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Analysis of the mechanical behavior and identification of failure type in sandwich structures with cork cores

CARVALHO, P.

Department of Mechanical Engineering Universidade Técnica de Lisboa, Instituto Superior Técnico

Av. Rovisco Pais, 1049-001, Lisboa, Portugal

Abstract

The goal of this thesis is to cover the study of properties and mechanical behavior of NL10 and NL30 cork agglomerates under compression, shear and three point bending according to ASTM test methods. The test specimens used in three point bending were manufactured by Resin Transfer Molding (RTM) process. Although the unsatisfactory results in NL30 agglomerate shear and bending tests, it is shown that it is more resistant than NL10 agglomerate due to its double size grains and superior density. As sandwich structures require low density cores, the NL30 agglomerate superior density is undesirable. The shear failure is mostly between grains, i.e., in the adhesive; the grains keep intact without affecting their integrity, clearing the way to an improvement of cork agglomerates to shear properties by creating a better adhesive and a better agglutination system to produce this agglomerates. �

Keywords: Cork agglomerate core, Experimental tests, Manufacture of sandwich test specimens, Resin Transfer Molding, Sandwich structures

1. INTRODUCTION

Being cork a natural raw material in large abundance in Portugal and with its exceptional mechanical and physical properties given by their particular characteristics, in the most recent years it had noticed an increasing interest on its applicability in alternative uses further than the well known cork stoppers and wall coverings from the traditional sectors. Cork applicability had been impelled by the sprouting of cork agglomerates and nowadays we can see it inserted in very distinct fields of application, such as in lifejackets, cigarette filters, sports, civil construction and automobile industry, among others. Cork utilizations and products have been increasing in the last years, mostly owing to R&D projects taken by research institutes and universities. Hence, it is intended the applicability study of cork based materials in structural applications, namely in aeronautical and aerospace industry, as core materials of sandwich structures thanks to its excellent thermal and acoustic isolation properties and the fact of being an ecological raw material. Sandwich structures are typically composed by two strong and stiff skins that carry most of the bending moments bonded to a low density core that bears the transverse shear and normal loads, conferring to this structure an excellent stiffness/weight ratio, [1].

Studies about the applicability of cork agglomerates as sandwich cores have been made recently, see [2,3,4], however their mechanical properties are very poor comparing to currently used sandwich core materials, like Honeycombs and Rohacell® which have several years of investigation and progress in that matter. Nevertheless, analyzing the failure mode of agglomerates subjected to shear, it can be seen that it is mainly adhesive, occurring separation between grains. Since grains are unaffected and remain intact, in such a way it opens perspectives of considerable developments with the intent of improvement. To carry on the study started by Pinto et al. [2] on characterizing NL10 and NL30 cork agglomerates, test methods are made to determine the flatwise compressive and the shear properties of sandwich cores and the flexural properties of sandwich constructions according to ASTM test methods. Sandwich specimens used in three point bending tests were produced by Resin Transfer Molding (RTM). This composite manufacturing process was employed because it is a promising process that presents lower environmental impact and less residual wastes than most processes, particularly open mold processes.

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2. TEST METHODS, MATERIALS AND SPECIMENS

2.1. Test methods

To determine the mechanical properties of cork agglomerates, three types of mechanical tests had been executed. The corresponding standards for each test and measured properties can be seen in Tab. 1.

Test Standard Measured Properties

Flatwise compression test

ASTM C 365

Compression strength Compression modulus

Core shear test ASTM C 273

Shear strength Shear modulus

Three point bending test

ASTM C 393

Core shear strength Skin shear strength

Sandwich flexural stiffness Tab. 1 - Test methods used to determine the mechanical

properties

Compression and three point bending tests had been carried out in an electromechanical Instron 3369 test machine with a 30 kN load cell, while for core shear tests a servohydraulic Instron 8502 test machine with a 30 kN load cell was used.

