Research Article Sandwich Structured Composites for ...Research Article Sandwich Structured...

Research ArticleSandwich Structured Composites for Aeronautics Methods ofManufacturing Affecting Some Mechanical Properties

Aneta Krzyhak1 MichaB Mazur2 Mateusz Gajewski2 Kazimierz Drozd2

Andrzej Komorek1 and PaweB PrzybyBek1

1Aeronautics Faculty Polish Air Force Academy Ulica Dywizjonu 303 No 35 08-521 Dęblin Poland2Mechanical Engineering Faculty Lublin University of Technology Ulica Nadbystrzycka 36 20-618 Lublin Poland

Correspondence should be addressed to Aneta Krzyzak akrzyzakwsosppl

Received 9 February 2016 Revised 10 May 2016 Accepted 16 May 2016

Academic Editor Linda L Vahala

Copyright copy 2016 Aneta Krzyzak et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Sandwich panels are composites which consist of two thin laminate outer skins and lightweight (eg honeycomb) thick corestructure Owing to the core structure such composites are distinguished by stiffness Despite the thickness of the core sandwichcomposites are light and have a relatively high flexural strength These composites have a spatial structure which affects goodthermal insulator properties Sandwich panels are used in aeronautics road vehicles ships and civil engineering The mechanicalproperties of these composites are directly dependent on the properties of sandwich components andmethod ofmanufacturingThepaper presents some aspects of technology and its influence on mechanical properties of sandwich structure polymer compositesThe sandwiches described in the paper were made by three different methods hand lay-up press method and autoclave use Thesamples of sandwiches were tested for failure caused by impact load Sandwiches prepared in the same way were used for structuralanalysis of adhesive layer between panels and coreThe results of research showed that themethod ofmanufacturingmore preciselythe pressure while forming sandwich panels influences some mechanical properties of sandwich structured polymer compositessuch as flexural strength impact strength and compressive strength

1 Introduction

In the case of modern engineering materials included inaeronautical materials apart from strength properties lowweight of the final element is a crucial aspect Such propertiesare directly connected with increasing operational propertiesof a given structure [1ndash3]Themost commonpurpose ofman-ufacturing sandwich structures [4] is to obtain the greateststiffness at minimum total density (average) All the men-tioned parameters can have satisfactory values provided thatthe following conditions are met produced structures will bedistinguished by low quantity (density) of faults and loweramount of resin in the material Improving these conditionsof production is the scientific objective of developing moreeffective and advancedmanufacturingmethodsThe continu-ity of the sandwich structure is especially significant in aero-nautics where a structural faultmay lead to the failure of a fly-ing object in consequence of subsequently happening events

Values of distinctive parameters of an engineering mate-rial influence possibilities of its later use A proper knowledgeof the materialrsquos characteristics allows for producing an ele-ment of desirable properties with using specified resources inrelation to the applied technology and the purpose of use in aspecific work environment The essential characteristics of astructuralmaterial are its physical andmechanical propertiesThe knowledge of these characteristics allows for estimatingthe materialrsquos reaction to mechanical loads which occurduring its later use in a specific environment Additionallya proper durability of use is ensured [5ndash8]

Sandwich structures are light materials which showconsiderable stiffness and high ratio of strength in relationto weight The main concept of the sandwich panel is thatexterior surfaces transfer loads caused by bending (flexuralload and compression) while the core transfers load causedby shearing Accordingly the work mode of the sandwich

Hindawi Publishing CorporationInternational Journal of Aerospace EngineeringVolume 2016 Article ID 7816912 10 pageshttpdxdoiorg10115520167816912

2 International Journal of Aerospace Engineering

Table 1 Relative characteristics of sandwich structures in relationto solid material [9] (see Figure 10)

Property Laminate(skin)

Sandwichstructure

Thicker sandwichstructure

Stiffness 10 70 370Flexural strength 10 35 92Weight 10 104 106

panel described macroscopically can be rightly compared totasks performed by I-beam [9ndash11]

Sandwich composite materials belong to the group ofanisotropic materials It means that their strength propertieschange depending on the applied load Using the knowledgeconcerning this anisotropy makes it possible to producecomposite materials which display specific properties indesired directions depending on needs They are developeddepending on requirements posed in relation to a givencomposite Moreover these requirements are directly con-nected with the application of a given structure The mostsignificant requirements are as follows stiffness strengthspecific volume thermoinsulating power acoustic resistanceability to absorb energy and hydrostatic weighing [1 12ndash15]

The effects of innovative research performed on sandwichcomposite structures can be illustrated by the development ofmaterials used in aeronautics Initially fillers in the form ofbalsa were used (military aircraft ldquode Havilland Mosquitordquo)Next cellulose acetate foams and later honeycomb fillers[11 16] were used They are used for military purposesdue to their nonmagnetic properties [11 16 17] Skins arecommonly made of standard materials used for structurallaminates based on thermosetting resins and glass or carbonreinforcement Skins can also be produced from thin platesmade of thermoplastic materials of high resistance to impactbut of low temperature of use [18] or metallic materials [19]The so-called low density spacer materials whose densityis far lower than water density are chosen to manufacturethe core These are most frequently polymer and ceramicfoams wood and ldquohoneycombsrdquo which are materials madeof elongated cells of a hexagonal or another shape

The sandwich structure is most widely used in applica-tions in which stiffness of an element is particularly signif-icant Redoubling the core thickness (Table 1 and Figure 10)influences the stiffness of the sandwich panel more thanseven times at barely 4 increase in the weight of theproduct Increasing the core thickness of a panel by fourtimes increases its stiffness more than 37 times at a barely6 increase in the weight Additionally flexural strengthimproves However its change is smaller than in the case ofcomparing the changes in the elementrsquos stiffness [9]

