Multi-material Composites Research Articles

Volumetric analysis and mesh generation of real and artificial microstructural geometries

Producing a viable finite element mesh of realistic microstructural structural geometry is a critical step in analyzing the thermo-mechanical behavior of complex multi-material composites. Advancements in imaging technology such as micro computed tomography have allowed modelers to access high resolution mesoscale geometries for direct numerical simulation. However, converting from voxel based 3D images to usable finite element meshes has been challenging. A robust method including algorithms and software scripts for generating finite element meshes from 3D imaged microstructures is presented. It includes a routine for inserting cohesive elements around material interfaces to enable modeling of interface properties including delamination and damage. The algorithms and procedures presented in this method leverage currently available software packages for processing surface based geometry into volume based meshes. In addition to converting real geometry from physical imaging systems, algorithms for producing numerically generated and statistically equivalent microstructural geometry are also included. These artificial microstructures can be a valuable resource for modelers when physical specimens do not exist or are limited in quantity.• Method establishes a workflow from voxel data to viable finite element mesh including interface information• Includes a method for synthetic geometry generation based on metrics from real microstructures

Direct 3D Printing of Hybrid Nanofiber-Based Nanocomposites for Highly Conductive and Shape Memory Applications

Three-dimensional (3D) printing with conductive polymer nanocomposites provides an attractive strategy for the "on-demand" fabrication of electrical devices. This paper demonstrates a family of highly conductive multimaterial composites that can be directly printed into ready-to-use multifunctional electrical devices using a flexible solvent-cast 3D printing technique. The new material design leverages the high aspect ratio and low contact resistance of the hybrid silver-coated carbon nanofibers (Ag@CNFs) with the excellent 3D printability of the thermoplastic polymer. The achieved nanocomposites are capable of printing in open air under ambient conditions, meanwhile presenting a low percolation threshold (i.e., <6 vol %) and high electrical conductivity (i.e., >2.1 × 105 S/m) without any post-treatments. We further find that this hybrid Ag@CNF-based nanocomposite shows a quick and low-voltage-triggered electrical-responsive shape memory behavior, making it a great candidate for printing electroactive devices. Multiple different as-printed Ag@CNF-based highly conductive nanocomposite structures used as high-performance electrical devices (e.g., ambient-printable conductive components, microstructured fiber sensors, flexible and lightweight scaffolds for electromagnetic interference shielding, and low-voltage-triggered smart grippers) are successfully demonstrated herein. This simple additive manufacturing approach combined with the synergic effects of the multimaterial nanocomposite paves new ways for further development of advanced and smart electrical devices in areas of soft robotics, sensors, wearable electronics, etc.

Inter-layer bonding characterisation between materials with different degrees of stiffness processed by fused filament fabrication

One of the main benefits of material extrusion additive manufacturing, also known as fused filament fabrication (FFF) or 3D printing, is the flexibility in terms of printing materials. Locally reinforced components can be easily produced by selectively combining reinforced with unfilled tough thermoplastics. However, such multi-material composites usually lack sufficient weld strength in order to be able to withstand operation loads. The present study attempts to close this gap by characterising the cohesion between the strands of two materials with different stiffness, namely neat PLA and short carbon fibre reinforced PLA (CF-PLA), produced by FFF using advanced fracture mechanical techniques. The full set of engineering constants of both materials were obtained under the assumption of transverse isotropy from tensile tests in combination with digital image correlation. Double cantilever beam (DCB) and cracked round bar (CRB) tests were used to determine the critical energy release rate (GIc). Both tests were in good correlation with each other and revealed that the interlayer PLA/CF-PLA bonding was at least as tough as the interlayer CF-PLA/CF-PLA bonding.

Locally addressable material properties in 3D micro-architectures

Heterogeneous materials with tailored, site-specific physical properties, ubiquitously found in biological systems, can reach optimal performances in applications. Compared to homogeneous materials or composite materials with random, disordered microstructures, materials with locally addressable properties achieve higher strength at lower density, or record-high shock absorption properties. A classic example of tailored, heterogeneous materials are functionally graded materials or materials where stiffness variations in bulk are controllable. However, current methods to locally control stiffness in a solid are mostly limited to geometrical variation of architected micro-structures, or to the creation of multi-material composites. Here, a novel approach to program stiffness variations within a single material system is presented. By subsequent exposure of a pre-crosslinked polymeric matrix using two-photon polymerization, arbitrary three-dimensional architectures with locally addressable stiffness variations are fabricated. Furthermore, the extension of the hierarchical level of three-dimensional micro-architectures by property variations in the constituent material is demonstrated. The programmable stiffness variations, in the sub-micron range, are either continuous or sharply confined, with a stiffness increase of up to 200% compared to the pre-crosslinked matrix. This site-specific control of stiffness and internal architecture opens the door for engineering materials with unprecedented mechanical properties.