2.2. Materials and test specimen

Tests were performed using two types of cork agglomerates identified by NL10 and NL30, provided by Corticeira Amorim - Indústria S.A. with the intent of study their behavior as sandwich structure cores. The first one has a density of 125 kg/m3 and single grain size of 2/3 mm and the second one has a density of 216 kg/m3 and double grain size of 2/3 and 3/4 mm. For testing sandwich structures, according to ASTM C 365 and C 273 standards, the test specimen can be simply the core of a sandwich construction, by other hand, in ASTM C 393 standard the test specimen must be a sandwich beam. The manufacturing process of the sandwich beams is described in section 2.3. Each test method requires different test specimens, as can be seen in Fig. 1. Test specimen dimensions are

specified in Tab. 2 according to the standards mentioned, where L is the length, b the width and d the thickness. The specimens used in three point bending tests have 1.5 mm of facing thickness and 12 mm of core thickness and a span length of 260 mm. To verify the scale effect in core shear tests two types of test specimen were used a larger one and a smaller one, as can be seen in Tab. 2.

Test ASTM Standard

Test Specimen, L x b x d [mm3]

Flatwise compression test C 365 50 x 50 x 50

Core shear test C 273 144 x 50 x 12

600 x 50 x 50

Three point bending test C 393 300 x 31 x 15

Tab. 2 - Dimensions of specimens and respective standards

2.3. Manufacture of sandwich specimen used in three point bending tests

The facings were laminated directly on the agglomerate cores resulting in sandwich plates, which were cut and polished in order to obtain the sandwich beams employed in three point bending tests. The sandwiches were manufactured using the conventional Resin Transfer Molding (RTM) processing technique in an ISOJET Piston 3L RTM machine. As matrix was used an unsaturated polyester resin with a resin/catalyst stequiometric rate of 100 parts of resin volume to 0.3 parts of methyl-ethyl-ketone peroxide (MEKP). As reinforcement were used two types of fibers: random fiberglass with 450 g/m2 of gramature, and bidirectional 0º/90º natural jute fiber with 310 g/m2, both compatible with unsaturated polyester resin. NL10 and NL30 cork agglomerates were used as sandwich cores. In this way it was possible to produce four distinct types of sandwich for the three point bending tests: two types of cores and two types of reinforced laminates.

Fig. 1 - Specimens used in: a) compression tests, b) flexural tests, c) shear tests

NL10 fiberglass

NL10 jute fiber NL30 NL10 b) a)

c)

NL10 large

NL30 small

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After stacking the reinforcements and the core in the mold by the sequence: fiber, core, fiber; the mold was placed in the 50º C heated press and closed with a pressure of 3 bar. The high temperature aims to increase the resin fluidity and to reduce its polymerization time. Then, the resin/catalyst mixture was poured into the injection cylinder and the resin outgassing was made with the vacuum pump at room temperature in a cycle of 10 minutes. The injection of resin was made with an injection pressure of 5 bar and the other parameters of each injection are shown in Tab. 3. Nine plates were produced, in which eight of them with NL10 and NL30 cores. From this eight plates where obtained at least five specimens of each type for the flexural tests.

Fig. 2 - Produced sandwich plates

In Fig. 2 it can be seen that plates 2, 3, 5, 6, 7 and 8 have a region without resin, where the reinforcement is dry since the mixture cannot go through. This is called the racetracking phenomenon, in which the resin flows better along the sides of the plate heading to the vent because of the imperfect fits between the core edges and the mold walls where the resistance is minor. Since the flow is very low the resin can soak partially the reinforcement in the central zone. This phenomenon can be better seen in Fig. 3.

The second plate had a large amount of dry reinforcement due to the two layers of fiber in each facing, what proved not to be very positive because the mold was too much filled hindering the resin flow.

Fig. 3 - Racetracking phenomenon where resin flows

better along the sides of the plate

Plates 1, 4 and 9 were the most satisfactory, without dry zones and less air voids than the ones that presented racetracking phenomenon. The first plate probably because the less amount of catalyst, which grants more fluidity to the resin. Though, after cutting it in order to obtain the test specimens, it was noticed that the laminate has not a very good adherence to the core. The remaining plates showed better adherence in the facings/core interface. The fourth plate success was in such a way random since the parameters of its manufacture were identical to used in other plates. The positive result of ninth plate was due to its core been composed by small parallelepipeds of cork, which allowed the resin to penetrate between open spaces filling the mold cavity and saturating all the fibers.

