Additive Manufacturing of Structural Cores and Washout Tooling for Autoclave Curing of Hybrid Composite Structures

This paper presents a study combining additive manufactured (AM) elements with carbon fiber-reinforced polymers (CFRP) for the autoclave curing of complex-shaped, lightweight structures. Two approaches were developed: First, structural cores were produced with AM, over-laminated with CFRP, and co-cured in the autoclave. Second, a functional hull is produced with AM, filled with a temperature- and pressure-resistant material, and over-laminated with CFRP. After curing, the filler-material is removed to obtain a hollow lightweight structure. The approaches were applied to hat stiffeners, which were modeled, fabricated, and tested in three-point bending. Results show weight savings by up to 5% compared to a foam core reference. Moreover, the AM element contributes to the mechanical performance of the hat stiffener, which is highlighted by an increase in the specific bending stiffness and the first failure load by up to 18% and 310%. Results indicate that the approaches are appropriate for composite structures with complex geometries

Effects of Low Velocity Impact on the Flexural Strength of Composite Sandwich Structures

The use of composite sandwich structures is rapidly increasing in the aerospace industry because of their increased strength-to-weight and stiffness-to-weight characteristics. The effects of low velocity impacts on these structures, however, are the main weakness that hinders further use of them in the industry because the damages from these loadings can often be catastrophic. Impact behavior of composite materials in general is a crucial consideration for a designer but can be difficult to describe theoretically. Because of this, experimental analysis is typically used to attempt to describe the behavior of composite sandwiches under impact loads. Experimental testing can still be unpredictable, however, because low velocity impacts can cause undetectable damage within the composites that weaken their structural integrity. This is an important issue with composite sandwich structures because interlaminar damage within the composite facesheets is typical with composites but the addition of a core material results in added failure modes. Because the core is typically a weaker material than the surrounding facesheet material, the core is easily damaged by the impact loads. The adhesion between the composite facesheets and the core material can also be a major region of concern for sandwich structures. Delamination of the facesheet from the core is a major issue when these structures are subjected to impact loads. This study investigated, through experimental and numerical analysis, how varying the core and facesheet material combination affected the flexural strength of a composite sandwich subjected to low velocity impact. Carbon, hemp, aramid, and glass fiber materials as facesheets combined with honeycomb and foam as core materials were considered. Three layers of the same composite material were laid on the top and bottom of the core material to form each sandwich structure. This resulted in eight different sandwich designs. The carbon fiber/honeycomb sandwiches were then combined with the aramid fiber facesheets, keeping the same three layer facesheet design, to form two hybrid sandwich designs. This was done to attempt to improve the impact resistance and post-impact strength characteristics of the carbon fiber sandwiches. The two and one layer aramid fiber laminates on these hybrid sandwiches were always laid up on the outside of the structure. The sandwiches were cured using a composite press set to the recommended curing cycle for the composite facesheet material. The hybrid sandwiches were cured twice for the two different facesheet materials. The cured specimens were then cut into 3 inch by 10 inch sandwiches and 2/3 of them were subjected to an impact from a 7.56 lbf crosshead which was dropped from a height of 38.15 inches above the bottom of the specimen using a Dynatup 8250 drop weight machine. The impacted specimen and the control specimen (1/3 of the specimens not subjected to an impact) were loaded in a four-point bend test per ASTM D7250 to determine the non-impacted and post-impact flexural strengths of these structures. Each sandwich was tested under two four-point bend loading conditions which resulted in two different extension values at the same 100 lbf loading value. The span between the two supports on the bottom of the sandwich was always 8 inches but the span between the two loading pins on the top of the sandwich changed between the two loading conditions. The 2/3 of the sandwiches that were tested after being impacted were subjected to bending loads in two different ways. Half of the specimens were subjected to four-point bending loads with the impact damage on the top facesheet (compressive surface) in between the loading pins; the other half were subjected to bending loads with the damage on the bottom facesheet (tensile surface). Theoretical failure mode analysis was done for each sandwich to understand the comparisons between predicted and experimental failures. A numerical investigation was, also, completed using Abaqus to verify the results of the experimental tests. Non-impacted and impacted four-point bending models were constructed and mid-span deflection values were collected for comparison with the experimental testing results. Experimental and numerical results showed that carbon fiber sandwiches were the best sandwich design for overall composite sandwich bending strength; however, post-impact strengths could greatly improve. The hybrid sandwich designs improved post-impact behavior but more than three facesheet layers are necessary for significant improvement. Hemp facesheet sandwiches showed the best post-impact bending characteristics of any sandwich despite having the largest impact damage sizes. Glass and aramid fiber facesheet sandwiches resisted impact the best but this resulted in premature delamination failures that limited the potential of these structures. Honeycomb core materials outperformed foam in terms of ultimate bending loads but post-impact strengths were better for foam cores. Decent agreement between numerical and experimental results was found but poor material quality and high error in material properties testing results brought about larger disagreements for some sandwich designs

