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Steven R. Nutt - One of the best experts on this subject based on the ideXlab platform.
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2.4 Out-of-Autoclave Prepreg Processing
Comprehensive Composite Materials II, 2020Co-Authors: Pascal Hubert, Timotei Centea, Steven R. Nutt, James Kratz, Lessa Grunefelder, Arthur LevyAbstract:The objective of this chapter is to provide an overview of the processing aspects of out-of-autoclave (OOA) Prepregs. This chapter serves as a design guideline for the definition of tooling, bagging configuration, and processing conditions for making parts with OOA Prepregs. Section 1 presents an overview of the OOA materials, including their application, resins, and fibers. OOA prepreg impregnation techniques are then discussed and typical properties of OOA composites are summarized. Section 2 covers OOA prepreg characterization methods, techniques to measure resin impregnation, thermochemistry, out-time, permeability, and bulk factor are presented. Section 3 describes the infrastructure used to cure OOA Prepregs, such as ovens, heating systems, tooling, and process diagnostic tools. Section 4 provides basic processing guidelines, covering bagging configuration, debulking methods, and cure cycles to make simple monolithic OOA laminates, while Sections 5 and 6 provide processing guidelines for sandwich panels and complex shape laminates. The cost analysis of the manufacturing process with OOA Prepregs is reviewed in section seven. Finally, section eight discusses future developments for OOA prepreg materials and processes.
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effects of resin distribution patterns on through thickness air removal in vacuum bag only Prepregs
Composites Part A-applied Science and Manufacturing, 2020Co-Authors: Sarah G.k. Schechter, Timotei Centea, Steven R. NuttAbstract:Abstract Prepregs with discontinuous resin patterns facilitate air removal and impart robustness to vacuum-bag-only processing of composites. However, optimal pattern characteristics have not yet been identified. A geometric model was developed to guide the fabrication of Prepregs with various discontinuous patterns and laminates with different orientations and ply counts. The model was used to evaluate metrics related to gas transport: projected surface area exposed, sealed interfaces, and tortuosity. Statistical analysis revealed that single layer surface area exposed and ply count had the greatest effect on projected surface area exposed; orientation had the greatest effect on sealed interfaces and tortuosity. From these insights, prototype Prepregs were fabricated to measure through-thickness permeability. Prepregs with a large percentage of sealed interfaces and high tortuosity exhibited lower permeability. The study demonstrated a methodology to differentiate/screen patterns for gas transport efficiency. The model can guide prepreg design and support robust production of composites via out-of-autoclave manufacturing.
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Thermoplastic prepreg with a partially polymerized matrix
International SAMPE Technical Conference, 2018Co-Authors: J.-h. Shin, D. Kim, Timotei Centea, Steven R. NuttAbstract:© 2018. Used by the Society of the Advancement of Material and Process Engineering with permission. Continuous fiber-reinforced thermoplastics feature key advantages over thermoset composites. However, extensive use of thermoplastic matrices has been restricted by processing challenges, including high melt viscosities and melting temperatures. In this work, we investigate an approach for improving thermoplastic prepreg processability and conformability by fabricating prepreg with partially polymerized thermoplastic matrix. We describe a case study of a prepreg composed of carbon fiber fabric and acrylic (polymethylmethacrylate) resin. The chemical and rheological kinetics of neat acrylic resin were investigated, followed by pre-polymer aging studies. Moreover, the optimal extent of polymerization (degree of monomer conversion) for the prepreg was determined by constructing a prepreg fabrication map. Prepreg samples were fabricated using lab-scale methods, and characterized to demonstrate tack at room temperature. Manufactured prepreg was thermoformed below the final glass transition temperature of the matrix with minimal pressure for porosity analysis. Polymerization kinetics data showed that auto-acceleration from the Trommsdorff effect plays a key role in both rate of polymerization and viscosity evolutions, while the aging studies indicated that refrigeration can delay room temperature aging of acrylic pre-polymer resin. Overall, the laminates thermoformed with partially polymerized thermoplastic Prepregs exhibited low porosity with no sign of bulk porosity or resin-deprived regions. This work highlights some potential advantages of the new thermoplastic prepreg, which emerges from the ability to utilize conventional thermoset prepreg processing capacities to manufacture thermoplastic composites, and identifies processing challenges that should be addressed.
