Fabrication of out-of-autoclave bismaleimide based composite laminates with embedded fiber optic sensors

Composites are becoming the material of choice in applications where weight savings are critical, like aerospace structures. The common composites used are- Carbon/Epoxy and Carbon/Bismaleimide (BMI). BMI based systems are preferred in applications which involve operating temperatures higher than conventional epoxies. Carbon/BMI laminates are traditionally fabricated in an autoclave, which is associated with high operating costs. In this work, a low cost out of autoclave (OOA) process is evaluated. It is desirable to have BMI OOA prepreg systems cure at reasonably low temperatures with sufficient degree of cure and green strength to maintain rigidity for subsequent freestanding post cure Carbon/BMI composite laminates are manufactured using an OOA compatible prepreg and the effect of varying base cure cycles on the green strength (strength before post cure) is investigated. In aerospace structures, Carbon/BMI composites are used in high temperature applications. Fiber optic sensors are a compact non-intrusive means of structural health monitoring under these conditions. Optical fiber based sensors have many advantages like their compact size, resistance to corrosion, immunity from electromagnetic interference, and multiplexing capabilities. Embedded fiber optic sensors are used to study stresses developed during cure of carbon/BMI composite laminates. The same sensors are then used to measure strain developed in the composite on the application of mechanical loads --Abstract, page iv

Performance evaluation of BMI resin system for thin-ply composites

Composites materials are increasingly being used in aerospace applications over the past few years. The unique properties like high strength to weight ratio, thermal stability, fatigue and corrosion resistance set them apart from the conventional materials. Composite materials are well suited for the applications where weight is the primary concern in the design. Composites structures are vulnerable to mechanical as well as thermal loadings. Transverse micro-cracking and delamination are the most common type of failures in composite materials. The thickness of the ply being used play a key role dictating the properties of the resultant composite structure. As the ply gets thinner the properties get better. Thick laminates are more susceptible to micro-cracking than thin laminates. Thereby, to manufacture laminates resistant to micro-cracking and delamination it is advised to use thinner plies. In this work, a BMI hardened prepreg system was used to prepare the laminated composites. Thin and thick ply laminates were used to make the composite panels .Mechanical testing was performed on the panels to evaluate the performance of thin-ply and thick-ply laminate system --Abstract, page iv

High temperature polymer composites using out-of-autoclave processing

High performance polymer composites possess high strength-to-weight ratio, corrosion resistance, and have design flexibility. Carbon/epoxy composites are commonly used aerospace materials. Bismaleimide based composites are used as a replacement for epoxy systems at higher service temperatures. Aerospace composites are usually manufactured, under high pressure, in an autoclave which requires high capital investments and operating costs. In contrast, out-of-autoclave manufacturing, specifically vacuum-bag-only prepreg process, is capable of producing low cost and high performance composites. In the current study, out-of-autoclave processing of high temperature carbon/bismaleimide composites was evaluated. The cure and process parameters were optimized. The properties of out-of-autoclave cured laminates compared well to autoclave manufactured composites. Numerical models were developed which simulate the curing process in composite laminates and used to optimize cycles and change processing parameters to obtain high-quality parts. The results were extended to enable manufacturing of high temperature composite sandwich structures. Sandwich structures were manufactured and thermo-mechanical properties were evaluated. Numerical models were built to simulate the effect of elevated temperatures on composite sandwich structures and validated using experiments. The results show that it is feasible to manufacture lab-scale high quality composites using the out-of-autoclave process. Also, numerical models are powerful tools which can be used to optimize cure cycles and simulate thermo-mechanical behavior of these composite parts --Abstract, page iv

