Auswahl der wissenschaftlichen Literatur zum Thema „Out-Of-Autoclave Consolidation“

Fundamentally, the mechanical characteristics of 3D-printed polymeric objects are determined by their fabrication circumstances. In contrast to traditional polymer processing processes, additive manufacturing requires no pressure during layer consolidation. This study looks at how a high-pressure autoclave chamber without oxygen affects layer consolidation throughout the fused deposition modelling process, as well as the mechanical qualities of the products. To attain high strength qualities for 3D-printed components such as injection-molded specimens, an experimental setup consisting of a 3D printer incorporated within a bespoke autoclave was designed. The autoclave can withstand pressures of up to 135 bar and temperatures of up to 185 °C. PLA 3D printing was carried out in the autoclave at two different pressures in compressed air and nitrogen atmospheres: 0 bar and 5 bar. Furthermore, injection molding was done using the same PLA material. Tensile, flexural, and Charpy tests were carried out on samples that were 3D printed and injection molded. In nitrogen, oxidation of the environment was prevented by autoclave preheating before printing, and autoclave pressure during printing considerably promotes layer consolidation. This imprinted mechanical strength on the 3D-printed items, which are virtually as strong as injection-molded components.

Contrary to other polymer processing methods, additive manufacturing processes do not require any pressure during the consolidation of layers. This study investigates the effect of high ambient pressure on the consolidation of layers during the FDM process and their analysis of mechanical properties. An experimental setup was arranged, consisting of a 3D printer integrated into a customized Autoclave, to achieve high strength properties for 3D printed parts as like injection-molded specimens. The autoclave can maintain 135 bar of pressure and a maximum temperature of 185 °C. 3D printing with PLA was carried out at 0 bar, 5 bar, and 10 bar. Tensile, flexural, and Charpy tests were conducted on printed specimens, and the effect of pressure and temperature on 3D-printed samples were analyzed. It could be shown that autoclave preheating before printing and autoclave pressure during printing improves the consolidation of layers immensely. The pressure inside the autoclave provokes a more intimate contact between the layer surfaces and results in higher mechanical properties such as yield strength, Young’s modulus, and impact strength. The properties could be raised 100%.

Three different out-of-autoclave manufacturing processes of CF/poly-ether-ether-ketone thermoplastic composites were characterized, including innovative laser-assisted automated fibre placement with in situ consolidation. Characterization techniques included differential scanning calorimetry, ultrasonic non-destructive testing and matrix digestion, in addition to 3D X-ray microcomputed tomography to investigate the void distribution, size and shape. The results revealed that in situ consolidation process can lead to the accumulation of large voids between the upper layers. Interlaminar shear, in-plane shear, tensile and flexure testing were used for mechanical evaluation. A reduction in the mechanical properties was observed for in situ consolidation laminates when compared to the other out-of-autoclave methods. The drop in mechanical properties of in situ consolidation laminates was mainly attributed to the differences found in void distribution and size. Optimization of processing parameters along with higher quality prepreg raw material could be of assistance for the improvement of mechanical properties of in situ consolidation structures.

Autoclaving is a process that ensures the highest quality of carbon fiber reinforced polymer (CFRP) composite structures used in aviation. During the autoclave process, consolidation of prepreg laminas through simultaneous elevated pressure and temperature results in a uniform high-end material system. This work focuses on analyzing in a fundamental way the applications of pressure and temperature separately during prepreg consolidation. A controlled pressure vessel (press-clave) has been designed that applies pressure during the curing process while the temperature is being applied locally by heat blankets. This vessel gives the ability to design manufacturing processes with different pressures while applying temperature at desired regions of the composite. The pressure role on the curing extent and its effect on the interlayer region are also tested in order to evaluate the consolidation of prepregs to a completely uniform material. Such studies may also be used to provide insight into the morphology of interlayer reinforcement concepts, which are widely used in the featherweight composites. Specimens manufactured by press-clave, which separates pressure from heat, are analytically tested and compared to autoclaved specimens in order to demonstrate the suitability of the press-clave to manufacture high-quality composites with excessively reduced cost.

This review considers the influence of resin-rich volumes (RRV) on the static and dynamic mechanical and physical behaviour of fibre-reinforced composites. The formation, shape and size, and measurement of RRV in composites, depending upon different fabric architectures and manufacturing processes, is discussed. The majority of studies show a negative effect of RRV on the mechanical behaviour of composite materials. The main factors that cause RRV are (a) the clustering of fibres as bundles in textiles, (b) the stacking sequence, (c) the consolidation characteristics of the reinforcement, (d) the resin flow characteristics as a function of temperature, and (e) the composite manufacturing process and cure cycle. RRV are stress concentrations that lead to a disproportionate decrease in composite strength. Those who are considering moving from autoclave consolidation to out-of-autoclave (OOA) processes should be cautious of the potential effects of this change.

