SIMULAITION AND ANALYSIS OF PROCESS-INDUCED DISTORTIONS FOR COMPOSITE STRUCTURES

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1 21 st Intenational Confeence on Composite Mateials Xi an, th August 2017 SIMULAITION AND ANALYSIS OF PROCESS-INDUCED DISTORTIONS FOR COMPOSITE STRUCTURES LIU Kai *1, ZHANG Boming 1, YE Jinui 1, Tang zhanwen 1, Jiang tian 1 1 School of Mateials Science and Engineeing, Beihang Univesity,Beijing , China liukai96200@yeah.net, web page: Keywods: Composite stuctues, Pocess-induced distotions, Finite elements ABSTRACT Pocess-induced distotions (PID) of fibe-einfoced polyme matix composites bing many poblems to pats assembly. A multi-level simulation method fo cue pocess of composite integated stuctue was developed to simplify calculation and impove accuacy of pediction. Panels and stiffened panels wee manufactued by autoclave pocess to veify the simulation method. It is found that numeical pedictions compaed quite well with expeimental measuements. Futhemoe, PID investigations wee caied out fo themoplastic composites poduced by themofoming pocess. Both pocess featues and mateial chaactes wee incopoated in simulation. Concening hemisphees with symmetic lay-ups, stamping induced stesses play a dominant ole in PID and detemine the defomation tend; fibe eoientations have an impotant effect on the themally induced defomations. 1 INTRODUCTION Advanced composite mateials have been widely used in aicaft, automobile and spoting goods. [1] Integated pocess is an impotant technique fo composites to educe pats and fastenes and meet the needs of stuctue pefomance. Cue-induced defomation is a key poblem to impove pocess quality of lage-scale and complex integated stuctue made of composite mateials. Fo educing cue-induced defomation, conventional method is to adjust and coect the skin of mold and cue system again and again accoding as expeiences and pocess tests. But this kind of tial and eo method inceases fabication cost. So it is impotant to develop a simulation method of pedicting cue-induced defomation fo composite stuctues, to guaantee poduct quality and educe fabication cost. [2] Investigations have been developed into cue-induced defomation and esidual stess distibution fo composites. Yi [3] et al. developed a nonlinea finite element model to analyze the esidual stesses in themoset composites duing cool-down afte cuing. Lee and Sohn [4] added the esidual stesses in the cuing peiod to the themal stesses in the cool-down peiod to make up the total esidual stess. Yoon and Kim [5] eseached the effect of themal defomation and chemical shinkage on the pocessinduced distotion of cabon/epoxy cuved laminates. Nui Esoy and Kevin Potte [6] consideed that composite pats manufactued by autoclave pocesses had manufactuing distotions due to unavoidable physical mechanisms that took place duing the pocess. Kim and Daniel [7] applied a moe diect method to measue the stain in the cue pocess. They used fibe Bagg gating and esistance stain chip buied inside the composite mateials to measue the changing of the stain. It was found that the ecipocity between mold and pat leads to evident stain in the cue pocess. Satish [8] applied shea laye to eplace the influence of mold on the composite mateial by simulation. The method can accuately pedict the cuing defomation geneated by the ecipocity between mold and pat. In this study, a multi-level simulation method fo cue pocess of composite stuctues was developed. Seveal simplified models wee involved in multi-level simulation method, including equivalent tempeatue load model and shea laye model. Themal defomation, cue shinkage and *Coesponding autho: LIU Kai. Tel.: ; addess: liukai96200@yeah.net.

2 LIU Kai, ZHANG Boming, YE Jinui mold effect wee consideed in analysis. Panels, panels and sandwich stuctues wee made to veify the simulation method. Futhemoe, PID investigations wee caied out fo themoplastic composites poduced by themofoming pocess. Both pocess featues and mateial chaactes wee incopoated in simulation. 2 MULTI-LEVEL SIMULATION METHOD Figue 1: Multi-level simulation method to pedict cue-induced defomation In multi-level simulation method, cue-induced defomation of diffeent kinds of composite stuctues was investigated in ode of stuctue complexity. Cue-induced defomation of complex stuctues was pedicted based on solutions obtained fom studies of simple composite stuctues. Additional factos wee taken into model to adapt to stuctual chaacteistic when the studied composite stuctue became moe complex. Totally, thee factos wee taken into consideation, including themal shinkage, cue shinkage and mold effect. But the defomation induced by these thee factos had diffeent atio with espect to diffeent kinds of stuctues. The objective of multilevel simulation method is to pedict cue-induced defomation of eal complex composite stuctues, so the model used must be as simplified as possible. Theefoe, subodinate effect factos could be neglected to each a balance between pedicting accuacy and time cost. And some semi-empiical models ae suitable to be selected. a) Equivalent tempeatue load model In autoclave pocessing, the shinkage of composite mateial (ε total ) consists of two pats: cue shinkage (denoted as ε h ) aoused by cue eaction of esin and themal shinkage (denoted as ε ) taking place in cooling stage afte esin was cued completely, as descibed in equation (1). [9] total h (1) ε depends on the themal expansion coefficient of composites. ε h is elated to the complex cue eaction of esin and is difficult to simulate. A semi-empiical model was poposed to egad ε h as an equivalent tempeatue load. In equivalent tempeatue load model, complex cue eaction is not taken into consideation and a lot of iteative computations wee avoided. T (2) h h T (3) then total ( T T h ) (4) whee α epesents the themal expansion coefficient of composites that have cued completely. T epesents the diffeence between cue tempeatue and oom tempeatue. T epesented the equivalent tempeatue diffeence. As h / T h / T (5)