Plate Core Reinforcement

(layers) Polyester [l] / catalyst [%]

Injection flow [cm3/min]

Polymerization time [h]

1 CAI-8123 Fiberglass (1) 1 / 0,25 35 2,5

2 NL10 Fiberglass (2) 1 / 0,3 40 1

3 NL10 Fiberglass (1) 1 / 0,3 35 1

4 NL30 Jute fiber (1) 1 / 0,3 35 1

5 NL10 Jute fiber (1) 1 / 0,3 35 1

6 NL30 Fiberglass (1) 1 / 0,3 35 1

7 NL10 Jute fiber (1) 1,2 / 0,3 35 1

8 NL30 Fiberglass (1) 1,2 / 0,3 30 1

9 NL10 Fiberglass / Jute (1) 1,5 / 0,3 30 1

Tab. 3 - Manufacturing parameters of each sandwich plate

1 2 3

4 5 6

7 8 9

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A larger flow causes a faster injection but by other hand can generate several problems to the produced parts such as more dry spots and air voids, fiber drag and the increase of dry reinforcement caused by the racetracking phenomenon. Another important factor is that the proper amount of resin must be transferred to the mold cavity. Theoretically, if it is transferred a larger amount of resin, even with the racetracking phenomenon, the fiber will be slowly impregnated by the resin but by other hand it is necessary to take precaution with the gel time of the resin.

In spite of some plates presented superficial defects proceeding from the production process, specimens where cut from areas without visible defects in order to perform the tests.

3. RESULTS AND DISCUSSION

3.1. Flatwise compressive tests

Three test specimens of each material had been tested at the constant speed of 2 mm/min, and for all the same behavior was observed (Fig. 4). In region (a) we can see the cell walls bending until the yield stress, where it initiates the corrugation on the walls, region (b), achieving about 70% of strain. From then on, in region (c), the walls start to contact to each other until it crushes. From this point it’s impossible to the cork recover its initial dimensions. In Tab. 4 it can be seen that NL30 cork agglomerate has greater compressive strength than NL10, mainly due to its superior density and double grain size allowing that the smaller grains fill the empty spaces between the larger ones.

Fig. 4 - Evolution of the behavior of NL10 and NL30 cork

agglomerates

Pmax [kN] �max [MPa] E [MPa]

NL10 934,5 0,3738 16,468

NL30 2311,2 0,9244 29,558

Tab. 4 - Average values obtained in the flatwise compressive tests

The comparisons between compressive modulus and between the ratio compressive modulus/density, i.e., specific compressive stiffness, of different sandwich cores are made in Fig. 5 and Fig. 6, respectively, where the values of the other sandwich cores were obtained by Lopes et al. [4] and Leite et al. [8]. CAI-8000 series are another type of cork agglomerate core.��

� Fig. 5 - Comparison between compressive modulus of

several types of cores

Fig. 6 - Comparison between specific compressive

stiffness of several types of cores

Compressive modulus of NL10 cork agglomerate is slightly inferior to that of polyurethane (PU) and Rohacell®, with about 25% of the value achieved by Honeycomb core. As NL30 modulus is about 46% of the value of Honeycomb core, it has the bigger compressive modulus, next to Honeycomb, among the nine core materials. Despite of the superior mechanical properties of NL30 agglomerate it is also heavier, and because of that its specific compressive stiffness decreases. In that way we can see that although NL30 has higher compressive modulus, it has a specific compressive stiffness very approximate of NL10. These two cork agglomerates have higher compressive modulus and specific compressive stiffness than the remaining agglomerates of CAI-8000 series, with about ten times the value of the specific stiffness achieved by CAI-8810 cork agglomerate.

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Fig. 7 - Failure modes. Core rupture: a) small specimen, b) large specimen; Adhesive failure: c) small specimen, d) large specimen

3.2. Core shear tests

Six test specimens of NL10 cork agglomerate, of which three small and three large, and sixteen of NL30, of which seven small and nine large, had been tested. Only four NL10 test specimens, two small and two large, exhibited valid results, i.e., the failure mode had been by core rupture, as it can be seen in Figs. 7 a) and b). The remaining ones reported adhesive failure, Figs. 7 c) and d), and cohesive/adhesive failure, becoming invalid tests, [6]. These poor results in shear tests were mainly due to the adhesive used in the bonding of the loading plates to cores. The adhesive is the same as Pinto et al. [2] used in his study, designated by PM1 and provided by Corticeira Amorim - Indústria S.A., although in his tests that wasn’t verified.

