To address the need for high-quality in-field repair of composite structures, a vacuum-bag-only (VBO) prepreg was designed, produced, and evaluated. The prepreg featured semi-preg formatting and a room-temperature-stable resin. The format provided a multitude of pathways with much shorter breathe-out distances relative to conventional, edge-breathing VBO prepregs, and thus enhanced through-thickness air permeability. A custom-built scarfed repair tool with an in-situ observation window was designed and employed to analyze the cure process during a repair. Microstructural quality, interlaminar shear strength, and glass transition temperature of semi-preg panels were compared to wet-laid epoxy panels processed with double vacuum debulking (DVD). The semi-preg formatting effectively reduced porosity for in-field scarf panels, and when used with the new material system, presents a viable alternative to DVD and wet layup.

During co-cure of honeycomb core sandwich panels, composite facesheets are cured and concurrently bonded to the core, introducing complex interactions that can lead to unsatisfactory bond-line formation. In this work, an in situ co-cure fixture is employed to directly observe the adhesive during processing and identify defect formation mechanisms specific to the bond-line. Relating fillet quality to imposed core pressure reveals the non-linear effect of core pressure. High pressures suppressed voids; intermediate pressures resulted in void growth and entrapment within deformed fillets; and low pressures led to void rupture and small, irregular fillets. Experimental results aided in developing a model to predict void growth in the bond-line. The findings presented here provide insight into the physics controlling the adhesive bond-line evolution during co-cure, which can inform manufacturing decisions to produce higher-quality honeycomb core sandwich structures.

The removal of inter-ply air is critical for limiting porosity in laminates. In this study, an in situ monitoring technique was employed to observe inter-ply air evolution during vacuum bag-only cure. Observations showed that reduced vacuum resulted in inefficient inter-ply air evacuation, a more rapid bubble expansion rate, and formation of new air bubbles. A modified tow impregnation model showed that resin infiltration was impeded at reduced vacuum conditions due to the presence of intra-tow air. However, the cured laminates showed that tows were fully impregnated in all cases, indicating that the entrapped intra-tow air migrated to inter-ply regions during cure. The interactions between intra-tow and inter-tow air at deficient vacuum conditions were revealed. Findings led to the conclusion that air remaining in intra-tow regions contributed more to the increase of inter-ply voids than the reduction in consolidation pressure difference associated with reduced vacuum.

Potential links between pressure conditions during co-cure of honeycomb sandwich panels, the extent of gas flow through facesheet and bond-line, and the level of permeability in the cured skin were evaluated. Half-sandwich structures comprised of fiber-reinforced polymer facesheets, film adhesive, and core were fabricated using a custom-built lab fixture. Autoclave, bag, and core pressures were varied to produce controlled, constant pressure differences during cure, and the resulting skins were tested for permeability using a fixture constructed to measure gas flow rate across the skins and to locate gas flow pathways. Facesheet cross-sections were analyzed to evaluate porosity. Porosity and the number of gas flow pathways were correlated to permeability, but significant gas flow was possible without high void content or with few channels, as pressure differentials led to complex variations in permeability. Overall, the study provides new insights into gas transport during composites processing and manufacturing, and the results provide guidance for modifying manufacturing processes to ensure part quality.

To address the need for increased efficiency in high performance composite processing, a vacuum bag only (VBO) semi-preg was designed, modeled, and evaluated. The semi-preg featured a vinyl hybrid resin formulated for rapid cure. A model was developed to describe the kinetic behavior of the resin, and then was employed to guide the design of efficient cure cycles. The semi-preg featured a discontinuous distribution of resin on the fiber bed. The format imparted high through-thickness air permeability by virtue of the multitude of air evacuation pathways with short breath-out distances relative to conventional out-of-autoclave prepregs (OoA). The kinetic model was used to create a test matrix of panels from the semi-pregs. Microstructural quality, interlaminar shear strength, and glass transition temperature were compared to a control panel with a longer, conventional cure cycle. The results demonstrated that fast-cure resins can be used in conjunction with cure modeling and semi-preg formats to design appropriate VBO cure cycles that consistently yield parts with low defect contents without autoclaves.

