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3.6 GE-90 and Derivative Fan Blade Manufacturing Design
3.6 GE-90 and Derivative Fan Blade Manufacturing Design Ram Upadhyay and Shatil Sinha, GE Global Research, Niskayuna, NY, United States r 2018 Elsevier Ltd. All rights reserved.
3.6.1 Introduction 3.6.1.1 History of PMC Fan Blade Development 3.6.2 Material Characterization 3.6.3 Process Modeling and Process Cycle Design 3.6.4 Tools for Producibility and Design 3.6.5 Future Challenges 3.6.6 Appendix References Relevant Websites
3.6.1
180 180 181 181 184 185 185 188 188
Introduction
The need for polymer composite fan blades for aircraft engines is primarily driven by weight. These materials have a density of roughly 1.5–1.7 g/cm3 compared to aluminum at 2.7 g/cm3 and steels at B7–8 g/cm3. In the polymer matrix composite (PMC) fan blade section, we will discuss the history of PMC development, followed by materials development, and the use of modeling and other productivity tools including analytics. Improving the propulsive efficiency of turbo-fan engines requires larger fans rotating at lower speeds, and the larger fans in modern engines account for an increasing portion of the total engine weight. For example, the CF6–80C2 has a bypass ratio of 5:1 and the fan accounts for 20% of the overall engine weight whereas the newer GEnx-1B engine has a bypass ratio of 10:1 and the fan makes up 30% of the engine weight.1 Furthermore, a 1 kg weight increase in the fan leads to additional increases of 1 kg for containment, 1/2 kg in the rotor, 1/2 kg in the engine structure and 1/4 kg in aircraft structure weight. Thus larger fans are driving the need for new lighter weight materials. As noted previously, the primary driver for use of PMC fan blades is weight reduction of the fan structure. Fiber reinforced carbon/epoxy composites offer very high specific modulus and specific strength as compared to metals,2 as shown in Fig. 1. The benefits of utilizing such light-weight materials for turbine fan blades are obvious.
3.6.1.1
History of PMC Fan Blade Development
In June 1967, Rolls-Royce was the first to offer a fan stage built of a new carbon fiber material called “Hyfil” developed at the Royal Aircraft Establishment (RAE) at Farnborough as part of RB211 engine. In May 1970, after having passed every other test, the fan stage could not pass the bird strike test which shattered the blade into pieces.3 This failure, although a setback for composites, emphasized the need for a tougher matrix material, and eventually led to development of HexPly 8551–7, an amine-cured
Fig. 1 Comparison of specific modulus and specific strength of metals and composites. Reproduced from Campbell, F.C., 2010. Introduction to composite materials. In: Structural Composite Materials. Materials Park, OH: ASM International (Chapter 1).
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Fig. 2 Historic perspective of composite fan blade development at GE.
toughened epoxy resin system.4 A preferred reinforcement for this resin is continuous IM7 graphite fibers, and consequently all current GE PMC fan blades in service have been made from IM7/8551–7 prepreg (pre-impregnated unidirectional ply). Fig. 2 shows the history of composite fan blades at GE starting with the un-ducted fan blade in 1980. The un-ducted fan concept never went in production, but it provided a good knowledge base for development of the GE-90 fan blade that went into commercial production in 1995. Since then there have been significant improvements in the aerodynamic design and a reduction in number of blades, and we are poised for further advances into the fourth generation. Composite fan blades have set a benchmark for future developments and further improvements in propulsive efficiency. Fig. 3 shows the typical steps of the blade manufacturing process. We start with a solid model of the blade driven by aerodynamic requirements. Blade volume is then divided into 3-D layers, each having the thickness of the prepreg sheet, using a process commonly referred as “peeling the onion.” Each layer, referred to as a ply, is then flattened into a 2-D shape and cut from a roll of prepreg with fiber direction being determined by the blade structural requirements. Plies are then laid up in a tool to build the intended volume of the part which is then cured under temperature and pressure. In the following sections we will look back at the development and commercialization of wide chord fan blades5 which included the utilization of a tougher matrix material, process modeling techniques, process design methods, and systems for manufacturing producibility. Each of these will be discussed in detail, followed by a summary with lessons learned and prospects for future developments.
3.6.2
Material Characterization
Understanding of material behavior during the forming process is critical to designing appropriate tooling and a process cycle that avoids producing internal defects. One of the challenges is to identify material properties that drive quality during the manufacturing process. From a manufacturing point of view, quality consists of acceptable internal defects like wrinkles, delaminations and porosity in addition to meeting dimensional tolerances defined by design. Acceptable defect sizes are set from mechanical and fatigue tests that limit the material property knockdowns that stay within design requirements. Characterization of curing, flow and modulus behavior is critical to understanding in situ behavior of the material during the consolidation process. We collaborated with the research team of Simon, McKenna, and Sindt6,7 at the National Institute of Standards and Technology (NIST) to characterize Hexply 8551–7, and developed mathematical models that predict the extent of cure and flow for any specified thermal cycle. These models were subsequently used for tooling and process design. Fig. 4 shows the evolution of the extent of cure and viscosity for a typical temperature cycle.
3.6.3
Process Modeling and Process Cycle Design
Driven by the aerodynamic need for accurate dimensions and surface finish, a matched die mold (referred to as “hard tooling”) was initially chosen. It was later shown that this tooling had implications for part internal quality. The molding was done in a 1000 t press specially designed to control closure to within 2.5 mm accuracy. Instead of a conventional single hydraulic ram there were four
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Fig. 3 Typical steps in a blade manufacturing process starting from a solid model to finished part.
