Georgios A. Pappas


Last Name

Pappas

First Name

Georgios A.

ORCID

0000-0003-1570-6885

Organisational unit

01159 - Lehre Maschinenbau und Verfahrenstechnik

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2019 - 201922020 - 202631
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Publications1 - 10 of 33
  • Towards the realization of composite metastructures: A failure analysis of connections
    Item type: Journal Article
    Gaultier, Victor; Pappas, Georgios A. (2024)
    Materials & Design
    Metastructures hold significant potential for applications such as adaptive structures and soft robotics. Architectures of fiber-reinforced polymer metastructures may relate to modular arrangement of straight and curved laminates, with their connections to resemble perfect cracks, thus susceptible to delamination. This study investigated geometrical effects on the load-carrying capabilities of these connections upon a global tensile deformation, as well as lean modeling tools to facilitate the development of architected composite metastructures. Numerical fracture mechanics approach on different connection geometries and thicknesses showed that connection delamination is a critical failure mode, but crack-driving-force has low dependence on connection shape for given ligament thickness (and stiffness). Adopted analytical models could capture either moment or force-driven delamination failure, while the intermediate regime necessitates numerical tools. First-ply failure may precede depending on shape and ligament stiffness. These trends were also verified on an exemplary rotating chiral composite geometry. Furthermore, interface load-carrying capability improvements were studied via design considerations including connection filler material and element variable thickness. Indicatively, the latter showed a 157 % increase in bending deflection (and global deformations), while reducing crack driving force by 38 % for a given load case. The conducted analysis offers valuable insights into the design of lightweight, load-carrying composite metastructures.
  • Manufacturing studies of a polymeric/composite heart valve prosthesis
    Item type: Journal Article
    Chen, Mary Jialu; Pappas, Georgios A.; Smid, Caroline C.; et al. (2024)
    Polymer Composites
    Current transcathether heart valves rely on metal mesh stents, resulting in discontinuous stent-leaflet interfaces that introduce stress concentrations, reducing valve lifetime. This work aims to investigate non-conventional methods to create a fully polymeric transcatheter heart valve, exhibiting a quasi-continuous interface with hemocompatible leaflets with high durability potential. Polyetheretherketone (PEEK) is of particular interest as a cardiovascular material due to hemocompatibility and mechanical resilience, highly relevant for catheter delivered valves. For increased reproducibility and design freedom, an autoclave process was used to manufacture thin-ply PEEK composite stents. We demonstrated that a suitable membrane material during manufacturing is essential to evenly distribute pressure around the stent. Meanwhile, a modified vacuum forming process was used to simultaneously form and attach PEEK leaflets to the stent using a heated mold. This simple and robust method enables rapid manufacturing of an integral PEEK-based valve design, resulting in improved stent-leaflet bonding, demonstrating an alternative to conventional dip-coating. The customized vacuum forming causes a controlled annealing effect in semicrystalline materials such as PEEK. Processing PEEK leaflets at higher mold temperatures results in higher crystallinity, elastic modulus, and bond strength. These processes enable greater design flexibility and promote composite materials for use in heart valve devices.
  • A comparative study of mode I delamination behavior of unidirectional glass fiber-reinforced polymers with epoxy and polyurethane matrices using two methods
    Item type: Journal Article
    Montenegro, Davi M.; Pappas, Georgios A.; Botsis, John; et al. (2019)
    Engineering Fracture Mechanics
  • Thin-ply thermoplastic composites: From weak to robust transverse performance through microstructural and morphological tuning
    Item type: Journal Article
    Schlothauer, Arthur; Pappas, Georgios A.; Ermanni, Paolo (2023)
    Composites Part B: Engineering
