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Natural ceramic composites present complex microstructures that lead to tortuous crack path and confer them high toughness. Current microreinforced composites do not yet reach the level of complexity found in natural microstructures. To achieve this, we developed an automated set up for controlling 2D microplatelet orientation using magnetic fields. The setup has 4 degrees of freedom and allows fine control of platelet angles in 3D as well as position. With our setup, we can thus build microstructured porous ceramic materials (alumina) with local orientation of microplatelets, at a concentration of about 40 vol%. Other work in our group has shown that the same setup can be used to process microstructured ceramics up to 95 vol%.
Among the infinite possible microstructures, a nacre-like helicoidal arrangement was selected due to its ability to tilt and twist the crack path. Indeed, we used an analytical model based on the laminate theory to compare energy dissipation due to mixed modes of failure. Nacre-like helicoidal arrangement was predicted to exhibit the highest energy dissipation. Then, we fabricated the hybrid microstructured porous specimen fabricated and infiltrated them with a silicone matrix. Using compression tests, our hybrid dissipated 1.6 times more energy as compared to nacre-like samples of same composition. The fabrication strategy proposed here could thus be a simple route to increase the toughness of microplatelet reinforced composites.



MATERIALS FOR HUMANITY (MH 21)

Energy dissipation in composites with hybrid nacre-like helicoidal microstructures

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

technical paper

The solutionization of Inconel 718 (IN718) is often terminated by rapid cooling at a rate of air cooling or faster. However, in this study, a slow cooling method utilizing furnace cooling was employed for selective laser melted (SLM) micron-TiC reinforced IN718 to investigate the interactions between the TiC particles and the matrix. Four holding temperatures of 975, 1075, 1175, and 1275 °C were chosen while the holding time was kept constant at 1 hour. It was found that the addition of micron-TiC facilitated the formation of nano-sized Ti-rich precipitates in as-printed samples. The difference in microhardness between reinforced and non-reinforced IN718 increased with increasing holding temperature, from 14.9% higher at 975 °C to 50.3% higher at 1275 °C. This was mainly attributed to the prevention of solute segregation to the grain boundary and the reduction of the susceptibility to hot cracking after adding TiC particles. The results indicate that the solute materials (Nb, Mo, and Si) were absorbed by the Ti-rich particles through Ostwald ripening mechanism instead of diffusing to the grain boundaries. The degree of absorption increased with increasing holding temperature. Moreover, grain coarsening at high solutionizing temperature was reduced substantially with the presence of Ti-rich particles due to Zener pinning. Thus, this provides a better understanding for choosing a suitable heat treatment profile for IN718 composite.

Mitigation of solute segregation after solutionization in selective laser melted Inconel 718 / micron-TiC composite

Electrification could provide many benefits towards the environment like lessening the dependence on non-renewable resources and reducing emissions when applied in the space of transportation. However, their battery packs take up a significant amount of space, decreasing the cargo capacity of EVs and payload of electric planes. Therefore, electrification of transportation options is driven largely by the advancement of battery technology and light-weighting composites with the goal of achieving a higher energy while reducing weight and saving space. 
To that end, the concept of providing structural power in the form of multifunctional structural batteries has been deemed as a potentially viable strategy to achieve both high energy density, light-weighting and space savings simultaneously. However, the scientific problems facing the development of multifunctional materials for structural batteries are especially hard to solve due to the seemingly incongruent material requirements for energy storage and structural stability. Structural battery constituents include the structural electrodes and the structural electrolytes. In particular, achieving good mechanical and conductivity properties on structural electrolyte is especially challenging due to the complicated electrochemical processes involved in batteries and the stringent mechanical requirements of structural components. In this work, a sulfonated polyether ether ketone, an engineering thermoplastic, grafted with polyethylene glycol (SPEEK-g-PEG) was studied for its multifunctional capabilities. A conductivity of 10-6 S cm-1 was achieved with SPEEK-g-PEG (SPEEK grafted with PEG 750) free-standing film paving the way for the use of engineering thermoplastics as multifunctional structural battery electrolytes.

Towards the design of multifunctional composites for structural battery applications

Polymer composites reinforced with a thin film made of carbon nanotubes (CNTs), also called as buckypapers, have been first reported in 2004 [1]. The CNTs in the films form an entangled, interconnected and porous network hold together by the van der Waals forces. The films retain most of the properties associated with CNTs, including high specific surface area, electrical and thermal conductivities, while they allow to bypass the agglomeration and processing issues faced by traditional processing methods based on melt compounding [2]. The reported applications of buckypapers include batteries, fuel cells, filters, strain sensing and lightning striking protection thus incorporating them into composites could allow for new multifunctional materials [3]. The main drawback of the buckypapers is their preparation method, which is currently based on filtration or Chemical Vapour Deposition, therefore making the preparation of the films slow, expensive and size restricted [4]. In this talk a new approach towards manufacturing of buckypaper composites will be presented. The method relies on making the thin films using a dispersion of carbon nanotubes directly on the epoxy layer, thus saving the time required for vacuum filtration. With the aid of data from Dynamic Mechanical Analysis tests, this presentation will explain the advantages and limitations of this manufacturing method. A future outlook of the buckypaper composites will be given, focusing on the multifunctionality, scalability and additive manufacturing of these materials.

