Characterization and optimization of the production process of

sandwich composite structures

Goncalo Maria de Franqueira Ferreira Gomesgoncalo.f.gomes@ist.utl.pt

Instituto Superior Tecnico, Universidade de Lisboa, Portugal

November 2016

Abstract

In the last few years, the use of composite materials in engineering structures has grownexponentially. Combining properties from different materials as the potential to create a finalstructure with superior characteristics to those presented by the individual parties. The need to usematerials that enable maximum performance of the structure in which they are inserted, inducedthe development of sandwich-type structures technologies, that are able to combine rigidity andmechanical strength of the materials, while maintaining the low weight of the composite. This isspecially important in industries where the weight of the structures is a major parameter, such asaerospace. Production represents a key process in the development of composite materials. Thisproject aimed to optimize the production process of a flat sandwich composite structure of low specificweight using natural materials, in which a cork agglomerate is used as the core material and a highdensity-rigidity material is used for the facing. The present work showed the importance of thecuring parameters used, in the bending performance of the sandwich structure. In this parameters areincluded the pressure applied during the curing cycle and the time and temperature at which this isoccurring, variables that influence majorly the degree of cross-linking present in the resins used as anadhesive and the infiltration, and consequently the amount of bonding present between the components.

Keywords: Composite materials, Sandwich structures, Cork core, Productive process

1. Introduction

The use of sandwich composite materials has in-creased significantly in industries like the auto-motive, nautical or aerospace, due to their highstrength-to-weight ratio. The increase in demandintroduced the need for production in a bigger scaleand for the optimization of production processes.

The manufacturing of composites incorporatesmany steps and variables through its cure processthat need to be controlled and optimized. Process-induced issues can degrade the structural perfor-mance of composites, and for that they need to besuppressed. The study of optimization of the pro-duction allows the enhancement of material prop-erties and also helps decreasing time and costs as-sociated.

The composite sandwich under study was createdby the company FRONTWAVE S.A., that devel-oped of a non-symmetrical sandwich composite ma-terial using a cork agglomerate as core and a stonepanel for the facing. However, the productive pro-cess lacks optimization since there is no standardprocedure for the production of sandwich compos-ites, being each case dependent on the properties of

the materials and configurations used.

Figure 1: Optimization work outline.

My contribution to this company, which resultedin this work, was the characterization and optimiza-tion of the productive process of this composite ma-terial. In each step of productive process there isa set of different variables that will affect the fi-nal characteristics of the structure, in which are in-serted the temperature and humidity present in thelamination, the pressure applied to the material in

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the cure and the time and temperature used. Inorder to improve the performance of the structureunder bending forces as well as the time and costinvolved in the manufacturing, various productionmethods, varying these different variables, were de-veloped and characterized, and taking into accountdesign constrains like the failure mechanism of thestructure, a final productive process was achieved.

2. BackgroundSandwich composites are becoming more and morepopular in structural design, mainly for their abil-ity to substantially decrease weight while maintain-ing mechanical performance. The need to mini-mize the components weight, without compromis-ing the essential mechanical properties in fields likethe aerospace, lead to the development of this typeof composites.

Due to their high resistance/weight relation,they are the main composite structure used in theaerospace [6] and nautical industries [8], where theweight of the structure has a huge influence in en-ergy consumption

2.1. Sandwich structure conceptA sandwich panel is composed by two or more ma-terials with different properties, in an arrangementof superimposed layers, linked with the use of anadhesive element. The simplest sandwich (fig. 2)consists of two thin stiff, strong sheets of dense ma-terial separated by a relatively thick layer of low-density material (named core), which may be muchless stiff and strong [1]. A large increase in thesecond moment of area is then imposed without aweight penalty, factor that has a large importancewhile designing an engineering structure.

Figure 2: Composite sandwich structure constituents.

2.2. Sandwich composites using cork agglomeratesas a core

Cork has specific physical, chemical and mechani-cal properties that make it a potential material forinnumerous applications. For example, due to itsthermal isolation properties, low combustion rateand shock absorption capacity, it has been chosenfor some aerospace applications - it is used for in-sulation in engines produced by NASA. When com-paring the specific compressive strength against thespecific modulus, cork has a better mechanical be-

havior than flexible polymer foams and comparableto some rigid polymer foams. Also, its low thermalconductivity combined with a reasonable compres-sive strength make it an excellent material for ther-mal insulation purposes as well as for applicationsin which compressive loads are present [7].