Study to investigate design, fabrication and test of low cost concepts for large hybrid composite helicopter fuselage, phase 1

The development of a frame/stringer/skin fabrication technique for composite airframe construction was studied as a low cost approach to the manufacture of large helicopter airframe components. A center cabin aluminum airframe section of the Sikorsky CH-53D helicopter was selected for evaluation as a composite structure. The design, as developed, is composed of a woven KEVLAR-49/epoxy skin and graphite/epoxy frames and stringers. To support the selection of this specific design concept a materials study was conducted to develop and select a cure compatible graphite and KEVLAR-49/epoxy resin system, and a foam system capable of maintaining shape and integrity under the processing conditions established. The materials selected were, Narmco 5209/Thornel T-300 graphite, Narmco 5209/KEVLAR-49 woven fabric, and Stathane 8747 polyurethane foam. Eight specimens were fabricated, representative of the frame, stringer, and splice joint attachments. Evaluation of the results of analysis and test indicate that design predictions are good to excellent except for some conservatism of the complex frame splice

Carbon Fiber Monocoque Development for a Formula SAE Racecar

Monocoque development of the 2015 Cal Poly Formula SAE racecar from design to competition

Feasibility of Hybrid Thermoplastic Composite-Concrete Load Bearing System

Thermoplastic composites have many advantages over thermoset composites such as being recyclable, rapidly manufacturable, and more impact resistant. The goal of this thesis is to assess the feasibility of using thermoplastic composites in structural applications through literature review, mechanical testing, design of a load-bearing hybrid composite-concrete structures, and the implementation of thermoplastic composites for tensile reinforcement of concrete. The study had four objectives covering the stated goal. Conduct a literature review to direct thermoplastic material selection Characterize thermoplastic material mechanical properties using standardized mechanical testing Design a hybrid composite-reinforced concrete beam, and Develop thermoplastic shear connectors to develop composite action between thermoplastic reinforcement and concrete Initially, thermoplastics that can be reinforced with E-glass fibers to be used as a structural part were investigated. Materials were selected for experimental characterization after extensive literature review based on performance, cost and manufacturing methods. Two industry accepted processes were selected for use in fabrication: vacuum infusion, a longstanding and highly accepted process traditionally used for the manufacturing of thermoset composites; and thermoforming, a fast production process that takes advantage of many properties of thermoplastic materials. Next, properties of these materials required for structural applications were quantified through mechanical testing. These properties include the modulus of elasticity, Poisson’s ratio and the ultimate strength in tension, compression and shear in principal material directions. Having a complete list of material properties is necessary in composite design. A design for a load-bearing composite-concrete beam was developed. In conventional construction, steel reinforcing bars are used to carry the tension in a concrete beam, but steel is susceptible to corrosion. These hybrid composite-concrete structures rely on the transfer of forces (composite action) between the thermoplastic composite, which acts as reinforcement, and the concrete section of the beam. The composite action is necessary for the composite reinforcement to develop tension through shear flow at the interface. The initial design to demonstrate the use of thermoplastic composites in this manner is the fabrication of a simple prismatic beam with the bottom-face reinforced with the composite. This provides a simple structure to demonstrate the feasibility of this technology for use in structural applications. Finally, the ability of the shear connectors developed to produce composite action in the proposed beam was experimentally assessed. Hybrid composite-concrete specimens were tested in compression to assess the feasibility of shear connectors (studs) to carry the shear flow at the interface between the thermoplastic reinforcement and concrete. Conclusions and recommendations are presented in Chapter 5. Recommendations for future work include the implementation of small-scale short-beam tests in four-point bending to further assess the degree of composite action being generated in the structure. Recommendations for future research on more effectively achieving composite action in hybrid thermoplastic composite-concrete members is also addressed