The most frequently tested mechanical properties ofsandwich composites are measurements of compressivestrength [11 20] three-point bending test [5 21ndash23] andimpact tests of a panel [7 24] In the case of materialsused in military technology additionally ballistic tests areconducted [25] Discovering values of these properties mea-sured in simulated conditions of the elementrsquos work givesthe possibility of relating them to real operation conditions

and unambiguously determining the usefulness of a givenmaterial Itmust be remembered that thematerialrsquos propertiesdetermined in the course of mechanical tests are dependenton conditions in which they have been determined Theobtained results are considerably influenced by the followingfactors the applied samples (their shape and dimensions)the strength machine used (fixtures for samples mountedon it and also the stiffness of the measurement system)and the speed of changing load The mentioned reasonsallow for a conclusion that values determined as a resultof mechanical tests are not of the characteristic materialconstants (coefficients) such as the density [26]Themechan-ical properties of a sandwich composite depend on partialproperties of components from which this composite hasbeen constructed Mechanical properties of foam and wooddepend to a large extent on the density of these materialsspeed of deformation temperature and humidity [27 28]Increased temperature of use and presence of steam have anadverse effect onmechanical properties of sandwich polymercomposites In the case of humidity water absorption occursin the material If the composite has fiber reinforcementabsorption occurs as diffusion on the separation boundarybetween the fiber and matrix (in the interface) Long time ofthematerialrsquos exposure to humidity can cause its degradationincluding cracking of thematrix and separation of fibers fromit [29]

The flexural test of sandwich panels can be conductedon the basis of ASTM C 393-00 or PN-EN ISO 141252001standards These documents specify the following factorsshape and dimensions of samples the way of conductingmeasurement and methodology of analysing results Theflexural test can be performed by means of either the three-or four-point method Four-point bending test is more rarelyused despite its advantagemdashlack of destructive influence ofthe pressure stamp on the top part of the tested compositeThis adverse influence occurs during a test conducted bymeans of the most commonly used method of three-pointbending [21] Consequently samples subject to four-pointbending test show increased flexural strength of the sandwichpanel [30] Failure of sandwich panels in the flexural testoccurs mainly in the core of the material It is a sliding crackin the core caused by shearing forces Ultimate shear strengthincreases proportionally to the density of the used foam coreand at decreasing the level of porosity The increase in foamthickness does not improve this characteristic

The increase in flexural strength of a sandwich panel canbe obtained by using a skin of a material which has a greaterstiffness or increasing its thickness [27] If the foam core haslow shear strength or is fragile its failure can be immediateand unpredictable When a material of high shear strength isused for the core decohesion causing delamination betweenthe core and outer structural layers of the panel can appear[31] Destroying the core during a flexural test usually occursdirectly under the load point that is a place where shearingforces and bending moment reach maximum values [32] Forthe mentioned reasons initiation propagation and stoppingthe development of cracking which leads to delamination arean essential aspect while testing sandwich structures [17]

International Journal of Aerospace Engineering 3

One of the faults in layeredmaterials including sandwichtype composites is low resistance to delamination as a resultof impact caused by small objects of low energy Such animpact can lead to the separation of the outer structural layerfrom the core which can facilitate the absorption of humidityby the material Consequently strength properties of a com-posite can significantly decrease In the sandwich materialthe laminate plays the most significant role in absorbingimpact energy On the other hand the thickness of thecore has influence on the decisive failure mechanism of thematerial [10] The quality of the connection between the coreand outer skin of the composite has a considerable influenceon the impact strength of the layered material It depends toa large extent on the used method of producing a panel [17]

Evaluation of the composite resistance to impact canbe performed with the use of a hammer drill according toCharpyrsquos method There is a test with recording total energyand a test with a separate recording of initiation and crackdevelopment energy [33ndash35] In the case of fiber compositesthe latter recording provides a lot of information concerningthe crack mechanism and gives a possibility of isolating theinfluence of components of a tested composite and structureof fibers on impact strength test [5]

2 Materials and Methods ofExperimental Tests

21 The Objective of Research and Used Materials Tests wereaimed at determining the influence exerted by the productiontechnology of sandwich laminates on chosen mechanicalproperties including structural characteristics occurring atthe boundary of core stages and sandwich panel skinsMoreover the objective of the tests was to analyse the processof developing cracks occurring after impact The knowledgeof such a process is indispensable for evaluating the degreeof damage in sandwich laminates and the usefulness oflaminates especially used in aeronautics

Sandwich panels of dimensions 500 times 210 times 10mm fromwhich samples for strength tests were cut were producedby means of three methods hand lay-up press method andautoclave use These methods were chosen due to their wideapplication for producing composites used in road transportand aeronautics Panels were built of a core in the formof polyurethane foam surrounded by a composite made ofepoxy resin Two types of materials were used for reinforcingthe resin Namely foam sheet S63 (Connector) used for thecore was characterized by the apparent density of 673 kgm3and compressive strength which equaled 0594MPa formeasurements in the direction of cell increase during itsproduction and 0309MPa for perpendicular directed mea-surements According to the manufacturerrsquos data the foamS63 with 93 cells of closed type showed water absorptionat 16 Water absorption test was described by means of thePN-EN ISO 1609 test standard

The following reinforcements were used in panel coverstwo types of mats EM 1002300125 and EM 1004300125which differed in the type of silane preparation (resp emul-sion andpowder) and two types of fabric STR015-200-110 and

STR 010-300-125 which had surface weight respectively 200and 300 gm2 The weave of both fabrics was the same typeldquoone to onerdquo It means that linear density of warprsquos rovingand weftrsquos roving was 200 or 300 depending on whether itwas the first or second fabric In all cases glass fibers of 12 120583mdiameter from E type glass produced by Krosglass SA wereused as reinforcement

The matrix of outer skins was made of low viscosityepoxy resin with a CES R70 symbol based on Bisphenol AF(modified by means of an active two-function diluter) andCES H71 hardener Such a composition is used to producelaminates from glass fibers creating glue connection andimpregnation of porous materials Dosing CES R70 + CESH71 composition is 100 54 of weight parts (or 100 56 of vol-ume parts)The estimated time of gelling 100 g of the mixtureat a temperature of 293K was approximately 30min Duringthe work with a mixture of resin and hardener the minimumtemperature of application was 281 K (optimal 291ndash293K) atrelative humidity not higher than 75The lifetime of 100 g ofCES R70 + CES H72 mixture was approximately 45 minutesat a temperature of 293K The density of hardened resin wasdetermined as 117 gcm3

22 Methods of Producing Material for Tests Every producedcomposite panel had a structure built of two glass-epoxy skinsand core made of polyurethane foam Every skin includedone layer of reinforcement in the polymer matrix In orderto mark samples symbols including one letter and onenumber were used The letter referred to the method ofcontact laminating (Tmdashcomposites reinforced with fabricsMmdashcomposites reinforced with mats) press method (P)and producing with the use of autoclave (A) The numberconcerned the reinforcement used in panel skins The fabriccalled STR 015-200-110 was replaced with number 2 whereasSTR 010-300-125 fabric is referred to by means of number 3EM 1002300125 and EM 1004300125 mats were markedwith numbers 2 and 4 respectively