When comparing cork with other core materialusually used, like Nomex (honeycomb) or Rohacell(foam), cork-epoxy agglomerates present a signifi-cantly better core shear stress limit, which reducesthe crack propagation region [4]. In terms of impactabsorption, cork presents higher values for load, anddemonstrates an extremely good recovery capacityand a high-energy absorption with minimum dam-age, due to its properties. For this, cork is becom-ing a good alternative to core materials in sandwichcomposites.

3. Process and process variablesMany factors influence the final properties of asandwich composite, in which are included the in-dividual properties of each component (faces, core,resins and fibers), the arrangement and ratio of thematerials and the way they connect between eachother into the part and also the way they are pro-duced and cure occurs. Various processing param-eters influence the quality and performance of thecomposite part such as the temperature and heatingrate, pressure applied, humidity and cooling rate.

The standard process to be optimized is now sum-marized,

• Material preparation

– Drying of the skin: stone arrives with acertain amount of humidity;

– Preparation of fibers: glass-fiber clothsare cut into the required dimensions;

– Epoxy preaparation: the two componentsare weighted. Then, the A component ismixed with the B component of the epoxysystem in order to start the chemical reac-tion of curing;

• Lamination process: the first layer of glass-fiber is laid-up over the skin panel and lami-nated with the epoxy resin. A plate of corkagglomerate is placed on top and the secondlayer of layer of glass-fiber+resin is applied;

• Gel time: the gel-time of the resins sets thetime waited until pressure application;

• Pressure application: pressure is applied tothe structure;

• Post cure: after pressure application, the postcure process starts:

– Heating: the material is inserted into theoven starting the post-cure process;

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– Cooling: The oven is turned-off and the ma-terial cools down to the ambient temperature.

Figure 3: Standard process.

The objective of this study was to optimize theproduction process previously described, in orderto both improve the mechanical characteristics ofthe composite in terms of bending stiffness and op-timize time and production costs. Each step de-scribed contains many variables that are importantin the cure process and influence the final cure stateand the final characteristics of the product.

The following variables can influence the processand are to be studied so that the best compromisebetween mechanical characteristics, production po-tential and productivity can be achieved and an op-timum profit can be withdrawn from the product.

3.1. TemperatureTemperature is very important in the cure processof epoxy resins. Cross-linking, glass transition tem-perature, viscosity and penetration depth dependstrongly on the curing temperature. The degree ofcure will be uniform as long as the temperature dis-tribution inside the composite is uniform.

The epoxy resins used should be processed withworkshop temperature conditions between 18-25◦Cin order to facilitate the mixing and the reinforce-ment impregnation.

3.2. HumidityDue to the hydrophobic characteristics of epoxyresins, the humidity present in the atmosphere andthe materials can represent a problem in the curingof the system. If humidity is present in the pores ofthe materials, infiltration of the resins can be com-promised, and so the adherence between differentmaterials. Also, since epoxy cure is an exother-mic process, water can evaporate contributing tothe onset of bubbles in the interface between differ-ent materials.

However, epoxy resins are not completely hy-drophobic. Studies made in the field revealed thatthe amount of water absorbed by a cured epoxy ad-hesive can vary from about 1% to 10% [5]. Thisabsorption affects negatively the cure as it softensepoxy resins, causes them to swell, and lowers theirglass transition temperature.

3.3. HardenersThe nature of the curing agent is known to be amajor factor in the rate of curing and, the time

that takes for the resin to cure plays a role in manyfactors of the curing process [9], like the density ofcross-linking, the amount of heat produced in theexothermic reaction and the depth of penetrationachieved.

3.4. PressureThe applied pressure provides the force needed tosqueeze excess resin out of the material and to uni-form adhesive distribution, to consolidate plies, tocompress vapor bubbles and to help the penetra-tion of the resin contributing to the adherence tothe materials. Pressure in cure should be appliedbefore the viscosity of the resin becomes sufficientlylow for resin flow to occur. This is the latest time atwhich the pressure should be applied, or the resinwont be able to flow and excess will build up insome regions.