Formula SAE Monocoque Chassis Development

Formula SAE is a collegiate competition hosted by SAE International with the primary goal being to design, manufacture, and race an open wheel race car. The Cal Poly Racing Formula SAE team strives for improvement every race season and has remained competitive as a result. The 2019-2020 management team determined that further research and development towards the chassis would yield the greatest performance benefit for future seasons, as the previous chassis platform limited packaging and mounting options for vehicle subsystems which interfaced with the chassis. A redesign of the Cal Poly Racing Formula SAE team’s carbon fiber reinforced polymer monocoque chassis was requested to improve subsystem integration, increase torsional stiffness, and reduce weight compared to the previous platform. Specifically, this senior project team focused on manufacturing process improvement and laminate design to meet these goals for the 2020 Formula SAE competition. This report details the design and manufacturing of such a chassis. Specific emphasis was placed on the geometry, laminate, and manufacturing process design. The geometry was designed using subsystem input for satisfactory integration of all subsystem components while maintaining a high specific torsional stiffness. The team also developed numerous analysis tools including spreadsheets and finite element models to design the asymmetric laminate of the chassis. Modular, multi-piece tooling was designed to produce a single-piece chassis and to allow for easy geometric changes in the future. Though two complete chassis were delivered to the Formula SAE team, the outbreak of COVID-19 prevented the collection of data that would have been used to validate the design. However, the Formula SAE team was made aware of the validation plan proposed in this report