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Effect of prepreg format on defect control in out-of-autoclave processing
Composites Part A: Applied Science and Manufacturing, 2017Co-Authors: Lessa Kay Grunenfelder, A. Dills, Timotei Centea, Steven R. NuttAbstract:Prepreg format plays a key role in part quality for composites produced using vacuum bag only (VBO) techniques. To date, however, VBO Prepregs have been produced by modifying existing autoclave formats. In this work, we introduce USCpreg, a prepreg format designed specifically for out-of-autoclave cure, featuring through-thickness permeability. We describe the fabrication and analysis of laminates processed with USCpreg, as well as laminates fabricated from traditional VBO prepreg formats. The through-thickness pathways for air transport in USCpreg result in near-zero internal porosity and defect-free surfaces in parts cured under VBO conditions, even under challenging processing conditions. Results highlight the fact that surface and internal porosity depend on prepreg format, and that through-thickness permeability is critical to achieving high quality parts in non-ideal manufacturing scenarios.
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Manufacturing cost relationships for vacuum bag-only prepreg processing
Journal of Composite Materials, 2016Co-Authors: Timotei Centea, Steven R. NuttAbstract:© SAGE Publications. Vacuum bag-only Prepregs enable the out-of-autoclave manufacture of high-performance composite structures and increase the material, part, and process selection space. However, manufacturing choices involve economic as well as technical considerations. To understand these relationships, we developed a technical cost model that captures the distinctive characteristics of vacuum bag-only prepreg processing (including vacuum-induced air evacuation and resin cure) and estimates the costs associated with materials, equipment, and labor. We applied the model to realistic manufacturing cases and used a parametric study to evaluate the effects of part characteristics, material use efficiency, and cure efficiency. The results indicate that prepreg cost, part size, prepreg waste, and the air evacuation capacity of the material have the strongest influence on part costs and demonstrate that cost modeling can guide efforts to improve or optimize processing by identifying the most economically valuable modifications.
Timotei Centea - One of the best experts on this subject based on the ideXlab platform.
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2.4 Out-of-Autoclave Prepreg Processing
Comprehensive Composite Materials II, 2020Co-Authors: Pascal Hubert, Timotei Centea, Steven R. Nutt, James Kratz, Lessa Grunefelder, Arthur LevyAbstract:The objective of this chapter is to provide an overview of the processing aspects of out-of-autoclave (OOA) Prepregs. This chapter serves as a design guideline for the definition of tooling, bagging configuration, and processing conditions for making parts with OOA Prepregs. Section 1 presents an overview of the OOA materials, including their application, resins, and fibers. OOA prepreg impregnation techniques are then discussed and typical properties of OOA composites are summarized. Section 2 covers OOA prepreg characterization methods, techniques to measure resin impregnation, thermochemistry, out-time, permeability, and bulk factor are presented. Section 3 describes the infrastructure used to cure OOA Prepregs, such as ovens, heating systems, tooling, and process diagnostic tools. Section 4 provides basic processing guidelines, covering bagging configuration, debulking methods, and cure cycles to make simple monolithic OOA laminates, while Sections 5 and 6 provide processing guidelines for sandwich panels and complex shape laminates. The cost analysis of the manufacturing process with OOA Prepregs is reviewed in section seven. Finally, section eight discusses future developments for OOA prepreg materials and processes.
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effects of resin distribution patterns on through thickness air removal in vacuum bag only Prepregs
Composites Part A-applied Science and Manufacturing, 2020Co-Authors: Sarah G.k. Schechter, Timotei Centea, Steven R. NuttAbstract:Abstract Prepregs with discontinuous resin patterns facilitate air removal and impart robustness to vacuum-bag-only processing of composites. However, optimal pattern characteristics have not yet been identified. A geometric model was developed to guide the fabrication of Prepregs with various discontinuous patterns and laminates with different orientations and ply counts. The model was used to evaluate metrics related to gas transport: projected surface area exposed, sealed interfaces, and tortuosity. Statistical analysis revealed that single layer surface area exposed and ply count had the greatest effect on projected surface area exposed; orientation had the greatest effect on sealed interfaces and tortuosity. From these insights, prototype Prepregs were fabricated to measure through-thickness permeability. Prepregs with a large percentage of sealed interfaces and high tortuosity exhibited lower permeability. The study demonstrated a methodology to differentiate/screen patterns for gas transport efficiency. The model can guide prepreg design and support robust production of composites via out-of-autoclave manufacturing.