The production process held in a factory can havethe need for a buffer between the lamination processand the application of pressure. In order to see if thewaiting period (gel-time) for pressure applicationhas a substantial influence in the properties of thefinal material, this should be studied.

Also, when it comes to pressure application in theprocess, three different variables can influence thecure. Which are, the amount of pressure applied,the time during which pressure is being applied andfinally the temperature under which this process oc-curs. This variables need to be studied together.

4. Specimens and equipmentTaking into account the forces that the compos-ite structure under study will withstand during itsworking life, the method that suits best our studyis the Standard Test Method ASTM C393 [2]. Thismethod covers the determination of the propertiesof flat sandwich constructions subjected to flatwiseflexure. The specimen is subjected to a three orfour-point-bending flexure test, in order to obtainthe various mechanical characteristics of the sand-wich composite material, like the bending stiffness,property that is to be optimized during this project.

Figure 4: Two-point load.

Due to the fact that this standard was developedfor symmetric composite sandwiches, it doesn’tfully apply to the one under study (it is not sym-metric) so, parameters like specimen dimensions,loading and testing conditions were adopted from

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this standard but a suitable method needs to bedeveloped - section 5.

4.1. SpecimenBased on the requirements from ASTM C393, thedimensions chosen are:

• Facing thickness, t: 5 mm;

• Core thickness, c: 10 mm;

• Sandwich thickness, b: 16 ± 0,5 mm;

• Sandwich width, w: 50 mm;

• Sandwich length, L: 300 mm.

• Span length, S: 250 mm;

Figure 5: Isometric and side view of the specimen [mm].

4.2. Specimen manufacturingThe base process, common to all specimens pro-duced can be defined:

1. The materials are prepared - The stone panel isdried up, the fibers are cut into the required di-mensions and the epoxy mixture is prepared;

2. A 305 x 305 mm stone plate is placed in the lami-nation area;

3. The first layer of fiber is placed on the stone plateand laminated using the epoxy mixture;

4. The cork agglomerate plate is placed on top of thefirst fiber-epoxy layer, aligned with the stone panel;

5. The second layer of fiber plus epoxy is laminatedon the top surface of the cork plate;

6. The epoxy starts its curing process from the mo-ment component A is mixed with B. The timeneeded to apply pressure is the gel-time and de-pends on the hardener used. When gel-time isachieved, pressure is applied;

7. After the pressure application stage, the post-cureprocess starts with the transfer of the material tothe oven. An heating ramp is applied until the re-quired temperature is achieved and stabilizes afterthe required amount of time of post-cure;

8. When the post-cure process in the oven finishes,temperature is turned off and the cooling processstarts, finishing when the ambient temperature isachieved;

For each test, five specimens were produced forthe mechanical test. The composite manufactur-ing described above produces a 305 x 305 x 21 mmplate. Then, the skin component is lowered 5 mmto meet the dimensions needed, and the plate is cutinto five 300 x 50 mm specimens, as represented infigure 6, finishing the specimen production.

Figure 6: Specimens cut from plate.

4.3. Equipments and SensorsElectromechanical Testing Machine

The 4-point-bending tests were performed usingInstron R© 3369 Dual Column Universal. This is anelectromechanical machine with a load cell capacityof 50 kN and a maximum speed of 500 mm/min.

Figure 7: Instron 3369 model.

Visual Image Correlation System - VICVIC is a system for measuring and visualizing

both strain and movement by means of image com-parison. It is based on the principle of Digital ImageCorrelation, providing full-field, 2 or 3-Dimensionaldisplacement and strain data for mechanical testingon planar specimens.

The system consists in one or two cameras (for2 or 3D analysis respectively) that capture imagesduring a mechanical test and a software that mapsand monitors every pixel subset within the area ofinterest. The method tracks the gray value pat-tern in small areas called subsets during deforma-tion. Using this apparatus its possible to recordthe displacement on every point of the specimen bytracking the pixels in the image and so, having amore precise information of what is happening inthe structure during the bending test.

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Hot Press

ISOJET R© hot press provided by Instituto Supe-rior Tecnico can deliver pressures up to 10 bar andits plates can be heated up to 250◦C, being able toapply high pressures and temperatures.