The test specimens that failed by core rupture have the behavior presented in Fig. 8 (such as the ones that presented cohesive/adhesive failure), in which the initial phase, region (a), where the behavior is predominantly elastic and the distortion grows up until it is near to 2% where the first crack appears next to the extremity of the core, region (b), and it is in this instant that the shear yield strength occurs. The plastic deformation begins after the yield strength, giving place to the progression of the crack through the center of the core reaching the maximum shear stress, region (c), where the rupture occurs. After this point the crack propagation is faster and the stress starts to decrease tending to zero till the separation of the core in two halves, region (d). Test specimens that presented adhesive failure has the behavior of Fig. 9: in region (e) the behavior is predominantly elastic, like it happened in the initial phase of previous case, and the stress grows till a maximum value where the adhesive fails causing the detachment of the loading plate to the core, region (f), and the shear stress falls immediately to zero.

Fig. 8 - Behavior in core shear tests of core rupture

failure

Fig. 9 - Behavior in core shear tests of adhesive failure

When the cross-head speed was reduced from 1.5 to 0.8 mm/min in the small NL10 specimen and increased from 1 to 1.5 mm/min in the large ones, so that the maximum load occur within 3 to 6 min according to the standard, [6], the failure mode altered from core rupture to adhesive failure. Such may happened due to those changes in the speed or due to some unsuccessful joining of the core to the loading plates. During the tests of NL30 agglomerate, the cross-head speed was varied in the interval 0.8 to 1.5 mm/min, however without any success of getting rupture of the core.

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Pmax [kN] ���� (Pmax) �max [MPa] �0,02 [MPa] G [MPa]

NL10 small 5,617 0,167 0,781 0,753 5,949

NL30 small (5,473) (0,275) (0,760) � 5,016

NL10 large 13,373 0,143 0,446 0,372 5,326

NL30 large (16,261) (0,106) (0,542) � 6,481

Tab. 5 - Average values obtained in core shear tests (between parenthesis are the maximum values of NL30 tests) The majority of failure modes obtained to NL30 specimens was adhesive and just two of the smaller test specimen returned cohesive/adhesive failure, with the speed of 1 mm/min. These results were due to the weak resistance of the adhesive comparing to the strength of the cork agglomerate.

Tab. 5 shows the average values of the valid tests (failure by core shear rupture) and, between parenthesis, the values of the specimens that presented the higher maximum load of NL30 tests. Test specimens that failed adhesively are considered invalid tests, but they have a valid shear modulus since in the initial zone of the test, until the failure, the shear strength is associated with the distortion of parallel planes to the facings generating the typical measure of elastic stiffness. When analyzing the fracture surfaces of the specimens who had rupture failure of the core, as well as those who had cohesive/adhesive failure, it is possible to check that the path of the crack has direction of 45º with the load line action, as it can be seen in Fig. 10. The rupture was essentially between grains like it has verified by Pinto et al. [2].

Fig. 10 - Fracture topology of NL10 test specimens: a) small, b) large

In Tab. 5 it is noticed that for the same core material, the smaller test specimens have higher properties than the larger ones because of their minor shear section, but that didn’ t happen with the shear modulus of NL30, like Pinto et al. [2] reported.

With respect to the larger specimens the NL30 cork agglomerate have higher shear strength than NL10, as expected, due to its superior density and double grain size, however it is not noticed in the smaller ones proving some incongruence in these. This situation can be caused by deviations in type and quantity of adhesive used in the agglutination process of the different cork agglomerates. Fig. 11 and Fig. 12 shows the shear modulus and the specific shear stiffness of different sandwich cores, where the data of sandwich cores were obtained by Pinto et al. [2], Lopes et al. [4] and Silva et al. [9]. �

�Fig. 11 - Comparison between shear modulus of several

types of cores

Fig. 12 - Comparison between specific shear stiffness of

several types of cores

NL10 and NL30 tested agglomerates have greater shear modulus than the ones tested by Pinto et al. [2], as well as CAI-8000 series agglomerates, though inferior to Rohacell® 51 and Honeycomb. NL30 agglomerate have

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a modulus of about 35% of Rohacell® 51 and about 17% of Honeycomb. In spite of the superior shear modulus presented by NL30 face to the other agglomerates, its specific shear stiffness is inferior to that of NL10, having this one the higher shear stiffness of all agglomerates. Its value is about 12% of the Rohacell® 51 and 5% of Honeycomb value of specific stiffness, while NL30 have about 8% of the first and 4% of the second one.