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Thermoplastic prepreg with a partially polymerized matrix
International SAMPE Technical Conference, 2018Co-Authors: J.-h. Shin, D. Kim, Timotei Centea, Steven R. NuttAbstract:© 2018. Used by the Society of the Advancement of Material and Process Engineering with permission. Continuous fiber-reinforced thermoplastics feature key advantages over thermoset composites. However, extensive use of thermoplastic matrices has been restricted by processing challenges, including high melt viscosities and melting temperatures. In this work, we investigate an approach for improving thermoplastic prepreg processability and conformability by fabricating prepreg with partially polymerized thermoplastic matrix. We describe a case study of a prepreg composed of carbon fiber fabric and acrylic (polymethylmethacrylate) resin. The chemical and rheological kinetics of neat acrylic resin were investigated, followed by pre-polymer aging studies. Moreover, the optimal extent of polymerization (degree of monomer conversion) for the prepreg was determined by constructing a prepreg fabrication map. Prepreg samples were fabricated using lab-scale methods, and characterized to demonstrate tack at room temperature. Manufactured prepreg was thermoformed below the final glass transition temperature of the matrix with minimal pressure for porosity analysis. Polymerization kinetics data showed that auto-acceleration from the Trommsdorff effect plays a key role in both rate of polymerization and viscosity evolutions, while the aging studies indicated that refrigeration can delay room temperature aging of acrylic pre-polymer resin. Overall, the laminates thermoformed with partially polymerized thermoplastic Prepregs exhibited low porosity with no sign of bulk porosity or resin-deprived regions. This work highlights some potential advantages of the new thermoplastic prepreg, which emerges from the ability to utilize conventional thermoset prepreg processing capacities to manufacture thermoplastic composites, and identifies processing challenges that should be addressed.
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In situ monitoring and analysis of void evolution in unidirectional prepreg
Journal of Composite Materials, 2018Co-Authors: Wei Hu, Lessa Kay Grunenfelder, Timotei CenteaAbstract:© 2018, The Author(s) 2018. In the layup of prepreg laminates, air is inevitably entrapped between adjacent prepreg plies, yet the removal of this inter-ply air under vacuum bag cure conditions is not well understood. In this study, an in situ visualization technique was developed to dynamically observe inter-ply air removal during the cure of an out-of-autoclave prepreg. The technique was used to investigate mechanisms of air removal and void evolution in unidirectional prepreg. Prepreg impregnation was also tracked by inspection of laminate cross-sections prepared at different times during the cure cycle. From these data, a three-stage air removal mechanism was documented based on the relationships between void content, resin properties, and tow impregnation as functions of time. Furthermore, a positive correlation was observed between the rate of evacuation and bubble elongation. Though discussed here in the specific context of voids in unidirectional laminates, the in situ observation technique developed for this work has broad potential to enhance understanding of processing phenomena associated with out-of-autoclave Prepregs.
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Effect of prepreg format on defect control in out-of-autoclave processing
Composites Part A: Applied Science and Manufacturing, 2017Co-Authors: Lessa Kay Grunenfelder, A. Dills, Timotei Centea, Steven R. NuttAbstract:Prepreg format plays a key role in part quality for composites produced using vacuum bag only (VBO) techniques. To date, however, VBO Prepregs have been produced by modifying existing autoclave formats. In this work, we introduce USCpreg, a prepreg format designed specifically for out-of-autoclave cure, featuring through-thickness permeability. We describe the fabrication and analysis of laminates processed with USCpreg, as well as laminates fabricated from traditional VBO prepreg formats. The through-thickness pathways for air transport in USCpreg result in near-zero internal porosity and defect-free surfaces in parts cured under VBO conditions, even under challenging processing conditions. Results highlight the fact that surface and internal porosity depend on prepreg format, and that through-thickness permeability is critical to achieving high quality parts in non-ideal manufacturing scenarios.
Suresh G. Advani - One of the best experts on this subject based on the ideXlab platform.