In additive manufacturing technologies, fused deposition modelling (FDM) is continuing its advancement from rapid prototyping to rapid manufacturing. However, effective usage of FDM is not performed due to the poor mechanical properties of the 3D-printed components. This drawback restricts their usage in many applications. Much research, such as reinforcing 3D-printed parts with fibers, changing printing parameters (infill density, infill concentration, extrusion temperature, nozzle diameter, layer thickness, raster angle, etc.) are aimed to increase the mechanical properties of 3D-printed parts. This research paper aims to investigate the effect of pressure and temperature on the mechanical properties and consolidation of layers of 3D-printed PLA (Polylactic Acid). Post-treatment was done using a customized autoclave. Autoclave has the capability to maintain 185 °C and 135 bar pressure. Three-dimensional-printed specimens were manufactured using the FDM process with two patterns. Later, the specimens were subjected to various post-treatment processes, then followed with testing and analysis of mechanical properties. Post-treatment process carried out by placing them in an autoclave at certain pressure and temperature conditions. To investigate the repeatability and tolerances, the test series includes a minimum of four to six test specimens. The results indicate that the concentric pattern yields the most desirable tensile, impact, and flexural strength due to the alignment of deposited rasters and better consolidation of layers with the loading direction. The pressure and temperature of the autoclave has a positive effect on the PLA samples, which helped them to reorganize the structure, hence strength properties were enhanced. The test results also compared with injection-molded samples for better understating.

A low-cost, low-waste manufacturing method for advanced thermoset composite parts could improve market penetration of composites compared to other engineering materials such as aluminum or steel. Such a method could combine some of the new trends in composites manufacturing such as resin infusion (eliminates need for prepreg), out-of-autoclave consolidation, and snap curing. The feasibility of a hybrid process with these characteristics has been demonstrated by uniting liquid composite molding, resin curing by electron beam irradiation, and high pressure consolidation with specialized elastomeric tooling. To demonstrate feasibility, a mold set was designed to make flat, square four-ply woven carbon fiber parts by (1) vacuum-infusing dry preforms with an electron beam–curable epoxy resin in minutes, (2) applying 690 kPa of uniform pressure and consolidating in seconds using an elastomer-faced specialized elastomeric tooling tool and simple hydraulic press, and (3) curing in seconds using a 3 MeV electron beam source. To better understand how various process parameters affect part performance, parameters are varied in a simple design of experiments, and flexural strength and stiffness, thickness distribution, fiber and void volume fractions, surface roughness, and cross-sectional characteristics (via microscopy) are measured and compared.