5. Experimental procedure

In therms of mechanical behavior, the objectivesare to maximize the bending stiffness of our mate-rial and also increase the maximum stress that thematerial (specimen) can withstand before fracture.

After producing the described specimens it is en-sured that they have the same weight (±1 %). Spec-imen dimensions -length, width and thickness- werealso measured, ensuring a precision of ±0.5 %.

5.1. Mechanical testing

For the mechanical testing, a four-point bendingtest using Instron 3369 was applied to each spec-imen. The assay was made at a constant speed of3mm/min using the third point loading distances(with a span of 250mm, the distance between eachone of the metal cylinders is 83,3(3)mm). Eachspecimen was tested until rupture.

The load that is being applied through the up-per actuator of the machine and its displacementthrough the test are recorded by the associated soft-ware. This data is then exported, providing the val-ues for the machine load and displacement at every0.05 seconds. The maximum load achieved beforespecimen failure is recorded. To transform the loadobtained, into the stress that is being applied to thematerial structure under bending forces, the follow-ing formula was used:

σ =3F (L− Li)

2bd2(1)

Where:

• F is the load -N;

• L is the length of the span - mm;

• Li is the length of the inner span - mm;

• b is the width of the specimen - mm;

• d is the thickness of the specimen - mm.

The Stress vs Extension graphic can then be plotedand analyzed.

5.2. Digital Image CorrelationWhile proceeding with the 4-point-bending test, the av-erage values for the displacements in the specimens midspan (region of maximum strain) are obtained using theVIC system cameras. Then, using the values of stressobtained in the 4-point-bending test and synchronizingwith the ones from the image correlation it is possibleto plot a precise graphic relating the stress being ap-plied through time and the strain that the specimen issubjected in its mid-section.

Figure 8: Displacements - VIC software.

5.3. Bending stiffness comparisonThe bending stiffness of our material is the resistancethat its structure offers against bending deformation.For a comparison purpose, for the first variables iter-ations, the different bending stiffnesses were comparedusing the rigidity inherent to the essay, which is theslope obtained from the plot regarding Stress versusExtension. This can easily be obtained using a smallMatlab program that neglects the initial and final non-linear areas and analyses the linear region allowing acomparison between different rigidities from the differ-ent curing processes.

Figure 9: Linear region slope obtained using Matlab.

5.4. Bending stiffness determinationThe method described above serves only for a compari-son purpose, due to the fact that it doesn’t characterizesthe real bending stiffness of the material.

To obtain the this value it is necessary to resort toanother method. This method allows the extractionof the bending stiffness and shear modulus of a non-conventional composite sandwich using several tests,through a geometrical ratio, testing the specimen withthe same type of loading but increasing the supportingspan (S=100mm, S=150mm, S=200mm, S=250mm).

The beam mid-span deflection is given by the follow-ing formula:

∆ =P (2S3 − 3SL2 + L3)

96D+

P (S − L)

4U(2)

Where:

• ∆ is the maximum deflection in mm;

• P is the maximum load, in N ;

• S is the span, in mm;

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• L is the loading distance, in mm;

• D is the flexural stiffness, in N −mm2;

• U is the transverse shear rigidity, in N .

Replacing L = S/3 (4-point-bending test with thirdpoint distance) in the mid-span deflection equation (eq.2), an equation of the type y = mx+b can be deducted:

(∆

S

1

P) =

1.7

96DS2 +

1

6U(3)

Taking m = 1.7/(96D) and b = 1/(6U) and solvingthem in order to the flexural stiffness (D) and transverseshear rigidity (U) respectively, we obtain:

D =1, 7

96m; U =

1

6b(4)

Where m is the slope of the graphic and b is the y-intercept.

For each supporting span, a maximum deflectionis obtained using the visual image correlation system.Knowing the maximum deflection, maximum load andspan for each test, a linear regression can be plotted andboth slope and y-intercept are calculated. Finally, withthe values obtained for m and b, the flexural stiffnessand the transverse shear rigidity are calculated usingequations 4.

6. Variables optimization, results and discus-sion

6.1. Standard processThe standard process follows the specimen manufactur-ing described in section 4.2. The hardener provides agel time of approximately 90 minutes and so, pressurewill be applied 90 minutes after lamination. For pres-sure, a value of 0.015 bar was applied overnight (timeinterval of 15±1h) at an ambient temperature between18 and 25◦C . The post-cure applied involves a heat-ramp from ambient temperature and stabilizes for thenecessary period of time.