    Thin-shell carbon fiber composites have great potential for structures that require large recoverable deformations, high stiffness and low weight, as in deployable space structures, biomedical devices and robotics. Despite being astonishingly flexible in fiber direction, such thin shells are highly sensitive to off-axis loading. This relates to the high manufacturing complexity and sensitivity to imperfections, revealing the need for in-depth understanding and enhancement of their transverse response. This paper provides crucial insights into influencing factors of thin-shell composites’ transverse strength using a highly controllable manufacturing technique to create novel thermoplastic thin-ply (35 μm) carbon fiber-PEEK laminas. The effects of fiber type, microstructure and polymer morphology as well as their interactions, are addressed towards a drastic increase in performance. A combination of microstructure tuning and isothermal crystallization can provide thin-shell composites with a more than 150% improved transverse performance compared to the state-of-the-art. The conducted analysis reveals the sensitivity to all related processing conditions and highlights the effect of their accurate control.
  • Assessment of leaflet designs for a novel fully-polymeric transcatheter heart valve
    Item type: Conference Poster
    Smid, Caroline Charlotte; Pappas, Georgios A.; Cesarovic, Nikola; et al. (2021)
    Approximately 30 million people from industrialized countries are estimated to be affected by heart valve diseases, which is most prevalent in the elderly [1]. Due to the increase in life expectancy worldwide, the prevalence of valvular heart disease is rising rapidly. By 2050, the demand is expected to exceed 850,000 heart valves annually [2]. Despite the continuous and considerable improvement of artificial heart valves, the replacement recurrently leads to a shift from the native valve disease to a prosthetic valve-induced disease due to design-inherent limitations and suboptimal prosthesis selection [3]. The improvement of artificial heart valves concerning functionality, durability, reliability, and cost can result in a shift from open-heart surgery as the first choice for heart valve replacement to minimally-invasive procedures and consequently, significantly reduce complications and therapy expenses for valvular disease. “Especially the reduction of the cost may drastically affect the accessibility of such devices for third world countries, where due to socio-economic circumstances the highest demand for artificial heart valves is observed (Zilla et al., 2008) and the medical equipment for open-heart surgery is not accessible.” (Schlothauer & Ermanni, 2019) Especially in the field of heart valves with large diameters, there are additional challenges, meaning that there is still a lot of need for research and development. A fully-polymeric transcatheter heart valve combining the advantages of the two main heart valve prosthesis types, namely the mechanical and the bioprosthetic heart valve, could address the current challenges. This new heart valve concept should be applicable to all anatomical heart valve diameters in order to be able to use them for a Total Artificial Heart as a superior goal. The hypothesis is that this concept can be enabled by a CF-PEEK composite stent and leaflets made of an ultra-stiff material. This ultra-stiff material, which is still unexplored for this application, seems promising for the durability, the hemocompatibility, and the possibility of larger atrio-ventricular heart valves. Since the material is approximately 350 times stiffer than the natural leaflet tissue, a new leaflet design is needed that reduces the flexural stiffness in order for the heart valve to exhibit good kinematics. Good kinematics is defined, among other things, by a low transvalvular pressure differential for the opening and a large orifice area in the open state. These characteristics will be investigated experimentally using a pulse duplicator. A pulse duplicator simulates the function of the heart, by generating a pulsatile flow through the heart valve placed in the circuit. In parallel, 'quasistatic' dynamic FEM simulations were performed to study the opening process of the new leaflet designs with different stiffnesses. Preliminary results show that the pulse duplicator mimics physiological conditions and that in the FEA the new designs open at a lower pressure differential than a conventional design mimicking the native leaflets, with the same stiffness. This new heart valve concept could become the next emerging technology. [1] Kadouch, J. and Labojka, D. (2017): Matters of the Heart: Valvular Heart disease today. Paris: The Art & Science of Risk [2] Yacoub, M. H.; Takkenberg, J. J. M. (2005): Will heart valve tissue engineering change the world? In Nature clinical practice. Cardiovascular medicine 2 (2), pp. 60–61. DOI: 10.1038/ncpcardio0112. [3] Pibarot, Philippe; Dumesnil, Jean G. (2009): Prosthetic heart valves: selection of the optimal prosthesis and long-term management. In Circulation 119 (7), pp. 1034–1048. DOI: 10.1161/CIRCULATIONAHA.108.778886.
  • Mechanical characterisation of GF-PET composite manufactured via in-situ Solid-State Polymerisation route
    Item type: Journal Article
    Vetterli, Oliver; Krüger, R.; Hentzen, S.; et al. (2025)
    Composites Part A: Applied Science and Manufacturing