Out-of-autoclave (OOA) prepreg processing is a new manufacturing method for high performance composites that seeks to reduce costs and increase production flexibility relative to traditional autoclave processing. OOA prepregs may be vacuum bag-only cured in conventional ovens to produce autoclave-quality parts with low void contents. To achieve such quality under vacuum consolidation, OOA prepregs feature dry areas that allow gas evacuation during the early stages of processing but must be subsequently infiltrated with resin. The present thesis describes a four-part investigation into the consolidation of such OOA prepregs, with particular emphasis on flow phenomena and flow-induced defects. First, a novel X-ray computed microtomography approach was successfully used to determine the microstructure of a representative OOA prepreg and track its evolution during processing. The results showed that, initially, the prepreg consisted of dry, micro-porous fibre tow cores surrounded by resin-rich areas and macro-pores, and that two phenomena occurred during processing: the progressive collapse of these macro-pores due to air evacuation at room temperature, and the impregnation of the dry tows at elevated temperature. Second, an analytical model for tow impregnation was developed for three OOA prepregs, and used in a parametric study to evaluate the effect of various material properties and process parameters. The results showed that towimpregnation can successfully occur in a wide variety of cases, but that deviations from ideal situations may lead to incomplete tow impregnation and flow-induced micro-porosity. Third, the effects of cure cycle and out-time on OOA prepreg consolidation were studied experimentally through material characterization, laminate manufacture and microstructural quality analysis. The results demonstrated that out-time had a detrimental effect on resin properties, process phenomena and tow micro-porosity, but that certain cure cycles mitigated or even eliminated such defects. These results were also compared to model predictions, which showed agreement with the observed trends and part quality at low out-times but under-predicted micro-void sizes at very high out-times. Finally, the effects of four deficient consolidation cases (repeated debulks, reduced ambient pressure, reduced vacuum and restricted air evacuation) were studied experimentally by laminate manufacture and final quality evaluation. The results established that reduced ambient pressure, reduced vacuum and restricted air evacuation had specific detrimental effects on consolidation phenomena and on macro- and micro-porosity within the final part, and clarified the allowable extent of such deviations for successful part manufacture. Overall, the thesis contributed to the knowledge on OOA prepreg processing in several ways. First, the key consolidation phenomena that occur during OOA prepreg consolidation were identified. Second, the effect of several material properties, process parameters and deviations from ideal conditions were evaluated to aid process optimization and define the allowable process windows for successful part manufacture. Finally, a new set of experimental and modelling tools were proposed to aid material characterization, process analysis and quality prediction.
La mise-en-forme de pré-imprégnés hors-de-l'autoclave est une nouvelle méthode de fabrication de composites haute-performance qui cherche à réduire les coûts et à augmenter la flexibilité par rapport à la fabrication traditionnelle par l'autoclave. Les pré-imprégnés hors-de-l'autoclave peuvent être mis en forme sous un sac à vide, dans un four conventionnel. Pour limiter la porosité, ils comportent des zones sèches qui permettent l'évacuation de gaz au début de la mise-en-forme et qui doivent ensuite être infiltrées par la résine. Cette thèse présente une investigation en quatre parties de la consolidation de tels pré-imprégnés, avec une emphase particulière sur le phénomène et les défauts induits par d'écoulement.En premier lieu, une nouvelle approche basée sur la microtomographie par rayons X a été utilisée avec succès pour déterminer la microstructure initiale d'un pré-imprégné hors-de-l'autoclave typique et pour suivre son évolution pendant la mise-en-forme. Les résultats ont démontré que, initialement, le pré-imprégné était constitué de noyaux de faisceaux de fibres entourés de zones riches en résine et de macro-pores, et que deux phénomènes ont prit place pendant la mise-en-forme: la disparition progressive des macro-pores due à l'évacuation des gaz à température ambiante, et l'imprégnation des faisceaux à températures élevées. En deuxième lieu, un modèle analytique décrivant l'imprégnation des faisceaux à été développé pour trois pré-imprégnés hors-de-l'autoclave, et utilisé pour une étude paramétrique afin d'évaluer l'effet de plusieurs propriétés des matériaux et paramètres de mise-en-forme. Les résultats ont démontré que l'imprégnation des faisceaux peut avoir lieu dans la majorité des cas, mais que des déviations des situations idéales peuvent causer une imprégnation incomplète et de la microporosité induite par l'écoulement. Ensuite, les effets du cycle de température et du temps d'exposition aux conditions ambiantes ont été étudiés par la caractérisation des matériaux, la fabrication de laminés et l'analyse de leur microstructure. Les résultats ont démontré que le temps d'exposition aux conditions ambiantes a un effet néfaste sur les propriétés de la résine, sur les phénomènes prenant lieu durant la mise-en-forme et sur la micro-porosité dans les faisceaux des pièces fabriquées; toutefois, ils ont aussi démontré que certains cycles de température ont mitigé ou même éliminé ces défauts. Ces résultats ont aussi été comparés aux prédictions du modèle, démontrant que ce dernier a capturé les tendances observées, mais qu'il a sous-prédit la taille des micro-porosités pour les cas d'exposition élevée aux conditions ambiantes. Finalement, les effets de quatre cas de consolidation déficiente (l'application de vide répétée, la pression ambiante réduite, le vide réduit et l'évacuation d'air restreinte) ont été étudiés expérimentalement par la mise-en-forme de laminés et l'évaluation de leur qualité. Les résultats ont établi que la pression ambiante réduite, le vide réduit et l'évacuation d'air restreinte ont des effets néfastes spécifiques sur les phénomènes de consolidation et sur la macro- et micro-porosité dans les pièces fabriquées, et clarifié l'ampleur permissible de telles déficiences pour des laminés d'une qualité acceptable. Globalement, cette thèse a contribué à des plus amples connaissances et une meilleure compréhension de la mise-en-forme de pré-imprégnés hors-de-l'autoclave. Premièrement, les principaux phénomènes prenant lieu pendant ce procédé ont été identifiés. Deuxièmement, les effets de plusieurs propriétés des matériaux, paramètres de mise-en-forme et déviations des conditions idéales ont été évalués afin d'aider à l'optimisation et de définir les fenêtres permissibles de variation pour une mise-en-forme réussie. Finalement, de nouvelles méthodes expérimentales et de modélisation ont été proposées pour aider la caractérisation des matériaux, l'analyse de procédés et la prédiction de qualité.