3.3. Three point bending tests

Twenty test specimens were tested, of which ten with NL10 agglomerate core and ten with NL30 core. In each ten test specimens, five have fiberglass laminates and the remaining five have natural jute fiber laminates as facings. All the experimental tests were made with cross-head speed of 6 mm/min. Shear stiffness of beams were calculated according to ASTM C 393 standard with the average shear modulus of larger dimension specimens obtained in core shear tests. In three point bending tests, the failure mode is generally by core rupture, [4,3], however in this study it can be seen four distinct failure modes: local wrinkling of the face, crush of the upper laminate, debonding of the laminate and rupture of the core.

Local wrinkling of the face (Fig. 13 a)) is an adhesive type failure and can be induced in facesheets subjected to in-plane compression or in-plane shear and the critical location normally is in the middle of the beam where the bending moment is greatest, [10]. Local crushing of the upper laminate in the load application place (Fig. 13 b)) is given when the thickness or the strength of the laminate is insufficient to carry the applied load. In this case it was due to the existence of small air spots derived from the production process weakening the beam in that zone. Failure caused by face/core debonding (Fig. 13 c)) is another adhesive failure and takes place when the strength in the adherence is minor than the strength of the core, causing it and spreading it. Rupture failure of the core (Fig. 13 d)) occurs only in some types of cores, and only if the laminate has good adherence to the core and its strength is sufficient to carry the applied load without crushing. In this case the crack begins between the application point of the load and the support of the beam, where the transverse loading is higher. While the deflexion increases the crack propagates until the edge of the specimen dividing it in two.

Fig. 13 - Failure modes: a) face wrinkling; b) crushing of the laminate; c) face debonding; d) rupture of the core

Most of the specimen with natural jute facings failed by local face wrinkling and only one of that type which have NL30 core failed by face/core debonding. All the specimens with fiberglass laminates and NL30 core showed face/core debonding failure. The specimens with the same facings but NL10 core presented three types of failure; one of them by local crushing of the laminate, two by core/face debonding and the last two by rupture of the core. Fig. 14 shows the typical behavior of specimens that failed by core rupture in the three point bending tests, but in most of the tests we can observe similar behavior with four distinct regions. When the test starts, region (a), we can see the initial loading increasing with the displacement of the cross-head until it achieves the maximum load value, region (b), where the failure occurs either by core rupture, face/core debonding or face wrinkling. Next, because of the failure propagation (crack, debond or wrinkle), the load decreases, region (c), until the specimen achieves a stable strength where the load remains about constant, zone (d), till failure reaches to the edge of the specimen.

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Pmax [N] �Pmax [mm] �max [MPa] �max [MPa] D [kN.mm2]

NL10 jute fiber 102,85 24,30 0,1229 10,650 1740,79

NL10 fiberglass 170,10 18,00 0,2032 17,613 4852,91

NL30 jute fiber 92,06 16,23 0,1100 9,532 2361,814

NL30 fiberglass 135,44 9,83 0,1618 14,024 7140,703

Tab. 6 - Test specimen that showed better performance, of each type

Fig. 14 - Evolution of the behavior in three point bending

of a specimen that failed by core rupture

The test specimens that achieved the higher maximum load in each set of experimental tests were the ones that presented the better performance and therefore the analysis refers to them, see Tab. 6 and Fig. 15.

Fig. 15 - Behavior of test specimen that showed better

performance, of each type

Flexural strength of fiberglass facings specimens is superior to that with natural jute, not only due to the higher gramature of its reinforcement, but because of the random disposition of their fibers, which grants it more flexural strength that if the fibers where bidirectional, like the jute ones. Test specimens with NL30 cores reached the maximum loads before their pairs with NL10 cores, i.e., maximum loads of NL30 specimens were achieved for inferior displacements than NL10 ones, and because of the failures were mostly adhesive it can be concluded that the strength in core/facings adherence in the first ones is very low, having some influence the fact of NL30 agglomerate have superior shear and flexural strength.