    The work reported in this short communication focusses on the impact of solid-state polymerisation (SSP) of glass fibre-poly(ethylene terephthalate) (GF-PET) composites, on polymer's, interface's, and eventually composite's performance, characterised through transverse tensile testing. Comparison with a state-of-the-art film stacking process revealed that similar, but also improved mechanical performance can be achieved by composites produced via the in-situ (i.e., at composite lamina level) SSP method. When the polymer is reacted at long intervals to achieve high molecular weights in-situ, a robust fibre-matrix interface is apparently formed, yielding fully cohesive failure of the composite (on fully desized fibres), while the improved polymer's ductility enhances further the transverse performance. Composite's ultimate transverse tensile strength saturation was found at a PET intrinsic viscosity of 0.82 dL/g, with values of ∼60 MPa, and ultimate strain of ∼0.82%, with the latter to show a slight increase when the intrinsic viscosity reached 0.95 dL/g, through even-longer SSP times. Thus, the prevalent challenges in thermoplastic reinforced composites, namely high processing viscosity and a weak fibre-matrix interface, are effectively addressed by the developed in-situ SSP route.
  • Novel Processing Route for PET-GF Composite Manufacturing Via SSP
    Item type: Other Conference Item
    Vetterli, Oliver; Pappas, Georgios A.; Ermanni, Paolo (2023)
    Proceedings of the 2023 International Conference on Composite Materials
  • Monitoring of Torque Induced Strain in Composite Shafts with Embedded and Surface-Mounted Optical Fiber Bragg Gratings
    Item type: Journal Article
    Konstantaki, Maria; Violakis, Georgios; Pappas, Georgios A.; et al. (2021)
    Sensors
    In this study, silica glass, optical fiber Bragg gratings (FBGs) are used for torque-induced strain monitoring in carbon fiber reinforced polymer (CFRP) hollow shafts toward the development of a methodology for structural load monitoring. Optical fibers with gratings are embedded during shaft manufacturing, by an industrial filament winding process, along different orientations with respect to its central axis and surface mounted after production. Experimental results are supported by numerical modeling of the shaft with appropriate boundary conditions and homogenized material properties. For an applied torque up to 800 Nm, the strain sensitivity of an embedded grating positioned along the reinforcing fibers’ direction winded under 55° is in the order of 3.6 pm/Nm, while this value is more than 4× times higher than the other examined orientations. The study also shows that surface-mounted optical fiber Bragg gratings along the reinforcing carbon fibers’ direction perform equally well in monitoring strains in composite shafts under torque.
  • Designing Polymeric Cardiovascular Biomaterials for Hemocompatibility and Mechanical Performance
    Item type: Conference Poster
    Chen, Mary Jialu; Pappas, Georgios A.; Massella, Daniele; et al. (2021)
    One of the greatest challenges facing polymeric cardiovascular devices is the issue of hemocompatibility. Devices such as polymeric heart valves potentially offer improved mechanical properties and quality of life compared to their animal tissue counterparts. However, they are still strongly limited by problematic interactions with blood. The reduction of platelet adhesion, thrombogenicity, and calcification have been addressed in a variety of surface and bulk modification methods, generally by increasing the hydrophilic character of polymers. However, most hydrophilization processes – oxygen plasma in particular – tend to offer limited longevity. The crystallinity of polymers has previously been observed to influence the extent of platelet adhesion, though the underlying mechanisms for this phenomenon are not clear. In this research, we report on the effect of crystallinity on hemolysis, thrombogenicity, and platelet adhesion in PEEK surfaces. By tailoring the bulk crystallinity, we demonstrate changes in the surface chemical composition and propose a potential strategy to achieve longer term surface modification for improved hemocompatibility. Additionally, we explore the influence of crystallinity on the mechanical properties of thin PEEK films, establishing the multi-dimensional impact of polymer crystallinity. The results shown here may have implications for the design of polymeric cardiovascular devices and considerations that should be taken during material selection.
  • Phase-field based fracture modeling at microscale: a case study on PEEK reinforced composites
    Item type: Other Conference Item
    Sangaletti, Simone; Pappas, Georgios A.; Ermanni, Paolo; et al. (2023)
    Proceedings of the 2023 International Conference on Composite Materials
Publications1 - 10 of 33