CURING OF THICK ANGLE-BEND THERMOSET COMPOSITE PART: CURING PROCESS MODIFICATION FOR UNIFORM THICKNESS AND UNIFORM FIBER VOLUME FRACTION DISTRIBUTION

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1 CURING OF THICK ANGLE-BEND THERMOSET COMPOSITE PART: CURING PROCESS MODIFICATION FOR UNIFORM THICKNESS AND UNIFORM FIBER VOLUME FRACTION DISTRIBUTION Malak I. Naji and Suong V. Hoa Conordia Centre or Compoite Department o Mehanial Enigneering Conordia Univerity Montreal, Quebe, Canada H3G 1M8 Summary: Thermoet ompoite part with angle-bend are more diiult to ure a ompared to traight part. The urvature at the bend uually reate iber miro-bukling, non uniorm iber ditribution aro the thikne, non uniorm iber ditribution along the length o the part, exitene o void, et. The proedure normally ued or the manuaturing o traight ompoite part thereore need to be modiied or part with angle bend. Thi paper preent the work done in the development o modiied proedure or the manuaturing o good thermoet part with angle bend. The work onit o two apet: numerial modeling and experimental. For numerial modeling, inite dierene method wa ued to model the governing equation and the urved geometry. A peial boundary ondition wa alo developed to handle the eet o bleeder material. Modiied proedure were developed rom the reult o the numerial model. Sample were manuatured uing the modiied proedure. AS4/ graphite/epoxy material were ued. Part with muh improved quality, a ompared to part made uing the manuaturer' reommended proedure, were obtained. Uniorm thikne ditribution and uniorm iber ditribution in the part made with modiied proedure were obtained. Keyword: angle-bend, thermoet, uring proe modiiation, uniorm iber ditribution, uniorm thikne. INTRODUCTION Autolave proeing i a manuaturing method ued to produe high perormane ompoite part. Varying temperature, preure and vauum yle in the autolave lead to dierent ure proee whih will end up with dierent inal part quality: (1) The degree o ure and the iber volume ration aet the mehanial propertie o the inal ompoite part. (2) The preure ditribution aet the ompation o the laminate and the void ormation. (3) Temperature gradient an introdue reidual tree and train. (4) Finally, or ot-eetive proe, the duration o the proeing yle hould be hort. There have been many tudie on the uring o thik thermoetting ompoite material. In Reerene [1,2], reearher developed ure imulation model that predit temperature and 1

2 degree o ure ditribution within the part a a untion o the autolave temperature hitory. Reearher who onidered the rein low model inlude Hojjati [3] who tudied uring o thik lat etion part, and Johnton et al [4] who howed, or urved etion, that rein low wa uneven reulting in non-uniorm part thikne. Alo, [4] ound that pringbak angle i inluened by two ator: hoie o tool material, and urae rition ondition. Many other worker a in [5,6] alo perormed tudie to ome up with optimized uring yle or their ompoite part. In a previou work [7], we howed how the reommended uring yle reulted in thikne and iber volume ration variation along the length and thikne o an angle bend ompoite part (Fig. 1). In thi work, a modiied uring proe wa developed that minimize the patial variation in thikne and iber volume ration along with the gradient in temperature and degree o ure. 2-tep and 3-tep uring yle were implemented experimentally and theoretially to tudy the variation in thikne and iber volume ration o an angle-bend piee made rom 50 layer o graphite/epoxy Herule AS4/ prepreg (ply thikne i approximately 0.16 mm and ha 36% rein ontent by weight). THICKNESS AND FIBER VOLUME FRACTION VARIATION To get a inal part that ha a onitent quality in term o thikne and v ditribution, the uring proe itel hould be altered. Reerene [8] wa the only one ound that mentioned how they ontrolled the abriation ondition to ahieve uniorm thikne in the orner o a lange-web. The peimen were manuatured uing hard tool on both ide. The oniguration o our lay up i hown in Fig. 2. In our ae, ine the lowet v value wa ound to be preent at the urved bend adjaent to the mold urae, then inerting a bleeder layer between the mold and the ompoite part prior to proeing would allow rein to eape at that urae and hene inreae the v value. Conequently, thi will reult into a lower variation in thikne rom etion to etion. (Thi ae will be reerred to a Cae (I).) Alternatively, adjuting the number o perorated releae ilm on the upper urae will allow ontrolling the amount o rein low out o the part. However, thi will produe a uniorm thikne but will not give a uniorm v ditribution. (Thi ae will be reerred to a Cae (II).) To imulate the preene o the lower bleeder layer and/or the upper perorated releae ilm, a generalized boundary ondition ormulation in term o a Robin preure boundary ondition wa impoed. Formulating preure boundary ondition or the Dirihlet or Neumann ae i a traight orward proedure. When preure value at the urae boundarie P are known, their value are diretly applied. Alo, when no low our at a urae boundary that ha outward P unit normal nˆ, then i et equal to zero. However, when low proeed through dierent nˆ media (a in the ae or rein low out o the laminate through the perorated releae ilm, bleeder and breather layer) and the preure value at thi boundary hange with rein build-up, neither Dirihlet nor Neumann ae will ueed in repreenting the real ituation. Thereore, a generalized boundary ondition ormulation or preure mut be derived or the ollowing reaon: 2