There is only one situation where the shear strength is valid, that is in the tests where failure occurred by rupture of the core, because this type of failure is given in the core agglomerate in contrast of the others. The average value of the maximum shear strength in these cases is of 196 kPa, well below the obtained in the core shear tests of 446 kPa. This difference is referred in [7] where it is said that the shear properties are best determined in accordance with ASTM C 273. It is interesting to relate the sandwich beams studied with the ones tested by Lopes et al. [4] in its tests despite of the significant differences between them, like in manufacturing process (autoclave) and in facings (carbon fiber composites with 1.58 mm thickness). However it is impossible to compare the core properties of both because the autoclave manufacturing process in addition to the diminishing of its core thickness increases the density becoming more resistant. The flexural stiffness of this specimens is much higher than the tested in this study, even the ones with agglomerate cores: the maximum value of flexural stiffness obtained in this study was of 7.1 N.m2 for the specimen with NL30 core and fiberglass facings and the values obtained by Lopes et al. [4] for agglomerates CAI-8000 series of 15 to 60 N.m2 and for Rohacell® 51 and Honeycomb of 84 and 172 N.m2, respectively.

4. CONCLUSIONS The laminates and sandwich plates produced and the experimentation done let conclude that parameters such as injection flow and pressure, amount of resin and temperature are very complex and are extremely important in the understanding of the RTM process.

The compressive and shear modulus confers to NL30 cork agglomerate better mechanical properties than NL10 due to its double grain size, which allows that the smaller grains fill the empty spaces between the larger ones conferring this agglomerate the major density as well as more joining surface between grains increasing its resistance. However there is an inconvenient; the increasing of the density is unfavorable to sandwich structures and, as NL10 and NL30 specific compressive stiffness are very close and the specific shear stiffness of NL10 is about 140% of

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NL30’ s with the advantage of NL10 to be less dense, it can be considered that NL10 cork agglomerate can do a better use as a sandwich structure than NL30, yet far away from the typical sandwich cores like Honeycombs and Rohacell®.

Both of the cork agglomerates tested proved that their mechanical properties are superior to the ones of CAI-8000 series tested by Lopes et al. [4]. This significant improvement in cork agglomerates properties shows the evolution in a short period of time in terms of used agglutinants, grain size and production of agglomerates however, like it was told in previous paragraph, far away from mechanical properties and lightness of the typical sandwich cores.

Test specimens that presented failure by rupture of the core in both types of tests, core shear and three point bending, showed that the failure is mainly adhesive, occurring between the adhesively joined cork grains. Since grains are unaffected and remain intact it is possible to improve these materials by using better agglutinants and new bonding techniques with the intent of getting cork agglomerates with more shear and flexural strength. Being cork an elastic and resistant raw material, it is believed that with more study and evolution of the cork grain joining technique it is possible to achieve higher strengths getting to the ones achieved by structural foams.

REFERENCES [1] BRENT STRONG, A.; (2007) Fundamentals of Composites Manufacturing: Materials, Methods and Applications, 2nd Edition, SME [2] PINTO, N.; REIS, L. e SILVA, A.; (2007) Investigação da Aplicabilidade de Derivados de Cortiça em Aplicações Sujeitas a Esforços de Corte. Tese de Mestrado, IST/UTL, Lisboa

[3] SOARES, B.; SILVA, A. e REIS, L.; (2007) Estruturas Sandwich com Utilização de Núcleos de Cortiça. Tese de Mestrado, IST/UTL, Lisboa [4] LOPES, J.; (2007) Estudo Sobre a Aplicabilidade de Aglomerados de Cortiça em Estruturas Sandwich. Tese de Mestrado, IST/UTL, Lisboa

[5] ASTM C 365; (2003) Standard Test Method for Flatwise Compressive Properties of Sandwich Cores. ASTM Standards Test Methods Volume 15.03: Space Simulation, Aerospace and Aircraft, High Modulus Fibers and Composites [6] ASTM C 273; (2000) Standard Test Method for Shear Properties of Sandwich Constructions. ASTM Standards Test Methods Volume 15.03: Space Simulation, Aerospace and Aircraft, High Modulus Fibers and Composites [7] ASTM C 393; (2000) Standard Test Method for Flexural Properties of Sandwich Constructions. ASTM Standards Test Methods Volume 15.03: Space Simulation, Aerospace and Aircraft, High Modulus Fibers and Composites [8] LEITE, M.; LOPES, J. E SILVA, A.; (2004) Estudo Experimental e Analítico, em Compressão, de Diferentes Espumas para Aplicação em Construção Sandwich. IST/UTL, Lisboa [9] SILVA, J. S.; MOREIRA, R. A. S. e RODRIGUES, J. D.; (2008) Application of Cork Compounds in Sandwich Structures for Vibration Damping. 8th International Conference on Sandwich Structures, FEUP, Porto [10] ASM HANDBOOK; (2001) Volume 21: Composites, ASM International