Vacuum bags in conjunction with autoclaves are currently employed to generate the consolidation pressures and temperatures required to manufacture aerospace level composites. As the scale of continuous fiber composite structures increases autoclaving becomes prohibitively expensive or impossible. The objective of this work is to develop flexible magnetic clamping structures to increase the consolidation pressure in conventional vacuum bagging of composite laminates, thereby obviating the need for an autoclave. A ferromagnetic rubber, which consists of rubber filled with iron, is being developed as a conformable and reusable vacuum bag that provides increased consolidation through attractive forces produced by electromagnets. Experiments and finite element modeling indicate that consolidation pressure in the range of 100 kPa can be generated by such a device with realistic power requirements. The effects of the magnetic clamping device process parameters on the consolidation pressure magnitude are modeled and characterized. In addition, a method for the efficient design of the magnetic clamping device is developed.

Les composites à fibres de carbone continues et matrice thermoplastique haute performance présentent un intérêt fort pour l’industrie aéronautique comparés aux traditionnels composites à matrice thermodurcissable, pour la diminution des temps de procédé ainsi que pour leur potentielle aptitude à être soudés et recyclés. C’est dans ce contexte que le projet collaboratif HAICoPAS (Highly Automatized Integrated Composites for Performing Adaptable Structures) porté par Hexcel et Arkema et regroupant un consortium industriel et académique, a vu le jour. Il a pour but de développer toute la chaîne de production, allant de la pré-imprégnation de la nappe jusqu’au démonstrateur, permettant de mettre en œuvre un composite fibres de carbone continues à matrice PEKK (Polyetherketoneketone) de la famille des PAEK (Polyaryletherketone) capable de répondre aux exigences industrielles, notamment de produire des pièces consolidées hors autoclave avec un taux de porosité inférieur à 1 %.Sachant que la résorption des porosités intra ou interplis demande un écoulement, certes local, mais d’ensemble du composite, des essais de rhéologie en squeeze flow ainsi que leur modélisation ont permis de mieux appréhender le comportement visqueux complexe du système fortement renforcé en fibres. Les paramètres de viscosité en loi de puissance du composite à 1 bar ont pu être identifiés, ainsi que leur augmentation avec la pression appliquée, corrélée au phénomène de shear banding. Puis, la mise en place d’essais de consolidation sous bâche à vide en étuve couplée à des essais modèles sous rhéomètre reproduisant le même cycle temps-température-pression, ont permis de mettre en évidence les relations procédé / microstructure / propriétés mécaniques induites. Une bonne dispersion de fibres et a fortiori peu de porosités confinées dans des zones très sèches en résine, est nécessaire pour faciliter la consolidation hors autoclave du composite. Aussi, une augmentation de la résistance au cisaillement interpli (ILSS), observée avec le temps de consolidation, s’est avérée être liée à l’homogénéisation de la répartition {fibres + matrice} aux interplis plutôt qu’au taux de porosité généralement considéré comme critère discriminant de la qualité de consolidation. Enfin, le rôle non négligeable joué par les volatils et le taux d’humidité en particulier, dans la consolidation du composite a pu être identifié
Carbon Fiber/High performance thermoplastic matrix composites are of great interest for the aeronautical industry, for the reduction of process times as well as for their potential ability to be welded and recycled compared to their thermoset matrix-based composites counterparts. In this context, the HAICoPAS (Highly Automatized Integrated Composites for Performing Adaptable Structures) collaborative project, built around an industrial and academic consortium and led by Hexcel and Arkema, aims to develop the entire production chain of a continuous carbon fiber reinforced composite with a PEKK (Polyetherketoneketone) matrix. This goes from the pre-impregnation of the tape to the welding of real parts capable of meeting industrial requirements, in particular, consolidate parts in out-of-autoclave (OOA) system with a porosity rate inferior to 1%.As the resorption of intra or interply voids requires a local flow of the whole composite, squeeze flow rheological tests have been modeled to understand the viscous behavior of this highly filled system. The power law viscosity parameters have been identified at 1 bar, as well as an unexpected increase of these parameters with the applied pressure which has been related to "shear banding". Then, consolidation experiments under vacuum bag in an oven, coupled with model rheological tests reproducing the same time-temperature-pressure cycle, have highlighted the process / microstructure / mechanical properties relationships induced. A good dispersion of fibers along with few porosities confined in dry areas is necessary to facilitate the out of autoclave consolidation of the composite. Also, an increase in the interlaminar shear strength (ILSS), observed with consolidation time, was found to be related to the homogenization of the {fibers + matrix} distribution at the interplies rather than to the more usually considered porosity rate. Finally, the important role played by the volatiles and the moisture content in particular, in the consolidation of the composite was identified

