ORIGINAL ARTICLE

A comparative study on the properties of GLARE laminatescured by autoclave and autoclave consolidation followedby oven postcuring

Sang Yoon Park & Won Jong Choi & Heung Soap Choi

Received: 10 November 2008 /Accepted: 27 October 2009 /Published online: 3 December 2009# Springer-Verlag London Limited 2009

Abstract In this study, a new curing technique for pro-ducing glass aluminum-reinforced epoxy (GLARE) lami-nates was explored to reduce autoclave processing time.GLARE2 5/4-0.4 laminates were fabricated with threedifferent approaches: full autoclave curing, autoclave con-solidation followed by oven postcuring, and full ovencuring. Two types of thermal analysis such as differentialscanning calorimetry and dielectric analyzer were perform-ed to provide a supplementary view of the cure behaviorsand also incorporated to understand the voids growth.Experimental results revealed that autoclave consolidationfollowed by oven postcuring laminates had greater interlam-inar shear strength than autoclave cure laminates. Addition-ally, this new curing technique could yield the comparablepart’s quality with minimal void content (<1%). The inter-ruption of cure cycle with an intermediate switch from auto-clave consolidation to oven postcuring resulted in shrinkageand collapse of voids.

Keywords GLARE . Cure behavior . Thermal analysis .

Autoclave

1 Introduction

Glass aluminum-reinforced epoxy (GLARE) laminates arehybrid materials and a kind of fiber metal laminates whichare built up as laminated materials of alternating thin metalsheets and composite prepreg layers [1, 2]. These laminateshave been developed by Delft University of Technology inNetherlands for their excellent fatigue behavior as com-pared to monolithic metals [2–4]. GLARE laminates havebeen lately introduced as the structural composite materialsfor high-performance aerospace application such as for theAirbus A380 upper fuselage [3–6].

For producing GLARE laminates, both thin metal sheetsand composite prepregs are built by layup in a mold andthen cured in an autoclave cycle with high pressure(maximum 6 bar) and cure temperature of 120°C [3, 7].The high-pressure environment of an autoclave facilitatesdissolution and thus removal of the voids present in thelaminate, allowing the part to satisfy the stringent mechan-ical property standards required by the manufacturer.Autoclave is a well-understood mature technology andprovides high-consolidation pressure using a programmedcure cycle, but is costly to operate with price competition ofthe final part [8]. Therefore, the reduction of manufacturingcosts is key element for the widespread usage of GLARElaminates in the aerospace industry [7]. There is also a needto investigate a feasible process window for producingsound GLARE laminates with the current generation ofmaterials and equipments.

During an autoclave processing, the consolidation of theprepregs is accompanied by curing reaction and rheologicalchange in the thermoset resin that strongly influence thefinal properties and the quality of the laminate. High-consolidation pressure (vacuum and autoclave pressures)involved in early stages of cure cycle produces parts with

S. Y. Park (*) :W. J. ChoiDepartment of Materials Engineering,Korea Aerospace University,200-1, Hwajon-dong, Deokyang-gu,Goyang, Gyeonggi-do 412-791, South Koreae-mail: hanavia@empal.com

H. S. ChoiKOREANAIR R&D Center,461-1, Jeonmin-dong, Yusung-gu,Taejon 305-811, South Korea

Int J Adv Manuf Technol (2010) 49:605–613DOI 10.1007/s00170-009-2408-x

high fiber content, which in turn leads to further reductionin voids growth [7, 8]. Void is one of important factors tocontrol during the consolidation process of GLARElaminates since the voids entrapped at the metal sheet–prepreg interface would be the critical location from thedelamination standpoint, which is the most frequentlygenerated in the composite laminates including the fibermetal laminates during their usage [2, 9, 10]. However, theneed for consolidation pressure is gradually diminishedbecause, once the thermoset resin is sufficiently cross-linked, the resin flow is negligible; a premature gelationcauses such a poor resin flow, which results in extensivevoid networks in the cured laminate.

In this study, the optimum and economic cure cycleconcept was benchmarked from the earlier research worksas shown in Fig. 1 [11, 12]. Materials are placed into anautoclave and then subjected to a partial curing reaction(autoclave consolidation stage). During an autoclave cycle,the materials are gelled and consolidated, and some ofexothermic energy is released. Subsequently, the cure cycleis interrupted with an intermediate cool down. The finalcuring reaction (oven postcuring stage) can be achieved in aconventional thermal oven, so that the oven postcuring stagecontributes to reduce the need for additional autoclave usage.