The most common methods of manufacturing sandwichstructured composites were chosenThis allowed for focusingon the practical aspect of applying these methods Howeverin order for such a comparison of methods to be possiblesuch types of resin and hardener were chosen to ensure thehardening of resin both at room temperature (hand lay-upand pressmethods) and at increased temperature (autoclave)The extent of resin crosslinking was not analysed

221 Hand Lay-Up For hand lay-up a mixture of epoxyresin and hardener in a ratio 100 54 of weight parts (a digitalscale with the accuracy of 001 g was used for measuringout) was prepared The liquid was mixed for approximately2 minutes until a clear consistency without visible streakswas obtained After applying a small quantity of liquid itwas spread by means of a wide brush and hard roller whichfacilitated squeezing air bubbles and even impregnation of thefabric

Producing one structure required the following actions(1) simultaneous parallel impregnation of two fragments

of the fabric


(2) impregnating the first element of reinforcement(3) placing and pressing the core to the first impregnated

fabric(4) turning over the crosslinked fabric with the core and

placing the second fragment of the fabric on the coreand precise pressing

222 Forming with the Use of the Press Method Panelsprepared by means of the contact method were subjectto pressing Next plates were placed one on another andseparated by means of stiff covers Pressure of approximately005MPa was used for pressing It was close to but nothigher that the foamrsquos compressive strength The pressurewas maintained until the resin was hardened that is forapproximately 24 hours

223 ProductionMethod with the Use of Autoclave Similarlyto pressing spacer structures were prepared for productionwith autoclave by means of hand lay-up Subsequentlytwo vacuum packages were prepared each of which wascomposed of the following layers

(1) formed composite (core and two skins)(2) perforated separating foil (with a surplus of 3 cm on

each side)(3) on each side two layers of absorbent fabric whose

purpose was to absorb surplus of resin(4) diaphragm foil from which a vacuum bag was

formed

Insulating the diaphragm foil was performed by meansof a special self-adhesive tape Two holes were made in thebag to which two valves were connected (their function wasto connect the vacuum system and pressure control) Thecurrent control of insulation in the vacuum package wasconducted by means of causing initial vacuum in the bagand observing changes in indications on the manometer Inthe case of changing indication on the manometer leakinesswas located and eliminated A detector (VacLeak LEQ-70)was used for accurately determining the place of leak Placingthe receiver closer to the source of leak resulted in obtaininggreater amplification of sound Prior to closing the autoclavethermocouples were attached to the created packages inorder to control temperature during the process (Figure 1)Crosslinking in the autoclave was conducted at a temperatureof 333 K and underpressure of 008MPa (08 bar) maintainedfor 10 hours

224 Procedure of Producing Sandwich Panels Thicknessmeasurement of composite panels showed that elementsmade of hand lay-up had the biggest thickness (Table 2)By comparing panels made of 200 gm2 (T2) fabric it wasdetermined that the thickness of composites produced insuch a way was bigger by 291 than in the case of thepress method and by 383 bigger than in the case of thoseproduced by means of the autoclave method In the caseof bigger basis weight reinforcement (T3) an increase in

Table 2 Measurement results of sandwich panel samples used fortests

Productionmethod

Type ofpanel

Averagethickness

mm

Standarddeviation

mm

Weight ratio ofreinforcement

Hand lay-up

T2 1096 020 1626T3 1100 014 2256M2 1242 013 1003M4 1247 018 984

Press P2 1064 006 2339P3 1084 005 2626

Autoclave A2 1054 004 2709A3 1066 004 3219

Figure 1 Vacuum packages before transport to autoclave

thickness by 145 (press method) and 418 (autoclave)was observed A visible increase in thickness was causedby a different quantity of resin in skins Hand lay-up addi-tionally caused obtaining a composite surface of sampleswhich also influenced the average thickness of laminate andwas reflected in a several times greater value of standarddeviation The press and autoclave technologies which wereconnected with exerting pressure and using a vacuum bagfacilitated a more even spreading of resin and easy removalof its surplus Examples of photographs showingmicroscopicsamples obtained by means of the hand lay-up (Figure 2)press method (Figure 3(a)) and autoclave use (Figure 3(b))present structures of an individual composite

Composite panels made of fabric reinforcement had agreater reinforcement weight ratio in comparison with mate-rial reinforcedmats (Table 2 Figure 3) Composites producedby means of the hand lay-up method had a lower ratio ofreinforcement than other laminates Sandwich compositeswere characterized by a relatively low reinforcement ratioin skins It was caused by the penetration of resin into theirregular cellular structure of the foam used for the core


(a)

Length = 10031120583m

100120583m

(b)

Length = 24626120583m

100120583m

Figure 2 A comparison of thickness of outer structural skins in composites produced by means of contact reinforcement method with (a)STR 015-200-110 fabric and (b) EM 1004300125 mat

250120583m 250120583m

(a) (b)

Figure 3 A comparison of structural skins in composites produced by means of (a) press method and (b) in the autoclave

Figure 4 Way of mounting the sample while performing three-point bending test

23 Methods of Determining Chosen Mechanical Properties

231 Compressive Strength Tests For the purpose of com-pressive strength tests samples with the following parameterswere used length of 100mm width ranging from 382to 406mm and thickness ranging from 109 to 128mmdepending on the type of material Samples were cut with aDEDRA DED7731 cut-off machine with a diamond circularsaw Zwick Roell Z100 device was used for the test Deflectionvelocity was 05mmmin

232 Flexural Strength Test Samples for the three-pointbending test (Figure 4) were prepared according to thePN-EN ISO 141252001 standard Cuboid-shaped samples

were used for the test with the following parameters length of160mm width ranging from 1332 to 1395mm and thicknessranging from 1072 to 1278 depending on the type ofmaterialFlexural strength tests were performed with the use of ZwickRoell Z100 device The test was conducted with a supportspacing of 100mm Velocity of the movement of the liftingbeam was 10mmmin whereas velocity while determiningthe flexuralmoduluswas 2mmminThe ray of used supportsand stamp forcing deformation was 5mm During the testthe stamp always exerted influence on the smooth side of thepanel