Sandwich core periodic cell topology effects

Les panneaux composites sandwich possédant une âme nid d'abeille permettent de disposer à la fois de propriétés statiques hors plan intéressantes (en raison de leur rigidité équivalente élevée) et de caractéristiques de masses faibles. Pour cette raison, ils sont largement utilisés dans les industries aérospatiale, automobile et navale. Les environnements dans lesquels ces matériaux sont utilisés mettent en jeu des efforts dans des gammes de fréquences larges. Si un rapport rigidité / masse élevé est profitable dans le domaine des basses fréquences, il conduit généralement à des comportements vibratoires et acoustiques médiocres lorsque la fréquence d’excitation augmente. La question abordée dans ce travail peut être formulée comme : comment les concepts périodiques peuvent-ils améliorer les signatures vibroacoustiques large bande et les performances de ces structures ? La plupart des solutions vibroacoustiques sont limitées en terme de bande de fréquences d’efficacité, et induisent généralement un ajout de masse. La prise en compte de règles de conception vibroacoustiques à un stade précoce du développement du produit est l'un des principaux objectifs de recherche en vue d’améliorer leurs performances et permettrait de concevoir des structures accordées sans aucune intervention ultérieure ou augmentation de masse. Ce travail se concentre donc sur l'étude des topologies de base de panneaux sandwich existants et a pour objectif de créer de nouvelles structures améliorées. La recherche a été menée en essayant de maintenir les propriétés structurelles souhaitées, ce qui justifie l'utilisation d'une telle solution en premier lieu, mais également en considérant son utilisation potentielle comme plate-forme pour la mise en place d’inserts de matériaux périodiques résonants. Ces noyaux cellulaires ont été fabriqués en utilisant la technique du Kirigami (qui est une variante de l'Origami) : il s’agit d’une ancienne technique japonaise qui consiste à créer des structures 3D en pliant et en découpant une feuille de matériau 2D. Cette technique de fabrication peut être utilisée comme un moyen systématique de produire des configurations générales en nid d'abeilles avec des composites à fibres longues par thermoformage et / ou autoclavage. Le principal indicateur utilisé ici afin d’évaluer les performances vibroacoustiques des topologies innovantes proposées est le nombre et la plage de bandes d'arrêt, également connues sous le nom de bandes interdites, qui décrivent les plages de fréquences dans lesquelles les ondes élastiques ne peuvent pas se propager dans la structure. Ce manuscrit est organisé en cinq chapitres. Le premier consiste en un bref aperçu des structures périodiques dans les différents domaines d'ingénierie. L'accent est mis sur les panneaux sandwich et leurs techniques de fabrication les plus populaires sera également décrit. Le deuxième chapitre présentera au lecteur le concept de propagation des ondes élastiques dans les milieux périodiques. De plus, des phénomènes comme les interférences de Bragg ou les bandes interdites résonantes seront présentés ainsi que la théorie de Floquet-Bloch appliquée aux structures à périodiques typiquement utilisées dans l’aéronautique. Cette dernière dérivation mathématique sera fusionnée avec l'approche d'analyse par éléments finis et mise en œuvre comme base pour les outils de prédiction numérique spécialement développés afin de permettre la réalisation d’investigations paramétriques sur des panneaux sandwich complets ou des cœurs nus. La théorie de Floquet-Bloch permet de récolter des informations cruciales sur le comportement dynamique de l’ensemble de la structure en n’effectuant l’analyse que sur une petite partie de celle-ci (cellule unitaire).[...]Honeycomb sandwich panels are well known to provide interesting static out of plane properties because of their high equivalent stiffness whilst containing mass and for this reason, they are widely used as a ‘building brick’ in the Aerospace, Automotive and Naval industries. The environment in which these materials operate involve external forces which excites them in the mid-low frequency range. However, while a high stiffness/mass ratio is a desirable static property, the vibration frequency domain is usually in the high range and therefore they become poor mechanical and acoustic insulators within the frequency range they are usually subjected to. The question addressed then is simple: how periodic concepts can improve the broadband vibroacoustic signatures and performances of those structures? Most of vibroacoustic solutions are frequency band limited, specific and usually include the addition of mass, which for certain engineering segments is disadvantageous. Including vibroacoustic design rules at early stage of product development is one of the main research targets to improve their performance and would allow to design tuned structures without any later intervention or mass increment. This work focuses on investigating existing sandwich panel core topologies and attempt to create novel improved structures. The research was carried out trying to maintain the desired structural properties which justifies the usage of such solution in the first place but also considering its potential use as a platform for Multiphysics resonating periodic material inserts. Such cellular cores were manufactured using Kirigami, which is a variation of Origami, an ancient Japanese technique that consists in creating 3D structures by folding a 2D sheet of material. This manufacturing technique can be used as a systematic way to produce general honeycomb configurations with off-the-shelf long fibre composites by thermoforming and/or autoclaving. The main indicator on which I will focus to evaluate the vibroacoustic performance of the proposed innovative topologies will be the number and range of stopbands, also known as a bandgaps, which describe the frequency ranges in which elastic waves are not transmitted within the structure, in combination with the constituent material and its damping properties. This manuscript is organised in five chapters. The first one consists of a brief overview on periodic structures in the various engineering domains. Emphasis on Sandwich panels and their most popular manufacturing techniques will also be described. The second chapter will introduce the reader to the concept of elastic wave propagation in periodic media. Also, phenomena like Bragg or resonant bandgaps will be explained as well as the Floquet-Bloch theory applied to macro-scale structures such as aeronautical cellular cores.[...

Design and Analysis of a Composite Monocoque for Structural Performance : a Comprehensive Approach

Indiana University-Purdue University Indianapolis (IUPUI)Lately numerous studies have been performed to design composite monocoques with high strength and low weight for various student level racing contests. The objective of this paper is to develop an insightful methodology to design and de veloped a light-weight composite monocoque. The monocoque is designed to pass the mandatory static load tests laid down by the International Automobile Feder ation (FIA)Formula 3. These Formula 3 tests are considered the baseline of the desired structural integrity of the composite monocoque. The presented design tech nique emphasises on a monocoque developed for Sports Car Club of America (SCCA) races. The three standard load tests performed on the monocoque are Survival Cell Side test, Fuel Tank test and Side Intrusion test. A sandwich layup of bi-directional woven carbon/epoxy prepreg and aluminium honeycomb is optimized for minimum weight while predicting the unknown properties of layup and ensuring the mono coque doesnt experience failure. The approach intends to achieve minimum weight with high torsional rigidity and is capable of being used for the design and analysis of any kind of formula type composite monocoque