Composite Design and Manufacturing Development for Human Spacecrafts

The Structural Engineering Division at the NASA Johnson Space Center (JSC) has begun work on lightweight, multifunctional pressurized composite structures. The first candidate vehicle for technology development is the MultiMission Space Exploration Vehicle (MMSEV) cabin, known as the Gen 2B cabin, which has been built at JSC by the Robotics Division. Of the habitable MMSEV vehicle prototypes designed to date, this is the first one specifically analyzed and tested to hold internal pressure and the only one made out of composite materials. This design uses a laminate base with zoned reinforcement and external stringers, intended to demonstrate certain capabilities, and to prepare for the next cabin design, which will be a composite sandwich panel construction with multifunctional capabilities. As part of this advanced development process, a number of new technologies were used to assist in the design and manufacturing process. One of the methods, new to JSC, was to build the Gen 2B cabin with Out of Autoclave technology to permit the creation of larger parts with fewer joints. An 8ply prepreg layup was constructed to form the cabin body. Prior to layup, a design optimization software called FiberSIM was used to create each ply pattern. This software is integrated with Pro/Engineer to allow for customized draping of each fabric ply over the complex tool surface. Slits and darts are made in the software model to create an optimal design that maintains proper fiber placement and orientation. The flat pattern of each ply is then exported and sent to an automated cutting table where the patterns are cut out of graphite material. Additionally, to assist in layup, a laser projection system (LPT) is used to project outlines of each ply directly onto the tool face for accurate fiber placement and ply buildup. Finally, as part of the OoA process, a large oven was procured to postcure each part. After manufacturing complete, the cabin underwent modal and pressure testing (currently in progress at date of writing) and will go on to be outfitted and used for further ops usage

High temperature polymer composites using out-of-autoclave processing

High performance polymer composites possess high strength-to-weight ratio, corrosion resistance, and have design flexibility. Carbon/epoxy composites are commonly used aerospace materials. Bismaleimide based composites are used as a replacement for epoxy systems at higher service temperatures. Aerospace composites are usually manufactured, under high pressure, in an autoclave which requires high capital investments and operating costs. In contrast, out-of-autoclave manufacturing, specifically vacuum-bag-only prepreg process, is capable of producing low cost and high performance composites. In the current study, out-of-autoclave processing of high temperature carbon/bismaleimide composites was evaluated. The cure and process parameters were optimized. The properties of out-of-autoclave cured laminates compared well to autoclave manufactured composites. Numerical models were developed which simulate the curing process in composite laminates and used to optimize cycles and change processing parameters to obtain high-quality parts. The results were extended to enable manufacturing of high temperature composite sandwich structures. Sandwich structures were manufactured and thermo-mechanical properties were evaluated. Numerical models were built to simulate the effect of elevated temperatures on composite sandwich structures and validated using experiments. The results show that it is feasible to manufacture lab-scale high quality composites using the out-of-autoclave process. Also, numerical models are powerful tools which can be used to optimize cure cycles and simulate thermo-mechanical behavior of these composite parts --Abstract, page iv

Advanced composites using non-autoclave processes: manufacturing and characterization

The objective of the present study is to develop non-autoclave processes to manufacture high performance composites for aerospace applications. In Paper 1, vacuum assisted resin transfer molding (VARTM) process was developed for elevated temperature composites. Use of VARTM process for fabricating high temperature resins presents unique challenges such as high porosity and low fiber volume contents. Two different vacuum bagging methods: Seeman Composite Resin Infusion Molding Process (SCRIMP) and Double Vacuum Bagging Infusion (DVBI) process were evaluated. Flow simulation tool was used to predict key flow parameters needed for the successful infusion. In Paper 2, honeycomb sandwich panels were manufactured using commercially available film adhesive and modified VARTM process. The resin incursion into the core openings is a major challenge for applying VARTM process to open cell core sandwich composites. Panels manufactured using the developed process did not show any resin accumulation in the core. The mechanical performance of the manufactured sandwich composites was evaluated. Results indicate that the VARTM process can be successfully used to manufacture honeycomb composite sandwich structures using currently available barrier adhesive films. In Paper 3, a new generation vacuum-bag-only cure out-of-autoclave (OOA) manufacturing process was studied. Physical and mechanical performance of the composites was evaluated. The influence of size, lay-up configuration, thickness and their interactions on the impact behavior of the composites was studied using Design of Experiments (DoE) --Abstract, page iv