Bio-inspired bimaterial composites patterned using three-dimensional printing

Multi-material composite structures exhibit highly enhanced mechanical properties by mimicking natural hierarchical materials such as nacre, crocodiles, armadillos, and turtles. Such composites have been reported to overcome the limitations of a single material. Numerous studies have developed bio-inspired composites with improved mechanical properties by using the patterns of natural materials. However, optimization of the patterns and configurations has not been studied comprehensively thus far. In this study, nacre-like laminated composites are designed, fabricated, and tested both experimentally and numerically to explore their energy absorption capacity. Nacre-like patterns are made by employing the Voronoi diagram, and produced into three-dimensional (3D) specimens of the structures by use of a dual extrusion 3D printer with two different plastic materials. Three-point bending tests are conducted to evaluate the energy absorption capacity of the specimen, which is found to be 11% larger than that of a single plastic specimen. The nacre-like structures are modeled reflecting the experimentally determined material properties, and the three-point bending tests are simulated by the finite element method. The experiment and numerical results of the tests are in good agreement. Multi-material composites with the natural hierarchical patterns are very promising to provide enhancement in mechanical properties. However, the adhesion between different materials in 3D printing still needs to be improved to fully harness the potential for further development.

Analysis of burr formation mechanisms when drilling CFRP-aluminium stacks using acoustic emission

Multimaterial composites are increasingly gaining importance in the design of aircrafts due to the demand for lightweight design in the aerospace industry. Riveting is used here as an important joining technique of individual structure components in aircrafts and requires the drilling of holes into the multimaterial composites. However, the individual layers in the stack place in part contrary demands on the optimum machining process and the design of drilling tools. To guarantee a sufficient drill hole quality, it is useful to safeguard the cutting process by monitoring the drill hole quality already during machining so that the reject rate of components may be reduced. Previous research works had shown that structure-borne sound could be used successfully for an online monitoring of drilling processes. Based on these investigations, this paper is focused on examining the burr formation mechanisms in CFRP-aluminium materials during the exit of the drill. By analysing patterns in the frequency spectra, it was shown that fundamental burr formation mechanisms could be detected by means of patterns in structure-borne sound signals. In addition, it was possible to infer the burr height produced in drilling from the analysis of the characteristic patterns in the structure-borne sound spectrum.

Powder-bed additive manufacturing for aerospace application: Techniques, metallic and metal/ceramic composite materials and trends

The current paper is devoted to classification of powder-bed additive manufacturing (PB-AM) techniques and description of specific features, advantages and limitation of different PB-AM techniques in aerospace applications. The common principle of “powder-bed” means that the used feedstock material is a powder, which forms “bed-like” platform of homogeneous layer that is fused according to cross-section of the manufactured object. After that, a new powder layer is distributed with the same thickness and the “printing” process continues. This approach is used in selective laser sintering/melting process, electron beam melting, and binder jetting printing. Additionally, relevant issues related to powder raw materials (metals, ceramics, multi-material composites, etc.) and their impact on the properties of as-manufactured components are discussed. Special attention is paid to discussion on additive manufacturing (AM) of aerospace critical parts made of Titanium alloys, Nickel-based superalloys, metal matrix composites (MMCs), ceramic matrix composites (CMCs) and high entropy alloys. Additional discussion is related to the quality control of the PB-AM materials, and to the prospects of new approaches in material development for PB-AM aiming at aerospace applications.

On the 3D printing of recycled ABS, PLA and HIPS thermoplastics for structural applications

Purpose Three-dimensional printing (3DP) is an established process to print structural parts of metals, ceramic and polymers. Further, multi-material 3DP has the potentials to be a milestone in rapid manufacturing (RM), customized design and structural applications. Being compatible as functionally graded materials in a single structural form, multi-material-based 3D printed parts can be applied in structural applications to get the benefit of modified properties. Design/methodology/approach The fused deposition modelling (FDM) is one of the established low cost 3DP techniques which can be used for printing functional/ non-functional prototypes in civil engineering applications. Findings The present study is focused on multi-material printing of primary recycled acrylonitrile butadiene styrene (ABS), polylactic acid (PLA) and high impact polystyrene (HIPS) in composite form. Thermal (glass transition temperature and heat capacity) and mechanical properties (break load, break strength, break elongation, percentage elongation at break and Young’s modulus) have been analysed to observe the behaviour of multi-material composites prepared by 3DP. This study also highlights the process parameters optimization of FDM supported with photomicrographs. Originality/value The present study is focused on multi-material printing of primary recycled ABS, PLA and HIPS in composite form.