Flexural strength 120590119891was calculated on the basis of the

following formula

120590119891=

3119865119871

2119887ℎ2 (1)

where120590119891is flexural strength (MPa) F is load (N) L is support

span (mm) h is sample thickness measured in the directionof force impact (mm) and b is sample width (mm)

For calculating the flexural modulus of elasticity flexuralmaximum deflections 1199041015840 and 11990410158401015840 were calculated Equationsused were

1199041015840=

1205761015840

1198911198712

6ℎ

11990410158401015840=

12057610158401015840

1198911198712

6ℎ

(2)

where 1199041015840 and 11990410158401015840 are flexural maximum deflections in the halflength of the beam (mm) and 1205761015840f and 120576

10158401015840

f are strain


Table 3 Measurement results of compressive strength tests

Method of producing composite Hand lay-up Press AutoclaveType of panel T2 T3 M2 M4 P2 P3 A2 A3119864modulus at compression MPa 62 91 11 112 154 675 119 125Standard deviation of modulus MPa 206 338 134 225 233 0315 115 1442Compressive strength MPa 0601 0589 0589 0601 0627 0621 0596 0610Standard deviation of strength MPa 00051 00424 00265 00168 00182 00077 00061 00122Deflection 23 24 20 26 12 19 18 23Standard deviation of deflection 02 05 05 04 06 01 08 06

Table 4 Measurement results of properties at flexural test

Method of producing composite Hand lay-up Press AutoclaveType of panel T2 T3 M2 M4 P2 P3 A2 A3Flexural strength MPa 367 372 622 726 303 354 318 402Standard deviation of strength MPa 054 031 094 126 040 018 015 021119864modulus MPa 359 350 301 301 343 350 360 402Standard deviation of modulus MPa 595 318 242 24 369 108 282 102Deflection at maximum strength 13 41 23 28 62 27 09 11Standard deviation of deflection 03 74 03 08 60 41 002 01

Themaximum deflections 1199041015840 and 11990410158401015840 (2) correspond to thefollowing adopted strain values 1205761015840

119891= 00005 and 12057610158401015840

119891= 00025

[23]Flexural modulus of elasticity was calculated by means of

the following equation

119864119891=

1198713

4119887ℎ3(

Δ119865

Δ119904

) (3)

where 119864119891

is flexural modulus of elasticity (MPa) Δs isdifference of flexural maximum deflections between 11990410158401015840 and1199041015840 and ΔF is difference between 11986510158401015840 load and 1198651015840 load atflexural deflection which equaled respectively 11990410158401015840 and 1199041015840

233 Impact Strength Test Impact test by means of Charpyrsquosmethod was conducted in accordance with the PN-EN ISO1792001 standard by using samples of the same dimen-sions as in the flexural test Samples used for measuringimpact test did not have a notch For the test a VEBWerkstoffprufmaschinen Leipzig type 40012 hammer witha support spacing of 70mm and a pendulum of 04 kJ energyimpact was used The blade of the used hammer was wedge-shaped with an internal angle of 30 plusmn 1∘ and a rounding ofan 119903 = 2 plusmn 05mm ray The test was performed at roomtemperature During the test energy used for the samplefailure was recorded Energy measurements during the testmade it possible to determine impact strength of thematerialwhich is work used for dynamic breaking of a sample withouta notch related to the initial cross-sectional area of the samplein the point of fracture Impact was calculated bymeans of theequation given below

119886119899=

119860119899

119887119905

sdot 103 kJm2 (4)

where 119860119899is impact energy used for breaking the sample kJ

b is sample width mm and t is sample thickness mm

24 Macroscopic Analysis of Structure Observations of thestructure were performed after tests of mechanical propertieswere conducted For this purpose Nikon SMZ 1500 stereo-scopic microscope with a magnification range from 075x to1125x was used By means of a Kodak Easyshare v803 digitalcamera mounted on a tripod stand photos visible in themicroscope lens were taken in the macro mode Tests madeit possible to evaluate the quality of produced materials andidentification of technological structural faults

3 Test Results and Their Analysis

31 Compressive Strength Test Table 3 presents average testresults obtained as a result of the compression trial Theobtained value of compressive strength for all panels wasin accordance with the one estimated by the foam pro-ducer and was approximately 059MPa (Table 3) A similardeformation independent of the composite type reachedapproximately a 2 level Nevertheless the influence ofreinforcement and method of producing a composite on theE modulus value at compression was proved The lowest Emodulus value at compression was recorded for compositesreinforced with fabrics which were produced by means ofhand lay-up

32 Flexural Strength Test Results of the flexural strength testare presented in Table 4 A greater stiffness is characteristicof materials whose outer structural skins include fabricreinforcement The greatest flexural strength was observedfor composites with mat reinforcement Probably it resultedfrom the occurrence of bigger amount of resin between


0

1

2

3

4

5

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(a)

0

1

2

3

4

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(b)

Figure 5 Examples of flexural characteristics of composite bending (a) T3 and (b) M2

loosely placed fibers (in fabrics they were tightly placed) Asshown by tests of the structure the thickness of the outerstructural layer made of mat was twice bigger than in caseof the outer layer made of fabric In mats fibers are placed inmany directions which can also contribute to this effect

Figure 5 presents examples of tension and deflectioncharacteristics obtained during a three-point bending trial forsome tested materials The beginning of the deflection curvewas an increasing linear function which then changed into aslightly falling curve (Figure 5(a)) At reaching a maximumvalue of load there was a failure of sample and change of thefunctionrsquos type into the nonlinear one was accompanied byfrequently quite rapid decrease in force value In some cases(Figure 5(b)) after lowering tension there was a nonlinearphase of its slight increase yet it failed to reach a value closeto the previous maximum oneThen the curve took the formof a decreasing function Composites produced by means ofthe autoclave method showed the greatest flexural strengthAdditionally they were distinguished by high repeatability ofresults (low standard deviation)

In most cases tested panels did not show decohesivefailure Nevertheless a failure of core foam in the placeof contact of the load stamp directly under the structurallayer (skin) and core deflection were observed Figure 6(a)presents an example of a sample reinforced with fabric afterthe flexural test

In several samples reinforcedwith amat failure caused bycracking of the core was observed Crack initiation occurredin the place where there was a connection core the structurallayer directly under the stamp which was a load on thesample Propagation of crack in the layer of connecting com-posites occurred It was further followed by delaminationAfter reaching a certain length of delamination there was a