3 1. to avoid the aumption o "ree bleeding", i.e. P = 0 that i aoiated by exluding the eet o the bleeder on the rein low behavior 1, 2. to overome diiultie in modeling the detailed low at the perorated releae ilm, bleeder and breather layer where rein would bleed out o the laminate reely at irt, and then will be retrited when the dierent loth are illed with rein 2, and 3. to quantiy the ombined reitane o releae, bleeder and breather material to low at that boundary. To ormulate a generalized boundary ondition or preure a well, Dary' law o permeability will be ued. Rein low rate at the laminate urae aording to thi law, i written a [10,11]: q n S p = µ P Sb P = nˆ µ nˆ (1) where S i the permeability in the nˆ diretion, and ubript p and b reer to the prepreg material and the bagging material (perorated releae ilm, bleeder and breather layer), repetively, a hown in Figure 3. The eond term in Equation 1 an be approximated by: Sb P µ nˆ S P b = µ P g (2) or Sb P µ nˆ where = ( P P ) Sb = µ g (3) i an average low oeiient that i analogou to the average heat traner oeiient h. i the thikne o the dierent bagging material. Hene, ombining equation 1 and 3, rein low an be written a: S p µ P nˆ = ( P P ) g (4) 1 A proedure ollowed by mot reearher. 2 A ituation that wa addreed in Reerene [9], and olved by etting P = 0 at the laminate top urae and P = 0. 5MPa at the edge. 3

4 or P = nˆ where ( F ) ( P P ) e ( F ) e g Sb = S p (5) h ( F ) e i an eetive low oeiient that i analogou to ued in heat low equation. k e When the value o ( F i mall, then the reitane to low rom laminate to boundary layer i ) e high, and vie vera. Now uing equation 5 will enable u to ormulate a generalized boundary ondition or preure, the ame a temperature. Thi i expreed a ollow: P a + bp + Pg = 0 or P, nˆ ( x z) on D (6) The three dierent boundary ondition that may be enored on the boundarie are ummarized in Table 1 below. Coe. a b Dirihlet Neumann Robin 1 ( ) e F ( F ) e Table 1: Generalized Boundary Condition Coeiient Figure 4 how the implementation o the generalized temperature and preure boundary ondition or the kind o lay-up that wa hown in Figure 2. For Cae (I), the boundary ondition will be: 1. Temperature: h! Top urae: = 10m 1 k e! Bottom urae: T = T (t)! Side urae: T = T (t) a a where T a (t) i the autolave temperature (ure yle). 2. Preure:! Top urae: ( ) = 1000 e ( ) = 100 F or 1 perorated releae ilm, F or 2 perorated releae ilm e 4