Due to the fact that a large number of variables is be-ing studied, as results are being obtained, some changessubsequent to the variation of the previous variables areintroduced in the new tests.

6.2. Gel time respectTheoretically gel-time is required due to the initial lowviscosity of the mixture, allowing resin infiltration be-fore pressure application and reducing the amount thatslips through the sides. With the aim of reducing thecycle time, a study was made to see if applying pres-sure right after lamination, would reduce the materialmechanical characteristics.

The average values for maximum stress and bendingstiffness comparison, for standard and gel-time disre-spect, are presented in the next tables.

As table 1 shows, the essay where pressure is appliedright after lamination obtains an increase in the averagefor maximum stress achieved before rupture around 9%.

For the bending stiffness comparison results, varia-tion can be neglected (0.5% increase).

Table 1: Average maximum stress obtained before ruptureof the 5 specimens tested for each case: gel-time respect anddisrespect.

Avg. (MPa) Std. Dev. (MPa)respected 35,6 ± 2,3 (6,4%)disrespected 38,8 ± 1,7 (4,4%)

Table 2: Average curve slope obtained for the 5 specimenstested for each case: gel-time respect and disrespect.

Average Std. Deviationrespected 1,98 ± 0,14 (7,0%)direspected 1,99 ± 0,08 (4,2%)

The value for gel time used (90min) for the epoxywas given by the supplier and obtained for a solutionof 70mL at 23◦C with a 4 cm high mix. However, forsandwich composites, this value depends on many fac-tors, like the materials properties, the degree of resininfiltration, the temperature present in the reaction,the area in direct contact with air and the volume ofresin present in the laminated area. Using the ASTMD 2471 [3] standard for the layer in direct contact withair, the gel-time for the epoxy system in our compositewas found to be higher, with a value of 380 minutes.

Figure 10 shows a representative plot of the resin vis-cosity through time. Spreading the same amount ofepoxy in a film slows down the cure as the rheology ofthe resin changes.

Figure 10: Viscosity vs time at ambient temperature forthe same amount of resin but with one - in black - in a 4cm high solution and the other - in blue - in a film withapproximately 0.5 mm thickness - representative.

Gel-time should then be waited to apply pressure ifwe are talking about a laminate (only fiber+resin) orif the structure is only composed by the resin to makea plastic material. This is the time where viscosity isbig enough for pressure to be applied without chang-ing the shape of the material and to uniform its struc-ture filling any gap. However, in this case, the variableresin infiltration as to be taken into account. If gel timeis achieved before pressure is applied, the infiltrationpresent will only be due to gravity, since epoxy isn’tfluid enough to penetrate, when pressure is applied.

As seen in the results obtained, a small increase inthe maximum stress before rupture is achieved and nochange in the average bending stiffness is observed. Thishappens because pressure is applied in a period of timewhere viscosity is at it’s lower value, allowing maximum

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penetration, contributing to the adhesion and consoli-dation between layers. With this, the waiting step iseliminated, reducing the overall time of manufacturing.

6.3. HardenerHardeners contribute to the velocity of the reactionsupon epoxy cure. In order to reduce production time,a faster hardener was introduced. The new hardenerprovides a much faster cure, which will enable a fasteroverall productive process, culminating in a higher pro-ductivity. The a gel-time, using ASTM D 2471 [3], wasfound to be 70 minutes.

Table 3 shows the average value for the maximumstress achieved. This values show an increase of about7% in the average.

Table 3: Average maximum stress obtained before ruptureof the 5 specimens tested for each set produced: using stan-dard hardener A and hardener B.

Avg. (MPa) Std. Dev. (MPa)A - standard 35,6 ± 2,3 (6,4%)B 38,1 ± 2,1 (5,5%)

For bending stiffness, the comparison results shownin table 4 demonstrate small increase of 1%. The re-sults obtained for both bending stiffness and maximumstresses demonstrate the viability of the hardener ex-change tested. This was expected due to the nature ofthe epoxy-hardener system - the new hardener only ac-celerates the process as it provides a higher polymericnetwork cross-linking rate.