(a)

(b)

Figure 6 Examples of panels produced after conducting flexuralstrength test of composites reinforced with (a) fabric and (b) mat

transverse cracking of the core at a 45∘ angle to the upperstructural layer and delamination between the core and thebottom skin of laminate Delamination occurred only incertain places and stopped after reaching a certain lengthFigure 6(b) illustrates failure of a sample reinforced with amat with a visible cracking of the core

33 Impact Test Values presented in Table 4 indicate that thegreatest value of resistance to impact was observed amongcomposites whose outer structural layers were reinforcedwith fabric and produced by means of the autoclave methodObtained from results higher standard deviation for impactstrength of materials made of fabric proves that there is agreater diversity in the quality of producing these samplesas compared to materials with mats It can be caused bythe occurrence of faults in the structure of these materialsComposites reinforced with a mat were distinguished bygrater thickness in relation to materials with fabric yet theirimpact strength was visibly lower (Table 5) It was probably


Table 5 Results of impact strength according to Charpyrsquos method

Method ofproducingcomposite

Type ofpanel

Average impactstrength kJm2

Standarddeviation kJm2

Hand lay-up

M2 1015 135M4 1042 193T2 1148 561T3 1287 447

Press P2 1904 305P3 1325 532

Autoclave A2 2030 237A3 1558 333

(a)

(b)

Figure 7 Examples of impact test failure in hand lay-up samplesreinforced with (a) STR 015-300-110 fabric and (b) EM 1002300125mat

caused by a lower ratio of the reinforcement weight to resinweight in structural layers (Table 2) Moreover the epoxyresin was a fragile material which together with a biggeramount of defects (air bubbles) influenced the much lowerimpact resistance of composites manufactured by hand lay-up method Additionally the surface of adhesive resin andfoam connection in composites produced by means of thepress and autoclave methods probably was bigger than inthe case of composites produced by hand lay-up methodDuring the manufacture by means of the press and autoclavemethods there appeared a force pressing outer layers of thereinforcement to the composite core

Both groups of materials (reinforced with mat and fabricproduced by hand lay-up method) mostly showed the sametype of failure as a result of which there was a delaminationof the sample from the place of impact to its end (Figure 7)Crack initiation occurred directly in the place of the ham-merrsquos impact on the boundary of the connection between thestructural laminate layer and core At this stage the crack wasof adhesive nature At certain length of its propagation therewas cracking of the core at a 45∘ angle to the skin and therewere decohesion and delamination of the bottom skin of thesandwich composite as shown in Figure 7(a) The presentedform of destruction is confirmed by [17]

As for composites reinforced with EM 1002300125Win a few cases there was a complete separation of bottomlayer of the laminate from the corematerial Simultaneously a

Figure 8 Impact failure observed for composites produced by theautoclave method (A2)

(a)

(b)

Figure 9 Samples of pressed composites P2 (a) and P3 (b) damagedduring impact strength test

decohesion of the core occurred inmany places (Figure 7(b))Lack of total separation of the laminate and core proves agood quality of the adhesive connection between epoxy resinand polyurethane core As for samples with EM 1004300125mat and STR 015-200-110 fabric in a few cases local failureof the core in the place of the hammerrsquos impact and asimultaneous delamination of the sample were observed

All composites produced by means of the autoclaveincluding a reinforcement with a fabric of a 200 gm2 (A2)basis weight showed a destruction of the skin with a lossof the core in the area of the hammerrsquos impact (Figure 8)On the lost parts of fabric an even layer of foam was alsoobserved Furthermore those composites were characterizedby the greatest average value of impact strength and the loweststandard deviation among those with fabric reinforcement(Table 5) The reason for the occurrence of a similar failuremechanism of samples and low variation of results was ahigh repeatability of producing composites in the autoclaveDistinctive failure of materials and greatest impact strengthcould have been caused by low basis weight of the fabric andlow content of resin in the reinforcement

During the impact test of composites with lower basisweight of the fabric the skin broke in two cases Simultane-ously there was no loss of the core and skin in the area ofthe hammerrsquos impact which occurred during the test of thecomposite reinforced with A2 material The average resultsshowed that the value of impact strength is lower by 2325than the similar average for A2 material The lower value ofimpact strength can be explained by a higher ratio of resin inskins

Materials produced by the press method independentlyof the type of used fabric were distinguished by visibledelamination on the boundary of the core and skin (Figure 9)Furthermore in pressed composites cracking of the compos-ite core parallel to the direction of impact was distinctive


t 2t 4t

Laminate (skin) Sandwich structure Thicker sandwich structure

Figure 10 Global structure of different types of laminates [9]

4 Conclusions

The strength of sandwich materials in relation to the impactstrength to a large extent depends on the properties of thelaminate in the structural skin and its connection with thecore of the sandwich composite The laminate plays the mostsignificant role in impact energy absorption during the trialOn the basis of conducted analyses it can be concluded thatmaterials with a reinforcement that has a higher ratio ofreinforcement weight to resin weight are distinguished by ahigher impact strength

For materials with a mat reinforcement higher values ofthe Emodulus were obtainedThe autoclave method allowedfor the production of composites distinguished by the highestvalues of impact strength and elasticity modulus Using theautoclave influenced obtaining materials distinguished bymaintained high repeatability Composites produced by thismethod were distinguished by nearly total lack of structuraldiscontinuity and visually high quality of surface (smooth-ness and homogeneity)

The presence of surface faults (air bubbles surface irreg-ularities) in the case of using the hand lay-up technologyresulted in obtaining strength test results characterized by ahigh variation Local lack of foam on the skin in the case ofsamples produced by means of the contact method indicatesan inaccurate connection of the core with the skin

The failure mechanism influenced by impact strengthforce pointed to the necessity of exchanging sandwich panelsAdditionally in the case of composites obtained by meansof the press method at low impact forces there occurreddelamination between the skin and core as well as a failureof the corersquos continuity Nevertheless the structure of theskin was not destroyed Such a lack of visible damage on thesurface of the laminate in some cases can be a beneficialphenomenon However in the case of composites producedby means of the autoclave method sudden contact impact ofhigh force caused a separation of the skin fragment from thecomposite in the place where the force occurred

The method of producing sandwich composites in aero-nautics is determined by labor intensity and quality of pro-ducing a composite Values of distinctive strength parameterspoint to an efficient use of the press method as a cheaperalternative to the autoclave method Mechanical propertiesof sandwich composites produced by means of both methodsare comparable