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Prediction of process-induced void formation in anisotropic Fiber-reinforced autoclave composite parts
International Journal of Material Forming, 2020Co-Authors: Bamdad Barari, Pavel Simacek, Shridhar Yarlagadda, Roger M. Crane, Suresh G. AdvaniAbstract:A numerical methodology is proposed to predict void content and evolution during autoclave processing of thermoset Prepregs. Starting with the initial prepreg void content, the void evolution model implements mechanisms for void compaction under the effect of the applied pressure, including Ideal Gas law compaction, and squeeze flow for single curvature geometries. Pressure variability in the prepreg stack due to interactions between applied autoclave pressure and anisotropic material response are considered and implemented. A parametric study is conducted to investigate the role of material anisotropy, initial void content, and applied autoclave pressure on void evolution during consolidation of Prepregs on a tool with single curvatures. The ability of the model to predict pressure gradient through the thickness of the laminate and its impact on void evolution is discussed.
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multi scale modelling of non uniform consolidation of uncured toughened unidirectional Prepregs
21st International ESAFORM Conference on Material Forming ESAFORM 2018, 2018Co-Authors: G Sorba, Christophe Binetruy, Elena Syerko, Adrien Leygue, Sebastien Comascardona, Jonathan P Belnoue, Oliver J Nixonpearson, Dmitry Ivanov, Stephen R Hallett, Suresh G. AdvaniAbstract:Consolidation is a crucial step in manufacturing of composite parts with Prepregs because its role is to eliminate inter- and intra-ply gaps and porosity. Some thermoset prepreg systems are toughened with thermoplastic particles. Depending on their size, thermoplastic particles can be either located in between plies or distributed within the inter-fibre regions. When subjected to transverse compaction, resin will bleed out of low-viscosity unidirectional Prepregs along the fibre direction, whereas one would expect transverse squeeze flow to dominate for higher viscosity Prepregs. Recent experimental work showed that the consolidation of uncured toughened Prepregs involves complex flow and deformation mechanisms where both bleeding and squeeze flow patterns are observed [1]. Micrographs of compacted and cured samples confirm these features as shown in Fig.1. A phenomenological model was proposed [2] where bleeding flow and squeeze flow are combined. A criterion for the transition from shear flow to resin bleeding was also proposed. However, the micrographs also reveal a resin rich layer between plies which may be contributing to the complex flow mechanisms during the consolidation process. In an effort to provide additional insight into these complex mechanisms, this work focuses on the 3D numerical modelling of the compaction of uncured toughened Prepregs in the cross-ply configuration described in [1]. A transversely isotropic fluid model is used to describe the flow behaviour of the plies coupled with interplay resin flow of an isotropic fluid. The multi-scale flow model used is based on [3, 4]. A numerical parametric study is carried out where the resin viscosity, permeability and inter-ply thickness are varied to identify the role of important variables. The squeezing flow and the bleeding flow are compared for a range of process parameters to investigate the coupling and competition between the two flow mechanisms. Figure 4 shows the predicted displacement of the sample edge with the multi-scale compaction model after one time step [3]. The ply distortion and resin flow observed in Fig.1 is qualitatively retrieved by the computational model.Consolidation is a crucial step in manufacturing of composite parts with Prepregs because its role is to eliminate inter- and intra-ply gaps and porosity. Some thermoset prepreg systems are toughened with thermoplastic particles. Depending on their size, thermoplastic particles can be either located in between plies or distributed within the inter-fibre regions. When subjected to transverse compaction, resin will bleed out of low-viscosity unidirectional Prepregs along the fibre direction, whereas one would expect transverse squeeze flow to dominate for higher viscosity Prepregs. Recent experimental work showed that the consolidation of uncured toughened Prepregs involves complex flow and deformation mechanisms where both bleeding and squeeze flow patterns are observed [1]. Micrographs of compacted and cured samples confirm these features as shown in Fig.1. A phenomenological model was proposed [2] where bleeding flow and squeeze flow are combined. A criterion for the transition from shear flow to resin b...