Experimentally Calibrated Abrasive Sliding Wear Model: Demonstrations for Rotary and Linear Wear Systems

Considerable effort has been made to model, predict, and mitigate wear as it has significant global impact on the environment, economy, and energy consumption. This work proposes generalized foundation-based wear models and a simulation procedure for single material and multimaterial composites subject to rotary or linear abrasive sliding wear. For the first time, experimental calibration of foundation parameters and asymmetry effects are included. An iterative wear simulation procedure is outlined that considers implicit boundary conditions to better reflect the response of the whole sample and counter-body system compared to existing models. Key features such as surface profile, corresponding contact pressure evolution, and material loss can be predicted. For calibration and validation, both rotary and linear wear tests are conducted on purpose-built tribometers. In particular, an experimental calibration procedure for foundation parameters is developed based on a Levenberg–Marquardt optimization algorithm. This procedure is valid for specific counter-body and wear systems using experimentally measured steady-state worn surface profiles. The calibrated foundation model is validated by a set of rotary wear tests on different bimaterial composite samples. The established efficient and accurate wear simulation framework is well suited for future design and optimization purposes.

Insights into Laser Ablation Processes of Heterogeneous Samples: Toward Analysis of Through-Silicon-Vias.

State-of-the-art three-dimensional very large-scale integration (3D-VLSI) relies, among other factors, on the purity of high-aspect-ratio Cu interconnects such as through-silicon-vias (TSVs). Accurate spatial chemical analysis of electroplated TSV structures has been proven to be challenging due to their large aspect ratios and their multimaterial composition (Cu and Si) with distinct physical properties. Here, we demonstrate that these structures can be accurately analyzed by femtosecond (fs) laser beam ablation techniques in combination with ionization mass spectrometry (LIMS). We specifically report on novel preparation approaches for the postablation analysis of craters formed upon TSV depth profiling. The novel TSV sample preparation is based on deep and material-selective reactive-ion etching of the Si matrix surrounding the Cu interconnects thus facilitating systematic focused-ion-beam (FIB) investigations of the high-aspect-ratio TSV structures upon ablation. The particular structure of the TSV analyte combined with the ⌀beam > ⌀Cu-TSV condition allowed for an in-depth investigation of fundamental laser ablation processes, particularly focusing on the redeposition of ablated material at the inner side-walls of the LIMS craters. This phenomenon is of imminent importance for the ultimate quantification in any laser ablation-based depth profiling. In addition, we have developed a new method which allows the unambiguous determination of the crossing-point of the Si/Cu||bare Si interface upon Cu-TSV depth profiling which is based on pronounced, depth-dependent changes in the mass-spectrometric detection of those Si xy+ species formed upon the LIMS depth erosion.

NUEVAS MEZCLAS CON BASE EN POLIPROPILENO DE BAJA VISCOSIDAD EN FUNDIDO OBTENIDAS MEDIANTE LA ADICIÓN DE CICLOBUTILENTEREFTALATO

In this work, polymeric blends based on polypropylene (PP) combined with cyclic butylene terephthalate (CBT) and maleic anhydride grafted polypropylene (PP-g-AM) have been prepared and studied for applications in thermoplastic composites and as multimaterial composites for automotive industry. The principal aim has been to reduce the melt viscosity of thermoplastic matrixes to values similar to thermosetting ones in order to be able to use more economic equipment for the thermoplastic composites development.. For that purpose, different polymeric blends of CBT/PP have been prepared, and the effect of the addition of a PP-g-AM compatibilizer in the fibrillar morphology and in the rheological behavior of the compounds has been analyzed. For compatibilized blends, apart from a substantial reduction of the size of CBT, a thixotropic behavior has been observed, with a marked reduction of the viscosity at the beginning of the shear test followed by a slight restoration at the conclusion of the shear stress. Stationary values as low as 20000 mPa.s have been registered which corresponds with a reduction greater than 90% from PP viscosity. Keywords: Polymer blending, thermoplastic composites, compatibilizer , fibrillar morphology, viscosity, thixotropic.