Void reduction during composites manufacturing is critical for successful processing. In this study, we perform a parametric study to determine the mechanisms of interply void evolution in unidirectional prepregs during vacuum bag-only cure and to identify the key factors that affect interply air removal. We employ an in situ visualization setup for direct, real-time observation of air removal for prepregs during cure. Results showed that super-ambient dwell at 50℃ effectively promoted interply air removal in unidirectional prepregs, reduced vacuum quality (80% vacuum) had negligible effects on part quality, and that an increase in moisture content of the laminate notably increased void content. Prepreg moisture content was tracked by the inspection of laminate water content at different times during the cure cycle, and the data was combined with a diffusion-based analytical model to predict void size and to improve the understanding of void evolution mechanisms. Results indicated that moisture content of the laminate decreased markedly as cure progressed, providing insights into bubble behavior (expansion and shrinkage) observed during cure. The modified model predictions aligned with experimental data, especially during the second stage, confirming that the observed void growth results from moisture diffusion.

High-performance composites are widely used in industry because of specific mechanical properties and lightweighting opportunities. Current automation solutions to manufacturing components from prepreg (pre-impregnated precursor material) sheets are limited. Our previous work has demonstrated the technical feasibility of a robotic cell to automate the sheet layup process. Many decisions are required for the cell to function correctly, and the time necessary to make these decisions must be reduced to utilize the cell effectively. Robot placement with respect to the mold is a significant and complex decision problem. Ensuring that robots can collaborate effectively requires addressing multiple constraints related to the robot workspace, singularity, and velocities. Solving this problem requires developing computationally efficient algorithms to find feasible robot placements in the cell. We describe an approach based on successive solution refinement strategy to identify a cell design that satisfies all constraints related to robot placement.

Extensive use of thermoplastic composites has been restricted by processing challenges emerging from high melt viscosities. We demonstrate the feasibility of thermoplastic prepreg with partially polymerized poly(methyl methacrylate) (PMMA) matrix and carbon fiber reinforcement. The low viscosity pre-polymer resin allowed part consolidation at low temperature and pressure. The chemical kinetics and rheology of PMMA polymerization were characterized, and an aging study was conducted to assess pre-polymer stability. Prepregs were fabricated using lab-scale methods, and a fabrication map was constructed to determine the optimal extent of polymerization for the prepreg. The prepregs were tested for tack and drape at ambient temperature, and thermoformed for microstructural and chemical analysis. The results show that auto-acceleration drives both the rate of polymerization and viscosity evolution, while refrigeration delays pre-polymer out-time effects. The thermoformed laminates exhibited near-zero porosity. This work establishes material and process development guidelines for reactive thermoplastic prepreg, and highlights potential advantages of the proposed prepreg.

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.

To improve the economics, flexibility, and speed of composite manufacturing, out-of-autoclave/vacuum bag-only (OoA/VBO) processing emerged as a low-cost alternative to traditional autoclave cure. While such techniques have been demonstrated to produce defect-free laminates under ideal conditions, OoA prepregs and VBO processing not sufficiently robust for widespread commercial adoption, particularly in the aerospace industry. This paper explores the relationship between prepreg format, throughthickness permeability, and robust OoA manufacturing of prepreg laminates. A method for producing USCpreg using a mask-and-press technique is described for the creation of prepregs with customized resin distribution. The format of uncured prepreg is characterized using light microscopy, and a custombuilt fixture is employed to confirm that USCpreg exhibits exceptional through-thickness permeability. Samples are cured from USCpreg and a conventional prepreg format under ideal and sub-optimal VBO conditions, and surface and bulk defects in the resulting laminate are characterized to determine the relationship between format and the sensitivity of part quality to non-ideal processing. These results are compared to conventional OoA prepregs, and results confirm a clear relationship between throughthickness permeability and reliable part quality. Defect formation mechanisms are identified and evidence is presented that shows that even minor differences in resin topography affect part quality.