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Void entrapment into air pathways in partially impregnated Prepregs in the Out-Of-Autoclave process
Proceedings of the American Society for Composites - 29th Technical Conference ASC 2014; 16th US-Japan Conference on Composite Materials; ASTM-D30 Mee, 2014Co-Authors: Thomas A. Cender, Volkan Eskizeybek, Suresh G. AdvaniAbstract:Out-of-Autoclave (OOA) thermoset prepreg manufacturing of aerospace quality parts is performed under low pressures, which makes it more susceptible to void formation and growth as compared to high pressure autoclave processing. Thus, OOA Prepregs are intentionally partially impregnated with resin, with the goal to distribute the resin such that a completely connected network of empty channels is formed in the initial material. This network serves as pathways for evacuation of gases entrapped in the laminate before consolidation and cure. This work investigates how mechanically entrapped air can be removed from partially impregnated OOA prepreg laminates before oven curing. First, a model of void evacuation time is derived to estimate the time necessary to apply vacuum to remove air from within the laminate before placing it in the oven. Next, a flow visualization technique is presented where the resin film of the partially impregnated OOA prepreg is pressed into the fabric, while recording the resin flow on the dry side. This quantifies the degree of resin impregnation with processing time. A relationship between degree of resin impregnation and gas permeability is presented which influences the evacuation time necessary to remove air from the system. A large panel (1.2m long) was fabricated in which low and high evacuation times were employed based on the model physics. The void content was quantified along the length via sectioning/polishing/image analysis and shown to have a void content of 1%. The results of this study should prove useful to develop optimal vacuum application times and temperature and pressure cycles for void reduction and removal during processing of Prepregs.
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resin film impregnation in fabric Prepregs with dual length scale permeability
Composites Part A-applied Science and Manufacturing, 2013Co-Authors: Thomas A. Cender, Pavel Simacek, Suresh G. AdvaniAbstract:Abstract Prepregs are precursor materials for composites processing. Most Prepregs are fully impregnated however some Prepregs are only partially impregnated with resin. This study characterizes the resin impregnation process in a prepreg consisting of a resin film laminated to one side of a woven fabric. Layers of such Prepregs are stacked on a tool surface to build thickness with low positive pressure. This assembly is then bagged and placed under vacuum pressure to remove the air and volatiles under vacuum before the resin impregnates these empty spaces. The assembly is then allowed to cure in an oven under vacuum pressure to form the composite. There is a need to understand resin flow in such fabrics with dual length scale permeability under the initial positive pressure and also the vacuum pressure. This paper focuses on understanding the under pinning flow mechanisms and presents an in situ method for characterizing fabric Prepregs under low positive pressures before vacuum bag processing. The proposed in situ method monitors the pattern of resin saturating the empty spaces between the fibers and the tows within the fabric over time, which allows for characterization of dual length scale permeability of such Prepregs. The experimental technique compresses a resin film into the fabric at a known temperature and pressure, during which the flow pattern is recorded from the opposite side through a transparent table with a CCD camera. The flow is modeled analytically to match the observed two stages: inter-fiber tow flow and intra-tow flow to characterize the dual length scale prepreg permeability.
Lessa Kay Grunenfelder - One of the best experts on this subject based on the ideXlab platform.
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In situ monitoring and analysis of void evolution in unidirectional prepreg
Journal of Composite Materials, 2018Co-Authors: Wei Hu, Lessa Kay Grunenfelder, Timotei CenteaAbstract:© 2018, The Author(s) 2018. In the layup of prepreg laminates, air is inevitably entrapped between adjacent prepreg plies, yet the removal of this inter-ply air under vacuum bag cure conditions is not well understood. In this study, an in situ visualization technique was developed to dynamically observe inter-ply air removal during the cure of an out-of-autoclave prepreg. The technique was used to investigate mechanisms of air removal and void evolution in unidirectional prepreg. Prepreg impregnation was also tracked by inspection of laminate cross-sections prepared at different times during the cure cycle. From these data, a three-stage air removal mechanism was documented based on the relationships between void content, resin properties, and tow impregnation as functions of time. Furthermore, a positive correlation was observed between the rate of evacuation and bubble elongation. Though discussed here in the specific context of voids in unidirectional laminates, the in situ observation technique developed for this work has broad potential to enhance understanding of processing phenomena associated with out-of-autoclave Prepregs.
