Metamaterials with their distinctive unit cell-based periodic architecture feature a wide range of possible properties with unusual characteristics and a high potential for optimization. Due to their complex interaction between unit cell geometry and material properties, as well as their inherent multi-scale nature, suitable optimization strategies need to be developed for metamaterials. One potential approach is to optimize the distribution of unit cells within a part to achieve a predefined deformation response. However, a significant challenge lies in determining the appropriate number and distribution of areas with varying properties (material sections) to facilitate an efficient optimization. In this study, a variable material section discretization scheme is presented, which is aimed at automatically updating the discretization to enhance the efficiency of metamaterial optimizations. This scheme is implemented as an extension to a previously proposed Finite Element simulation-based optimization framework for unit cell-based metamaterials. The framework includes a numerical homogenization method and interpolation scheme for efficiently correlating unit cell parameters with homogenized material properties, coupled with a black-box optimization method. In the present study, the previously proposed framework was extended to incorporate a scheme for monitoring and adjusting the material section discretization during the optimization process. To assess the effectiveness of the implemented routine, it was tested in conjunction with a genetic algorithm for optimizing the parameter distribution of a 2D tri-anti-chiral metamaterial to match a predefined lateral deformation under load.

PurposeA key characteristic of powder bed fusion for polymers is that also the non-processed powder in the powder bed is exposed to elevated temperatures. This alters the properties of the remaining powder, which is compensated by refreshing the used powder with new powder. Nonetheless, it is discarded after a certain number of process iterations, which is economically and ecologically highly disadvantageous. Research works intensively to analyse and reduce the concurring effects responsible for powder ageing. This study aims to give a comprehensive overview of the cumulative changes in the powder and the printed parts when conducting several subsequent build cycles.Design/methodology/approachNew polyamide powder (PA12) was used in a total of nine subsequent build cycles with constant sintering parameters and without powder refreshing. After each iteration, the powder and parts were tested for their morphological, thermal and rheological properties.FindingsThe results are related to three main changes in the powder during the build cycles: decreasing bulk density (through agglomeration), increasing melt viscosity (through polyamide post-condensation) and increasing melting peak and onset temperatures (through thermal annealing of the powder).Originality/valueEven though the ageing of PA12 powder in powder bed fusion is well-known, it is not yet fully understood. Studies are not complete and due to different ageing conditions only partially comparable. The detailed study aims to help understand the related effects of powder ageing for process-relevant properties and to show which factors require control to limit the powder ageing.

In this work, the fatigue behaviour of Polyetheretherketone (PEEK) under tensile (mode I) and out of plane shear (mode III) loading was examined by using razor blade notched cylindrical bars. By varying the applied mode I and mode III loadings, fatigue fracture curves of pure mode I and mixed mode I/III were established. The aim was to find a suitable method to describe the behaviour of both pure mode I and mixed mode I/III loading via a singular equivalent stress intensity factor. This was found possible by increasing the coefficient of the mode III contribution by more than 500 % compared to previous work on other thermoplastic materials. This significant change of the mode III contribution was mainly attributed to the different thermo-mechanical state of PEEK at the testing temperature of 23 degrees C, as well as the good friction properties of PEEK which leads to a transferal of mode III contributions to mode I contributions due to crack flank sliding of the formed factory roof formations on the fracture surfaces.

Mechanical metamaterials have gained a lot of research interest over the last years due to their unusual mechanical properties and potential use for structural applications. However, the design and analysis of mechanical metamaterials remains challenging and time-consuming. Herein, we present a software framework for automated generation, finite element modeling and analysis of extruded mechanical metamaterials based on simple closed curves. By generalizing extruded unit cells with pores defined by simple closed curves, a wide variety of existing and novel metamaterials can be created and analyzed. Each pore can either be empty or filled with one or more materials, resulting in single- or multi-material metamaterials. Since part of the mechanical response of a metamaterial is defined by the geometric parameters of a unit cell, parameter studies are directly integrated into the framework. Examples of well-established mechanical metamaterials were used as benchmark structures and compared to their analytical solutions. We also demonstrate how generalized curves can further improve the mechanical properties of these structures, for example the load bearing capabilities and range of Poisson's ratios. Furthermore, newly developed single- and multi-material designs of mechanical metamaterials with tunable Poisson's ratio are presented and analyzed with the proposed framework. The framework is implemented in Python, executed via ABAQUS and allows for unit-cell based homogenization and full-size 2D and 3D simulations. The scripts are made open source and publicly available.

