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The present paper takes a novel approach to production of fibre-reinforced thermoplastic tubes. The method begins with the raw materials, reinforcing fibre and thermoplastic granulate which are processed to tapes through a newly developed direct impregnation process. It is followed by consolidation of fibre-reinforced thermoplastic tubes using infrared (IR) emitters in the filament winding process. This process employs various angles and utilizes a rotatable consolidation axis. The winding process operates at a constant speed, addressing the challenge of bending the fibre-reinforced tapes in the angle reversal areas near the tube ends. Experiments have confirmed that the process can run at speeds reaching approximately 470 mm/min. The design of the impregnation line takes into account the properties of the thermoplastic and the roving, allowing for a speed of up to 1 m/s.
Test Setup for Investigating the Impact Behavior of Biaxially Prestressed Composite Laminates
(2024)
Instrumented impact testing and compression-after-impact testing are important to adequately qualify material behavior and safely design composite structures. However, the stresses to which fiber-reinforced plastic components are typically subjected in practice are not considered in the impact test methods recommended in guidelines or standards. In this paper, a test setup for investigating the impact behavior of composite specimens under plane uniaxial and biaxial preloading is presented. For this purpose, a special test setup consisting of a biaxial testing machine and a specially designed drop-weight tower was developed. The design decisions were derived from existing guidelines and standards with the aim of inducing barely visible impact damage in laminated carbon fiber-reinforced plastic specimens. Several measurement systems have been integrated into the setup to allow comprehensive observation of the impact event and specimen behavior. A feasibility test was performed with biaxially prestressed carbon fiber-reinforced plastic specimens in comparison with unstressed reference tests. The compressive-tensile prestressing resulted in lower maximum contact forces, higher maximum deflections, higher residual deflections and a different damage pattern, which was investigated by light microscopic analysis. Finally, the functionality of the experimental setup is discussed, and the results seem to indicate that the test setup and parameters were properly chosen to investigate the effect of prestresses on the impacts behavior of composite structures, in particular for barely visible subsequent damages.
Review of elasto-static models for three-dimensional analysis of thick-walled anisotropic tubes
(2023)
Most shell or beam models of anisotropic tubes under bending have no validity for thick-walled structures. As a result, the need to develop three-dimensional formulations which allow a change in the stress, strain and displacement distributions across the radial component arises. Basic formulations on three-dimensional anisotropic elasticity were made either stressor displacement-based by Lekhnitskii or Stroh on plates. Lekhnitskii also was the first to expand these analytical formulations to tubes under various loading conditions. This paper presents a review of the stress and strain analysis of tube models using three-dimensional anisotropic elasticity. The focus lies on layered structures, like fiber-reinforced plastics, under various bending loads, although the basic formulations and models regarding axisymmetric loads are briefly discussed. One section is also dedicated to the determination of an equivalent bending stiffness of tubes.
Although many composite structures are inconsistently curved, such as the leading edges of aircraft wings, the variety of research in impact engineering is almost limited to the impact performance of plates or cylindrically curved specimens. It is not known whether the findings obtained from standardized tests can be transferred to curved structures or which adaptions are required. Therefore, a deeper understanding of the deformation and damage behavior of inconsistently curved structures is essential to transfer the observed impact behavior of flat specimens to general curved structures and therefore to utilize the full lightweight potential of a load-specific design. An accurate description of the procedure as well as the results of the experimental and numerical study of the low-velocity impact behavior of differently single-curved elliptic specimens is presented. To close the research gap of the impact behavior of geometries with curvatures between the plates and simplified leading edges, novel specimens geometries have been derived from established impact test standards. Glassfiber-reinforced specimens are subjected to an instrumented impact test at constant impact energy. This is numerically investigated by a stacked-layer model, which used cohesive zone modeling to enable the simulation of matrix cracking, fiber fracture and delamination. The resulting projected damage areas, as well as the force and deflection histories, were evaluated and section cuts were examined to discuss the damage morphology, formation and propagation process. Significant effects on maximum deflection, compliance and dynamic behavior on the size and morphology of damage were found.
Additive Fertigung hat sich in zahlreichen industriellen Anwendungen etabliert und bildet eine wichtige Schlüsseltechnologie. Im Gegensatz zu metallischen Werkstoffen, haben additiv gefertigte Bauteile aus Kunststoffen geringere Festigkeit und Steifigkeit, sodass sich ihre Verwendung als lasttragende Strukturen schwierig gestaltet. Insbesondere der Einsatz von endlosen Verstärkungsfasern kann die mechanischen Eigenschaften additiv gefertigter Strukturen signifikant verbessern und die Fertigung hochbelastbarer Faserverbundstrukturen im 3D-Druckverfahren ermöglichen. Daher gilt es aktuell notwendige Anlagen und Prozessketten für den Fertigungsprozess aber auch Vorgehensweisen für die belastungsoptimierte Auslegung der Faserverläufe innerhalb des Bauteils zu entwickeln, um so das Themengebiet „Endlosfaserverstärkter 3D-Druck“ weiter voranzutreiben. Für mehr räumliche Freiheit bei der Positionierung der Druckbahnen können industrieroboterbasierte Systeme eingesetzt werden, um so das Potenzial gerichteter Bauweise von Faserverbundstrukturen auch im additiven Fertigungsprozess vollumfänglich ausschöpfen zu können. Dabei ermöglicht ihr Einsatz auch eine räumliche Ablage der Faserverstärkung, wobei für die Materialablage ein geeigneter 3D-Druckkopf erforderlich ist. Für die Implementierung der Faserverstärkung ist zudem eine dem Lastfall entsprechende Auslegung des Bauteils sowie die Ermittlung einer sinnvollen Faserpositionierung innerhalb des Bauteils erforderlich, wobei unterschiedliche Variablen aus den Bereichen Material, Struktur und Fertigungsprozess berücksichtigt werden müssen. Im Rahmen des Vortrages werden die Herausforderungen der Technologieentwicklung des endlosfaserverstärkten 3D-Drucks aufgegriffen sowie auf eine belastungsorientierte Faserpositionierung näher eingegangen. Aktuelle Erkenntnisse werden diskutiert sowie eine Entwurfsmethodik für die Prozesspfadgenerierung vorgeschlagen.
Die Erforschung von faserverstärkten Kunststoffen (FVK) mit thermoplastischer Matrix ist aufgrund der Schweißeignung als Verbindungstechnologie, der thermischen Umformbarkeit, der Recyclebarkeit sowie der verkürzten Prozesszeiten im Vergleich zu den vorwiegend eingesetzten faserverstärkten Duroplasten erstrebenswert. Die Wechselwirkung zwischen den physikalischen, mechanischen und geometrischen Parametern erhöht den Komplexitätsgrad des Herstellungsprozesses. Ziel ist daher die Entwicklung einer Imprägniertechnik, die eine Fasertränkung mittels eines Thermoplastschmelzbades ermöglicht.