Development of multi-layered epoxy composites with carbon nanotube buckypaper reinforcement

Carbon fiber veils are beneficial materials to functionalize the adhesively bonded joints which are damaged when they need to be debonded. The effects of three different carbon fiber veils on the mechanical, thermal and electrical characteristics of epoxy adhesive systems were examined. In comparison to the neat epoxy configurations, carbon fiber veil interleaving enhanced storage modulus, thermal diffusivity and lap shear strength (LSS) of the adhesive joints while lowering specific heat capacity (Cp) and glass transition temperature (Tg). Fourier-transform infrared spectroscopy (FTIR) analysis showed no chemical changes occurred in epoxy after heating at 100°C and interleaving a carbon fiber veil layer between two epoxy film adhesive layers. Surface roughness and water contact angle measurements were conducted to investigate the wettability of GFRPadherends. ANSYS coupled thermal-electric simulations displayed good agreement with Joule heat experiments. Thermomechanical debonding via Joule heating provided high debonding effectiveness in terms of requiring low force and time while providing no fiber-tearing on the surface of the adherends and heating the bonded region of the joints selectively.

Debonding of carbon fiber veil interleaved adhesively bonded glass fiber reinforced polymer (GFRP) joints

Fiber reinforced thermoplastic (FRTP) composites are gaining popularity in many industries because of their potential to be recycled, among other reasons. Filament winding is a manufacturing process suitable for the fabrication of axisymmetric components. For the filament winding of FRTP composites, the consolidation process can be carried out in-situ, where the FRTP is heated during the winding process. This enables the manufacturing process to be completed in a single step, thus allowing for higher productivity. In this study, the filament winding of glass/polyamide-6 composite using a customized in-situ consolidation setup is investigated. Instead of heating only at the nip point, infrared heaters were used to supply heat across the length of the composite component. This setup is intended to increase the exposure time of the thermoplastic to the heat source. The effectiveness of the in-situ consolidation setup is studied by evaluating the physical and mechanical properties of the consolidated FRTP components. First round of results show that the customized process can produce composite specimens with low void content (below 5%) at a line speed of 2 m/min.

Filament winding of glass/polyamide-6 commingled fibers with customized infrared in-situ consolidation

Prediction of mechanical properties using multiscale manufacturing simulation and micromechanical models is the focus of this paper. An integrated platform is designed to cover the whole chain of analysis and perform the properties' calculation and transfer them between the modules in a smooth mapping procedure. The manufacturing step is the particular problem of coupled dual-scale deformation-flow process such as the one arising in RTM, Vacuum Assisted Resin Infusion (VARI) and Vacuum Bag Only (VBO) prepregs. A finite element formulation of porous media theory framework is employed to predict the element-wise local volume fractions and the deformation of a preform in a press forming process. This formulation considers coupling effects between macro-scale preform processes and mesoscale ply processes as well as coupling effects between the solid and fluid phases. The outcome of the manufacturing simulation is the deformed mesh and element wise volume fractions to be used in micromechanical model to calculate mechanical properties. Structural performance of the "deformed" geometry is then evaluated in mechanical analysis. Structural performance of composites is directly dependent on its ingredient's properties, ply configurations and the manufacturing effects. These are evaluated in the integrated simulation chain and are defined as inputs. The contribution is concluded with a numerical example, where a compression-relaxation test of a planar fluid filled prepreg at globally un-drained condition is considered followed by a mechanical loading analysis. The development is user friendly and interactive and is established to enable design and optimization of composites.
This research is supported by the Singaporean Agency for Science, Technology and Research (A*STAR) under Polymer Matrix Composites Programme (PMCP) project (WP1-1) through the grant no. R265000673305.


Integrated simulation chains for composites mechanical performance and manufacturing effect of defects

The toughness of epoxy resin has a significant influence on the performance of fiber-reinforced composites. Pure epoxy resin has poor toughness and usually needs to be toughened by adding elastomer. However, the epoxy resin is usually a liquid with very low viscosity at the mixing temperature. When the high molecular weight elastomer is directly used, the viscosity of the two materials is quite different, and it is difficult for the shear force to be transmitted from the matrix phase to the dispersion phase. On the other hand, when using a liquid rubber type with low viscosity to toughen, not only the toughening effect is not good, but also the heat resistance and strength of epoxy resin will be significantly reduced. Therefore, to prepare high-performance fiber-reinforced composite materials, we use dynamic covalent bonds to construct a reversible molecular weight polyurethane elastomer toughening agent to achieve reversible changes in the viscosity of the elastomer at different temperatures and its applying for epoxy resins toughening. The reversible bond polyurethane elastomer has low viscosity at higher temperature and is easy to disperse in epoxy resin; after the temperature decreases, the toughening agent returns to high molecular weight elastomer. The phase separation of the composite material system can be controlled on a larger scale by changing the temperature. This technology is expected to be further promoted and applied in multiphase composite material systems.