Abstract Composite repairing is an issue because of the size of the parts, the cost of the production equipment and the poor consolidation between the parent part and the repairing patches. In this study an out-of-autoclave procedure is proposed where the agglomeration pressure is provided by shape memory foams. These foams can be shaped in the desired form by means of a thermos-mechanical cycle, and provide additional pressure if constrained during recovery. Shape recovery is achieved by using the same thermal cycle which is necessary for the cure of the patches. In this study, thermoplastic shape memory foams have been characterized to evaluate the agglomeration pressure they can provide as a function of the compression strain. In order to show the feasibility of their application, the cure of a composite laminate has been performed by using pre-shaped shape memory foams. Results show that a good consolidation is achieved despite of the absence of a vacuum bag.

Abstract. Carbon fiber reinforced (CFR) composite rings with and without interlaminar carbon nanotubes (I-CNTs) were manufactured starting from high performance prepregs used in aeronautics. Nano-particles deposition was obtained by an innovative easily scalable procedure which does not affect composite processability. A CNTs-solvent solution was sonicated for an established time to reduce bulk CNTs agglomerates. The solution was poured on prepreg stripes and left at room temperature for solvent evacuation. The prepreg stripes were wrapped on steel pipe to obtain the annular configuration and manufactured by an out-of-autoclave (OOA) process. The consolidation pressure was applied through a thermo-shrinkable tube and curing occurred in oven. Stereomicroscopy and mechanical tests were performed on CFR and I-CNTs/CFR rings. A good adhesion among the plies was observed as well as the positive effects of CNTs deposition; an increase of 34%, 15% and 17% was obtained for the stiffness, the peak load and the fracture energy of the functionalized rings, respectively.

Reinforcement compaction sometimes referred as consolidation process and is one of the key steps in various composite manufacturing processes such as autoclave and out-of-autoclave processing. The prepregs consist of semi-cured thermoset resin system impregnating the fibers. hence, the prepreg shows strong viscoelastic compaction response, which strongly depends on compaction speed and stress relaxation. modeling of time-dependent response is of utmost importance to understand the behavior of prepregs during different stages of composites manufacturing processes. The quasilinear viscoelastic (QLV) theory has been extensively used for the modeling of viscoelastic response of soft tissues in biomedical applications. In QLV approach, the stress relaxation can be expressed in terms of the nonlinear elastic function and the reduced relaxation function. The constitutive equation can be represented by a convolution integral of the nonlinear strain history, and reduced relaxation function. This study adopted a quasilinear viscoelastic modeling approach to describe the time dependent behavior of uncured-prepregs under compression. The model was modified to account for the compaction behavior of the prepreg under a compressive load. The deformation behavior of the prepreg is usually characterized by the fiber volume fraction, V . In this study, the material used was a 2/2 Twill weave glass prepreg (M26T) supplied by Hexcel® Industries USA. We performed a compaction experiment of the uncured prepreg at room temperature at different displacement rate and subsequent relaxation to describe the viscoelastic behavior of the prepreg. The model parameter calibration was performed using the trust-region-reflective algorithm in matlab to a selected number of test data. The calibrated model was then used to predict the rate dependent compaction and relaxation response of prepregs for different fiber volume fractions and strain rates.

The Tailored Universal Feedstock for Forming (TuFF) material is an aligned, discontinuous carbon fiber material with high fiber volume fraction up to 63% and mechanical performance equivalent to continuous fiber, unidirectional composites. The short fiber material allows at least 40% in-plane extension during processing enabling metal-like forming approaches simplifying composites manufacturing significantly. Traditionally, TuFF preforms are produced at areal weight (AW) of ~8 grams per square meter (gsm), stacked and impregnated with thermoset or thermoplastic polymer to create prepreg followed by curing/consolidation in an autoclave or stamp forming process resulting in high-performance structural parts. Here, the impregnated TuFF prepreg can be handled the same way as traditional continuous fiber prepreg. In contrast, to enable liquid composite molding (LCM) processes with TuFF material, the unimpregnated (dry) short fiber TuFF preforms must be stabilized for handling and preforming purposes. This paper details an electrospun veil approach as shown in Figure 1 to stabilize the individual TuFF sheets while maintaining the in-plane extensibility for complex geometry parts. Electrospun TPU fibers are applied onto the TuFF surface and then consolidated via a combination of heating and pressure, formingtrials were carried out using the stabilized preforms and composites werefabricated using LCM. Tensile tests show ~90-95% property retention versus theunstabilized baseline. The approach allows fabrication of stabilized TuFF fabricsfor the first time enabling the use of LCM processes for complex geometry parts.