Multi-material Design Research Articles

Multi-scale topology optimization of structures with multi-material microstructures using stiffness and mass design criteria

Nowadays, there is a great interest on the part of the automotive and aerospace industry to design environmentally-friendly structures. To that purpose, stiffness-oriented designs are proposed here by extending previous work on multi-scale topology optimization to the multi-material setting reformulating the problem to include appropriately mass constraints and discussing different design domain parametrizations and algorithmic strategies.On the macroscale, the problem of minimizing the compliance subject to a global mass constraint is addressed. On the microstructure scale, the multi-material design is carried out by solving the problem of minimizing the local complementary strain energy density with mass density constraint. As a result, very efficient structures composed of spatially varying porous and multi-material microstructures are obtained. The optimal design of the multi-material microstructure can be done either in a pointwise manner or in larger subdomains, to promote design uniformity. These parametrizations are here compared and discussed. Moreover, two different algorithmic strategies to solve the multi-scale problem are proposed, and their pros and cons discussed. They differ in the way the macro and micro design variables are related and updated. The macro design variables consider the mass density distribution along the structure, while the micro variables define the microstructure's topology using a multi-material SIMP interpolation scheme.The results show very efficient structures with locally optimized multi-material microstructures, which can outperform their single-material counterparts with regards to stiffness while maintaining the same mass. Additionally, the maximum stress verified on multi-material structures tends to be lower than the one obtained in the single-material counterparts.

Competing fracture modes in Al-steel resistance spot welded structures: Experimental evaluation and numerical prediction

Multi-material design has been increasingly used in the automotive industry for structural lightweighting. A novel resistance spot welding technique has been developed to join aluminium to steel with acceptable joint strength. However, aluminium to steel resistance spot welds can fail in either interfacial (IF) or pull-out (PO) fracture modes. In the present study, coupon-level tensile shear and coach peel samples are tested under quasi-static condition, and component-level T-joint structures are tested under impact load. Experimental results show that in the IF fracture mode, weld failure occurs due to the facture of the intermetallic layer, while in the PO fracture mode, weld failure occurs due to a combination of different mechanisms including through-thickness tearing of the weld nugget and possibly partial fracture of the intermetallic layer. In the finite element analysis, two force-based fracture criteria are implemented via mesh-independent connector element considering a general loading condition, and whichever fracture potential reaches unity first determines the fracture mode. The proposed method is validated by experiments and has the following features: firstly, it can capture the behaviour of fracture mode transitioning from IF to PO as weld diameter increases; secondly, it can simulate the maximum load and deformation at fracture; thirdly, it can indicate the global response and absorbed energy in impact analysis with reasonable accuracy. Furthermore, a distinct advantage of the model is that it is free of non-physical calibration. The parameters in the model are obtained from geometric and material properties of the weld directly. The developed method shows desirable performance and is potentially suitable for applications in large-scale automotive structural analysis and design.

Bending behavior of structured steel sheets with undercuts for interlocking with Al die-cast metal

Due to the current changes in mobility, lightweight design concepts continue to be of particular interest to the automotive industry. One form is the multi-material design, in which the advantageous properties of different materials are combined in one component. In this work, a component made of a steel sheet with stiffening structures of cast aluminum is considered. The joint is created by channel structures with undercuts on the surface of the steel sheet, into which the molten aluminum can flow. After solidification, an interlocking connection is created. The aim of this work is to investigate the influence of a bending operation on the surface structure before the die casting process. Numerical simulations and experimental validations were performed with different bending angles and radii as well as orientations between the channel structure and the punch. The results show that the undercuts on the outer radius are reduced by up to 75% by the bending operation, thus weakening the resulting joint. On the inner radius, the channel opening width narrows by up to 73% and can thus impede the filling with the melt.

Towards 3D-printed alumina-based multi-material components with enhanced thermal shock resistance

A novel architectural design is introduced which utilizes the layer-by-layer capabilities of the vat photopolymerization 3D printing process to fabricate multi-material ceramic components with improved thermal shock resistance. The combination of 3D-printed alumina-zirconia (ZTA) with alumina (A) layers generates compressive residual stresses in the embedded alumina regions during cooling down from sintering. Thermal shock tests in water are performed on samples at different maximum temperatures and the strength degradation of the multi-material design is investigated and compared to the reference monoliths. Experimental results show that the retained strength of the multi-material ceramic after thermal shock is twice as high as that of the monoliths, associated with the crack arrest capability of the embedded layers. The concept is demonstrated on 3D-printed multi-ceramic blades for potential high temperature applications, showing enhanced “damage-tolerance” against thermal shock cracks. These findings open the path for fabricating reliable ceramic components using the vat photopolymerization process.