Adaptable elastomer and its application in toughening epoxy resin based on dynamic bonds

Applications of fiber-reinforced composites in engineering structures sometimes require joining with other materials such as metal components or other composites. Various joining methods such as adhesive bonding and bolting can be adopted depending on the required loading conditions, types of materials, surface conditions, and environmental factors. This study aims to model and analyze mechanical joints between carbon fiber reinforced thermoplastic composite plates with metallic materials under out-of-plane load. Conical metallic inserts are used to transfer the load to the composite plate. The influence of the insert geometry and the adhesion parameters to the strength of the joint were studied. It should be noted that in reality the adhesion parameters, which include the strength and toughness of the interface between the composite surface and the metallic insert, can potentially be modified by using various surface treatments to the metallic insert. The results show that both the geometry of the insert and the adhesion parameters influences the strength of the joint, indicating that the mechanism of load transfer is not only through mechanical locking between the two but also through their adhesion.

Analysis of hybrid joints between carbon fiber reinforced thermoplastic composite plates and metallic inserts

Coherent interaction of light beams in the presence of a thin absorber allows to operate between the regimes of total light absorption and total light transmission. It was demonstrated that such absorbers (coherent perfect absorber, CPAs) provide nonlinear input-output signal dependencies and enables light-by-light control without requiring intrinsic nonlinearity. All-optical logical gates, signal amplification and dark pulse generation were demonstrated with CPAs and classical (laser) light. Beyond this, recent experimental studies showed that CPAs enable manipulation of quantum states of light including single photon and NOON states. 
Here, we extend quantum regimes of CPAs by interfering bosonic and fermionic states of entangled photons. In a single experiment we compare interference of bosonic and fermionic states in the absence and presence of dissipation. Lossless interference results in the Hong-Ou-Mandel effect where bosons coalesce and fermions anti-coalesce. In contrast, lossy interference with CPAs has the opposite outcome with anti-coalescent bosons and ‘coalescent’ fermions. We provide analysis of this ‘reversed’– anti-Hong-Ou-Mandel effect, and show that it originates from the phenomena of probabilistic two-photon absorption for bosonic pairs and deterministic one-photon absorption for fermionic pairs. The ability to manipulate and control the behaviour of photons and their output statistics with CPAs offers novel approaches for quantum information and metrology applications.

Anti-Hong-Ou-Mandel effect with coherent perfect absorbers

Silicon photonics is an enabling technology which has facilitated the drive towards continuous miniaturization of photonic integrated systems, by providing low cost and highly scalable solutions leading to faster and more compact devices. This has led to an increasing modal size mismatch between nanoscale on chip waveguides and standard optical fibers used in data communications today. As a result, one of the key challenges faced by current telecom industry has been to efficiently couple light between waveguides and external light sources such as fibres and free space optics, without additional fabrication post-processing and complex optical alignment. Among various coupling platforms, in-plane grating couplers (GC) have been widely preferred, mainly because of low fabrication costs, ease of alignment and high level of flexibility in circuit design. 
However, these couplers are passive photonic components, thus their coupling efficiency (CE) is fixed once the device is fabricated. Incorporating a switching functionality into an efficient GC design where the CE can be dynamically tuned, would allow for a multifunctional component enhancing its potential for a diverse reconfigurable range of applications as well as allowing post-fabrication trimming of silicon photonic circuits. 
Using inverse design techniques, we demonstrate CMOS compatible, reconfigurable phase change chalcogenide-on-insulator based apodised GC having maximized CE of more than 50% at λ=1580nm when the phase of the chalcogenide is in amorphous state. When the phase is switched to crystalline state, a near zero CE is shown allowing the design to be reversibly reconfigurable with highest transmission modulation contrast of more than 50db. 
We further demonstrate that the centre frequency for which the GC design has been optimized for maximum CE, can be shifted by varying geometrical parameters such as etch depth and thickness to tune the operational window between E to L telecom bands.

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Energy dissipation in composites with hybrid nacre-like helicoidal microstructures

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Mitigation of solute segregation after solutionization in selective laser melted Inconel 718 / micron-TiC composite

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Mitigation of solute segregation after solutionization in selective laser melted Inconel 718 / micron-TiC composite