Fig. 4 Degree of cure, cure rate, and viscosity of Epoxy 8551–7 for a typical heating ramp.
independently controlled rams, one for each corner. Internal temperature, pressure, viscosity, and flow were simulated using state of the art Finite Element Analysis (FEA) methods, and were then related to part defects like wrinkling, delamination, and porosity. One of the major defects was out-of-plane wrinkles, as shown in Fig. 5, that significantly reduce the structural properties. Models and experiments (see Appendix for details) were used to assess the effect of various geometry, thermal/closure, and material parameters on wrinkles measured as fiber waviness on tapered specimens with fixed ply layup sequences.8 Fig. 6 shows the sensitivity of fiber waviness to various parameters. Concentrated bulk has large influence on waviness, but low bulk can lead to lack of consolidation and porosity. Lower root temperature is good for wrinkles but can lead to porosity. Material effects are strongly coupled with process parameters. Nevertheless, higher viscosity or lower flow is better for wrinkles, but can lead to surface quality issues. Based on this and other data, the cure cycle was designed for the blade and the models were extended to the full blade geometry. Fig. 7 shows the effect of additional bulk (i.e., the amount of compaction during the consolidation step) due to higher per-plythickness of the prepreg used to build the volume. The models clearly show larger pressure gradients for higher bulks that were found to correlate with an increased degree of wrinkling. During fabrication, the parts were laid up in one tool and then transferred to a mold for consolidation, making it necessary to trim the edges of the layup to fit the mold. Process models were then used to understand internal pressure distribution and
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Fig. 5 Typical out-of-plane wrinkles in fan blade.
Fig. 6 Sensitivity of geometric parameters: uniform and concentrated bulk; thermal parameters: out-time and thermal gradient; and material parameters: viscosity and reaction rate on fiber waviness.
Fig. 7 Effect of additional bulk (amount of compaction) on the pressure distribution during consolidation due to higher per ply thickness.
develop trimming specifications to minimize wrinkles. The effects of the thermal boundary conditions were also modeled to set the temperature cycle. The molding press was designed to control individual platen temperatures and root and tip temperatures independently. A typical axial resin velocity distribution during consolidation in the part is shown for cold and hot root conditions in Fig. 8.9 There is significant difference in the upper root area where higher velocities produced resin rich areas which led to delamination due to resin shrinkage. A cold root temperature difference between the root and platen of B251F was correlated with the occurrence of delamination in root section. Specifications for the temperature cycle were developed based on this analysis. The temperature/pressure cycle was designed with the intent of minimizing internal defects, which were primarily wrinkles in the root or airfoil regions, porosity or delamination. The dimensions were always within tolerance due to the choice of hard tooling. The complex interaction of material physical and chemical variability, temperature and pressure boundary conditions, and the temperature/pressure cycle made it extremely hard to overcome all defects simultaneously. There was always a tradeoff between wrinkles, porosity and delamination. We learned, after conducting many DOEs and producing hundreds of parts, that the inherent entitlement yield of the process was significantly reduced due to not allowing dimensions to have variation. Entitlement yield is defined as the percentage yield of good parts when same process and material specifications are used. Due to inherent variations, the entitlement yield tends to be much smaller than the desired yield of, for example, 95%.
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Fig. 8 Hot and cold root spanwise resin velocity during consolidation. Higher velocity leads to resin rich areas.
Fig. 9 Methods and major historic milestones to eliminate wrinkles in PMC fan blades.
One of the enduring lessons for composites is that variation in volume due to material must be handled in tool design and accounted for in the process. It is a three-way trade-off between dimensions, internal quality, and performance.10 As a result of low yield entitlement from the hard tooling configuration, we redesigned the tooling so it has an open mold with a flexible caul, referred to as the “soft tooling concept.” This tool could accommodate volume variation in the prepreg without creating the large pressure gradients that caused wrinkles. Parts were then cured and consolidated in an autoclave under nearisotropic pressure conditions. This change of tooling greatly decreased both porosity and delamination, but ply wrinkles were still a major cause of rejects. The wrinkles were both out-of-plane and in-plane at 45 degree from the reinforcement orientation. Numerous other improvements were required to eliminate these wrinkles, many of which are shown in the timeline in Fig. 9. Initial improvements were based on controlling the movement of material within the mold. Additional improvements came from controlling the various geometric factors of the tooling, and by instituting in-line measurements and volume adjustments to the layup kit in order to adapt for prepreg thickness variation.
3.6.4
Tools for Producibility and Design
As we worked through the tough challenge of understanding the key quality drivers and trade-offs necessary to improve our yield entitlement, it was necessary to develop a platform where we could integrate all material, process and quality data, thereby enabling fast interactive analyses to better understand the underlying process behavior. Fig. 10 shows the fundamental architecture
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Fig. 10 Architecture of the GE Informatics system that is being used as a producibility tool and feedback mechanism from manufacturing to design.
of the GE Informatics system that is currently being used for fan blade processing and for most other composite components for GE engines. The primary objective of the system is to provide tools and methods to almost automatically integrate all data, convert it into information, generate knowledge in terms of mathematical relationships and enable corrective action. Overall the system minimizes the “learn and adapt time” for a given process. It is extremely important to develop a feedback mechanism so that important manufacturability data is easily available to the next generation of component designs. Manufacturing provides crucial feedback for design, such as feature producibility and bonded assembly stack-ups. Reductions in the number of geometric measurements needed for airfoil sections are realized by using statistical sampling based on correlations between sections.11 This helps designers make robust designs that are producible and at the same time maximize weight reductions without compromising long-term performance. The GE Informatics system is starting to make the linkage stronger and is a key piece in the design for manufacturability puzzle.