Efficient multi-material topology optimization design with minimum compliance based on ResUNet involved generative adversarial network

Efficient multi-material topology optimization design with minimum compliance based on ResUNet involved generative adversarial network

Topology optimization of multi-material structures considering anisotropic yield strengths

Multi-material structures offer the superior performance and broader designable freedom, and have gained increasing interest benefitting from the advances in additive manufacturing (AM) technique. However, the effect of anisotropic yield strength of these AM-fabricated parts on topology optimization of multi-material structures has so far received little attention. In the current work, an anisotropic strength-based topology optimization method for multi-material structures is developed for maximizing strength performance under total mass constraint, which allows for the consideration of directional dependence and tension-compression asymmetry of yield strengths based on the Tsai-Wu yield criterion. Another challenge for multi-material structures design is that the possible permutations of numerous candidate materials are unlimited, it is impossible that all combinations of multiple candidate materials are tested for the optimal strength performance of multi-material structures. To solve this problem, we propose a preselection method for the inclusion or exclusion of candidate materials based on their elastic properties, yield strengths and densities before optimization. Afterwards, our topology optimization method can automatically find the optimal combination of volume fractions of different candidate materials. Several 2D and 3D numerical examples are investigated for minimizing the maximum of failure indexes of strength for fixed total mass. The advantages of multi-material structures are verified by comparing to single-material designs. The effectiveness of the preselection method of candidate materials is also verified.

A 3D Printing‐Enabled Artificially Innervated Smart Soft Gripper with Variable Joint Stiffness

AbstractThe manufacturing industry has witnessed advancements in soft robotics, specifically in robotic grippers for handling fragile or irregular objects. However, challenges remain in balancing shape compliance, structural rigidity, weight, and sensor integration. To address these limitations, a 3D‐printed multimaterial gripper design is proposed. This approach utilizes a single, nearly fully automated 3D printing process to create a universal gripper with almost no assembly work. By processing functional polymer, polymer nanocomposite, and metal wire simultaneously, this technique enables multifunctionality. The gripper achieves different gripping configurations by adjusting joint stiffness through Joule heating of conductive polylactic acid material, ensuring shape conformance. Embedded metal wires, created using an in‐house wire embedding technique, form reliable high‐current‐loading interconnections for the conductive joints acting as the heater. Additionally, an integrated soft sensor printed in thermoplastic polyurethane (TPU) and conductive TPU detects compression levels and discerns handled samples. This study showcases the potential of 3D multimaterial printing for on‐demand fabrication of a smart universal gripper with variable stiffness and integrated sensors, benefiting the automation industry. Overall, this work presents an effective strategy for designing and fabricating integrated multifunctional structures using soft, rigid, and conductive materials, such as polymer, polymer nanocomposite, and metal through multimaterial 3D printing.

Light-based Three-Dimensional Printing of Complex-Shaped Photonic Colloidal Glasses.

Colloidal glasses display angle-independent structural color that is tunable by the size and local arrangement of submicron particles. While films, droplets and microcapsules with isotropic structural color have been demonstrated, the shaping of colloidal glasses in three dimensions remains an open manufacturing challenge. Here, we report a light-based printing platform for the shaping of colloidal glasses into three-dimensional objects featuring complex geometries and vivid structural color after thermal treatment. Rheology, photopolymerization and calcination experiments are performed to design the photoreactive resins leading to printable colloidal glasses. With the help of microscopy, scattering and optical characterization, we show that the photonic properties of the printed objects reflect the locally ordered microstructure of the glass. The capability of the platform in creating three-dimensional objects with isotropic structural color is illustrated by printing lattices and miniaturized sculpture replicas with unique shapes and multimaterial designs. This article is protected by copyright. All rights reserved.