Table 4: Average curve slope obtained for the 5 specimenstested for each set produced: using standard hardener A andhardener B.

Average Std. DeviationA - standard 1,98 ± 0,14 (7,0%)B 2,0 ± 0,06 (3,1%)

The initial viscosity, after mixing the epoxy sys-tem components, for the new hardener is close to 50%higher. Also, this new hardener has a faster curing ratecomparing with the previous, and achieves higher curetemperatures. When the adhesive curing-rate is slower,the cross-linking network will from slower, preventingthe mobility of some unreacted groups as they are im-peded from migrating to the remaining open reactivesites, which will result in a more expanded structure.Faster curing rates achieve higher temperatures and willprovide sufficient kinetic energy to initiate the chemi-cal reactions at every locations and give the moleculesenough mobility to fully network, creating a high cross-linking density that will allow the best possible mechan-ical and physical properties.

6.4. Cure process - pressure, time & temperatureTheoretically, an increment in pressure would increasethe level of infiltration, affecting beneficially the prop-erties of the composite structure. To study this effect

on the bending properties of the sandwich, the 0.015bar applied in the standard process was increased 20times. Maintaining the overnight application at an am-bient temperature (18-25◦C), a pressure of 0.3 bar wasintroduced. Figure 11 shows that an increase in bothrigidity and maximum stress was obtained.

Figure 11: Stress-displacement curves relative to the 4-point-bending test for the set of 5 specimens produced usinga pressure of 0.015 bar and 0.3 bar in the curing stage. Eachone of the two curves show the behavior of the most repre-sentative specimen tested for each case.

In fact, an increase of 10% in average maximum stressand about 16% for the bending stiffness, are obtainedwith this increment in pressure - tables 5 and 6.

Table 5: Average maximum stress obtained before ruptureof the 5 specimens tested for each condition: pressure appli-cation of 0.015 bar and 0.3 bar.

Avg. (MPa) Std. Dev. (MPa)0,015 bar 35,6 ± 2,3 (6,4%)0,3 bar 39,3 ± 2,2 (5,5%)

Table 6: Average curve slope obtained for the 5 specimenstested for each case: pressure application of 0.015 bar and0.3 bar.

Average Std. Deviation0,015 bar 1,98 ± 0,14 (7,0%)0,3 bar 2,36 ± 0,07 (3,1%)

The improvements obtained with a higher value ofpressure come to confirm what was already predicted:a higher level of penetration is obtained that leads toa higher layer adhesion, contributing to a better struc-tural performance under bending.

The increase in pressure produces desirable results.However, to further continue with this study, the timevariable needs to decrease in the process. The nexttestes were made in order to reduce time of pressure ap-plication, and also, due to the interconnection betweentime and temperature, this two variables were studiedtogether.

It was discovered that the gel-time for the adhesivenow used (using the faster hardener B) is around 70minutes. Knowing that in order to obtain the desired

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structural properties, pressure should be applied beforeand for a certain amount of time after gel-time, to per-mit good infiltration and layer consolidation, a time pe-riod of 90 minutes was tested.

After that, a 60◦C temperature was introduced usingthe same 90 minutes to test the viability of higher tem-peratures. Because temperature accelerates the cureprocess, the gel time at this temperature was found tobe 12 minutes, and for this reason, time of pressure ap-plication was reduced until 20 minutes.

The results of these tests are next presented:

Table 7: Average maximum stress obtained before ruptureof the 5 specimens tested for each condition: cure cycle us-ing 0.015 bar of pressure with ambient temperature for 90minutes and overnight, and using 60◦C of temperature for90, 50 and 20 minutes.

Avg.(MPa) Std. Dev.(MPa)Tamb-overnight 38,1 ± 2,1 (5,5%)Tamb-90min 38,6 ± 1,43 (3,7%)60◦C-90min 40,4 ± 4,2 (10,5%)60◦C-50min 39,8 ± 3,1 (7,9%)60◦C-20min 39,5 ± 2,05 (5,2%)

The average maximum stress achieved for each case,as well as the standard deviation are presented in thenext table. The values obtained are very similar be-tween each other, having a maximum variation of 6%,that is not of great importance due to the fact that theyare all inside the same margin of error.