3.6.5
Future Challenges
PMCs represent a mature technology that has been in use for aircraft engines for over 20 years, but significant challenges still remain in low-cost manufacturing and reducing the cycle time to introduce new components. One of the key challenges is developing design rules that allow the inherent trade-off between dimensions, internal quality, and performance. We are just now, after decades of application, beginning to build a foundation for manufacturable designs. The tools for this are: (1) process models capable of exploring the entire material and process space, (2) an Informatics system for fast feedback and adaptive control, and (3) robust feedback to design. Future challenges to introducing new composite components faster is to develop tools for design, manufacturing and testing that can incorporate inherent variation in raw materials and directional placement of reinforcements. This requires a fundamental rethinking of design practices that account for composite materials with inherent variation rather than simply replacing metal designs with composite materials. In the last decade, there has been great progress in new modeling techniques using Proper Generalized Decomposition (PGD) method that has been pioneered by Francisco Chinesta at Ecole Centrale Nantes.12 The novel idea behind PGD is to treat all input parameters coming from material, geometry, and process as additional dimensions beyond x–y–z space dimensions and produce a single solution that contains the entire parametric space. This parametric solution can be used as an engine to explore the entire process design space in real-time and also be used to adapt for incoming variation during manufacturing process. Application of this revolutionary method has been applied to an airfoil shaped outlet guide vane13 where a standard finite element method solution that took 6 h of computing time could be obtained in less than 20 s. This might truly accelerate introduction of new components in much shorter period than has been possible.
3.6.6
Appendix
A tapered geometry was used to conduct initial process design (see Fig. 11) using experiments and simulation. Part quality is described in terms of four factors: fiber waviness, porosity, delamination, and lack of consolidation. In this study, focus was on the formation and control of fiber waviness.
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Fig. 11 Tapered geometry used for initial process design. Also shown are a snapshot of temperature and viscosity distribution during the processing stage predicted by process models.
Fig. 12 A typical schematic of press motion and temperature cycle.
The study was divided into four major tasks: material characterization, material modeling, add-on subroutine development, and development of correlation between the process and quality parameters. Material evolution was simulated for various sets of geometric and process conditions, and sensitivity of the part quality was evaluated with respect to the process conditions. Initial cure ranged from 20% (bottom) to 10% (top) varying in a quasi-linear fashion. The layup sequence was 745/0/90 degrees. Top and bottom of the part was divided into two heating zones while left and right sides had independent heating. A schematic of temperature and tool closure history used in this study is shown in Fig. 12. Initial temperature and pressure change rate is chosen so that there is sufficient time for material redistribution and reduction in the stresses generated during the material redistribution. Then, in the second stage, the temperature and pressure is maintained constant to allow for the stresses to relax and the material to reach complete cure state. In the third stage, part is cooled in a semicontrolled fashion to ensure that the temperature gradient due to cooling is such that the transient thermal stresses generated do not lead to any failure, especially delamination. Fig. 11 is a snapshot of temperature distribution in the part during the heating part of the cure cycle. In a traditional material, this cycle at this time would show a higher temperature at the boundaries compared to the interior. However, in the present case, due to the exothermic nature of the epoxy, the interior temperature is higher relative to the surface temperature. During curing, mechanical and thermal properties of the material are dependent upon the temperature and the history, and after full cure, they become a sole function of temperature. During the cure, parts develop transient stresses which depend upon the temperature and cure and their distribution, material properties, ply layup, and ply orientation. Cumulative effect of these stresses and the stresses generated due to the applied pressure affect the degree of consolidation, porosity, material redistribution, and fiber waviness. During the cooling stages, the effective stresses are function of cooling rate and the visco-elastic properties of the material. An inappropriate cooling rate leads to high transient stress which could cause delamination.
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Fig. 13 Fiber waviness for different set of processing conditions: (a) extreme wrinkles, (b) marginal wrinkles, and (c) wrinkle free.
