AN APPROACH TO CURE THICK FIBER REINFORCED COMPOSITE LAMINATES.

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1 AN APPROACH TO CURE THICK FIBER REINFORCED COMPOSITE LAMINATES. Mr. K. Vijaya Kumar 1, Dr. Mir Safiulla 2, Dr. A.N Khaleel Ahmed 3 1 Research Scholar, Gousia College of Engineering. Ramnagaram., INDIA vijaykmr3764@gmail.com 2 Prof. & Head (R&D), Department of Mechanical Engineering, Gousia College of Engineering.INDIA 3 Prof. & Head, Department of Mechanical Engineering & Vice Principal, KNS Institute of Technology, Bangalore.INDIA, khaleeldme@yahoo.com Abstract Autoclave curing process of carbon fiber reinforced thick composite structures is complex and involves multiple intermediate precompaction processes to achieve high quality and reliable parts. Even today it is observed the thick composite structures are manufactured in several stages with precompaction process in autoclave or in oven for every 12 layers ( 3 mm thickness) which results in longer lead time and higher production cost, hence this paper focus to develop a single process to cure these thick parts made of Hexply G939 carbon epoxy prepeg layers so as to reduce the lead time and cost. This developed cure cycle avoids the intermediate precompaction process and found an appreciable savings up to 20% of the curing cost per part with the same quality. Keywords: thick laminates, precompaction, composite, cure cycle, prepeg 1.INTRODUCTION The fiber reinforced composite structures are extensively used in aerospace ( civil and military ), automobile sectors and in infrastructure ( buildings and bridges) due to the appreciable weight savings and high Specific strength. In future the thick composite monolithic parts play a vital role in the applications that require thick sections like satellite platform boards, stiffeners, columns, and foot bridges etc. That can meet demanding service conditions [1]. There are several inherent difficulties in processing of thick thermosetting polymer composites because of their low thermal conductivity across the thickness and the presence of voids encountered during the manufacturing of these composite structures [2-3]. During a typical cure cycle the interior temperature of a thick section may lag far behind the surface temperature, causing the surface regions to cure more quickly. If they cure too quickly and gel before the interior is consolidated, the excess resin in the interior is trapped. This leads to nonuniform fiber volume distribution and excessive void growth in the resin-rich interior. In order to efficiently manufacture quality parts, on-line control and process optirnization are necessary, which in turn require a realistic model of the entire process [4]. Earlier research work has been done to predict and to analyze the exothermic behavior of Cytech glass fiber composites and also research has been done to cure these thick composite laminates with multiple heating and cooling steps which requires more time and continuous monitoring to control the sequence of heating and cooling operations practically during curing, earlier research has not defined the exact parameters like the heating rate at different stages of curing, pressure application rate, exact time of applying the pressure and cooling rate at different stages of curing. This leads to a lack of information to apply this work practically. Less information is available to cure thick laminates made of Hexcel 939 Carbon epoxy prepeg layers. Perhaps the cure cycle developed found to be satisfactory for one material in some conditions, may not be appropriate in other conditions, in particular for thick composite structures. The variation in material properties due to laminate thickness can partially be attributed to differences in processing conditions [5], hence there is a need to propose an optimum cure cycle for curing thick G939 carbon epoxy composite structures since the existing recommended two step cure cycles will not suit for the curing of thick composite structures. 2. EXPERIMENTAL WORK Laminates were fabricated using Hexply 913/ G939 carbon prepegs as shown in the table 1. Along with these laminates a reference laminate of 2 mm thickness is prepared and cured as per the existing two step cure cycle shown in fig 1. and tested for DT & NDT as per the DIN standards. Table1. Details of the laminates. 158