The objective of the current study is to manipulate thecure cycle for producing GLARE laminate through amethod of autoclave processing time reduction, andcompare the overall physical and mechanical properties ofthe fabricated GLARE laminates. Physical properties suchas void content, fiber content, metal volume fraction(MVF), and glass transition temperature were analyzedand compared according to the different processing routes.Two types of thermal analysis such as differential scanningcalorimetry (DSC) and dielectric analyzer (DEA) werecarried out to provide a supplementary view of the cure

behavior. Finally, the interlaminar shear strength tests wereperformed for the mechanical property characterization, andthe results were correlated with the physical properties.

2 Experimental

2.1 Fabrication of specimens

The GLARE laminates were prepared by stacking alternat-ing layers of 2024-T3 aluminum sheets (Alcoa Inc., USA)and glass fiber/epoxy prepregs (HG181, Hankuk Fiber Co.,South Korea). The layup scheme of GLARE laminate wasGLARE2 5/4-0.4 where 5/4-0.4 represents five layers ofaluminum sheets (0.4 mm in thickness per sheet) and fourlayers of glass/epoxy prepregs [3, 5, 7]. All aluminumsheets used in the production of GLARE laminates wereanodized in chromic acid electrolyte and coated with acorrosion inhibiting BR®127 primer (Cytec Industries Inc.,USA) prior to an autoclave cure [3, 13]. Details ofmethodology and procedure are described in the literatures[7, 14, 15]. All plates were fabricated to the size ofapproximately 250×250 mm. Figure 2 shows a photographof GLARE laminate manufactured in the study.

Fig. 2 Photograph of GLARE2 5/4-0.4 laminate prepared in this study

Fig. 1 Schematic view of anautoclave consolidation withoven postcuring, heat (dashedarrow), consolidation pressure(solid arrow)

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2.2 Curing procedures

Three different approaches were chosen in order to inves-tigate the effect of cure cycle on the properties of GLARElaminates and are described below.

2.2.1 Autoclave curing

For autoclave curing, GLARE laminates consisting ofGLARE2 5/4-0.4 were prepared. The manufacture’s rec-ommended cure cycle (FA) was employed for autoclavecuring as shown in Fig. 3. A consolidation pressure of 3 bar(304.0 kPa) was applied throughout the cure cycle with fullvacuum of −1 bar (−107.9 kPa).

2.2.2 Autoclave consolidation followed by oven postcuring

As outlined in Table 1, the cure cycles were set to inves-tigate the effects of two settings for autoclave consolidation

temperatures; 90°C dwell for AO-1 process and 100°Cdwell for AO-2 process, respectively. In AO-1 process, thelaminates were subjected to partial curing reaction in anautoclave at 90°C for 45 min as shown in Fig. 4a. Onremoval from an autoclave, the complete cure was obtainedin a conventional thermal oven, so that oven postcuring (at127°C for 90 min) without usage of additional autoclavecould be explored as a method of autoclave processing timereduction as shown in Table 1. In the same manner, AO-2laminates were partially cured at 100°C for 45 min andthermally postcured in a conventional oven as shown inFig. 4b. For both AO-1 and AO-2 processes, the consolida-tion pressure of 3 bar (304.0 kPa) was applied throughoutautoclave consolidation stage with full vacuum. The totalautoclave cycle time was 110 min for AO-1, 120 min forAO-2 as compared to typical autoclave cycle time (260 min).

2.2.3 Oven curing

GLARE laminates were manufactured using a conventionalthermal oven according to the manufacture’s recommendedcure cycle as mentioned above in Section 2.2.1. Full ovencuring (FO) was employed in order to simulate the auto-clave cure using vacuum pressure alone with absence of anyapplied consolidation pressure. Full vacuum was appliedthroughout the cure cycle.