When injection molding semi-crystalline polymers, it is known that the resulting morphological and mechanical properties are strongly dependent on the processing conditions. Selectively influencing the performance of the final part as early as during processing is of great interest for industrial applications. To investigate the processing-structure-property correlation for a POM hompolymer resin, tensile specimens were manufactured under conventional injection molding conditions following a Design of Experimets (DoE). Morphological characterization was carried out by microscopy and X-ray measurements. Crystallinity was also measured by differential scanning calorimetry (DSC). Additionally, the (fracture) mechanical behaviour of the samples was assessed. Influences of the processing parameters on the crystalline structure and orientations within the material were identified. Furthermore, tensile tests were found to be sensitive to changes in morphology and processing, whereas the fracture behaviour did not change significantly despite the structural changes within the samples.

Herein, copper particles are deposited on additively manufactured surfaces and investigations are performed to determine the mechanical properties and the impact of copper tracks on the surfaces. The basic investigation covers quasistatic tests, namely, tensile and three point bending, for three different printing orientations and two infill variations (+45°/−45° and 0°/90°), which shows no remarkable differences. In addition, a copper track is sprayed via atmospheric pressure plasma spraying (APPS) onto the polymeric samples and characterized regarding hardness and electric conductivity. Furthermore, a specific application for the copper track on the polymer substrate is recreated by a cyclic three point bending with a novel sample geometry (T‐shaped). The sprayed copper track has 60% of the hardness and 40% of the indentation modulus of bulk copper. Depending on the substrates’ topography, the electric conductivity varies from 7% to 18% of bulk copper. The lifetime of the copper track (i.e., conductivity) is strongly dependent on the deformation and the fracture of the polymer underneath. The underlying failure mechanism is triggered either by the topography of the polymer substrate or as a consequence of the damage in the copper track, leading to superficial cracks in the polymer surface.

The use of photolatent transesterification catalysts provides a unique way to locally control dynamic bond exchange reactions in vitrimers.

Orthogonal photoreactions provide a unique way to locally and independently control (thermo)mechanical properties and functionality of polymer networks simply by choice of the wavelength. Herein, a library of acrylate functional coumarin monomers is synthesized, which are cured by sequence-dependent wavelength orthogonality. In the presence of a long wavelength absorbing photoinitiator, the monomers undergo rapid curing by visible light induced radical chain growth polymerization. Subsequent irradiation with light in the UV-A region selectively initiates the [2+2] photocycloaddition of the coumarin chromophores, which is confirmed by FTIR and UV-vis experiments. Through a well-targeted design, acrylate-based and thiol-acrylate resin formulations are prepared, whose fast curing rate, low viscosity, and prolonged storage stability enable the one-step fabrication of multi-material structures by digital light processing (DLP) 3D printing. By using a dual-wavelength printer, which operates at two different wavelengths (405 and 365 nm), objects comprising soft (epsilon = 22%, sigma = 7.5 MPa) and stiff (epsilon = 2%, sigma = 8.3 MPa) domains are printed with a single resin vat. Along with tensile properties, the wavelength selective change in the network structure features a local control of the glass transition temperature (Delta T-g = 17 degrees C) in the 3D-printed objects. Soft active devices are fabricated by dual-wavelength DLP 3D printing, with distinct domains having a higher T-g and the local programming of multi shapes is demonstrated.

Different designs for mechanical metamaterials with tunable normal-strain shear coupling effect have been demonstrated over the last years. Their adjustable shear deformation makes them suitable as building blocks for soft robotics applications or structures with a desired deformation behavior, such as shape morphing structures. Herein, we present a modified 2.5D and 3D chiral-based mechanical metamaterials with tunable normal-strain shear coupling effect and Poisson's Ratios close to zero. Advancing from conventional chiral-based metamaterials by introducing additional geometric freedoms into the design of the unit cell, a broad range of shear deformations, compression moduli and porosities can be achieved. 2.5D specimens with selected geometric parameters were additively manufactured with polypropylene using Fused Filament Fabrication. Compression tests were performed to investigate the mechanical properties and shear deformation. Two different numerical models were employed using ABAQUS to study the influence of the geometric parameters onto the mechanical properties and were verified by the experiments. The numerical material models were based on three-point-bending test data. The three-dimensional design was investigated with numerical simulations based on a homogenization approach to cover a broad range of geometric parameters.

Metamaterials are a class of materials with a distinctive unit cell-based periodic architecture, often resulting in unique mechanical properties. The potential of metamaterials can be further improved by using gradients of unit cell parameters and thereby creating a specific distribution of material properties in a part. This design freedom comes with the challenge of finding new suitable design and optimization strategies. In this study, a Finite Element simulation-based optimization framework to create a predefined deformation behavior of unit cell-based metamaterials is presented. The framework consists of a numerical homogenization method to create an efficient linear elastic material representation, an interpolation scheme for a fast correlation of unit cell parameters with homogenized material properties and a black-box based optimization part. The framework is tested on a tri-anti-chiral metamaterial with additional geometric parameters and a newly developed transition unit cell. Several specified lateral deformations of simple 2D rectangular test specimens under tensile load are used as primary optimization targets and the homogenized results are validated against Finite Element simulations of fully modeled tri-anti-chiral structures. In addition, the effect of the design space and the number of design variables is studied and several combinations of optimization objectives and constraints are tested.