Competing Interests

The authors declare that they have no competing interests

References

[1] F C Campbell Manufacturing Technology for Aerospace Struc-tural Materials Elsevier London UK 2006

[2] A Krzyzak and D Valis ldquoSelected safety aspects of poly-mer composites with natural fibresrdquo in Safety and ReliabilityMethodology and Applications T Nowakowski M MłynczakA Jodejko-Pietruczuk and S Werbinska-Wojciechowska Edspp 903ndash909 Taylor amp Francis Group London UK 2015

[3] M Landowski M K Budzik and K Imielinska ldquoWpływmetody wytwarzania na własciwosci laminatow poliestrowoszklanych do budowy małych jednostek pływającychrdquoInzynieria Materiałowa vol 5 pp 868ndash872 2001

[4] ASTM Standard C 274-99 Standard Terminology of StructuralSandwich Constructions American Society for Testing Materi-als 2000

[5] A I Boczkowska Kompozyty Oficyna Wydawnicza Politech-niki Warszawskiej Warszawa Poland 2003

[6] W Krolikowski Polimerowe Kompozyty KonstrukcyjneWydawnictwo Naukowe PWN Warszawa Poland 2012

[7] H Leda Kompozyty Polimerowe z Włoknami Ciągłymi Wyt-warzanie Własciwosci Stosowanie Wydawnictwo PolitechnikiPoznanskiej Poznan Poland 2006

[8] D Zuchowska Polimery Konstrukcyjne WydawnictwoNaukowo Techniczne Warszawa Poland 2000

[9] F C Campbell Structural Composite Materials ASM Interna-tional Novelty Ohio 2010

[10] A Muc and R Nogowczyk ldquoFormy zniszczenia konstrukcjisandwiczowych z okładzinami wykonanymi z kompozytowrdquoComposites vol 5 no 4 pp 31ndash36 2005

[11] S Ochelski and T Niezgoda ldquoKompozytowe konstrukcjepochłaniające energię uderzeniardquo Przegląd Mechaniczny vol 1pp 21ndash28 2007

[12] F C CampbellManufacturing Processes for Advanced Compos-ites Elsevier London UK 2004

[13] M A Dweib B Hu A OrsquoDonnell H W Shenton and R PWool ldquoAll natural composite sandwich beams for structuralapplicationsrdquo Composite Structures vol 63 no 2 pp 147ndash1572004

[14] A Jungert ldquoDamage detection in wind turbine blades usingtwo different acoustic techniquesrdquo in Proceedings of the 7th fibPhD Symposium Journal of Nondestructive Testing StuttgartGermany September 2008


[15] A P Mouritz and A G Gibson Fire Properties of PolymerComposite Materials Springer 2006

[16] L J Gibson and M F Ashby Cellular Solids Structure andProperties Cambridge University Press 1997

[17] R Wojtyra and K Imielinska ldquoBadania pękania udarowego wkonstrukcjach przekładkowych poliestrowo-szklanych z rdze-niem z pianki PVCrdquo Kompozyty vol 7 no 3 pp 140ndash144 2007

[18] H Ning G M Janowski U K Vaidya and G HusmanldquoThermoplastic sandwich structure design and manufacturingfor the body panel ofmass transit vehiclerdquoComposite Structuresvol 80 no 1 pp 82ndash91 2007

[19] A G Mamalis K N Spentzas N G Pantelelis D EManolakos and M B Ioannidis ldquoA new hybrid concept forsandwich structuresrdquo Composite Structures vol 83 no 4 pp335ndash340 2008

[20] ASTM ldquoStandard test method for flatwise compressive prop-erties of sandwich coresrdquo ASTM Standard C 365-03 AmericanSociety for Testing Materials 2005

[21] ASTM Standard C 393-00 Standard Test Method for FlexuralProperties of Sandwich Constructions American Society forTesting Materials 2000

[22] T S Gates X Su F Abdi G M Odegard and H M HerringldquoFacesheet delamination of composite sandwich materials atcryogenic temperaturesrdquo Composites Science and Technologyvol 66 no 14 pp 2423ndash2435 2006

[23] ISO ldquoCompositematerials reinforced with fiberMarking prop-erties at flexural testrdquo PN-EN ISO 141252001 ISO 2001

[24] ISO ldquoMarking impact by means of Charpyrsquos methodrdquo PN-ENISO 1792001 2001

[25] J Christopherson M Mahinfalah G N Jazar and M RAagaah ldquoAn investigation on the effect of a small mass impacton sandwich composite platesrdquoComposite Structures vol 67 no3 pp 299ndash306 2005

[26] M Blicharski Inzynieria Materiałowa WydawnictwoNaukowo-Techniczne Warszawa Poland 2014

[27] S Y Shen F J Masters H L Upjohn and C C FerraroldquoMechanical resistance properties of FRPpolyol-isocyanatefoam sandwich panelsrdquo Composite Structures vol 99 pp 419ndash432 2013

[28] M Osei-Antwi J De Castro A P Vassilopoulos and TKeller ldquoShear mechanical characterization of balsa wood ascore material of composite sandwich panelsrdquo Construction andBuilding Materials vol 41 pp 231ndash238 2013

[29] F Aviles and M Aguilar-Montero ldquoMechanical degradationof foam-cored sandwich materials exposed to high moisturerdquoComposite Structures vol 92 no 1 pp 122ndash129 2010

[30] A Corigliano E Rizzi and E Papa ldquoExperimental character-ization and numerical simulations of a syntactic-foamglass-fibre composite sandwichrdquo Composites Science and Technologyvol 60 no 11 pp 2169ndash2180 2000

[31] G Belingardi M P Cavatorta and R Duella ldquoMaterial charac-terization of a composite-foam sandwich for the front structureof a high speed trainrdquo Composite Structures vol 61 no 1-2 pp13ndash25 2003

[32] A C Manalo ldquoBehaviour of fibre composite sandwich struc-tures under short and asymmetrical beam shear testsrdquo Compos-ite Structures vol 99 pp 339ndash349 2013

[33] D Feng and F Aymerich ldquoDamage prediction in compositesandwich panels subjected to low-velocity impactrdquo CompositesPart A Applied Science and Manufacturing vol 52 pp 12ndash222013