Fig. 11 also shows a snapshot of viscosity distribution in the part. Viscosity is a function of temperature and degree of cure. In the initial stages, viscosity is low and the applied pressure causes material to flow which reduces porosity and increases its strength. However, a strong flow can cause large shear stresses at the ply interfaces, which can lead to fiber waviness. During the curing process, stress state continuously changes and requires trade-offs among the processing parameters for an optimum quality characteristic. Fig. 13 shows the range of waviness under different conditions where parts would be totally unacceptable to a good part. Key factors that directly or indirectly affect part quality can be classified as: geometric, rheo-kinetics, and thermo-mechanical. Sensitivity of these factors on the fiber waviness are summarized in Fig. 6. The parameters studied are concentrated and uniform bulk sensitivities as geometric factors, initial cure, temperature gradient, and closure time as thermo-mechanical factors, and rate of reaction and viscosity as rheo-kinetic factors. Here uniform bulk is a measure of the volume change, and concentrated bulk is a measure of the surface roughness. In this study, the geometric shape and variation in the material principal direction, i.e., ply layup are not considered as they were prespecified to fulfill the design objectives. The two geometric parameters, uniform and concentrated bulk change have compensating effect on the fiber waviness. Fiber waviness is low at low uniform bulk but it reduces consolidation leading to porosity or delamination failure. A low concentrated
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bulk seems to increase the fiber waviness. Thermo-mechanical and rheo-kinetic factors also needed optimization due to their opposing effect on fiber waviness and consolidation. An increase in viscosity and reaction rate shows a decrease in fiber waviness. In the lower range of reaction rate, the fiber waviness is not highly sensitive to the reaction rate, however, as the reaction rate increases so does the sensitivity of fiber waviness upon it. Effect of viscosity is convoluted with the effect of reaction rate, closure time, degree of initial cure, and consolidation pressure. At high viscosity, even higher consolidation pressure does not necessarily create any flow induced shear stresses which could cause waviness, however, the high viscosity reduces the consolidation leading to porosity or delamination. Effect of thermo-mechanical factors on the fiber waviness is presented in Fig. 6. A high temperature gradient along the length of the part creates fiber waviness which is highly sensitive to the temperature gradient at its higher values. Fiber waviness decreases linearly with increase in the degree of pre-cure or the out-time. However, a large pre-cure decreases consolidation and increases porosity. Effect of closure time did not show any significant effect on the fiber waviness for the studied range of parameters. The observations presented here, together with the sensitivities of the porosity with respect to all the factors, were used to determine an optimum value for the process parameters for an optimum quality. Fig. 13 shows different levels of fiber waviness in an experimental part as the process parameters were refined. Fig. 13(a) shows the initially obtained level of fiber waviness; Fig. 13(b) shows a degree of fiber waviness for a sub-optimized process conditions; and Fig. 13(c) shows a part obtained using the optimum conditions. The sensitivity study in Fig. 6 shows a relationship between the control parameters and part quality. This relationship was established using an intermediate correlation term. All the control parameters are first related to an interim parameter. Then a relationship is established between this interim parameter and the quality metric parameter used for measuring the fiber waviness. The correlation between the interim parameter and the fiber waviness in our study is found to be linear. This relationship was validated and used in all the work presented here for relating control (process) parameters to the fiber waviness. Composites of thick and complex nature tend to show two types of defects, namely porosity or lack of consolidation and fiber waviness. Maximizing one type of them can lead to a reduction in the other type. Here, the quality control specifically related to the fiber waviness is discussed. It is to be noted that the total quality is an optimization of all the quality measures rather than that of one parameter at a time.
See also: 3.1 Certification and Compliance Considerations for Aircraft Products with Composite Materials. 3.2 Scaling Crucial to Integrated Product Development of Composite Airframe Structures. 3.3 The Impact of Large Integrated and Bonded Composite Structures on Future Military Transport Aircraft. 3.4 Composites in Missiles and Launch Vehicles. 3.5 Design, Manufacture and Test of Cryotank Components. 3.7 Application of the A-VaRTM to the MRJ's Empennages. 3.9 Lessons Learned on Supportability and Repair of Composites. 3.23 Polymer Matrix Composite Thermal Materials
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Worthoff, F., 2013. System requirements flowdown to components influencing manufacturability. In: Composite Manufacturing Symposium, GE Global Research, April 16. Campbell, F.C. (Ed.), 2010. Introduction to composite materials. In: Structural Composite Materials. Materials Park, OH: ASM International, pp. 1–30 (Chapter 1). Rolls-Royce RB211. Available at: http://en.wikipedia.org/wiki/Rolls-Royce_RB211#cite_note-bird_ingestion-8. HEXCEL. Prepreg. Available at: http://www.hexcel.com/Resources/DataSheets/Prepreg. Murphy, G.C., Furhmann, B.J., 1992. Wide chord fan blade, US Patent US 5141400 A, August. Simon, S.L., McKenna, G.B., Sindt, O., 2000. Modeling the evolution of the dynamic mechanical properties of a commercial epoxy during cure after gelation. Journal of Applied Polymer Science 76, 495–508. Prasatya, P., McKenna, G.B., Simon, S.L., 2001. A viscoelastic model for predicting isotropic residual stresses in thermosetting materials: Effects of processing parameters. Journal of Computational Mathematics 35, 826–848. Upadhyay, R.K., Pandy, R.K., 1999. Design of manufacturing process for complex composite. In: SEM Conference, Society for Experimental Mechanics. Upadhyay, R.K., Sinha, S., Bushko, W., Kirpatrick, B., 2006. Manufacturing process simulation. In: Progress Report GE-Boeing Collaboration Program, January. Upadhyay, R.K., 2013. Uniqueness of composites from manufacturing perspective. In: Composite Manufacturing Symposium, GE Global Research, April 16. Lednicky, T., 2013. Case study of successful design manufacturing interaction. In: Composite Manufacturing Symposium, GE Global Research, April 16. Chinesta, F., Keunings, R., Leygue, A., 2014. The proper generalized decomposition for advanced numerical simulations – A primer. Heidelberg: Springer International Publishing, Springer Briefs in Applied Science and Technology, ISBN 978-3-319-02865-1. Aguado, J.V., Borazacchiello, D., Ghanatios, C., et al., 2017. A simulation app based on reduced order modeling for manufacturing optimization of composite outlet guide vanes. Advanced Modeling and Simulation in Engineering Sciences. doi:10.1186/s40323-017-0087-y.