Direct fabrication of compositionally graded Ti-Al2O3 multi-material structures using Laser Engineered Net Shaping

Laser Engineered Net Shaping (LENS™), which is a laser based additive manufacturing method, was utilized to fabricate Ti-Al2O3 compositionally graded structures. The Ti-Al2O3 graded composites consisted of different sections −Ti6Al4V alloy, Ti6Al4V + Al2O3 composites, and pure Al2O3 ceramic. After LENS™ processing, microstructural characterization, phase analysis, elemental distribution, and microhardness measurements were performed on the cross sections of Ti-Al2O3 graded composites. Each section had their unique microstructures and phases. Moreover, hardness measurements demonstrated that the pure Al2O3 section had the highest hardness of 2365.5 ± 64.7 HV0.3. Conventional ceramic processing requires extensive post-processing including high temperature sintering, which makes it difficult for direct fabrication of metal-ceramic multi-layer structures. The results demonstrate that LENS™ can be utilized to process multi-material metal ceramic composites in a single step while maintaining the size, shape and compositional variations based on computer aided design files. Since this is a first-generation work, and limited research results are available in published literature related to LENS™ processing of both metals and ceramics in one operation, the demonstration of this work is expected to inspire future studies on manufacturing of multi-material composites using AM.

Process monitoring on drilling fiber-reinforced plastics and aluminum stacks using acoustic emissions

In order to meet the lightweight demands in aviation industry, more and more multi-material composites are processed in aircrafts’ structural parts. A common connection to join these stack materials to other parts of the components is to use rivets. For that type of connection it is necessary to drill precise holes in the stack material. Due to the fact of the multi-layer design with different material properties and therefore a different machinability of the components, high demands on tool performance and process stability are set. When drilling carbon fiber reinforced plastics (CFRP), flaws, such as delamination and fringes as well as a poor bore hole quality, have to be avoided regarding the demands of the manufactured workpiece. In contrast, when processing the metal layer, there is a demand of burr-free holes after the machining process. This leads to high challenges in optimizing the drilling process and the tool geometry in order to accomplish long and reliable tool service times when processing generally expensive aircraft components.In order to monitor the drilling process of CFRP-aluminum stacks, this work shows the application of acoustic emission (AE) measurement technology to investigate the cutting mechanisms and enable a prediction for bore hole quality. For this purpose, the spectrum of the acoustic emission of the cutting process is examined for the various stages when cutting the different layers of the stack. Here, varying cutting mechanisms in regard to chip, burr formation and jamming of chips are observed and discussed. In wear-out tests the tool service times, the development of tool wear and the generated workpiece quality are recorded and the applicability of AE for process control is shown. Afterwards, suitable approaches are introduced in order to relate the tool wear state to the burr height by tracking the acoustic emission.

Tunable 3D printed multi-material composites to enhance tissue fidelity for surgical simulation

Tunable 3D printed multi-material composites to enhance tissue fidelity for surgical simulation

Metasilicone

We present a method for designing and fabricating MetaSilicones ---composite silicone rubbers that exhibit desired macroscopic mechanical properties. The underlying principle of our approach is to inject spherical inclusions of a liquid dopant material into a silicone matrix material. By varying the number, size, and locations of these inclusions as well as their material, a broad range of mechanical properties can be achieved. The technical core of our approach is formed by an optimization algorithm that, combining a simulation model based on extended finite elements (XFEM) and sensitivity analysis, computes inclusion distributions that lead to desired stiffness properties on the macroscopic level. We explore the design space of MetaSilicone on an extensive set of simulation experiments involving materials with optimized uni- and bi-directional stiffness, spatially-graded properties, as well as multi-material composites. We present validation through standard measurements on physical prototypes, which we fabricate on a modified filament-based 3D printer, thus combining the advantages of digital fabrication with the mechanical performance of silicone elastomers.

3D-printed multimaterial composites tailored for compliancy and strain recovery

Co-continuous multimaterial composites are novel types of multifunctional structures. This study focuses on numerical and experimental investigation of the mechanical behavior of 3D periodic single-material cellular D-structure and the corresponding co-continuous composite. Different volume fractions of desired geometry were fabricated by multimaterial fused deposition modeling (FDM) technology and compressive mechanical properties of the samples were obtained by mechanical tests. It was observed that embedding a hyperelastic material to the cellular structure dramatically hindered the shearing bands in localized regions to develop, thereby made it feasible for composite material to undergo larger deformations without failure. Furthermore, it was demonstrated that the soft phase in multimaterial composite induces a homogeneous deformation to cellular structure, which enhances the load-bearing capacity and flexibility of the whole composite. In this paper, it was shown that the co-continuous multimaterial composite provides a well-balanced approach between desired flexibility and load-bearing which is referred to as compliancy. A strain recovery between 82 and 93% was also measured when unloading for multimaterial composite. These integrated properties could be valuable to various engineering applications such as synthetic limbs, soft robotics, and wearable structures as shoes and splints.