2.3 Microscope observation and physicalproperties measurement

Void contents were examined from scanning electron micro-scope (SEM) micrographs, 4.1×0.9 mm, at five locations foreach specimen [16]. An S-2400 SEM instrument (HitachiCo. Japan) was used for all void content analyses. Voidcontents (areal percent) were determined using Counter™

Fig. 3 Manufacturer’s recommended cure cycle (autoclave)

Table 1 Comparison of processing time; breakdown by operation for laboratory scale processes

Production step Full autoclavecure

Autoclave consolidation followedby oven postcuring

Full ovencure

FA AO-1 AO-2 FO

Tooling preparation 25 min 25 min 25 min 25 min

Prepreg/metal sheet cutting 30 min 30 min 30 min 30 min

Surface treatments 390 min 390 min 390 min 390 min

Hand layup 30 min 30 min 30 min 30 min

Vacuum bagging 25 min 25 min 25 min 25 min

Autoclave cure cycle 260 min 110 min 120 min –

Oven cure cycle – 160 min 160 min 260 min

Debagging 10 min 10 min 10 min 10 min

Total production time 770 min 780 min 790 min 770 min

Autoclave processing time(% difference respect to the autoclave cure, FA)

260 min 110 min (−57.69%) 120 min (−53.85%) 0 min

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image analysis software (Mirero Inc., South Korea) ac-cording to the MIL-HDBK-17-1F [17]. The number of pixelwhich can be counted on the voids was computed based onthe total number of pixels in the image. The fiber contentwas determined by the acid digestion method according tothe American Society for Testing Materials (ASTM) D3171 standard [18]. The cured GLARE laminate thicknesswas measured by a micrometer with ±0.001 mm resolution(Mitutoyo Co., Japan), and the thickness of each separateprepreg layer was measured with image analysis software.The MVF was defined as ratio of the sum of thickness ofindividual metal layer and total thickness of laminate [19].The use of MVF is useful for the prediction of mechanicalstrength properties in the GLARE laminate [19, 20]. TheMVF was defined by using the following equation:

MVF ¼Xt

p

tal=tlam ð1Þ

where; tal is thickness of each separate metal sheet, tlam isGLARE laminate thickness, and p is the number of metalsheets [19]. The glass transition temperature (Tg) was

measured by a DSC instrument. The glass transition wasdefined as a change in heat capacity as epoxy resin wastransformed from a glassy state to a rubbery state accordingto the ASTM E 1269 standard [21]. The heating rate usedwas 5°C/min to 250°C.

2.4 Prepreg characterization

2.4.1 DSC analysis

A DSC 50 instrument (Shimadzu Co., Japan) was used tocharacterize the thermal behaviors of prepreg resin and itscure kinetics. Degree of cure (α) is determined by dividingheat evolved at time (HT) by total heat (HU) generatedduring the entire reaction [22, 23]:

a ¼ HT

HUð2Þ

An autocatalytic model, which has first-order differentialequation as a function of a nondimensional variable ofdegree of cure, was used to investigate cure kinetics ofprepreg resin using the following equation:

@a@t

¼ K 1� að Þnam ð3Þ

where superscripts n and m are reaction orders, andtemperature-dependent reaction rate, K, is commonlydescribed by the Arrhenius equation:

K ¼ A exp$E

RT

� �ð4Þ

where A is pre-exponential factor, R is universal gas con-stant, E is activation energy, and T is resin cure temperature,respectively.

2.4.2 DEA analysis

The viscosity profiles of prepreg resin throughout a cure cyclewere investigated using a DEA 2970 (TA Instrument Co.,USA) in ceramic parallel plate mode. The viscosity informa-tion is useful in determining when to switch from autoclaveconsolidation to oven postcuring stage. The relative highviscosity should be accomplished prior to switching to post-cure stage in a thermal oven [24]. Approximately 0.5 mg ofsample was placed between two gold electrodes, and the gapsize was 0.5 mm. A frequency range of 0.1 Hz–1.0 kHz wasused. The actual state of the materials during curing reactionwas monitored in situ by the dielectric sensors [25, 26].

2.5 Interlaminar shear strength tests

The bonding quality was evaluated by interlaminar shearstrength through a short-beam bending test by the ASTM D

Fig. 4 Autoclave consolidation followed by oven postcuring (ovenpostcuring). a Autoclave consolidation stage at 90°C isothermal dwell.b Autoclave consolidation stage at 100°C isothermal dwell

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2344 standard [27]. Interlaminar shear strength can be usedfor quality control and process specification purpose. Thecrosshead speed was 1.3 mm/min. The specimens weretested in a servohydraulic MTS 810 testing machine (MTSSystems Corporation, USA) at a constant deformation rateup to ultimate failure. All results were treated with theadvanced general aviation transport experiments procedure[28]. In this study, the discussion on the results was limitedto A-basis value which requires a minimum of 55 testspecimens per cure cycle condition.