Performance Evaluation Of Composite Sandwich Structures With Additively Manufactured Aluminum Honeycomb Cores With Increased Bonding Surface Area

Modern aerostructures, including wings and fuselages, increasingly feature sandwich structures due to their high-energy absorption, low weight, and high flexural stiffness. The face sheet of these sandwich structures are typically thin composite laminates with interior honeycombs made of Nomex or aluminum. Standard cores are structurally efficient, but their design cannot be varied throughout the structure. With additive manufacturing (AM) technology, these core geometries can be altered to meet the design requirements that are not met in standard honeycomb cores. This study used a modified aluminum honeycomb core, with increased surface area on the top and bottom, as the core material in sandwich panels. The modified honeycomb core was produced through the laser powder bed fusion method. The behavior of the modified sandwich composite panels was evaluated through three-point bend, edgewise compression, and impact tests, and their performance was compared to that of a conventional honeycomb core sandwich panel. The three-point bend test results indicated that the sandwich structure\u27s ultimate shear strength improved by 12.6% with the modified honeycomb core. Additionally, the displacement at the failure of the structure increased by 11%. The edgewise compression tests showed that the ultimate edgewise compressive strength improved by 19.1% when using the modified core. The impact test results revealed that the peak force increased by 8% and the energy-absorbing capacity of the sandwich structure increased by 20% with the use of the modified honeycomb core

An experimental technique to characterize interply void formation in unidirectional prepregs

Out-of-autoclave prepreg processing requires evacuation of volatiles in the early stages of processing to achieve an acceptable final void content. In this study, single prepreg plies were laid-up onto a glass tool to simulate a ply–ply interface, to gain an understanding of initial air entrapment and eventual removal mechanisms. The contact was recorded during processing with various edge breathing configurations to identify the relationship between evacuation pathways and contact evolution. The existence of preferential flow channels along the fibre direction of the material was demonstrated by characterizing the prepreg surface. Gas evacuation in those channels prevented contact during an extended ambient temperature vacuum hold. The contact between the prepreg and glass tool equilibrated around 80% during the ambient vacuum hold, and reached full contact at elevated temperature after a brief loss in contact due to moisture vaporization, when the resin pressure decreased to below the water vapour pressure. </jats:p

Hydrokinetic turbine composite blades and sandwich structures: Damage evaluation and numerical simulation

“Composite materials are gaining interest due to their high strength to weight ratio. This study deals with both experimental and numerical approaches to cover the aspects of the failure of composite materials in hydrokinetic turbine applications. In Part I, the location and magnitude of failure in the horizontal axis water turbine carbon fiber-reinforced polymer (CFRP) composite blades with different laminate stacking sequences were investigated. Two lay-up orientations were adopted for this work ([0⁰]4 and [0⁰/90⁰]2s). A finite element analysis model was generated to examine the stresses along the blade. Five angles were introduced to study the effect of pitch angle on the CFRP blades. The numerical results showed very good agreement with the experimental results. In Part II, an experimental setup was developed to test the delamination progression in CFRP blades under hydrodynamic loads in a water tunnel. Thermography analysis was employed to scrutinize the propagation of delamination. In addition, a computational fluid dynamics and one-way fluid-structure interaction were developed to predict the stresses along the blade. The unidirectional ([0⁰]4) blades showed the best performance while the cross-ply blades ([0⁰/90⁰]2s) are prone to delamination. In Part III, the effect of increasing the contact area between the core and facesheet was studied. Two tests (impact and flat-wise tension) were carried out to examine the integrity of the structure. A finite element model was developed to study the damage due to localized load, such as impact load. The results obtained from both the tests (impact and flatwise tension) showed that increasing surface area had improved the structural integrity in regards to damage resistance due to impact, and delamination resistance between the facesheet and the core due to tension”--Abstract, page iv