3 21 st Intenational Confeence on Composite Mateials Xi an, th August 2017 then T h T h / (6) h / could be obtained fom fibe Bagg gating expeiments. Finally, the equivalent tempeatue diffeence was detemined accoding to equation (6). Figue 2: Bagg gating expeiment esults b) Shea laye model Thee is a lage body of evidence confiming the occuence of tool pat inteaction. The themal expansion coefficient diffeence between composites and mold will contibute to cue-induced defomation. A shea laye was added to the suface of composites in tough with mould. The shea laye is pat of the composites and acts in the tempeatue ising peiod afte the gelling point of composites. Because of the shea laye, thee was a stess gadient in the thickness diection of composites. Shea laye has two paametes: thickness (t s ) and themal expansion coefficient (α s ). Shea laye paametes (t s and α s ) wee detemined fom unidiectional laminate expeiments ([0] 3, [0] 4, [0] 5, [0] 6 ). Figue 3: unidiectional laminate expeiments Figue 4: ts and αs detemination

4 LIU Kai, ZHANG Boming, YE Jinui 3 SIMULATION VERIFICATION 3.2 Laminates by autoclave pocess Figue 5: Asymmetic panels Label Stack sequence Expeimental maximum wapage /mm Simulation /mm Eo /% 1 [45/90 2 /-45 2 /0 2 /45] [0 3 /45 4 ] [0/0/45/45] [0/0/45/0] [0/90/-45 2 /90/0/45 2 ] [0/0/45/-45] [90/90/45/45] [90/45/45/0] [90/90/90/45] [90/90/45/-45] Table 1: Simulation and expeimental esults of asymmetic panels Ten laminates with asymmetic stack sequence wee manufactued to veify the multi-level simulation method. Laminate geomety was 150*75 mm. Simulation and expeimental esults ae shown in Table 1. Value of cue induced defomation lied between 2 to 30 mm. Pedicting accuacy of multi-level simulation method was 85% moe o less. 3.3 Stiffened panels by autoclave pocess Unit:mm Figue 6: geomety of stiffened panels.

5 21 st Intenational Confeence on Composite Mateials Xi an, th August 2017 Figue 7: Simulation and actual defomation of stiffened panels unde co-cue Figue 8: Simulation and actual defomation of stiffened panels unde co-bonding with cued stinge and uncued skin Figue 9: Simulation and actual defomation of stiffened panels unde co-bonding with cued skin and uncued stinge Pocess The maximal displacement of skin Eo% Expeimental Simulation Co-cue Co-bonding(cued stinge) Co-bonding(cued skin) Table 2: The maximal displacement of skin at diffeent cuing scheme Simulation and expeimental esults of stiffened panels ae displayed in Figue. 9-11and Table. 3. Defomation tend pedicted by multi-level model is consistent with expeimental esults. Pediction accuacy is moe than 80%. With egad to stiffened panels, a special poblem is cuing scheme. Diffeent cuing schemes lead to defomation vaiation, but this kind of diffeence is limited. A stess tansfe model was utilized to descibe the effect of cuing scheme on defomation. [10] Due to limited space, the stess tansfe model is not intoduced hee. 3.3 themoplastic composite hemisphees by themofoming pocess Both pocess featues and mateial chaacteistics should be embedded in PID calculation. In the past decades, most studies on the PID subject wee caied out fo themosetting composites manufactued by autoclave pocess, RTM, hot pess etc. Reseaches on themoplastic composites ae scacely available, especially those on themoplastic composite stuctues poduced by themofoming. Diffeent fom taditional pocesses, the 2D blank in themofoming is stamped into a 3D shape by extenal foces. Fibe eoientations may take place if doubly-cuved stuctues ae encounteed and