Table 8: Average curve slope (representing bending stiff-ness) and respective standard deviation obtained for the 5specimens tested (in a 4-point-bending test) for each case:cure cycle using 0.015 bar of pressure with ambient tem-perature for 90 minutes and overnight, and using 60◦C oftemperature for 90, 50 and 20 minutes.

Average Std. DeviationTamb-overnight 2.0 ± 0,06 (3,12%)Tamb-90min 2.04 ± 0,07 (3,6%)60◦C-90min 2.11 ± 0.15 (7.1%)60◦C-50min 2.09 ± 0.12 (5.7%)60◦C-20min 2.07 ± 0.07 (3.6%)

Regarding the slope obtained for the graphic (rela-tive bending stiffness) the same conclusion can be with-drawn as the results are close to the standard with asmall maximum increase of 5.5%.

Analyzing the results, some conclusions and advancesin the process can be made:

Reducing the time of application to 90 minutes, pro-duces a small increase in the maximum stress and bend-ing stiffness of 1.3% and 2% respectively. However, thisvariation is so small that it is probably due to the errorassociated with the essay. This results enable a great re-duction in the time used for cure, since a period of moreor less 15 hours (overnight) is shortened to 90 minutes.

When temperature is increased to 60◦C, the improve-ment achieved in properties rises up to 6%. This isachieved with a 90 minutes cure time and decreases lit-tle with the cut introduced in time (up to 20 minutes).These results can be explained due to the cross-linkingrate, that is increased with the use of temperature cul-minating in a higher cross-link density and consequentlybetter performance of the resins. Also, it can be noticedthat, as time decreases also the standard deviation goesto lower values, which shows that, although the prop-erties with a longer cure produce better results, thiscure is not so uniform through the surface. Neverthe-less, the fact is that the results obtained are very closeto each other, implies that the improvements achievedwith these tests relate to the optimization of time in thecure process, that is reduced largely.

The results presented triggered new studies regardingeven higher pressures. Two new pressure values wereused - 1 and 3 bar, while temperature was maintained at60◦C as it provides good results and it is feasible in ourprocess. With this temperature, a good rate of cure canbe achieved providing short curing periods. Regardingtime of cure, at 60◦C, it was found that 20 minutes isthe minimum time required for cure, however, to makesure that all the components are fully cured and also tobetter understand the influence of heat combined withpressure, a 60 minutes cure was also applied.

Figure 12: Stress-displacement curves relative to the 4-point-bending test for the set of 5 specimens produced foreach case: standard process method and cure cycle with60◦C of temperature using a pressure of 1 and 3 bar for20 and 60 minutes. Each one of the two curves show thebehavior of the most representative specimen tested for eachcase.

Figure 12 shows a large increase, referent to the stan-dard, both in graphic slope and in maximum stress.The results for average stress and standard deviationare shown in table 9 - an increase from 20 to 35% in theaverage stress was achieved.

For the bending stiffness comparative study, an in-crease of 41,4% relatively to the standard was achieved.

The results show that an increase of at least 20%in both maximum stress and bending stiffness was ob-tained for all cases. However, the failure mechanism ofsome specimens differ from each other and can intro-duce a risk to the material. The failure of specimensoccurred, usually, by failure of the facing (see fig. 13).At a certain amount of applied load, the skin is not ableto support the stresses involved and breaks. This occurs

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Table 9: Average maximum stress obtained before ruptureof the 5 specimens tested for each condition: cure cycle with60◦C of temperature using a pressure of 1 and 3 bar for 20and 60 minutes.

Avg. (MPa) Std. Dev. (MPa)Standard 35,6 ± 2,3 (6,4%)1 bar - 20min 47,1 ± 1,37 (2,9%)1 bar - 60min 48,2 ± 3,32 (6,9%)3 bar - 20min 42,7 ± 0,8 (1,9%)3 bar - 60min 43,9 ± 2,5 (5,6%)

Table 10: Average curve slope obtained for the 5 specimenstested for each case: cure cycle with 60◦C of temperatureusing a pressure of 1 and 3 bar for 20 and 60 minutes.

Average Std. DeviationStandard 1,98 ± 0,14 (7,0%)1 bar - 20min 2,8 ± 0,026 (0,9%)1bar - 60min 2,72 ± 0,12 (4,5%)3 bar - 20min 2,4 ± 0,026 (1,2%)3 bar - 60min 2,6 ± 0,17 (6,4%)

with no delamination between layers, which indicates agood adherence between the different materials.