Relevant Websites http://www.cfmaeroengines.com/press/lufthansa-places-1-0-billion-leap-1a-engine-order/745 CFM International. http://www.geaviation.com/press/ge90/ge90_20131119a.html GE Aviation.
3.6.1 Introduction 3.6.1.1 History of PMC Fan Blade Development 3.6.2 Material Characterization 3.6.3 Process Modeling and Process Cycle Design 3.6.4 Tools for Producibility and Design 3.6.5 Future Challenges 3.6.6 Appendix References Relevant Websites
3.6.1
180 180 181 181 184 185 185 188 188
Introduction
The need for polymer composite fan blades for aircraft engines is primarily driven by weight. These materials have a density of roughly 1.5–1.7 g/cm3 compared to aluminum at 2.7 g/cm3 and steels at B7–8 g/cm3. In the polymer matrix composite (PMC) fan blade section, we will discuss the history of PMC development, followed by materials development, and the use of modeling and other productivity tools including analytics. Improving the propulsive efficiency of turbo-fan engines requires larger fans rotating at lower speeds, and the larger fans in modern engines account for an increasing portion of the total engine weight. For example, the CF6–80C2 has a bypass ratio of 5:1 and the fan accounts for 20% of the overall engine weight whereas the newer GEnx-1B engine has a bypass ratio of 10:1 and the fan makes up 30% of the engine weight.1 Furthermore, a 1 kg weight increase in the fan leads to additional increases of 1 kg for containment, 1/2 kg in the rotor, 1/2 kg in the engine structure and 1/4 kg in aircraft structure weight. Thus larger fans are driving the need for new lighter weight materials. As noted previously, the primary driver for use of PMC fan blades is weight reduction of the fan structure. Fiber reinforced carbon/epoxy composites offer very high specific modulus and specific strength as compared to metals,2 as shown in Fig. 1. The benefits of utilizing such light-weight materials for turbine fan blades are obvious.
3.6.1.1
History of PMC Fan Blade Development
In June 1967, Rolls-Royce was the first to offer a fan stage built of a new carbon fiber material called “Hyfil” developed at the Royal Aircraft Establishment (RAE) at Farnborough as part of RB211 engine. In May 1970, after having passed every other test, the fan stage could not pass the bird strike test which shattered the blade into pieces.3 This failure, although a setback for composites, emphasized the need for a tougher matrix material, and eventually led to development of HexPly 8551–7, an amine-cured
Fig. 1 Comparison of specific modulus and specific strength of metals and composites. Reproduced from Campbell, F.C., 2010. Introduction to composite materials. In: Structural Composite Materials. Materials Park, OH: ASM International (Chapter 1).
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Fig. 2 Historic perspective of composite fan blade development at GE.
toughened epoxy resin system.4 A preferred reinforcement for this resin is continuous IM7 graphite fibers, and consequently all current GE PMC fan blades in service have been made from IM7/8551–7 prepreg (pre-impregnated unidirectional ply). Fig. 2 shows the history of composite fan blades at GE starting with the un-ducted fan blade in 1980. The un-ducted fan concept never went in production, but it provided a good knowledge base for development of the GE-90 fan blade that went into commercial production in 1995. Since then there have been significant improvements in the aerodynamic design and a reduction in number of blades, and we are poised for further advances into the fourth generation. Composite fan blades have set a benchmark for future developments and further improvements in propulsive efficiency. Fig. 3 shows the typical steps of the blade manufacturing process. We start with a solid model of the blade driven by aerodynamic requirements. Blade volume is then divided into 3-D layers, each having the thickness of the prepreg sheet, using a process commonly referred as “peeling the onion.” Each layer, referred to as a ply, is then flattened into a 2-D shape and cut from a roll of prepreg with fiber direction being determined by the blade structural requirements. Plies are then laid up in a tool to build the intended volume of the part which is then cured under temperature and pressure. In the following sections we will look back at the development and commercialization of wide chord fan blades5 which included the utilization of a tougher matrix material, process modeling techniques, process design methods, and systems for manufacturing producibility. Each of these will be discussed in detail, followed by a summary with lessons learned and prospects for future developments.
3.6.2
Material Characterization
Understanding of material behavior during the forming process is critical to designing appropriate tooling and a process cycle that avoids producing internal defects. One of the challenges is to identify material properties that drive quality during the manufacturing process. From a manufacturing point of view, quality consists of acceptable internal defects like wrinkles, delaminations and porosity in addition to meeting dimensional tolerances defined by design. Acceptable defect sizes are set from mechanical and fatigue tests that limit the material property knockdowns that stay within design requirements. Characterization of curing, flow and modulus behavior is critical to understanding in situ behavior of the material during the consolidation process. We collaborated with the research team of Simon, McKenna, and Sindt6,7 at the National Institute of Standards and Technology (NIST) to characterize Hexply 8551–7, and developed mathematical models that predict the extent of cure and flow for any specified thermal cycle. These models were subsequently used for tooling and process design. Fig. 4 shows the evolution of the extent of cure and viscosity for a typical temperature cycle.
3.6.3
Process Modeling and Process Cycle Design
Driven by the aerodynamic need for accurate dimensions and surface finish, a matched die mold (referred to as “hard tooling”) was initially chosen. It was later shown that this tooling had implications for part internal quality. The molding was done in a 1000 t press specially designed to control closure to within 2.5 mm accuracy. Instead of a conventional single hydraulic ram there were four
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Fig. 3 Typical steps in a blade manufacturing process starting from a solid model to finished part.
