Search

Fibre-matrix interface performance is essential in fibre reinforced polymer composites, leading to important efforts in quantification and optimization. This is even more relevant for thermoplastics, since interfacial bonding happens only via physical interactions. Standardised mechanical tests provide homogenized composite properties, but fail to isolate the contribution of the interface. Micromechanical ones are designed for this exact purpose, but need complex set-ups and sample preparation. This study adopts a novel multiscale approach to measure mechanical properties of polycarbonate-glass fibre composites, manufactured under different interfacial conditions (sized & desized). This is enabled by use of focused ion beam in precise manufacturing and post-mortem analysis of specimens. The results show an evident difference between tested conditions at the macroscale (mode I), where sized specimens outperform desized ones. At the microscale (mode II), these differences are less pronounced due to the high ductility of the matrix resulting in a cohesive failure of the composite., Proceedings of the 20th European Conference on Composite Materials: Composites Meet Sustainability, ISBN:978-2-9701614-0-0

ISSN:0003-6951 ISSN:1077-3118

Proceedings of SPIE, 10170, ISSN:0277-786X, Health Monitoring of Structural and Biological Systems 2017, ISBN:978-0-8194-8546-5

The concept of shape-morphing is to embed control in a design to broaden its band of functionality. Whether it is for cars, planes, buildings, or personal medicine, the goal is to achieve more with less. That is more efficiency, more functions, more force, and more range with less energy, and less weight. The design of these new types of structures is however complex, as all the variables of a system are intertwined. Existing designs are either simple or small to reduce the number of design variables, although this affects the achievable deformation range of a structure. There is a need for a design method that can handle the complexity of the search space of shape-morphing structures without compromising the breadth of their design space or the accuracy of their deformation. This thesis proposes four design methods, one for compliance-controlled shape-morphing and three for actuator-controlled shape-morphing. First, compliance-control is achieved through optimization of the distribution of materials with varying stiffness in a 2D lattice structure. The method is implemented for geometric and material linearity and nonlinearity to observe how the choice of model affects the accuracy of the deformations. The method is verified by optimizing the material distribution for a 2.5D NACA target shape and replicating the results experimentally using multimaterial 3D printing. Then, the modeling and four fabrication methods for soft pneumatic actuators are compared, as they are necessary to experimentally verify the actuator-controlled shape-morphing methods. The final actuator design is adapted to the constraints of soft lithography, the fabrication method that delivers the most robust and predictable actuators of the four. Actuator-control is achieved by optimizing the actuator layout within a lattice structure with the aim of achieving a target deformation. The three methods show how reducing the search and design space of a structure improves the computational efficiency and accuracy for 2D, 2.5D, and 3D deformation. The first method assumes static and kinematic determinacy and small displacements; the second assumes small displacements in overdeterminate structures; the third can achieve large shape changes in overdeterminate structures. All three methods are verified through finite element analysis and experimentally. Finally the implications of the findings related to the four shape-morphing design methods are discussed. The achievable search and design space of each method are compared. The designer is free to chose the method best adapted to their needs, as each one is a compromise of accuracy, range, and computational efficiency. The methods can be employed for different types of actuation, but their application to industrial fields is hindered by the mechanical properties of the material that can currently be 3D printed. The advent of several new manufacturers of compliant and multimaterial 3D printers promises further industrial developments for the field of shape-morphing.

Active structures have the ability to change their shape, properties, and functionality as a response to changing operational conditions, which makes them more versatile than their static counterparts. Despite recent advances in engineering, materials science, and fabrication processes, the systematic design and fabrication of active structures is still a challenge as many structures are designed by hand in a trial and error process and thus limited by engineers' knowledge and experience. This thesis aims to systematically design, fabricate, and test novel and active structures that can change their shape and mechanical properties. The structures overcome known limitations such as the reversibility of deformations triggered by one-way active materials and transforming into multiple states and shapes. This is achieved by combining mechanical principles, material knowledge, multi-material 3D printing, and computational design methods. First, fundamental properties of lattice mechanics are exploited to design structures at the verge of determinacy. By tailoring the topology and geometry of structures, kinematic deformation modes with a specified target shape and a single degree of freedom can be directly integrated. These shape morphing deformations are, by design, reversible and can feature multiple, independent deformation modes for different input displacements. The concept of determinacy is further used to design structures that can switch between both extremes of cellular structures, bending-dominated and stretch dominated behavior, in a single structure. This is achieved by combining a distinct topological and geometric design with a 3D-printed, heat-responsive shape-memory polymer (SMP), which enables topology transformation and provides direct control over the mechanical properties of the structures. To overcome the tedious programming step and one-way actuation of many SMPs and make active shape-morphing deformations reversible, the combination of two different 3D-printed SMPs in a single structure is explored to encode shape-morphing behavior under global heating and a single input actuation. By design, the structures can have two different, mechanically stable states, where the topology remains intact. To achieve multiple target states with a single structure and only one input actuation, local heating is explored in the final part of this thesis. Along with a novel topology optimization approach that ensures fabricability of the structures and compatibility with the drop-in, copper coil heating elements, different target states for a single input actuation are encoded in the structure. A material dithering scheme based on multi-material 3D printing and sequential heating are used to control the thermo-mechanical properties of the structures and switch between the different deformation modes. While some of the proposed concepts are limited by current 3D printing processes, the generality of both the underlying principles and the computational methods makes them directly applicable to future advances in materials science and fabrication technologies. As such, the findings in this thesis provide a first step towards the integrated design and fabrication of active structures and the development towards industrial applications across many length scales and fields such as the shape-morphing wings of aircraft, car panels, and building facades.