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Effect of prepreg format on defect control in out-of-autoclave processing
Composites Part A: Applied Science and Manufacturing, 2017Co-Authors: Lessa Kay Grunenfelder, A. Dills, Timotei Centea, Steven R. NuttAbstract:Prepreg format plays a key role in part quality for composites produced using vacuum bag only (VBO) techniques. To date, however, VBO Prepregs have been produced by modifying existing autoclave formats. In this work, we introduce USCpreg, a prepreg format designed specifically for out-of-autoclave cure, featuring through-thickness permeability. We describe the fabrication and analysis of laminates processed with USCpreg, as well as laminates fabricated from traditional VBO prepreg formats. The through-thickness pathways for air transport in USCpreg result in near-zero internal porosity and defect-free surfaces in parts cured under VBO conditions, even under challenging processing conditions. Results highlight the fact that surface and internal porosity depend on prepreg format, and that through-thickness permeability is critical to achieving high quality parts in non-ideal manufacturing scenarios.
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The influence of prepreg architectureon part quality for vaccum bag onlyprocessing
International SAMPE Technical Conference, 2015Co-Authors: Lessa Kay Grunenfelder, G. Riddle, Timotei Centea, Steven R. NuttAbstract:Copyright 2015. Used by the Society of the Advancement of Material and Process Engineering with permission. Conventional vacuum bag only (VBO) Prepregs are designed with engineered vacuum channels (EVaCs) to facilitate in-plane air removal during low-pressure cure. However, through-thickness permeability in such Prepregs is negligible, requiring air removal almost exclusively via the EVaCs. In this work, a new prepregging technique is employed which results in increased through-thickness permeability, promoting removal of inter-ply air and gasses. The prepreg architecture features enhanced through-thickness permeability, in comparison with conventional VBO Prepregs. We describe the fabrication and characterization of composite laminates made with this prepreg system, as well as control samples fabricated using a traditional prepreg design. Results show that surface and internal porosity are highly dependent on prepreg architecture, and that the introduction of through-thickness permeability can improve part quality in a range of manufacturing situations.
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A review of out-of-autoclave Prepregs - Material properties, process phenomena, and manufacturing considerations
Composites Part A: Applied Science and Manufacturing, 2015Co-Authors: Timotei Centea, Lessa Kay Grunenfelder, Steven R. NuttAbstract:Out-of-autoclave (OoA) prepreg materials and methods have gained acceptance over the past decade because of the ability to produce autoclave-quality components under vacuum-bag-only (VBO) cure. To achieve low porosity and tight dimensional tolerances, VBO Prepregs rely on specific microstructural features and processing techniques. Furthermore, successful cure is contingent upon appropriate material property and process parameter selection. In this article, we review the existing literature on VBO prepreg processing to summarize and synthesize knowledge on these issues. First, the context, development, and defining properties of VBO Prepregs are presented. The key processing phenomena and the influence on quality are subsequently described. Finally, cost and environmental performance are considered. Throughout, we highlight key considerations for VBO prepreg processing and identify areas where further study is required.
Pascal Hubert - One of the best experts on this subject based on the ideXlab platform.
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2.4 Out-of-Autoclave Prepreg Processing
Comprehensive Composite Materials II, 2020Co-Authors: Pascal Hubert, Timotei Centea, Steven R. Nutt, James Kratz, Lessa Grunefelder, Arthur LevyAbstract:The objective of this chapter is to provide an overview of the processing aspects of out-of-autoclave (OOA) Prepregs. This chapter serves as a design guideline for the definition of tooling, bagging configuration, and processing conditions for making parts with OOA Prepregs. Section 1 presents an overview of the OOA materials, including their application, resins, and fibers. OOA prepreg impregnation techniques are then discussed and typical properties of OOA composites are summarized. Section 2 covers OOA prepreg characterization methods, techniques to measure resin impregnation, thermochemistry, out-time, permeability, and bulk factor are presented. Section 3 describes the infrastructure used to cure OOA Prepregs, such as ovens, heating systems, tooling, and process diagnostic tools. Section 4 provides basic processing guidelines, covering bagging configuration, debulking methods, and cure cycles to make simple monolithic OOA laminates, while Sections 5 and 6 provide processing guidelines for sandwich panels and complex shape laminates. The cost analysis of the manufacturing process with OOA Prepregs is reviewed in section seven. Finally, section eight discusses future developments for OOA prepreg materials and processes.