3 Results and discussion

The results for the physical and mechanical properties aregiven in Table 2, where the most significant properties suchas void and fiber contents, cured laminate thickness, MVF,and interlaminar shear strength are listed. The physicalproperties were averaged over eight specimens. Table 2shows the effect of cure cycle on the physical propertiesand as a consequence, the interlaminar shear strength ofGLARE laminates.

The oven postcuring process (AO) is effective inreducing the void content and increasing the fiber content.Although a noticeable difference was not observed betweenautoclave and oven postcuring laminates, the fiber contentof the laminates manufactured by oven postcuring (57.23%,AO-2) was slightly higher than those by autoclave lami-nates (54.25%, FA). The lower fiber content in the ovenlaminate (FO) could correlate to an increase in the laminatethickness of the manufactured panels. But the oven post-curing could consolidate the plies as compared to autoclavecure. Another property of particular interest is the variation

of prepreg layer thickness between metal sheets. The meanthickness of each separate prepreg layer for autoclave curelaminates was 0.256 mm as shown in Fig. 5. Those for theoven postcuring laminate, however, were found to be 0.246(AO-1) and 0.243 mm (AO-2), respectively. This indicatedthat the oven postcuring laminates had greater uniformitythan those by either autoclave or oven counterparts. Theprepreg layer as present in the laminate should behomogenized to form a reliable GLARE laminate construc-tion. A noticeable difference in the MVF results was notfound.

Figures 6 and 7, which show typical cross-sectionalmicrographs, were taken at ×60 and ×100 magnificationsfor autoclave, autoclave consolidation followed by oven

Fig. 5 Mean thickness distribution of each separate glass/epoxyprepreg layer in GLARE2 5/4-0.4 laminate according to cureconditions (1st ply is bag side, 4th ply is tool side, see Fig. 1)

Table 2 GLARE2 5/4-0.4 panel properties fabricating from autoclave, autoclave consolidation followed by oven postcuring, and oven(percentage difference respect to the autoclave cure, FA)

Properties Full autoclave cure Autoclave consolidation followed by oven postcuring Full oven cure

FA AO-1 AO-2 FO

Physical propertiesa

Void content (%) 1.51±0.04 0.95±0.03 (−37.09%) 0.89±0.01 (−41.06%) 3.58±0.07 (137.09%)

Fiber content (%) 54.25±1.15 53.15±0.82 (−2.03%) 57.23±0.75 (5.49%) 51.26±1.03 (−5.51%)

Mean laminate thickness (mm) 3.05±0.05 3.02±0.03 (−0.98%) 2.86±0.02 (−6.23%) 3.19±0.04 (4.59%)

MVF 0.67±0.08 0.67±0.05 (0.00%) 0.71±0.02 (5.97%) 0.64±0.07 (−4.48%)

Mechanical properties (interlaminar shear strength)a

Mean (MPa)b 61.07±3.39 59.86±1.69 (−1.98%) 63.25±2.05 (3.57%) 55.09±2.17 (−9.79%)

C.V. (coefficient of variance)c 5.56% 2.83% 3.24% 3.93%

A-basis value (MPa) 54.83 53.74 (−1.99%) 56.78 (3.56%) 49.45 (−9.81%)

a Sample mean±sample standard deviationbMean value of 55 sampling sizec C.V. is sample coefficient of variation (in percent)

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postcuring, and oven, respectively. In these figures,porosity was shown as white particles, and most of voidswere spherical bubble geometry. The autoclave cure cycleproduces a low void laminate (1.51%), and this result canaccount for the effectiveness of pressurization with void

reduction. The presence of voids at the metal sheets–prepreg interface will create relatively high stress concen-trations at the interface during interlaminar shear strengthtest, leading to a premature failure of GLARE laminate. Itwas also noted that oven postcuring process, without the

Fig. 6 Cross-sectional views ofGLARE laminates at ×60magnification

Fig. 7 Cross-sectional views ofGLARE laminates at ×100magnification

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assistance of consolidation pressure for a longer time, couldlead to the comparable part’s quality with the lowest voidcontent (below 1.0%). To summarize, the physical propertyresults demonstrated the importance of cure cycle tailoringfor GLARE laminate cured via oven postcuring process.