Hybrid preforms are used in thermoplastic composite manufacturing processes to reduce the potentially long consolidation times caused by the high viscosities of thermoplastic melts. Darcy's law for fluid flow through a porous medium indicates that the negative effects of high viscosities on impregnation time can be offset by reducing the maximum distances the thermoplastic melts must flow for complete consolidation. In thermoplastic composite preforms, the total impregnation length is reduced by increasing the degree of mingling between reinforcement and matrix. Currently, mingling in such preforms is found on the level of the laminate down to the level of the yarn, but may occur on any of the hierarchical tiers found in fibre-reinforced composite materials. The level and quality of mingling in existing arrangements - such as organosheets, commingled yarns, powder-impregnated yarns and fibre impregnated thermoplastics (FITs), co-woven yarns or stacked laminates - greatly influence the flow lengths, cycle times, achievable part complexity, raw material costs, and suitable manufacturing routes. Given the limited selection of commercially available preform architectures, manufacturers must choose between the low cycle times of organosheets and the better drapeability of unconsolidated hybrids, e.g. commingled yarns, in thermoforming. The development of a material architecture which combines the fast processing of fully impregnated products with the flexibility of unconsolidated preforms would render thermoplastic composites significantly more attractive to high volume production markets, e.g. automotive parts. This thesis proposes hybrid bicomponent fibres - which consist of continuous reinforcement fibres individually sheathed in a thermoplastic polymer - as a new class of preform materials for thermoplastic composites. By reducing the scale of mingling between the reinforcement and matrix materials to the level of the fibre, a full wet-out of the fibres is ensured while the unconsolidated nature of the material allows the fibres to shift and deform with respect to each other to ensure drapeability even at room temperature. It is hypothesized that preforms made from hybrid bicomponent fibres can be stamp formed with cycle times similar to those of pre-consolidated blanks. Furthermore, it is expected that the void content of laminates stamp formed from hybrid bicomponent fibre preforms is greatly influenced by sintering mechanisms and the removal and/or collapse of air pockets. The presented research aims to answer these hypotheses by developing suitable methods to manufacture such hybrid bicomponent fibres and by processing them into consolidated laminates. The basic idea of hybrid bicomponent fibres is motivated and introduced in further detail in part I. Part II moves on to discuss materials and their corresponding processing methods for their suitability in realizing bicomponent fibres. A fibre forming approach based on glass-melt spinning combined with an in-line coating process is chosen. Multiple versions of the latter are investigated empirically, namely dip-coating of newly spun glass fibres in either a polymer solution or a sparsely nanofilled polymer melt, as well as the so-called kiss-roll coating method. It is found that drawing glass monofilaments of finite length over a rotating roll which is partially immersed in a dilute polymer solution yields a coating method which can endure high fibre velocities while ensuring the deposition of a sufficiently thick thermoplastic sheath for down-stream conversion into a structural grade composite laminate. The validity of this strategy is proven in the realization of a pilot plant which employs solution kiss-roll coating in-line with melt-spinning of a glass monofilament for the continuous fabrication of bicomponent fibres. Unidirectional layups of specimens of aluminium borosilicate glass fibres clad in polycarbonate produced with the pilot plant were characterized for their consolidation behaviour, the results of which are reported in part III. Supported by theoretical treatments on issues related to void collapse and autohesion, a parameter study on rapid stamp forming of these preforms was performed and complemented with stamp forming trials processing cross-ply layups of different thicknesses. All experiments yielded excellent laminate qualities with void contents