2 Temperature in De Mr. K. Vijaya Kumar et al. / IJAIR ISSN: Laminate No. No. of layers Laminate Thickness in mm Laminate Laminate Laminate Laminate Laminate It is observed that the thermocouple connected to 50 layer laminate is lagging than all the other thermocouples as shown in cure chart fig. 3, later at 110 deg. C it has drastically shot up indicated exothermic reaction, thereby the curing process is stopped and the laminates were sent to the DSC evaluation to analyze the enthalpy reactions. After evaluating the DSC values it is found the laminate with 50 layers has a peak exothermic reaction. Fig.1 Existing two step cure cycle. ( for < 3 mm thick parts ) Time in Minutes Fig.2 Developed multistep cure cycle for 40 layers ( for 10mm thick parts) 2.1 Curing of laminates - Trial -1 Multi Step Cure Cycle A multiple cure cycle is designed as shown in the above fig 2. with intermediate dwells and the curing parameters are calculated and set as follows. Exothermic Reaction Fig 3. Cure Chart of Trial -1 curing 2.2 Curing of laminates - Trial -2 In this trial the similar specimens of 8, 20, 30 & 40 layers were prepared and in this trial the thickness is restricted up to 40 layers. The curing is done as per the similar cure cycle as designed for the trial -1 but the heating rate during the ramp up time between 90 deg.c to 110 deg.c has been reduced from 1.6 deg.c to 1.0 deg.c since as the heat up rate is more the material exposes to sudden exothermic reactions and it is observed that still the exothermic reaction noticed in 40 layer laminate as it reaches to 110 deg.c and over a period of time it has increased drastically up to 120 deg.c which is the peak cure set point for the 913 resin and the thermocouple reached back to 110 deg.c over a period of time within 8-10 min which indicates that the reaction is completed and then the temperature raised to 135 deg.c and cured as shown in the cure chart fig.4, After curing the laminates were tested for DT and NDT. Heating rate : 1-2 deg.c/min Dwell Time : First dwell : 20 min at 75 deg.c, dwell : 25 min at 90 deg.c, Third dwell : 20 min at 110 deg.c, Fourth dwell : 40 min dwell at 135 deg.c. Second Air Pressure : 1 bar up to 75 deg. C and increased to 2 bar and then increased to 3 bar after first dwell throughout the cure cycle. Cooling rate : 1-2 deg.c/min.( The low cooling rate is better to relieve residual stresses completely [6]) The autoclave is programmed as per the above parameters and the laminates are cured., Exothermic Reaction Fig. 4 Cure Chart of Trial -2 Curing 2.3 Curing of laminates - Trial

3 Since in the second trial as the laminate with 40 layers experienced the exothermic reaction at 110 deg.c again the analysis has made to rectify this with an attempt to cure the laminates with less heating rate in trial -3, hence again the laminates were prepared and cured with low heating rate between 90 deg.c to 110 deg.c with deg.c/min there by the curing is completed as per the cycle, it is observed that the curing is completed without any exothermic reaction as shown in the cure chart fig.5, this proved that the thick laminates up to 40 layers ( 10 mm) can be cured at a stretch without precompactions. Where the F = Peak Load, Newton s, b = Width of the specimen, mm & t = Thickness of the specimen, mm The table.2 below shows the ILSS values of the specimens cured with the developed multi step cure cycle and the two step cure cycle. Specimen No. Multi Step Cure Cycle ILSS ( mpa) Two Step Cure Cycle ILSS ( mpa) Fig.5 Cure chart for the trial -3 curing. 3. RESULTS & DISCUSSIONS After curing the laminates were tested for visual and NDT tests like A-Scan and the Ultrasonic C-Scan to check for the voids, internal cracks, FODs, etc. By the Ultrasonic C scan report it is observed that the DB variation is uniform throughout the thickness and ensured the absence of any internal defects and then the specimens were cut in dimensions of 20x10x2mm as per the DIN ( Dutch Institute for Norms (DIN-29971)) Standards to test the ILSS ( Interlaminar Shear Strength ) values multi step two step Fig. 6 ILSS values of Multi step and two step cure cycle. Length L= 20 mm width b= 10 mm, thickness t= 2 mm Fig.7. Ultrasonic C scan View of Master specimen ILSS CALCULATIONS : Strength = 3/4 F / ( b x t). N/mm² ILSS - Inter Laminar Shear 160