Fig. 4 Degree of cure, cure rate, and viscosity of Epoxy 8551–7 for a typical heating ramp.
independently controlled rams, one for each corner. Internal temperature, pressure, viscosity, and flow were simulated using state of the art Finite Element Analysis (FEA) methods, and were then related to part defects like wrinkling, delamination, and porosity. One of the major defects was out-of-plane wrinkles, as shown in Fig. 5, that significantly reduce the structural properties. Models and experiments (see Appendix for details) were used to assess the effect of various geometry, thermal/closure, and material parameters on wrinkles measured as fiber waviness on tapered specimens with fixed ply layup sequences.8 Fig. 6 shows the sensitivity of fiber waviness to various parameters. Concentrated bulk has large influence on waviness, but low bulk can lead to lack of consolidation and porosity. Lower root temperature is good for wrinkles but can lead to porosity. Material effects are strongly coupled with process parameters. Nevertheless, higher viscosity or lower flow is better for wrinkles, but can lead to surface quality issues. Based on this and other data, the cure cycle was designed for the blade and the models were extended to the full blade geometry. Fig. 7 shows the effect of additional bulk (i.e., the amount of compaction during the consolidation step) due to higher per-plythickness of the prepreg used to build the volume. The models clearly show larger pressure gradients for higher bulks that were found to correlate with an increased degree of wrinkling. During fabrication, the parts were laid up in one tool and then transferred to a mold for consolidation, making it necessary to trim the edges of the layup to fit the mold. Process models were then used to understand internal pressure distribution and
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Fig. 5 Typical out-of-plane wrinkles in fan blade.
Fig. 6 Sensitivity of geometric parameters: uniform and concentrated bulk; thermal parameters: out-time and thermal gradient; and material parameters: viscosity and reaction rate on fiber waviness.
Fig. 7 Effect of additional bulk (amount of compaction) on the pressure distribution during consolidation due to higher per ply thickness.
develop trimming specifications to minimize wrinkles. The effects of the thermal boundary conditions were also modeled to set the temperature cycle. The molding press was designed to control individual platen temperatures and root and tip temperatures independently. A typical axial resin velocity distribution during consolidation in the part is shown for cold and hot root conditions in Fig. 8.9 There is significant difference in the upper root area where higher velocities produced resin rich areas which led to delamination due to resin shrinkage. A cold root temperature difference between the root and platen of B251F was correlated with the occurrence of delamination in root section. Specifications for the temperature cycle were developed based on this analysis. The temperature/pressure cycle was designed with the intent of minimizing internal defects, which were primarily wrinkles in the root or airfoil regions, porosity or delamination. The dimensions were always within tolerance due to the choice of hard tooling. The complex interaction of material physical and chemical variability, temperature and pressure boundary conditions, and the temperature/pressure cycle made it extremely hard to overcome all defects simultaneously. There was always a tradeoff between wrinkles, porosity and delamination. We learned, after conducting many DOEs and producing hundreds of parts, that the inherent entitlement yield of the process was significantly reduced due to not allowing dimensions to have variation. Entitlement yield is defined as the percentage yield of good parts when same process and material specifications are used. Due to inherent variations, the entitlement yield tends to be much smaller than the desired yield of, for example, 95%.
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Fig. 8 Hot and cold root spanwise resin velocity during consolidation. Higher velocity leads to resin rich areas.
Fig. 9 Methods and major historic milestones to eliminate wrinkles in PMC fan blades.
One of the enduring lessons for composites is that variation in volume due to material must be handled in tool design and accounted for in the process. It is a three-way trade-off between dimensions, internal quality, and performance.10 As a result of low yield entitlement from the hard tooling configuration, we redesigned the tooling so it has an open mold with a flexible caul, referred to as the “soft tooling concept.” This tool could accommodate volume variation in the prepreg without creating the large pressure gradients that caused wrinkles. Parts were then cured and consolidated in an autoclave under nearisotropic pressure conditions. This change of tooling greatly decreased both porosity and delamination, but ply wrinkles were still a major cause of rejects. The wrinkles were both out-of-plane and in-plane at 45 degree from the reinforcement orientation. Numerous other improvements were required to eliminate these wrinkles, many of which are shown in the timeline in Fig. 9. Initial improvements were based on controlling the movement of material within the mold. Additional improvements came from controlling the various geometric factors of the tooling, and by instituting in-line measurements and volume adjustments to the layup kit in order to adapt for prepreg thickness variation.
3.6.4
Tools for Producibility and Design
As we worked through the tough challenge of understanding the key quality drivers and trade-offs necessary to improve our yield entitlement, it was necessary to develop a platform where we could integrate all material, process and quality data, thereby enabling fast interactive analyses to better understand the underlying process behavior. Fig. 10 shows the fundamental architecture
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Fig. 10 Architecture of the GE Informatics system that is being used as a producibility tool and feedback mechanism from manufacturing to design.
of the GE Informatics system that is currently being used for fan blade processing and for most other composite components for GE engines. The primary objective of the system is to provide tools and methods to almost automatically integrate all data, convert it into information, generate knowledge in terms of mathematical relationships and enable corrective action. Overall the system minimizes the “learn and adapt time” for a given process. It is extremely important to develop a feedback mechanism so that important manufacturability data is easily available to the next generation of component designs. Manufacturing provides crucial feedback for design, such as feature producibility and bonded assembly stack-ups. Reductions in the number of geometric measurements needed for airfoil sections are realized by using statistical sampling based on correlations between sections.11 This helps designers make robust designs that are producible and at the same time maximize weight reductions without compromising long-term performance. The GE Informatics system is starting to make the linkage stronger and is a key piece in the design for manufacturability puzzle.
