Timotei Centea

Characterization Methodology of Thermoset Resins for the Processing of Composite Materials - Case Study: CYCOM 890RTM Epoxy Resin

The resin characterization is a key element in the manufacturing of composite materials. Resin processing properties and their associated constitutive models are essential in order to define and optimize the processing parameters and predict the final properties of a composite structure. In this article, a comprehensive methodology is presented to characterize the main processing properties of a thermoset resin system. As a case study, the thermal, chemorheological, and thermomechanical properties of the CYCOM 890RTM epoxy resin were investigated. A cure kinetics model taking into account the diffusion was found to accurately predict resin cure kinetics behavior within the processing condition range. The developed resin rheological model accurately predicted the onset of resin gelation and the evolution of resin viscosity with temperature and degree-of-cure. The glass transition temperature and instantaneous elastic modulus were determined using also a rheometer in a solid torsion mode. Finally, volumetric changes, resin chemical shrinkage and coefficient of thermal expansion were investigated taking into account the chemical and thermal effects. In general, the detailed procedure and techniques presented in this work can be applied to the intensive characterization of a wide range of thermoset resin systems.

Out-of-autoclave prepreg consolidation under deficient pressure conditions

The present study investigated the effects of three pressure-related process deviations (reduced ambient pressure, reduced vacuum and restricted air evacuation) on the consolidation and quality of flat laminates manufactured by out-of-autoclave prepreg processing. The evolution of laminate thickness, measured in-situ, was correlated with thickness and porosity data for woven and unidirectional fibre bed composites. The process deficiencies are shown to have had distinctive detrimental effects on the rate of thickness change and on the amount, distribution and morphology of voids, and to have been more pronounced for the woven fabric prepreg format due to its higher initial bulk factor.

Manufacturing cost relationships for vacuum bag-only prepreg processing

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.

Out-of-autoclave prepreg consolidation: Coupled air evacuation and prepreg impregnation modeling

Out-of-autoclave prepregs produce low-porosity parts through a complex consolidation process that includes air evacuation through a partially impregnated microstructure and subsequent resin infiltration flow. In this article, we propose, develop, and validate the first model describing this consolidation process by computing the thickness change of the material during processing. The air evacuation period is first simulated as a function of fiber packing and pressure conditions. Then, the cure period is described by modeling flow and compaction phenomena. Model predictions are compared to several experimental case studies, which show that the effect of most material properties and process parameters is well captured, and used in parametric studies to identify key trends.

Scaling Challenges Encountered with Out-of-Autoclave Prepregs

The traditional manufacturing method for flight-critical aerospace structures made of composite materials is the autoclave. Autoclave processing is robust and well-understood, but involves high acquisition and operation costs. Out-of-autoclave materials and techniques are increasingly considered as cost-effective replacements to autoclaves; however, their capacity to accommodate scale-up issues commonly encountered when manufacturing larger parts have not yet been thoroughly investigated. The present study considers two such issues for a representative out-of-autoclave prepreg: the effects of resin out-time at room temperature and the material’s ability to evacuate entrapped air. Room-temperature outtime is shown to affect the resin flow phenomena that occur during processing and lead to dramatic increases in tow porosity; however, different temperature cure cycles are shown to mitigate this issue. The material’s permeability is shown to be adequate in-plane but practically non-existent through-thickness in the as-received condition; however, modifications are shown to increase this permeability to acceptable levels and consequently reduce porosity.

In situ monitoring and analysis of void evolution in unidirectional prepreg

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.

In Situ Observations and Pressure Measurements for Autoclave Co-Cure of Honeycomb Core Sandwich Structures

In this article, we describe an experimental method for investigating the autoclave co-cure of honeycomb core composite sandwich structures. The design and capabilities of a custom-built, labscale “in situ co-cure fixture” are presented, including procedures and representative results for three types of experiments. The first type of experiment involves measuring changes in gas pressure on either side of a prepreg laminate to determine the prepreg air permeability. The second type involves co-curing composite samples using regulated, constant pressures, to study material behaviors in controlled conditions. For the final type, “realistic” co-cure, samples are processed in conditions mimicking autoclave cure, where the gas pressure in the honeycomb core evolves naturally due to the competing effects of air evacuation and moisture desorption from the core cell walls. The in-situ co-cure fixture contains temperature and pressure sensors, and derives its name from a glass window that enables direct in-situ visual observation of the skin/core bond-line during processing, shedding light on physical phenomena that are not observable in a traditional manufacturing setting. The experiments presented here are a first step within a larger research effort, whose long-term goal is to develop a physics-based process model for autoclave co-cure.

Effects of thermal gradients on defect formation during the consolidation of partially impregnated prepregs

We describe the effects of thermal gradients on the consolidation of partially impregnated prepregs. Laminates were cured on a heated tool in isothermal and nonisothermal conditions. Key process parameters were varied, including thermal gradient magnitude, air evacuation direction, and vacuum quality. Laminate quality was assessed using microscopy of polished cross-sections and X-ray computed tomography, and interpreted relative to the evolution of resin and prepreg properties during cure. The results show that thermal gradients influenced the rate of impregnation of the prepreg and the rate of gas transport, and affected the amount and distribution of porosity when air was not fully evacuated. Temperature distributions that led to cold regions at the ply boundaries were advantageous, typically exhibiting lower porosity than isothermal baselines. Conversely, gradients resulting in hotter-than-average part perimeters effectively sealed air within the laminate, degrading quality. The results clarify fundamental defect formation mechanisms for partially impregnated prepregs and other processes reliant on air evacuation through an unsaturated preform and provide guidelines for part, tool, and process design.

2.4 Out-of-Autoclave Prepreg Processing

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.