Influence of High-Pressure Die Casting Process Parameters on the Compound Strength of Hybrid Components with Undercut Sheet Metal

Reducing the weight of parts through lightweight designs impacts CO2 emissions, especially in the automotive and transportation sectors, which have significant fuel and electric energy consumption. Using multi-material design approaches, specific material properties can be combined to achieve effective lightweighting. Commonly used metals include aluminum, which is known for its high specific strength, and steel, which is valued for its strength and structural integrity. However, joining aluminum and steel presents challenges given their different thermophysical properties and the potential formation of brittle intermetallic phases, making common joining techniques like fusion welding unsuitable. In this study, a hybrid casting process for the production of a complex workpiece from dissimilar materials was investigated. Aluminum was die-cast around a steel sheet insert. Surface structures with undercuts on the steel sheet were applied through modified cold rolling, allowing molten aluminum to flow into the channels and interlock during solidification. It was found that elevated temperatures of the melt and tool were beneficial for the interlocking, resulting in a 30% increase in compound strength. Furthermore, a reduction in both the piston position at the changeover point, between the pre-filling and cavity filling, and the melt velocity at the gate reduced the compound’s strength by 41% and 30%. Up- and downstream processes did not show any significant influence on the conducted experiments. Based on this, two main detrimental effects were observed: pre-solidification of the aluminum melt and gas entrapment.

Multi-material isogeometric topology optimization in multiple NURBS patches

In design optimization of structures, design domains are often described by multiple non-uniform rational B-spline (NURBS) patches. To perform multi-material topological design in such design domains, an effective Nitsche-based multi-material isogeometric topology optimization (ITO) method is proposed in this paper. Specifically, the Nitsche's method is utilized to effectively couple the interfaces between adjacent NURBS patches. In order to preserve the coercivity of the bilinear form, the stabilization parameters are estimated locally according to the element size. Meanwhile, the discrete material optimization (DMO) approach is employed to describe the mixing of candidate materials. Analytical computation of sensitivities for coupled elements is developed as well. The effectiveness of the proposed method is demonstrated via several 2D and 3D numerical examples, where the design domains are constituted by conforming and non-conforming NURBS patches, respectively.

Crack Growth Simulations With SMART Method in Adhesively Bonded Joints

Due to the important advantages of adhesive joints, such as their suitability for multi-material designs, their use has been increasing in the last decade. Determining the fracture behavior of structural adhesive bondings is essential for structural durability. In crack propagation analyses, adaptive meshing has drawn considerable attention because of its improvements in terms of complex preprocessing and time management. This paper presents a recently introduced separating morphing and adaptive remeshing technology (SMART) innovative crack growth simulation for adhesively bonded joints, considering static and cyclic cases. For the static case, an R-curve was obtained for the bonding joints of carbon steel and Araldite 2015. For the cyclic case, the Carbon Fiber Reinforced Polymer (CFRP) bonding joints were analyzed under constant-amplitude loading conditions. The crack-propagation rate and the number of cycles were estimated. Crack propagation simulations were validated using experimental data. Acceptable agreement was achieved between the experimental and estimated results.

Development of a highly various warp yarn manipulation system for the fabrication of 2D net-shape non-crimp-fabrics for highly material efficient concrete reinforcements

Textile reinforcements have revolutionized many industries, most of all aerospace, automotive and building. Due to its high performance and the significant increased lightweight potential, they have established themselves as an efficient and sustainable alternative to conventional materials. However, in view of the increasing demand of an ever growing population in contrast to a scarcity of resources and urge of material efficiency in the face of climate change, novel technologies for highly material efficient products in large-scale productions are required. In this regard, a major research field involves the multiaxial warp-knitting technique for the production of high performance textile preforms. In several steps, this technology has been further developed to enable the production of application-specific and bionic-inspired textile preforms, which are characterized by a force compliant and multi-material design. Yet the structural possibilities are still limited. This article presents a novel developed warp yarn manipulation system for multiaxial warp-knitting machines. The system enables the fabrication of 2D net-shape non-crimp-fabrics (NCF) made of up to 16 single warp yarns that specify by an alternating diagonally offset and overlapping edge-strand. This structure is suitable for the use as textile lattice girder for reinforcing concrete slab structures Hereby the developed warp yarn manipulation system allows highly various structural parameters of the 2D net-shape NCF for highest material efficiency and product variety. The technological development is discussed in means of the constructive and electronic control design as well as a specific application example for new net-shape reinforcement structures.