Figure 13: Failure of the face occurring in the specimensunder 4-point-bending test.

However, for the two sets of 3 bar pressure, differentfailures have occurred. The first due to delaminationof the skin-fiber interface (see fig. 14). The high valueof pressure applied makes a great amount of resin toslip through the sides impeding the infiltration neededfor correct bounding between the different components,mainly between the skin and fibers.

Figure 14: Delamination failure occurring in the specimensunder 4-point-bending test.

Another type of failure observed implies the shearfailure of the core. The cork agglomerate isn’t able to

sustain the forces involved and fails causing the skinalso to break (see fig. 15).

Figure 15: Failure of the core occurring in the specimensunder 4-point-bending test.

It was observed that when a 3 bar pressure is appliedfor 20 minutes, a reduction of 1mm is observed on thecore thickness and a 2mm reduction in the case of 60minutes (fig. 16). This shows that, with such highvalue of pressure, in the presence of temperature, thecork agglomerate isn’t able to recover and its thicknessis reduced. Moreover, if the time of application is higherthe reduction accentuates.

This reduction in thickness is related both with thecompressive strength of the agglomerate, the tempera-ture in the cure and the cure process itself. When thishigh value of pressure allied with a 60◦C temperatureis applied, the core thickness reduces. Simultaneously,the resin impregnates in the agglomerate and infiltratesthe empty pores present and cures inside them. Whenpressure is removed the resin has highly increased itsviscosity and rigidity, impeding the recovery of the coreto its initial thickness. As a result of this phenomenon,the cork agglomerate will lose some of its compressibil-ity properties, becoming more rigid which will result inits rupture under high bending forces.

Figure 16: Specimen cured using a 3 bar pressure for atime interval of 60 minutes. A reduction of 2mm is presentin the cork agglomerate.

The failure modes observed for the 3 bar pressureshow that, although the values obtained for maximumstress and bending stiffness represent a desirable in-crease in these properties relatively to the standard pro-cess, this amount of pressure is too high if a stable com-posite material under bending forces is to be achieved.Nevertheless, for both bending stiffness and maximumstress, 1 bar of pressure optimizes the most our com-posite material under bending forces.

The results using 60 minutes, obtain very close valuescomparing with the ones using 20 minutes, but with amuch higher standard deviation. This, allied with thefact that the 20 minutes cure is three times smaller thanthe second one, makes it the most desirable cure process.

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6.5. Final resultsPresenting the values obtained for the best processachieved -pressure of 1 bar applied for 20 minutes ata temperature of 60◦C- it is possible to see the increaseobtained in maximum stress achieved before ruptureand in bending stiffness.

Table 11: Average maximum stress and bending stiffnessfor the standard and optimized productive process.

Bending stiffness Maximum stressStandard 79,7 x 106 N.mm2 35,6 MPaFinal 117,9 x 106 N.mm2 47,1 MPa

An increase of 48% in the bending stiffness of thematerial and 32,3% for the maximum stress before frac-ture are achieved, that represents a great optimizationin therms of bending characteristics for the compositestructure.

7. ConclusionsFrom the studies made through this work it was possibleto achieve the goals initially set.

The bending stiffness was increased 48% and themaximum stress achieved in the specimens before rup-ture rase 32,3%, relative to the standard cured materialinitially tested.

The developments made in the process allowed for abig time cycle reduction. Table 12 shows the reductionin time achieved for the curing process of the composite.The post curing cycle remains unchanged and representa 7 hour plus time interval.

Table 12: Time required for production in the initial andfinal processes (not including post-cure).

Initial FinalLamination 10 min 10 min

Gel time 90 min -Cure 15 h 20 minTotal 16h40min 30 min

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[3] D. ASTM. 2471-00, standard test method for geltime and peak exothermic temperature of reactingthermosetting resins. American Society for Testingand Materials, Philadelphia, PA.

[4] O. Castro, J. M. Silva, T. Devezas, A. Silva, andL. Gil. Cork agglomerates as an ideal core mate-rial in lightweight structures. Materials & Design,31(1):425–432, 2010.

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