3.6.5
Future Challenges
PMCs represent a mature technology that has been in use for aircraft engines for over 20 years, but significant challenges still remain in low-cost manufacturing and reducing the cycle time to introduce new components. One of the key challenges is developing design rules that allow the inherent trade-off between dimensions, internal quality, and performance. We are just now, after decades of application, beginning to build a foundation for manufacturable designs. The tools for this are: (1) process models capable of exploring the entire material and process space, (2) an Informatics system for fast feedback and adaptive control, and (3) robust feedback to design. Future challenges to introducing new composite components faster is to develop tools for design, manufacturing and testing that can incorporate inherent variation in raw materials and directional placement of reinforcements. This requires a fundamental rethinking of design practices that account for composite materials with inherent variation rather than simply replacing metal designs with composite materials. In the last decade, there has been great progress in new modeling techniques using Proper Generalized Decomposition (PGD) method that has been pioneered by Francisco Chinesta at Ecole Centrale Nantes.12 The novel idea behind PGD is to treat all input parameters coming from material, geometry, and process as additional dimensions beyond x–y–z space dimensions and produce a single solution that contains the entire parametric space. This parametric solution can be used as an engine to explore the entire process design space in real-time and also be used to adapt for incoming variation during manufacturing process. Application of this revolutionary method has been applied to an airfoil shaped outlet guide vane13 where a standard finite element method solution that took 6 h of computing time could be obtained in less than 20 s. This might truly accelerate introduction of new components in much shorter period than has been possible.
3.6.6
Appendix
A tapered geometry was used to conduct initial process design (see Fig. 11) using experiments and simulation. Part quality is described in terms of four factors: fiber waviness, porosity, delamination, and lack of consolidation. In this study, focus was on the formation and control of fiber waviness.
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Fig. 11 Tapered geometry used for initial process design. Also shown are a snapshot of temperature and viscosity distribution during the processing stage predicted by process models.
Fig. 12 A typical schematic of press motion and temperature cycle.
The study was divided into four major tasks: material characterization, material modeling, add-on subroutine development, and development of correlation between the process and quality parameters. Material evolution was simulated for various sets of geometric and process conditions, and sensitivity of the part quality was evaluated with respect to the process conditions. Initial cure ranged from 20% (bottom) to 10% (top) varying in a quasi-linear fashion. The layup sequence was 745/0/90 degrees. Top and bottom of the part was divided into two heating zones while left and right sides had independent heating. A schematic of temperature and tool closure history used in this study is shown in Fig. 12. Initial temperature and pressure change rate is chosen so that there is sufficient time for material redistribution and reduction in the stresses generated during the material redistribution. Then, in the second stage, the temperature and pressure is maintained constant to allow for the stresses to relax and the material to reach complete cure state. In the third stage, part is cooled in a semicontrolled fashion to ensure that the temperature gradient due to cooling is such that the transient thermal stresses generated do not lead to any failure, especially delamination. Fig. 11 is a snapshot of temperature distribution in the part during the heating part of the cure cycle. In a traditional material, this cycle at this time would show a higher temperature at the boundaries compared to the interior. However, in the present case, due to the exothermic nature of the epoxy, the interior temperature is higher relative to the surface temperature. During curing, mechanical and thermal properties of the material are dependent upon the temperature and the history, and after full cure, they become a sole function of temperature. During the cure, parts develop transient stresses which depend upon the temperature and cure and their distribution, material properties, ply layup, and ply orientation. Cumulative effect of these stresses and the stresses generated due to the applied pressure affect the degree of consolidation, porosity, material redistribution, and fiber waviness. During the cooling stages, the effective stresses are function of cooling rate and the visco-elastic properties of the material. An inappropriate cooling rate leads to high transient stress which could cause delamination.
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Fig. 13 Fiber waviness for different set of processing conditions: (a) extreme wrinkles, (b) marginal wrinkles, and (c) wrinkle free.