Multi-Material 3D-Printing Nozzle Design Based on the Theory of Inventive Problem Solving and Knowledge Graph

Fused deposition modeling (FDM) technology is an emerging technology with promising applications, with the nozzle playing a crucial role in extrusion, heating, and material ejection. However, most current extrusion-based 3D printers handle only single-material printing, making the integration of multiple materials through a single nozzle challenging due to compromised quality and clogging risks. This paper introduces a method to design multi-material 3D printing nozzles using the Theory of Inventive Problem Solving (TRIZ) and knowledge graph (KG). By optimizing design and leveraging TRIZ’s contradiction resolution principle, this study addressed bottlenecks and complexities in multi-material nozzle design, providing insightful recommendations. A patent knowledge graph focused on spray nozzles was created, storing material properties, design elements, and constraints for enhanced knowledge sharing. Building on identified challenges and recommendations, the study utilized keyword searches and associative paths in the knowledge graph to guide designers in generating innovative solutions. Validation was achieved through two distinct nozzle design models resulting from guided innovations. The TRIZ-KG methodology presented in this paper provides designers with a systematic cognitive framework to empower designers in overcoming technical obstacles and proposing precise solutions.

A punching process to join metal sheets and fibre reinforced polymer composites by mechanical interlocking

In the multi-material lightweight design of structural components for the automotive industry, the joint between different materials plays a significant role in reducing vehicle weight without compromising performance or safety. Conventional technologies to mechanically join metals and carbon fibre reinforced polymers result in either drilling a hole in the composite material or increasing the weight of the part because of the fasteners employed. This work presents a new, simple, cost-efficient and non-weight penalizing mechanical joining technology for metal sheets and fibre reinforced polymer prepregs. It consists of a single-step punching process where the metallic sheet is completely perforated, but the prepreg is not. The punch pushes the carbon fibres through the hole in the metal sheet with no or minimal fibre breakage, generating a mechanical interlock which, in turn, increases the shear strength and absorbed energy of the co-cured joint.

Mechanical and Microstructure Characterisation of the Hypoeutectic Cast Aluminium Alloy AlSi10Mg Manufactured by the Twin-Roll Casting Process

Multi-material designs (MMD) are more frequently used in the automotive industry. Hereby, the combination of different materials, metal sheets, or cast components, is mechanically joined, often by forming joining processes. The cast components mostly used are high-strength, age-hardenable aluminium alloys of the Al–Si system. Here, the low ductility of the AlSi alloys constitutes a challenge because their brittle nature causes cracks during the joining process. However, by using suitable solidification conditions, it is possible to achieve a microstructure with improved mechanical and joining properties. For this study, we used the twin-roll casting process (TRC) with water-cooled rollers to manufacture the hypoeutectic AlSi10Mg for the first time. Hereby, high solidification rates are realisable, which introduces a microstructure that is about four times finer than in the sand casting process. In particular, it is shown that a fine microstructure close to the modification with Na or Sr is achieved by the high solidification rate in the TRC process without using these elements. Based on this, the mechanical properties increase, and especially the ductility is enhanced. Subsequent joining investigations validate the positive influence of a high solidification rate since cracks in joints can be avoided. Finally, a microstructure-property-joint suitability correlation is presented.

Programming and physical realization of extreme three-dimensional responses of metastructures under large deformations

Structures and materials with programmable mechanical responses are desirable for many applications. Great advancement has been achieved and led to the discovery of metastructures/metamaterials with unconventional programmed properties. While most established studies focus on two-dimensional (2D) or pseudo-three-dimensional systems, certain complex deformations and functionalities can only be realized in three-dimensional (3D) space. The lack of comprehensive exploration of programmable large-deformation kinematics in 3D space could leave out a plethora of 3D deformation mechanisms, structural geometries, and complex responses that may lead to unprecedented mechanical functionalities. Based on multimaterial inverse design by topology optimization, this study systematically investigates several precisely programmed nonlinear extreme responses in 3D structures under finite deformations. Sophisticated 3D geometries with unique deformation capabilities are discovered. Under monotonic loading, extreme behaviors such as self-recovering counter-rotation and sequential lateral expansion–contraction are created, which are beyond the reach of 2D structures. The associated mechanisms fully exploit 3D space and optimally harness free-form geometries, material nonlinearity, the large disparity in material properties, and large rotations to deliver the target responses. Some discovered mechanisms are heterogeneous in space and asynchronous in time, spatially consisting of a series of sub-mechanisms. Albeit with complex geometries, the optimized structure with multi-phase and heterogeneous mechanisms is accurately fabricated through a proposed hybrid fabrication method tailored for 3D geometries combining 3D printing and casting, and the design’s unique programmed behavior is validated. The experimentally measured response shows high agreement with the prescribed target and numerically programmed responses. The discovered unique 3D deformation patterns and designs, underlying mechanisms, tailored 3D fabrication approach, and experimental procedures could provide meaningful mechanics insights and guidelines for realizing function-oriented mechanical metastructures/metamaterials that fully harness 3D space.