5 P! Bottom urae: = 0 η P! Side urae: = 0 η where the autolave preure wa P a = MPa and the bag preure wa P g = 0 Pa. Figure 5 how the model reult o applying thi preure boundary on the bottom urae Cae (I). A hown, the thikne variation wa ontrolled and kept to a negligible value. Alo, a expeted, the v variation wa negligible or Cae (I) ( v 68% ) a hown in Figure 6. COMPARISON WITH EXPERIMENT Experiment were done in order to invetigate the modiied ure proe uggeted above. A 50 layer ample wa prepared rom Herule AS4/ prepreg and ured uing the 2-tep modiied uring proe. Two (2) perorated releae ilm were laid at the top urae o the laminate. Two bleeder layer were laid at the top urae o the mold (under the laminate). One over the mold urae rom φ = 20! to φ = 70!, the other one over the whole urae. The inal thikne ater uring wa almot 7mm a hown in Figure 7. CONCLUSIONS Thi reearh demontrated the ability o getting a uniorm iber ditribution aro the thikne o a part with a urved angle-bend hape. By introduing a generalized preure boundary ondition in the imulation model, the real behavior o rein low wa modeled. Thikne variation over the part length were negligible and a uniorm thikne wa obtained. Thi wa poible though modiying the uring proe by inorporating bleeder layer at the lower urae between the mold and the ample. Thi extra layer wa able to aborb the rein and hene dereae the thikne and inreae the iber volume ration. However, the urae inih wa not o the ame appearane a that o a part proeed without thi extra layer. REFERENCES 1. Twardowki, T.W., Lin, S.E. and Geil, P.H., "Curing in Thik Compoite Laminate: Experiment and Simulation," J. Compoite Material, Vol. 27, 1993, pp Bogetti, T.A. and Gillepie, J.W., "Proe-Indued Stre and Deormation in Thik Setion Thermoetting Compoite Laminate," 21t SAMPE Tehnial Conerene, New Jerey, Sept Hojjati, M. and Hoa, S.V., "Curing Simulation o Thik Thermoetting Compoite," Compoite Manuaturing, Vol. 5, No. 3, 1994, pp Johnton, A., Hubert, P., Fernlund, G., Vaziri, R. and Pourartip, A., "Proe Modeling o Compoite Struture Employing a Virtual Autolave Conept," Siene and Engineering o Compoite Material, Vol. 5, 1996, pp Ciriioli, P.R., Wang. Q. and Springer, G.S., "Autolave Curing - Comparion o Model and Tet Reult," J. Compoite Material, Vol. 26, No. 1, 1992, pp

6 6. Kim, J.S. and Lee, D.G., "Development o an Autolave Cure Cyle with Cooling and Reheating Step or Thik Thermoet Compoite Laminate," J. Compoite Material, Vol. 31, No. 22, 1997, pp Naji, M.I. and Hoa, S.V., "Curing o Thik Angle-Bend Thermoet Compoite Part: Curing Cyle Eet on Thikne Variation and Fiber Volume Fration, " Submitted to Journal o Reinored Plati and Compoite, April Kan, H.P., Bhatia, N.M. and Mahler, M.A., "Eet o Poroity on Flange-Web Corner Strength," Compoite Material: Fatigue and Frature, Vol. 3, 1991, pp Young, W.-B., "Compating Preure and Cure Cyle or Proeing o Thik Compoite Laminate," Compoite Siene and Tehnology, Vol. 54, No. 3, 1995, pp Atrom, B.T., Pipe, R.B. and Advani, S.G., "On Flow through Aligned Bed and it Appliation to Compoite Proeing," J. Compoite Material, Vol. 26, No. 9, 1992, pp Gebart, B.R., "Permeability o Unidiretional Reinorement or RTM," J. Compoite Material, Vol. 26, No. 8, 1992, pp ! φ( ) Figure 1: Thikne variation along urved part proeed with the reommended 2-tep and 3-tep ure yle [7]. 6

7 Figure 2: The laminate geometry and lay-up equene ued. 7

8 Figure 3:Rein low out o the laminate through the ombined releae, bleeder and breather material. Figure 4: Temperature and preure boundary ondition. 8

9 Figure 5: Thikne variation along the urved part or Cae (I). Figure 6: v variation aro the thikne obtained rom experiment or the 2-tep modiied proe. 9

10 Figure 7: Thikne variation along the laminate length obtained rom the 2-tep modiied. ure proe Cae (I) experiment. 10