Fig. 11 also shows a snapshot of viscosity distribution in the part. Viscosity is a function of temperature and degree of cure. In the initial stages, viscosity is low and the applied pressure causes material to flow which reduces porosity and increases its strength. However, a strong flow can cause large shear stresses at the ply interfaces, which can lead to fiber waviness. During the curing process, stress state continuously changes and requires trade-offs among the processing parameters for an optimum quality characteristic. Fig. 13 shows the range of waviness under different conditions where parts would be totally unacceptable to a good part. Key factors that directly or indirectly affect part quality can be classified as: geometric, rheo-kinetics, and thermo-mechanical. Sensitivity of these factors on the fiber waviness are summarized in Fig. 6. The parameters studied are concentrated and uniform bulk sensitivities as geometric factors, initial cure, temperature gradient, and closure time as thermo-mechanical factors, and rate of reaction and viscosity as rheo-kinetic factors. Here uniform bulk is a measure of the volume change, and concentrated bulk is a measure of the surface roughness. In this study, the geometric shape and variation in the material principal direction, i.e., ply layup are not considered as they were prespecified to fulfill the design objectives. The two geometric parameters, uniform and concentrated bulk change have compensating effect on the fiber waviness. Fiber waviness is low at low uniform bulk but it reduces consolidation leading to porosity or delamination failure. A low concentrated
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bulk seems to increase the fiber waviness. Thermo-mechanical and rheo-kinetic factors also needed optimization due to their opposing effect on fiber waviness and consolidation. An increase in viscosity and reaction rate shows a decrease in fiber waviness. In the lower range of reaction rate, the fiber waviness is not highly sensitive to the reaction rate, however, as the reaction rate increases so does the sensitivity of fiber waviness upon it. Effect of viscosity is convoluted with the effect of reaction rate, closure time, degree of initial cure, and consolidation pressure. At high viscosity, even higher consolidation pressure does not necessarily create any flow induced shear stresses which could cause waviness, however, the high viscosity reduces the consolidation leading to porosity or delamination. Effect of thermo-mechanical factors on the fiber waviness is presented in Fig. 6. A high temperature gradient along the length of the part creates fiber waviness which is highly sensitive to the temperature gradient at its higher values. Fiber waviness decreases linearly with increase in the degree of pre-cure or the out-time. However, a large pre-cure decreases consolidation and increases porosity. Effect of closure time did not show any significant effect on the fiber waviness for the studied range of parameters. The observations presented here, together with the sensitivities of the porosity with respect to all the factors, were used to determine an optimum value for the process parameters for an optimum quality. Fig. 13 shows different levels of fiber waviness in an experimental part as the process parameters were refined. Fig. 13(a) shows the initially obtained level of fiber waviness; Fig. 13(b) shows a degree of fiber waviness for a sub-optimized process conditions; and Fig. 13(c) shows a part obtained using the optimum conditions. The sensitivity study in Fig. 6 shows a relationship between the control parameters and part quality. This relationship was established using an intermediate correlation term. All the control parameters are first related to an interim parameter. Then a relationship is established between this interim parameter and the quality metric parameter used for measuring the fiber waviness. The correlation between the interim parameter and the fiber waviness in our study is found to be linear. This relationship was validated and used in all the work presented here for relating control (process) parameters to the fiber waviness. Composites of thick and complex nature tend to show two types of defects, namely porosity or lack of consolidation and fiber waviness. Maximizing one type of them can lead to a reduction in the other type. Here, the quality control specifically related to the fiber waviness is discussed. It is to be noted that the total quality is an optimization of all the quality measures rather than that of one parameter at a time.
See also: 3.1 Certification and Compliance Considerations for Aircraft Products with Composite Materials. 3.2 Scaling Crucial to Integrated Product Development of Composite Airframe Structures. 3.3 The Impact of Large Integrated and Bonded Composite Structures on Future Military Transport Aircraft. 3.4 Composites in Missiles and Launch Vehicles. 3.5 Design, Manufacture and Test of Cryotank Components. 3.7 Application of the A-VaRTM to the MRJ's Empennages. 3.9 Lessons Learned on Supportability and Repair of Composites. 3.23 Polymer Matrix Composite Thermal Materials
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Worthoff, F., 2013. System requirements flowdown to components influencing manufacturability. In: Composite Manufacturing Symposium, GE Global Research, April 16. Campbell, F.C. (Ed.), 2010. Introduction to composite materials. In: Structural Composite Materials. Materials Park, OH: ASM International, pp. 1–30 (Chapter 1). Rolls-Royce RB211. Available at: http://en.wikipedia.org/wiki/Rolls-Royce_RB211#cite_note-bird_ingestion-8. HEXCEL. Prepreg. Available at: http://www.hexcel.com/Resources/DataSheets/Prepreg. Murphy, G.C., Furhmann, B.J., 1992. Wide chord fan blade, US Patent US 5141400 A, August. Simon, S.L., McKenna, G.B., Sindt, O., 2000. Modeling the evolution of the dynamic mechanical properties of a commercial epoxy during cure after gelation. Journal of Applied Polymer Science 76, 495–508. Prasatya, P., McKenna, G.B., Simon, S.L., 2001. A viscoelastic model for predicting isotropic residual stresses in thermosetting materials: Effects of processing parameters. Journal of Computational Mathematics 35, 826–848. Upadhyay, R.K., Pandy, R.K., 1999. Design of manufacturing process for complex composite. In: SEM Conference, Society for Experimental Mechanics. Upadhyay, R.K., Sinha, S., Bushko, W., Kirpatrick, B., 2006. Manufacturing process simulation. In: Progress Report GE-Boeing Collaboration Program, January. Upadhyay, R.K., 2013. Uniqueness of composites from manufacturing perspective. In: Composite Manufacturing Symposium, GE Global Research, April 16. Lednicky, T., 2013. Case study of successful design manufacturing interaction. In: Composite Manufacturing Symposium, GE Global Research, April 16. Chinesta, F., Keunings, R., Leygue, A., 2014. The proper generalized decomposition for advanced numerical simulations – A primer. Heidelberg: Springer International Publishing, Springer Briefs in Applied Science and Technology, ISBN 978-3-319-02865-1. Aguado, J.V., Borazacchiello, D., Ghanatios, C., et al., 2017. A simulation app based on reduced order modeling for manufacturing optimization of composite outlet guide vanes. Advanced Modeling and Simulation in Engineering Sciences. doi:10.1186/s40323-017-0087-y.
Relevant Websites http://www.cfmaeroengines.com/press/lufthansa-places-1-0-billion-leap-1a-engine-order/745 CFM International. http://www.geaviation.com/press/ge90/ge90_20131119a.html GE Aviation.