Mono-material product design with bio-based, circular, and biodegradable polymers

Mono-material product design with bio-based, circular, and biodegradable polymers

A microfluidics-assisted photopolymerization method for high-resolution multimaterial 3D printing

3D printing and bioprinting are recognized as key technologies for the construction of complex micro-devices, micro-environments, and culture models. Thanks to their potential to produce precisely heterogeneous and 3D architectures, multimaterial printing methods, which enable the production of functional 3D structures integrating multiple materials, have attracted specific attention. Nevertheless, combining multimaterial and high-resolution printing is still a major challenge, and the available technologies do not provide simultaneously the resolution and multiplexing capabilities required to create heterogeneous 3D environments. In this work, we introduce the 3D-FlowPrint concept, which involves an opto-microfluidic printhead immersed in a liquid and moved above a surface. This technology combines the convenience of microfluidics in terms of the handling and delivery of small volumes of materials with the resolution provided by laser lithography. Delivered materials are hydrodynamically confined under the printhead owing to controlled aspiration of the injected material, ensuring a continuous supply of material and avoiding cross-contamination issues. Combining microfluidics with photo-polymerization provides unique advantages as it separates the polymerization process from the material delivery, permitting high-resolution polymerization (down to 10 μm) and multimaterial handling (switching time below 60 s). We present a first proof-of-concept using poly(ethylene glycol) diacrylate (PEGDA)-based hydrogels as a photosensitive material model, along with a detailed investigation of the influence of exposure parameters, printhead velocity, and hydrodynamic parameters on the fabrication of 2D and 3D heterogeneous structures. 3D-FlowPrint allows the creation of sub-millimetric to millimetric scale objects with multimaterial designs. A first validation was performed to show the potential of the approach in biology for the creation of engineered microenvironments for cell culture.

A mechanical interlocking joint between sheet metal and carbon fibre reinforced polymers through punching

The joint between different lightweight materials plays a significant role in multi-material design of structural components for the automotive industry, aiming to reduce the vehicle’s weight without compromising performance or safety. Yet, conventional mechanical joining technologies between metals and Carbon Fibre Reinforced Polymers (CFRP) result in either a hole being drilled in the composite material, leading to damages which reduce the load bearing capacity, or the weight of the part being increased due to the incorporation of fasteners. At the same time, alternative mechanical joining methodologies involve complex and costly processing, hindering their industrial application. This work presents a new, simple, cost-efficient and non-weight penalizing mechanical joining technology between a metal sheet and fibre reinforced polymer prepregs consisting of a single-step punching process. In this process, the metallic sheet is completely perforated, while the prepreg is not. The punch pushes the carbon fibres through the metallic hole, with no, or minimal fibre breakage, generating a mechanical interlock. The shear strength and the absorbed energy of the co-cured joint increase with the incorporation of the mechanical interlocking joint.

Nonlinear Multimaterial Architecture for Greater Soft Material's Toughness and Delaying Damage Propagation.

Designing soft robots that have greater toughness and better resistance to damage propagation while at the same time retaining their properties of compliance is fundamentally important for soft robotics applications. This study's main contribution is proposing a framework for nonlinear multimaterial architectural design of soft structures to increase their toughness and delay damage propagation. What are the limits when combining significantly different materials in one structure that will delay crack propagation while significantly maintaining postdamage toughness? Through this study, we observed that there is a very dynamic interplay when combining significantly different materials in one structure; this interplay could weaken or strengthen the multimaterial structure's toughness. In biological evolutionary terms, the Pangolin, Seashell, and Arapaima have found their answer for deflecting the crack and maintaining strength in their bodies. How does nature put these multimaterial structures together? Our research led us to find that the multimaterial toughness limits depend largely on the following parameters: components' relative morphology, architecture, spatial distribution, surface areas, and Young's Modulus. We found that a linear geometry, when it comes to morphology and/or architecture relative to surface area in multimaterial design, significantly reduces total toughness and fails to delay crack propagation. In contrast, incorporating geometric nonlinearities in both morphology and architecture significantly maintains higher total toughness even after damage, and significantly delays crack propagation. We believe that this study can open the door to further research and ultimately to promising and wide applications in soft robotics.