A novel projection based electro-stereolithography (PES) process for production of 3D polymer-particle composite objects

PurposePolymer-particle composites, which have demonstrated wide applications ranging from energy harvesting and storage, biomedical applications, electronics and environmental sensing to aerospace applications, have been investigated for decades. However, fabricating polymer-particle composites with controlled distribution of particles in polymer continues to be a fundamental challenge. As to date, a few additive manufacturing (AM) technologies can fabricate composites, however, with a limited choice of materials or limited dispersion control. Against this background, this research investigated a hybrid polymer-particle composite manufacturing process, projection electro-stereolithography (PES) process, which integrates electrostatic deposition and projection based stereolithography (SL) technologies.Design/methodology/approachIn PES process, a photoconductive film collects charged particles in the regions illuminated by light. Then, collected particles are transferred from the film to a polymer layer with defined patterns. Lastly, a digital mask is used to pattern the light irradiation of the digital micromirror device chip, selectively curing the photopolymer liquid resin and particles of that layer. By transferring particles from the photoconductive film to the photopolymer in a projection-based SL system, multi-material composites with locally controlled dispersions could be produced. A proof-of-concept PES testbed was developed. Various test cases have been performed to verify the feasibility and effectiveness of the developed approach.FindingsChallenges in this novel AM process, including process design, particle patterning and transferring, are addressed in this paper. It is found that particles can be transferred to a layer of partially cured resin completely and accurately, by using the stamping approach. The transferring rate is related to stamping force and degree of conversion of the recipient layer. The developed hybrid process can fabricate polymer-particle composites with arbitrary dispersion pattern, unlimited printable height and complicated geometries.Originality/valueAlthough an electrostatic deposition process has been investigated as a 3D printing technology for many years, it is the first attempt to integrate it with projection SL for fabricating multi-material polymer composite components. The novel hybrid process offers unique benefits including local dispersion control, arbitrary filling patterns, wide range of materials, unlimited printable height and arbitrary complicated geometries.

Study on steady-state thermal conduction with singularities in multi-material composites

Increasing demand in material and mechanical properties has led to production of complex composite structures. The composite structures, made of different materials, possess a variety of properties derived from each material. This has brought challenges in both analytical and numerical studies in thermal conduction which is of significant importance for thermoelastic problems. Therefore, a unified and effective approach would be desirable. The present study makes a first attempt to determining the analytical symplectic eigen solution for steady-state thermal conduction problem of multi-material crack. Based on the obtained symplectic eigen solution (including higher order expanding eigen solution terms), a new symplectic analytical singular element (SASE) for numerical modeling is constructed. It is concluded that composite structures composed of multi-material with complex geometric shapes can be modeled by the developed method, and the generalized flux intensity factors (GFIFs) can be solved accurately and efficiently.

Introducing a level-set based shape and topology optimization method for the wear of composite materials with geometric constraints

The wear of materials continues to be a limiting factor in the lifetime and performance of mechanical systems with sliding surfaces. As the demand for low wear materials grows so does the need for models and methods to systematically optimize tribological systems. Elastic foundation models offer a simplified framework to study the wear of multimaterial composites subject to abrasive sliding. Previously, the evolving wear profile has been shown to converge to a steady-state that is characterized by a time-independent elliptic equation. In this article, the steady-state formulation is generalized and integrated with shape optimization to improve the wear performance of bi-material composites. Both macroscopic structures and periodic material microstructures are considered. Several common tribological objectives for systems undergoing wear are identified and mathematically formalized with shape derivatives. These include (i) achieving a planar wear surface from multimaterial composites and (ii) minimizing the run-in volume of material lost before steady-state wear is achieved. A level-set based topology optimization algorithm that incorporates a novel constraint on the level-set function is presented. In particular, a new scheme is developed to update material interfaces; the scheme (i) conveniently enforces volume constraints at each iteration, (ii) controls the complexity of design features using perimeter penalization, and (iii) nucleates holes or inclusions with the topological gradient. The broad applicability of the proposed formulation for problems beyond wear is discussed, especially for problems where convenient control of the complexity of geometric features is desired.

Smart Material Composites Substitute Monolithic Structures

The present paper approaches possible advantages of hybrid constructions compared to monolithic design. Hybrid constructions represent multi-material composites where each of the materials employed are optimally utilized. Therefore, materials consumption decreases which leads to material, energy and cost efficiency and finally contributes to sustainability.The investigations targets on a possible substitution of a heavy iron casted pump housing by a metal-polymer hybrid light weight construction and on the achievable total mass reduction. Multi-material composites are prerequisite for lightweight design and promise a huge mass reduction potential.