Aerospace sandwich structures are often manufactured using an autoclave co-cure process, in which prepreg facesheets are cured and bonded to honeycomb core simultaneously. Co-cure involves coupled physical phenomena, including gas migration, prepreg consolidation, resin/adhesive flow and crosslinking, and potential mechanisms of defect formation. Due to the “black box” nature of the process, however, scientific understanding of the complex and interacting phenomena that cause defects remains incomplete. We address this challenge using an in situ visualization method that enables direct observation of the skin/core bond-line in realistic autoclave conditions. Five cases are presented, which span a range of process conditions leading to various defect-formation phenomena, and show the effectiveness of in-bag pressurization for preventing bond-line defects. We demonstrate that in situ diagnostics can eliminate much of the trial-and-error typically involved in process troubleshooting, by providing insights into the physics of co-cure that would otherwise be difficult to obtain.

We propose a hybrid human-robot cell for reducing labor in multi-layer prepreg composite sheet layup process. The human expert performs initial experiments to assess the process sequence, and the direction of the layup. The expert knowledge is then transferred to the system in the form of high level commands which are converted into low-level collision free trajectories for the robots in the cell. This paper demonstrates the feasibility of the composite sheet layup using the robotic manipulators along with the design of end-effector tools that are used during the layup process. We give a comparison between manual and hybrid cell for execution times in different steps involved. Finally, we give an overview of process consistency benefits of the hybrid cell.

Vacuum bag only processing has been studied for decades as a potential alternative to high cost and low-efficiency autoclave cure. Low pressure cure, however, steers from a lack of robustness. While autoclave quality parts can be produced under ideal conditions, lay-up and cure must be carefully controlled when high compaction pressures are removed. With current out-of-autoclave prepregs, large parts and complex geometries pose manufacturing challenges, with the inability to evacuate trapped air leading to unacceptably high void contents. These drawbacks can be eliminated through the introduction of through-thickness permeability, which facilitates rapid air removal over short distances. In this work, a through-thickness permeable prepreg, termed USCpreg, is compared to a range of commercially available vacuum bag only prepregs on the basis of permeability, bulk factor, and cured laminate quality.

Effective strategies for the reuse and recycling of in-process prepreg waste are needed to reduce economic and environmental costs. In this paper, we investigate the compression molding of prepreg waste converted into scrap “chips” (or strands). Material is randomly distributed within a lab-scale closed mold and cured with control of temperature and pressure. Material properties and process parameters such as chip geometry, fiber bed reinforcement, resin state, and cure cycle are varied and shown to influence porosity and thickness. These experiments clarify the phenomena governing microstructural quality and identify manufacturing pathways for high-quality parts. In addition, mechanical properties are measured for laminates with high and low defect levels. The study demonstrates the viability of prepreg reuse. Furthermore, the resulting insights provide a basis for practical science-based optimization of the reuse of production prepreg waste.

Polymer film dewetting on a substrate (independent of fiber bed architecture) was explored, developed, and demonstrated as a method to produce out-of-autoclave, vacuum bag-only (OoA/VBO) prepregs with high transverse permeability and process robustness. The dimensions of the surface openings created by dewetting were measured,and thepercent surface area exposed was calculated. Prepregs were fabricated with continuous and dewetted (discontinuous) films to producetrial laminates. The laminates were cured under both standard and sub-optimal conditions, and were characterized before, during, and after cure.Laminates fabricated with dewetted resin consistentlyachieved near-zero porosity. In contrast,laminates with continuous filmdisplayed high levels of porosity,particularly during sub-optimal cure. The findings demonstrate that dewetting can be used effectively to produce OoA prepregs with high through-thickness permeability, whichcan yieldporosity-free laminates via VBO processing. Furthermore, these results elucidate aspects of resin dewetting that are critical in thecreation ofrobust OoA prepregs.

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.

The co-cure of honeycomb sandwich structures involves complex physical phenomena and poses challenges for manufacturing process development. This paper describes results from a multi-year research project on co-cure, during which in situ observations, processing trials, and microstructural analysis were used to understand key physics and defect formation mechanisms. Results show that co-cure is governed by interactions between core pressure evolution, bond-line formation, and facesheet consolidation, and that defect evolution is path-dependent and challenging to control. However, results also demonstrate a manufacturing strategy for successful and reliable co-cure. Overall, the paper clarifies the fundamental science associated with co-cure of honeycomb sandwich structures, and provides a viable pathway for optimizing manufacturing processes.