6 LIU Kai, ZHANG Boming, YE Jinui stamping induced stesses ae peseved in the stuctues. Thus, a pioi, those featues may have an influence on the PID of composites. Uniaxial pepeg, appoximately 0.25 mm thick, composed of a poly-popylene (PP) homopolyme matix containing 60% volume faction E-glass fibes, has been used. The themofoming setup is shown in Fig. 10 (a). Befoe themofoming, the pepeg was cut into 240*240 mm squaes and stacked into a blank, which was attached to the blank-holde fame by spings using clips. The stacking consequence of the blank was [0/90/0/90]s. Figue 10 Fig. 1 (a) themofoming set-up; (b) hemispheical mold; (c) blank-holde An example of a themofomed pat is shown in Fig. 11(a). Because of the anisotopy of the composite mateial and the effect of the spings, the pat takes on a petal shape. It was finally timmed into a hemisphee, as shown in Fig. 11(b). Obvious pocess-induced distotions ae obseved. The coss-section pofile of the hemisphee is not the same as the mold shape. It is an ellipse athe than a cicle, with a majo axis 124.5mm (U=0.25 mm) in the 0 diection and a mino axis 123 mm (U=-0.5 mm) in the 90 diection. PID is denoted as U, which is calculated fom the axis (L). Outwad defomation occus when U is positive and inwad distotion is epesented by negative U. U ( L 124 mm) / 2 (7) Figue 11 (a) a themofomed pat; (b) a finally timmed hemisphee; (c) the coodinate to descibe defomations; (d) fibe oientations in 45 diection; (e) shea angle measuement; (f) fibe oientations in 0 diection.

7 21 st Intenational Confeence on Composite Mateials Xi an, th August 2017 Figue 12 (a) pedicted distotions of a themofomed pat befoe timming; (b) simulation esults of PID of a hemisphee; (c) the pedicted PID neglecting stamping esidual stesses; (d) the pedicted PID neglecting both stamping esidual stesses and fibe eoientations. (Defomation scale facto 10) It is shown in Fig. 11 that obvious fibe eoientations occu afte themofoming. In the diection of 0 and 90, the shea angle is appoximately equal to zeo, showing that no shea defomation takes place in these egions; in the diections of +45, the absolute values of shea angles each thei maximums, shea defomation takes place in busts in these egions. To evaluate the effect of FR quantitatively, the total pocess induced distotions (PID) ae divided into themally induced defomation (TID) and stamping induced defomation. Fig. 12 (c) demonstates the themally induced distotions of hemisphees. Obvious TID is obseved and vaies with diffeent diections. Howeve, unifom shinkage manifests and TID is in a negligibly small amount when fibe eoientations ae neglected, as demonstated in Fig. 12(d). This is because both the stiffness popeties and the expansion behavios of the hemisphee change locally as a function of the fibe eoientation. An inhomogeneous distibution of these popeties leads to non-unifom shinkage and hence to futhe poduct distotions. This non-unifom shinkage plays a cucial ole fo TID of symmetic layup hemisphees. It is poved in Figue 12(b) and Figue 11(b) that the simulation method could accuately pedict shape distotions of hemisphees. In the 0 diection, the contou of the hemisphee expands outwad. Howeve, the contou of the hemisphee spings inwad if the stamping esidual stesses ae ignoed. Theefoe, stamping induced stesses play a dominant ole in PID fo hemisphees with symmetic layups and detemine the defomation tend. 4 CONCLUSION A multi-level simulation method fo cue pocess of composite integated stuctue was developed to simplify calculation and impove accuacy of pediction. Equivalent tempeatue load model and shea laye model wee intoduced. A mass of expeiments wee caied out to veify the multi-level simulation method, including panels, L-shape beams, stiffened panels and sandwich stuctues. Results show that the adopted simulation method can be used to pedict the cue-induced defomation fo diffeent kinds of stuctues, with accuacy of 80%.

8 LIU Kai, ZHANG Boming, YE Jinui REFERENCES [1] R. J. Mogan, E. E. Shin, J. Lincoln, et al, Oveview of polyme matix composites pefomance and mateials development fo aeospace applications, SAMPE Jounal, 37, 2001, pp [2] Yue Guangquan, Study on simulation and contol method of cue-induced defomation fo integated composite panel [D], Habin, Habin Institute of Technology, [3] S. Yi, K. S. Chian and H. H. Hilton, Nonlinea viscoelastic finite element analyses of themosetting polymeic composites duing cool-down afte cuing, Jounal of Composite Mateials, 36, 2002, pp [4] S. S. Lee and Y. S. Sohn, Viscoelastic analysis of esidual stesses in a unidiectional laminate, Stuctual Engineeing and Mechanics, 4, 1994, pp [5] K. J. Yoon and J. S. Kim, Effect of themal defomation and chemical shinkage on the pocess induced distotion of cabon/epoxy cuved laminates, Jounal of Composite Mateials, 35, 2001, pp [6] N. Esoy, K. Potte, M. R. Wisnom, An expeimental method to study the fictional pocesses duing composites manufactuing, Composites Pat A: Applied Science and Manufactuing, 36, 2005, pp [7] Kim, Daniel, Cue cycle effect on composite stuctues manufactued by esin tansfe molding, Jounal of Composite Mateials, 36, 2002, pp [8] K. B. Satish and V. S. Lloyd, A linea finite element model to pedict pocessing-induced distotion in FRP laminates, Composites Pat A: Applied Science and Manufactuing, 36, 2005, pp [9] Tang zhanwen, Zhang boming, Pediction of cuing defomation in integated design and manufactue of composites, Aeonautical Manufactuing Technology, 15, 2014, pp [10] Jiang tian, Zhangboming, Simulation and veification of cue-induced defomation by stages fo integated composite stuctue, Acta Mateiae Compositae Sinica, 30, 2013, pp