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Polymers have the potential to replace metallic or bioprosthetic heart valve components due to superior durability and inertness while allowing for native tissue-like flexibility. Despite these appealing properties, certain polymers such as polyetheretherketone (PEEK) have issues with hemocompatibility, which have previously been addressed through assorted complex processes. In this paper, we explore the enhancement of PEEK hemocompatibility with polymer crystallinity. Amorphous, semi-crystalline and crystalline PEEK are investigated in addition to a highly crystalline carbon fiber (CF)/PEEK composite material (CFPEEK). The functional group density of the PEEK samples is determined, showing that higher crystallinity results in increased amount of surface carbonyl functional groups. The increase of crystallinity (and negatively charged groups) appears to cause significant reductions in platelet adhesion (33 vs. 1.5% surface coverage), hemolysis (1.55 vs. 0.75%∙cm−2), and thrombin generation rate (4840 vs. 1585 mU/mL/min/cm2). In combination with the hemocompatibility study, mechanical characterization demonstrates that tailoring crystallinity is a simple and effective method to control both hemocompatibility and mechanical performance of PEEK. Furthermore, the results display that CFPEEK composite performed very well in all categories due to its enhanced crystallinity and complete carbon encapsulation, allowing the unique properties of CFPEEK to empower new concepts in cardiovascular device design., Biomaterials Advances, 146, ISSN:2772-9508

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., Composites Part B: Engineering, 261, ISSN:1359-8368, ISSN:1879-1069

In this work, the impact of high stress gradients, found in bending of thin unidirectional fiber reinforced shells (~0.1 to 1 mm), on compressive micro-buckling failure, was analyzed. Such thin shells show increased resistance to compressive failure under high curvatures, which may even allow tensile fiber damage to drive ultimate failure for very low thickness (e.g., Composites Science and Technology, 216, ISSN:0266-3538, ISSN:1879-1050

Very thin carbon fiber composite shells can withstand large bending curvatures without failure. The resulting high tensile and compressive strains require accurate modeling of the fiber-dominated non-linear effects to predict the mechanical response. To date, no universal modeling technique can precisely capture the behavior of such structures. In this work, successful representation of composite’s response was achieved by utilizing single fiber tension and compression experimental data, implemented to extend a basal-plane-realignment based non-linear carbon fiber material model. Numerical techniques were adopted to model the bending behavior of unidirectional carbon fiber composites that was recorded in a comprehensive experimental campaign. Observations show that high material non-linearity leads to a non-negligible neutral-axis shift and drastic reduction of bending modulus due to compressive softening. Tensile fiber failure is the driving mechanism in thin shells flexure allowing for elastic compressive strains of up to 3% without micro-buckling. As a result, a remarkable flexibility in thin shells is realized. With increasing thickness, the elastic flexibility is reduced as the failure-driving mode switches to compressive micro-buckling., Composites Science and Technology, 199, ISSN:0266-3538, ISSN:1879-1050

Large design and manufacturing effort for high load carrying composite structures results from anisotropic material behavior, tedious curing or forming conditions as well as high sensitivity to manufacturing defects. Such challenges limit the design freedom and result in large cost and time effort. A novel design approach is proposed to realize load carrying structures based on the utilization of the outstanding flexibility of thin composite shells and the “complexity for free” approach of additive manufacturing. To this purpose, highly integrated structures are created by folding cured and thin composite shells around additively manufactured internal core topologies. The developed structures do not require complex molding approaches, while maintaining a high degree of manufacturing quality. A multidisciplinary design optimization is used to fully exploit the design freedom and the load carrying capabilities of the structure. Following the design concept, a UAV wing structure that carries more than 100 times its own weight is developed, optimized and tested to validate the design approach and demonstrate load carrying ability and manufacturing quality., Composite Structures, 250, ISSN:0263-8223, ISSN:1879-1085

Scientific Reports, 9 (1), ISSN:2045-2322

In fiber reinforced composite materials, the manufacturing process induces residual stresses and distortions that decrease the mechanical performance of the structure and affect its geometry, especially in thin laminates. Multi-physics simulations were performed to assess the spring-in effect in ultra-thin composite parabolic solar reflectors. For this purpose, a resin kinetic model has been developed by means of differential scanning calorimetry experiments. The kinetic relation has been implemented into the finite element software in order to correctly predict the evolution of the composite degree of cure during the manufacturing process. Specimens were produced in an autoclave and their final geometries were measured by means of a non-contact measuring system and compared with numerical predictions, showing very good agreement.

High strain composite (HSC) shells made from carbon fiber (CF) reinforced polymers can enable considerable mass savings in deployable space structures. Minimizing the packaged volume while maintaining structural stiffness, is a key challenge in the design of HSC structures and requires accurate prediction of a structure’s behavior during folding. A major unknown in the design of such structures is the non-linear material behavior and its effects on the stress distribution in composite laminates. This paper addresses this challenge by experimentally quantifying and modelling non-linear material behavior in unidirectional CF-Polyether ether ketone (PEEK) composites and extending the investigations to laminate level by means of an extended laminate theory. It was found that fiber non-linearity has a positive effect on laminate's flexibility, especially in cross-ply laminates. Here, neutral axis shift delays the formation of high strains on the tensile side and can mitigate critical transverse tensile stresses in off-axis plies. The susceptibility of a lamina to transverse cracking, however, limits design choices and flexibility. This highlights the importance of a suitable matrix system for these composites, which offers sufficient transverse strength to cope with off-axis stresses. The findings are put into context of foldable structures by considering the design and manufacturing of a lenticular boom structure.