ISBN:978-3-907234-12-9

The use of composite materials in structural applications is limited by the high costs associated with the production of the components. The research of cost-effective solutions leads to the exploration of innovative manufacturing approaches. One of those is the combined prepreg / liquid composite molding (LCM) process. This manufacturing technique consists in the combination of prepreg materials with dry fabrics, subsequently impregnated by the injection of a liquid resin. The main ad-vantage of the combined process is the possibility to exploit the characteristics of the two manufacturing approaches in different regions of the same component. This shall enable the manufacturing of structures with a high level of integration and increase the effectiveness of the production. New challenges arise when combining different manufacturing techniques. A characteristic phenomenon of the combined prepreg/LCM process is the bleeding of the prepreg resin into the dry fabric. Such consolidation problem has a relevant influence on the properties of the cured laminate and needs to be understood to guarantee a good reliability of the process. The main objective of this thesis is the description and study of the mechanisms driving the consolidation of a stack of reinforcement plies. The first part of the work aims at establishing the manufacturing procedures for the combined out-of-autoclave (OOA) prepreg / vacuum assisted resin transfer molding (VARTM) process. Following an experimental approach, the main characteristics of the process are described and the influence of the process parameters on the composite quality is assessed. The derived procedures are then validated with the production of a representative aerospace demonstrator component. In the second part of the thesis, the material properties sup-porting the modeling of the consolidation process, i.e. fiber bed through-thickness compaction and permeability, are investigated. A new approach for the characterization of the quasi-static and dynamic (time-dependent) compaction response of fiber beds is proposed and validated for different conditions. Furthermore, an experimental setup and procedure for the derivation of the “true” fiber bed through-thickness saturated permeability is established. The characterization method is validated exploiting a numerical model to assess the deformation the fiber bed sample during the experiment. Finally, the derived material properties are implemented in coupled finite element models describing the consolidation of reinforcement plies and the resin flow in composite manufacturing processes. The simulation of a saturated consolidation is validated against experimental data measured in a dedicated setup. The observations made in this thesis contribute to a better understanding of consolidation phenomena in composite manufacturing processes. The insight shall support the definition of manufacturing and design guidelines, aiming at increasing the reliability and robustness of the production of composite components.

ISBN:978-3-906916-50-7

This thesis presents and explores a set of novel approaches for the development of hybrid, integrated, lightweight, structures by combining additive manufacturing (AM) with fiber-reinforced polymers (FRP) in layup processes. The presented methods combine the strengths of both technologies: While AM takes a forming, structural and functional role, FRPs primarily contribute with their outstanding mechanical properties at low weight to the performance of the structure. From these characteristics three major design concepts are derived, including AM tooling for composite fabrication, AM structural sandwich cores with additional functionalities and AM load introduction elements. The understanding of materials and manufacturing approaches is a precondition for the embodiment of the design concepts. Therefore, various manufacturing routes are investigated including AM technologies such as fused deposition modeling, binder jetting and selective laser sintering. The thermo-mechanical stability of polymeric AM parts is crucial for autoclave curing of hybrid high-performance structures. To successfully design complex AM elements for the curing process of FRP, the thermo-mechanical creep properties of polymeric AM materials are characterized in three-point bending creep and tensile tests. Alternative concepts include sand and salt structures to produce sacrificial tooling. The embodiment of the three design concepts comprises two directions: designing for the performance of the part during service operation and designing to support the manufacturing of the structure. In this thesis both directions are addressed: Design for Performance: The specific mechanical performance of AM-CFRP structures is assessed in comparison to state-of-the Art structures by comparing the weight, the first failure load, the breaking load and the bending stiffness. Hat-stiffeners with structural cores made by AM were over-laminated with CFRP prepregs, cured in an autoclave and tested in three-point bending. AM core designs include honeycombs, trusses oriented along principal stresses, hollow structures filled with salt and a machined PMI foam core with a local load introduction element made by AM. AM-CFRP hat-stiffeners exhibit an increase in the specific fracture load ranging from 54% to 107% compared to the reference. Weights vary from an increase by up to 55% to a reduction of 5%, and the specific bending stiffness is increased by up to 41%. Results thereby confirm the mechanical competitiveness of AM-CFRP structures. Design for Processing: Additive manufacturing can support the production of composite parts along the process chain ranging from tooling, layup, handling, curing and post-processing. Four major design principles are presented and classified into integrated positioning and fixation elements, layup and handling aids, structural curing aids and post processing aids. Case studies show that the consideration of the processing during the design phase can reduce the deformation of the part during curing, the number of parts and the number of work steps and even eliminate drilling operations for the post-processing of inserts. The AM-CFRP approach is validated by incorporating the material data, the design principles and concepts into three components on system level. First, a novel aircraft instrument panel consisting of a multi-functional sandwich core shows that AM-CFRP can reduce the structural weight by 40%, the part count by 50% and the assembly steps by 50%. The second case study validates the mechanical performance of the AM-CFRP approach on component level by assessing the ultimate and the fatigue strength of a novel AM-CFRP prosthetic knee. The third case study consists of a robot leg structure and yields in weight reductions by 54% compared to state-of-the art references. The combination of AM with CFRP thus is a suitable approach for the manufacturing of individualized, lightweight, and geometrically-complex structures with integrated functionalities for low-volume applications, e.g. in the field of aerospace, flying vehicles, robotics and biomedical structures.

ISBN:978-3-909386-46-8

Ingenieurwissenschaften, ISBN:978-3-89963-565-2