[34] A Mostafa K Shankar and E V Morozov ldquoInsight into theshear behaviour of composite sandwich panels with foam corerdquoMaterials and Design vol 50 pp 92ndash101 2013

[35] P Qiao and M Yang ldquoImpact analysis of fiber reinforcedpolymer honeycomb composite sandwich beamsrdquo CompositesPart B Engineering vol 38 no 5-6 pp 739ndash750 2007

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Active and Passive Electronic Components

Control Scienceand Engineering

Journal of



RotatingMachinery


Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design



Shock and Vibration


Civil EngineeringAdvances in

Acoustics and VibrationAdvances in



Electrical and Computer Engineering

Journal of

Advances inOptoElectronics


Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of


Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014


Chemical EngineeringInternational Journal of Antennas and

Propagation




Navigation and Observation



DistributedSensor Networks



Table 1 Relative characteristics of sandwich structures in relationto solid material [9] (see Figure 10)

Property Laminate(skin)

Sandwichstructure

Thicker sandwichstructure

Stiffness 10 70 370Flexural strength 10 35 92Weight 10 104 106

panel described macroscopically can be rightly compared totasks performed by I-beam [9ndash11]

Sandwich composite materials belong to the group ofanisotropic materials It means that their strength propertieschange depending on the applied load Using the knowledgeconcerning this anisotropy makes it possible to producecomposite materials which display specific properties indesired directions depending on needs They are developeddepending on requirements posed in relation to a givencomposite Moreover these requirements are directly con-nected with the application of a given structure The mostsignificant requirements are as follows stiffness strengthspecific volume thermoinsulating power acoustic resistanceability to absorb energy and hydrostatic weighing [1 12ndash15]

The effects of innovative research performed on sandwichcomposite structures can be illustrated by the development ofmaterials used in aeronautics Initially fillers in the form ofbalsa were used (military aircraft ldquode Havilland Mosquitordquo)Next cellulose acetate foams and later honeycomb fillers[11 16] were used They are used for military purposesdue to their nonmagnetic properties [11 16 17] Skins arecommonly made of standard materials used for structurallaminates based on thermosetting resins and glass or carbonreinforcement Skins can also be produced from thin platesmade of thermoplastic materials of high resistance to impactbut of low temperature of use [18] or metallic materials [19]The so-called low density spacer materials whose densityis far lower than water density are chosen to manufacturethe core These are most frequently polymer and ceramicfoams wood and ldquohoneycombsrdquo which are materials madeof elongated cells of a hexagonal or another shape

The sandwich structure is most widely used in applica-tions in which stiffness of an element is particularly signif-icant Redoubling the core thickness (Table 1 and Figure 10)influences the stiffness of the sandwich panel more thanseven times at barely 4 increase in the weight of theproduct Increasing the core thickness of a panel by fourtimes increases its stiffness more than 37 times at a barely6 increase in the weight Additionally flexural strengthimproves However its change is smaller than in the case ofcomparing the changes in the elementrsquos stiffness [9]

The most frequently tested mechanical properties ofsandwich composites are measurements of compressivestrength [11 20] three-point bending test [5 21ndash23] andimpact tests of a panel [7 24] In the case of materialsused in military technology additionally ballistic tests areconducted [25] Discovering values of these properties mea-sured in simulated conditions of the elementrsquos work givesthe possibility of relating them to real operation conditions

and unambiguously determining the usefulness of a givenmaterial Itmust be remembered that thematerialrsquos propertiesdetermined in the course of mechanical tests are dependenton conditions in which they have been determined Theobtained results are considerably influenced by the followingfactors the applied samples (their shape and dimensions)the strength machine used (fixtures for samples mountedon it and also the stiffness of the measurement system)and the speed of changing load The mentioned reasonsallow for a conclusion that values determined as a resultof mechanical tests are not of the characteristic materialconstants (coefficients) such as the density [26]Themechan-ical properties of a sandwich composite depend on partialproperties of components from which this composite hasbeen constructed Mechanical properties of foam and wooddepend to a large extent on the density of these materialsspeed of deformation temperature and humidity [27 28]Increased temperature of use and presence of steam have anadverse effect onmechanical properties of sandwich polymercomposites In the case of humidity water absorption occursin the material If the composite has fiber reinforcementabsorption occurs as diffusion on the separation boundarybetween the fiber and matrix (in the interface) Long time ofthematerialrsquos exposure to humidity can cause its degradationincluding cracking of thematrix and separation of fibers fromit [29]

The flexural test of sandwich panels can be conductedon the basis of ASTM C 393-00 or PN-EN ISO 141252001standards These documents specify the following factorsshape and dimensions of samples the way of conductingmeasurement and methodology of analysing results Theflexural test can be performed by means of either the three-or four-point method Four-point bending test is more rarelyused despite its advantagemdashlack of destructive influence ofthe pressure stamp on the top part of the tested compositeThis adverse influence occurs during a test conducted bymeans of the most commonly used method of three-pointbending [21] Consequently samples subject to four-pointbending test show increased flexural strength of the sandwichpanel [30] Failure of sandwich panels in the flexural testoccurs mainly in the core of the material It is a sliding crackin the core caused by shearing forces Ultimate shear strengthincreases proportionally to the density of the used foam coreand at decreasing the level of porosity The increase in foamthickness does not improve this characteristic

The increase in flexural strength of a sandwich panel canbe obtained by using a skin of a material which has a greaterstiffness or increasing its thickness [27] If the foam core haslow shear strength or is fragile its failure can be immediateand unpredictable When a material of high shear strength isused for the core decohesion causing delamination betweenthe core and outer structural layers of the panel can appear[31] Destroying the core during a flexural test usually occursdirectly under the load point that is a place where shearingforces and bending moment reach maximum values [32] Forthe mentioned reasons initiation propagation and stoppingthe development of cracking which leads to delamination arean essential aspect while testing sandwich structures [17]














of the fabric










formed




Productionmethod

Type ofpanel

Averagethickness

mm

Standarddeviation

mm


Hand lay-up

T2 1096 020 1626T3 1100 014 2256M2 1242 013 1003M4 1247 018 984

Press P2 1064 006 2339P3 1084 005 2626

Autoclave A2 1054 004 2709A3 1066 004 3219





(a)

Length = 10031120583m

100120583m

(b)

Length = 24626120583m

100120583m


250120583m 250120583m

(a) (b)








following formula

120590119891=

3119865119871

2119887ℎ2 (1)