A DSC experiment was used to characterize the curestatus of prepreg resin during autoclave consolidation stage.Figure 8 shows a comparison of curing exotherm anddegree of cure when the prepregs were subjected to the90°C and 100°C dwell temperatures. It was noted that theprepreg resin had undergone an exothermic chemical

reaction by measuring the degree of cure (α): 73% for90°C dwell and 90% for 100°C dwell, respectively. Theincrease in postcure temperature to 127°C has allowed foradvanced curing reaction to occur and further complete the

Fig. 10 Probability density function of interlaminar shear strengthautoclave cure of FA (⊙), autoclave consolidation with ovenpostcuring of AO-1 (■), AO-2 (●), and oven cure of FO (△)

Fig. 9 DEA curves during their respective autoclave consolidationstage simulation. a 90°C isothermal dwell. b 100°C isothermal dwell

Fig. 8 DSC analysis results. a Degree of cure ( ) and curing reactionrate (—) at 90°C isothermal dwell. b Degree of cure (⊙) and curingreaction rate (- - -) at 100°C isothermal dwell

Table 3 Comparison of glass transition temperature according toautoclave, autoclave consolidation followed by oven postcuring, andoven (percentage difference respect to the autoclave cure, FA)

Cure cycle Tg (°C)

Autoclave (FA) 131.25±1.12

Oven post-curing (AO-1) 129.45±0.98 (−1.37%)

Oven post-curing (AO-2) 131.08±0.99 (−0.13%)

Oven (FO) 131.06±0.86 (−0.14%)

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curing reaction (>95% degree of cure). A parameter whichis sensitive to the advancement of the cure process andindicative of the thermoset resin network density is theglass transition temperature. The comparable glass transi-tion temperature was measured in both case of autoclave(Tg=131.3°C) and oven postcuring (129.5°C for AO-1 and131.1°C for AO-2) processes as listed in Table 3. Thisindicated that autoclave cure cycle time reduction achievedwith oven postcuring, therefore, does not adversely affectthe degree of cure for the thermoset resin.

It is noteworthy that the resin viscosity throughout a curecycle plays an important role in the ultimate quality of thelaminate produced [29]. The lower viscosity over theduration of the cure cycle accounts for a lower voidcontent. The obtained DEA signals are shown in Fig. 9.The complex viscosity initially decreases with increasingtemperature prior to onset of curing reaction. The decreasein complex viscosity in this region is due to increase in themobility of component molecules. As curing reactionfurther proceeds, the complex viscosity passes through theminimum and then abruptly increases due to increasedmolecular weight and more extensive network formation.Figure 9a, b also highlighted the significance of loweredviscosity over autoclave consolidation period. An autoclaveconsolidation temperature allowed the viscosity to remainrelatively low over a longer period of time before theviscosity rose significantly.

Probability density functions of the interlaminar shearstrength for the variously fabricated laminates are shown inFig. 10. The mean strength and A-basis values are listed inTable 2. As expected, the apparent interlaminar shearstrength was reduced by increased void content. Thereduction in void content from 1.51% (FA) to 0.89%(AO-2) could account for 3.6% increase in interlaminarshear strength of AO-2-cured sample. This improvement inspecific interlaminar shear strength was postulated to bedue to the lowering of the resin viscosity over the durationof the cure, resulting in better wet through of fibers by resinand improved interfacial adhesion between prepreg–prepregand metal sheet–prepreg. The improved interfacial adhe-sion has been reported previously for GLARE laminatesby using interlaminar shear strength tests, for example[30, 31].

4 Conclusions

The objective of the current study is to manipulate the curecycle for producing GLARE laminate through a method ofautoclave processing time reduction. Two types of thermalanalysis were performed to provide a supplementary viewof the cure behavior and also incorporated to understand thevoids growth. Interlaminar shear strength tests were carried

out to support the evaluation of the proposed curingtechniques, and the results were correlated to the physicalproperties.

GLARE laminates cured by autoclave consolidation at100°C dwell with oven postcuring (AO-2) had the highestinterlaminar shear strength. The reduction in void contentfrom 1.51% (FA) to 0.89% (AO-2) could account for about3.6% increase in the interlaminar shear strength of AO-2laminate. This enhancement in interlaminar shear strengthvalue was attributed to an adhesion improvement at metalsheet–prepreg interface. As demonstrated using the ovenpostcuring process, a low viscosity during autoclaveconsolidation stage consolidates the plies with fiber wet-out and reduces the voids in a cured laminate. In addition,the interruption of the cure cycle in autoclave with anintermediate cool down resulted in shrinkage and collapseof voids, and this result is in congruence with earlier reportedresult for collapse of voids [11]. The results presented in thisstudy underline the potential of new curing technique inimproving GLARE laminate processing and manufacture.This proposed curing technique could be as a method ofautoclave processing time reduction by 53% (or greater)with comparable qualities and properties.

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