4 Process time in Hrs Autocl ave curing cost in Set up cost Cost Ist stage precompaction after 12 layers ,500 IInd stage Precompaction after 24 layers ,500 IIIrd stage Precompaction after 36 layers ,500 Final curing after 40 layers ,500 Cost 32,000 /- Fig.8 Ultrasonic C scan View of the specimens Fig. 7 & 8 shows the ultrasonic C scan view of the Master laminates and the laminates cured with the Developed Multistep cure cycle, it is observed that the Values are within the limits. Thereby it is evident that There are no voids and the internal defects in the cured Laminates. Process Multi step curing for 40 layers time in Hrs Autoclav e curing cost in Rs. Set up cost Cost ,500 Cost 12,500 /- 3.1 Cost Analysis In two step cure cycle shown in fig. 1 consists of two ramps and dwell stages which are used generally to cure the composite structures within (12 15 layers) 2 to 3 mm thickness and takes almost 6 hours to complete the process. But the same cure cycle is not used for curing thick parts which are above 6 mm thickness like Hub Plate, Spar pack, Wedge Packs, flex beam etc of the advanced light choppers due to the chances of voids and the exothermic reaction during curing. Hence after every 12 layers it needs to be pre-compacted in the autoclave under pressure. This process takes almost 3-4 hours to complete. After precompaction, again 12 layers will be laid up and taken for the second stage pre-compaction in the autoclave and then taken for the final curing. Hence the total curing cost of the existing procedure for curing a part consisting of 40 layers is about Rs. 32,000/- Rs. And the duration is 17 hrs as tabulated in the table. 3, the same can be implemented in the developed cure cycle and the total cost is reduced to only Rs. 12,500/- and the total time taken is only 6.5 hrs. as shown in the table 4. Table.3 Cost analysis of the existing process Table.4 Cost analysis of the Multistep process Savings: 1) Curing process duration reduced up to: = 10.5 Hrs. 2) Curing process cost reduced up to: 32,000-12,500 = Rs 19,500 /- 4. CONCLUSIONS 1. The cure cycle developed will be used to cure the 913 resin thick composite components up to 40 layers efficiently. 2. About 60% of the cost and 61% of lead time is saved with the developed cure cycle compared to the existing procedure of curing thick parts. 3. The dwell time to be increased up to 70 mins at 135 deg. C to achieve better quality& complete curing of intricate shaped composite structures. 4. The cooling rate during the cooling stage should be kept low as the thickness of the composite structure increases to relieve residual stress completely. 161

5 5. The pressure is increased from 3 bar to 5 bar after 90 Dec. C for better strength and the compaction of the thick composite parts. 6. For curing more than 40 layers the cure cycle consisting of more dwell steps and the more dwell time in the initial steps with a lower heating rate is developed. 7. The 913 resin carbon components are always prone to exothermic during Dec. c there by the thermal gradient (heating rate) & the pressure application plays a vital role during this stage. 8. Vacuum can be reduced to minimum after applying the maximum external pressure during the cure cycle. 9. It is necessary to establish a minimum acceptable criteria of voids in relation to the thickness of the laminates. 5. REFERENCES 1. W.R Broughton, Thick composites, NPL Report CMMT(A)263, Rubin A.M., Jerina K.L., Evaluation of porosity in composite aircraft structures., Mechanics of Composite Materials, Vol. 30, No 6, PP , Kardos J.L., Dudukovic M.P, Dave R., Void Growth and transport during Processing of Thermosetting-Matrix Composites:. Advances in Polymer Science, Vol.80, PP , Optimization of Curing Cycles for Thick-wall Products of the Polymeric Composite Materials Oleg Dmitriev and Sergey Mischenko, Tambov State Technical University,Russia, textbook on Advances in Composites Materials - Ecodesign and Analysis. Page no Wisnom,M. Size effects in the testing of fiber composite materials,journal of composite Science and Technology, Vol. 59 (1999) pp Curing stresses in thick polymer composite components part-ii : Management of residual stresses by L. G. Stringer1, R. J. Hayman1, M. J. Hinton1, R.A.Badcock,1 and M. R. Wisnom2,UK. 162