiments.

The buckling load of the hard loadapplication is exactly in the area ofthe fatigue limit found in theexperiments. It was assumed that atthe fatigue limit no more buckling isoccurring, and so in the experimentsthe load at the fatigue limit is exactlythe buckling load. In these cases, the

Figure 9: Imperfections included in the simulations (illustrated exaggeratedly)

Figure 10: Glue on Honeycomb cells modelled by different layers

Figure 8: Fatigue strength diagrams for L und W samples

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References[1] Sharma, N.; Gibson, R.F.; Ayorinde, E.O.: "Fatigue of Foam

and Honeycomb Core Composite Sandwich Structures: ATutorial", Journal of Sandwich Structures and Materials 20068: pp 263-319

[2] Blitzer, T.: "Honeycomb Technology, Materials, design,manufacturing, applications and testing", first edition,Chapman & Hall, 1997

[3] Bauer, J.: "Ermittlung von Schwingfestigkeitseigenschaftenfür Leichtbaupaneele mit Wabenstruktur", University ofLuxembourg, Faculty of Science, Technology andCommunication, PhD-FSTC-5-2008, 2008

[4] Hänel, B.; Haibach, E.; Seeger, T.: "RechnerischerFestigkeitsnachweis für Maschinenbauteile aus Stahl,Eisenguss- und Aluminiumwerkstoffen – FKM-Richtlinie", 5thedition, VDMA Verlag GmbH, 2003

[5] Schijve, J.: "Fatigue of Structures and Materials", first edition,Kluver Academic Publishers, 2001

Table 3: Comparison: buckling load / fatigue limit

Table 4: Life comparison: FKM / experiment

endurance limit can be predictedwith an accuracy of approximately10%.

In Table 3, nonlinear analysis means,that the contact surface is changingwith load and that the deflectionscan grow at the buckling loadnonlinearly.

Shear failureWhen no buckling is occurring, afatigue analysis is performed usingthe FKM-guideline [4].

The hard load application leads tocore indentation: here, only the softload application is examined. InTable 4, the results of an L and a W-specimen are compared with thetest results.

The lifetime predictions for the twocases examined in Table 4 areconservative. The durability found inthe tests was higher than theprediction by a factor of two. Thiscan be considered to be a goodprediction [5].

ConclusionsTwo different failure modes of thehoneycomb core structure wereexamined: core indentation andshear failure. Core indentationinduces buckling of severalhoneycomb cells. This results in hightensile forces, which will quicklyinitiate cracks. In practice,components should be designed sothat no buckling occurs. Thebuckling load can be calculatedeasily with a Finite Elementsimulation.

The shear failure mode can beanalysed by doing a static FiniteElement Analysis. Afterwards alifetime analysis can be done usingthe FKM-guideline. There were onlysmall differences between thefatigue predictions and theexperiments.

The differences between thepredicted and the tested lifetimesare only 10% based on stress andonly a factor of two in real-life.

AcknowledgementThe materials were sponsored by Eurocomposites, Echternach, Luxembourg. Many thanks to them.

AUTHOR INFORMATIONLaurent Wahl I University of LuxembourgLaurent.Wahl@uni.lu

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