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A stochastic approach to model void formation during out-of-autoclave prepreg consolidation
ICCM International Conferences on Composite Materials, 2015Co-Authors: Rhena Helmus, Pascal Hubert, Roland HinterhölzlAbstract:© 2015 International Committee on Composite Materials. All rights reserved. Material variability is always present in prepreg materials. Accompanied by fluctuations in process parameters, handling and environmental conditions, these inherent uncertainties can lead to significant variations in void content within the final part. This is especially critical for processing thermoset Out-of-Autoclave (OoA) Prepregs since the safeguard provided by high autoclave pressure no longer exists. For a more realistic simulation of void formation during OoA processing, contingencies in input parameters have to be considered. Stochastic models are suitable to account for random input factors in order to quantify process output variations. This is a prerequisite to reduce the risk of unserviceable quality of parts to a minimum. The following study attempts to allocate different input variabilities to three types of voids in OoA Prepregs and highlights stochastic approaches to model each of them individually.
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anisotropic air permeability in out of autoclave Prepregs effect on honeycomb panel evacuation prior to cure
Composites Part A-applied Science and Manufacturing, 2013Co-Authors: James Kratz, Pascal HubertAbstract:Abstract Air evacuation is crucial to achieve low porosity for vacuum-bag-only manufacturing of out-of-autoclave Prepregs. In this paper, the air permeability of composite skins was evaluated for in-plane and transverse air evacuation. The air permeability and microstructure were evaluated for three common prepreg fabric architectures: unidirectional, plain weave, and 5 harness satin. Since prepreg permeability is anisotropic, a 3-D pressure gradient can arise in honeycomb skins. In-order to calculate the effective air permeability coefficients of honeycomb skins, the computational fluid dynamics software FLUENT was used to determine the 1-D pressure gradient and corresponding area normal to flow. The air permeability coefficients were correlated to the prepreg microstructure using micro-CT imaging. The results showed that the in-plane air permeability was higher for fabrics with larger visible dry tow areas. The transverse air permeability was higher for Prepregs that formed connected macro-porosity networks after lay-up, compared to Prepregs with isolated macro-pores.
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Anisotropic air permeability in out-of-autoclave Prepregs: Effect on honeycomb panel evacuation prior to cure
Composites Part A: Applied Science and Manufacturing, 2013Co-Authors: James Kratz, Pascal HubertAbstract:Air evacuation is crucial to achieve low porosity for vacuum-bag-only manufacturing of out-of-autoclave Prepregs. In this paper, the air permeability of composite skins was evaluated for in-plane and transverse air evacuation. The air permeability and microstructure were evaluated for three common prepreg fabric architectures: unidirectional, plain weave, and 5 harness satin. Since prepreg permeability is anisotropic, a 3-D pressure gradient can arise in honeycomb skins. In-order to calculate the effective air permeability coefficients of honeycomb skins, the computational fluid dynamics software FLUENT was used to determine the 1-D pressure gradient and corresponding area normal to flow. The air permeability coefficients were correlated to the prepreg microstructure using micro-CT imaging. The results showed that the in-plane air permeability was higher for fabrics with larger visible dry tow areas. The transverse air permeability was higher for Prepregs that formed connected macro-porosity networks after lay-up, compared to Prepregs with isolated macro-pores. © 2013 Elsevier Ltd. All rights reserved.
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Measuring the impregnation of an out-of-autoclave prepreg by micro-CT
Composites Science and Technology, 2011Co-Authors: Timotei Centea, Pascal HubertAbstract:Resin flow into dry reinforcement regions is the main microstructural change during the processing of out-of-autoclave Prepregs and influences air evacuation and void suppression. Such impregnation flow was investigated experimentally during the processing of a second-generation out-of-autoclave prepreg. First, laminates were partially processed to different stages of a simple cure cycle. Then, samples from each laminate were scanned using X-ray microtomography (micro-CT) to obtain 3D microstructural data. This data was used to investigate the initial microstructure of the material and measure the extent of impregnation at each processing stage, the rate of impregnation, and the evolution of macro-porosity within the material. © 2010 Elsevier Ltd.