1199041015840=

1205761015840

1198911198712

6ℎ

11990410158401015840=

12057610158401015840

1198911198712

6ℎ

(2)


10158401015840

f are strain







119891= 00005 and 12057610158401015840

119891= 00025



119864119891=

1198713

4119887ℎ3(

Δ119865

Δ119904

) (3)

where 119864119891



119886119899=

119860119899

119887119905

sdot 103 kJm2 (4)








0

1

2

3

4

5

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(a)

0

1

2

3

4

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(b)






(a)

(b)







Type ofpanel



Hand lay-up

M2 1015 135M4 1042 193T2 1148 561T3 1287 447

Press P2 1904 305P3 1325 532

Autoclave A2 2030 237A3 1558 333

(a)

(b)






(a)

(b)







t 2t 4t



4 Conclusions






Competing Interests


References







































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation






















of the fabric










formed




Productionmethod

Type ofpanel

Averagethickness

mm

Standarddeviation

mm


Hand lay-up

T2 1096 020 1626T3 1100 014 2256M2 1242 013 1003M4 1247 018 984

Press P2 1064 006 2339P3 1084 005 2626

Autoclave A2 1054 004 2709A3 1066 004 3219





(a)

Length = 10031120583m

100120583m

(b)

Length = 24626120583m

100120583m


250120583m 250120583m

(a) (b)








following formula

120590119891=

3119865119871

2119887ℎ2 (1)




1199041015840=

1205761015840

1198911198712

6ℎ

11990410158401015840=

12057610158401015840

1198911198712

6ℎ

(2)


10158401015840

f are strain







119891= 00005 and 12057610158401015840

119891= 00025



119864119891=

1198713

4119887ℎ3(

Δ119865

Δ119904

) (3)

where 119864119891



119886119899=

119860119899

119887119905

sdot 103 kJm2 (4)








0

1

2

3

4

5

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(a)

0

1

2

3

4

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(b)






(a)

(b)







Type ofpanel



Hand lay-up

M2 1015 135M4 1042 193T2 1148 561T3 1287 447

Press P2 1904 305P3 1325 532

Autoclave A2 2030 237A3 1558 333

(a)

(b)






(a)

(b)







t 2t 4t



4 Conclusions






Competing Interests


References







































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation


















formed




Productionmethod

Type ofpanel

Averagethickness

mm

Standarddeviation

mm


Hand lay-up

T2 1096 020 1626T3 1100 014 2256M2 1242 013 1003M4 1247 018 984

Press P2 1064 006 2339P3 1084 005 2626

Autoclave A2 1054 004 2709A3 1066 004 3219





(a)

Length = 10031120583m

100120583m

(b)

Length = 24626120583m

100120583m


250120583m 250120583m

(a) (b)








following formula

120590119891=

3119865119871

2119887ℎ2 (1)




1199041015840=

1205761015840

1198911198712

6ℎ

11990410158401015840=

12057610158401015840

1198911198712

6ℎ

(2)


10158401015840

f are strain







119891= 00005 and 12057610158401015840

119891= 00025



119864119891=

1198713

4119887ℎ3(

Δ119865

Δ119904

) (3)

where 119864119891



119886119899=

119860119899

119887119905

sdot 103 kJm2 (4)








0

1

2

3

4

5

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(a)

0

1

2

3

4

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(b)






(a)

(b)







Type ofpanel



Hand lay-up

M2 1015 135M4 1042 193T2 1148 561T3 1287 447

Press P2 1904 305P3 1325 532

Autoclave A2 2030 237A3 1558 333

(a)

(b)






(a)

(b)







t 2t 4t



4 Conclusions






Competing Interests


References







































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










(a)

Length = 10031120583m

100120583m

(b)

Length = 24626120583m

100120583m


250120583m 250120583m

(a) (b)








following formula

120590119891=

3119865119871

2119887ℎ2 (1)




1199041015840=

1205761015840

1198911198712

6ℎ

11990410158401015840=

12057610158401015840

1198911198712

6ℎ

(2)


10158401015840

f are strain







119891= 00005 and 12057610158401015840

119891= 00025



119864119891=

1198713

4119887ℎ3(

Δ119865

Δ119904

) (3)

where 119864119891



119886119899=

119860119899

119887119905

sdot 103 kJm2 (4)








0

1

2

3

4

5

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(a)

0

1

2

3

4

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(b)






(a)

(b)







Type ofpanel



Hand lay-up

M2 1015 135M4 1042 193T2 1148 561T3 1287 447

Press P2 1904 305P3 1325 532

Autoclave A2 2030 237A3 1558 333

(a)

(b)






(a)

(b)







t 2t 4t



4 Conclusions






Competing Interests


References







































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation















119891= 00005 and 12057610158401015840

119891= 00025



119864119891=

1198713

4119887ℎ3(

Δ119865

Δ119904

) (3)

where 119864119891



119886119899=

119860119899

119887119905

sdot 103 kJm2 (4)








0

1

2

3

4

5

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(a)

0

1

2

3

4

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(b)






(a)

(b)







Type ofpanel



Hand lay-up

M2 1015 135M4 1042 193T2 1148 561T3 1287 447

Press P2 1904 305P3 1325 532

Autoclave A2 2030 237A3 1558 333

(a)

(b)






(a)

(b)







t 2t 4t



4 Conclusions






Competing Interests


References







































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0

1

2

3

4

5

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(a)

0

1

2

3

4

Flex

ural

stre

ngth

(MPa

)

1 2 3 40

Deflection ()

(b)






(a)

(b)







Type ofpanel



Hand lay-up

M2 1015 135M4 1042 193T2 1148 561T3 1287 447

Press P2 1904 305P3 1325 532

Autoclave A2 2030 237A3 1558 333

(a)

(b)






(a)

(b)







t 2t 4t



4 Conclusions






Competing Interests


References







































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation












Type ofpanel



Hand lay-up

M2 1015 135M4 1042 193T2 1148 561T3 1287 447

Press P2 1904 305P3 1325 532

Autoclave A2 2030 237A3 1558 333

(a)

(b)






(a)

(b)







t 2t 4t



4 Conclusions






Competing Interests


References







































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










t 2t 4t



4 Conclusions






Competing Interests


References







































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation

































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









Research Article Sandwich Structured Composites for ...Research Article Sandwich Structured...

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