Dielectric elastomer actuators rely on Coulomb forces for their actuation. They are promising candidates when it comes to the manufacture of electrically driven soft and lightweight robotic devices, which can undergo large movement. However, their commercial availability and hence their technological relevance are still rather limited due to several reasons: in-depth material knowhow in terms of electrical and mechanical behavior required; materials have to be available in thin sheets; complicated and error prone fabrication; limited design freedom. In order to change this, this paper presents a soft dielectric actuator fully produced by fused filament fabrication. Additive manufacturing with conventional fused filament fabrication machines offers the potential for the production of personalized dielectric actuators which are easily accessible and comparably cheap. The only requirement is a fused filament fabrication printer with multi-material capability. Focusing on a mass customization of the 3D printed actuators, exclusively commercially available devices and filaments without any modifications are used. In particular, the 3D printed prototypes are characterized in terms of the maximum displacement depending on the electrode printing direction. As a showcase example, a soft dielectric gripper made of two 3D printed actuators is developed. Comparable with other dielectric elastomer actuators, a maximum displacement of 42% is reached.

Artificially structured materials, also called metamaterials, can achieve very unique properties due to the fact that their attributes are mainly derived from the design of the structures. This work gives an overview of the mechanical behavior of four differently designed and 3D-printed metamaterial structures. For each structure, both, the printing technique and the materials used for printing were varied. In total four different polymers were investigated and processed with three different 3D-printing techniques. Their mechanical behavior was studied by compression tests, which showed the wide range of material properties that can be achieved by utilizing the different materials/3D-printing techniques. In addition, bending tests of 3D-printed bending specimens were conducted to provide material data for subsequent finite element simulation of the structures. The comparison of experiments and numerical simulations gave further insight into local deformation mechanisms and allowed for a better understanding of the overall deformation behavior.

In this work, a very wide set of quiescent isothermal and non-isothermal calorimetric experiments are carried out in order to analyze POM crystallization kinetics also under cooling rates comparable to those experienced by the polymer during processing. To investigate the effect of flow on the POM crystallization behavior, also some Linkam shearing tests are conducted. An enhancement of the POM crystallization process is observed under flow. A Kolmogoroff-Avrami-Evans (KAE) model for quiescent crystallization is proposed. It is based on nucleation (considering both a homogeneous and a heterogeneous process) and growth mechanisms. Model parameters of nucleation and growth are determined, and the overall model is able to describe the POM crystallization process in the whole cooling rate range adopted for the experiments. This allows to obtain a reliable description of the POM crystallization evolution during processing and provides important knowledge for managing the POM injection molding process and the final physical properties of POM products.

This study examines the mechanical responses of three semi-crystalline POM polymers under uniaxial tension and compression loadings with various loading histories and at different temperatures. The studied POM polymers show variations in their molecular weight and degree of crystallinity with insignificant differences in the lamella thickness. Experimental results indicate that mechanical loadings, even at relatively low strain levels, cause permanent deformations and hysteretic responses, which are attributed to changes in the microstructures of polymers. Tensile and compressive loadings lead to different microstructural changes and elevated temperatures accelerate the changes. The POM polymers also show pronounced viscoelastic responses. A new mathematical model, based on a multi-network approach, is formulated to describe the macroscopic thermo-mechanical response of viscoelastic POM polymers incorporating the net effect of microstructural changes. The model assumes that the polymer has different stress-free configurations associated with different microstructures and the microstructural changes are governed by the deformations of the polymers. The model is shown capable of describing mechanical responses of POM polymers under various loading conditions, loading histories, and temperatures.

To date, the 3D printing of polymers with heterogeneous and locally controlled material properties is still a challenging area in additive manufacturing. In terms of vat photopolymerization 3D printing, the fabrication of multi-material objects typically relies on an automatic material exchange of different resin vats. However, along with the high complexity of the printing equipment, this technique suffers from a low build speed and often yields 3D printed objects with week interlayer adhesion across the various material interfaces. Herein, we use chemo-selective wavelengths to fabricate objects with multi-material properties by dual-wavelength vat photopolymerization 3D printing employing a single vat. The photopolymers' stiffness and flexibility are conveniently controlled by two photoreactions working at two different wavelengths. In particular, a dual photocurable resin is applied containing multi-functional acrylates, which are cured by a radical induced chain growth reaction at 405 nm, and bi-functional epoxy monomers, which additionally undergo cationic curing upon UV exposure (365 nm). FT-IR experiments confirm the wavelength selective network formation whilst dynamic mechanical analysis and tensile tests give evidence of the distinctive difference of the related mechanical properties. By being able to produce soft (epsilon = 24%, sigma = 1.0 MPa) and stiff (epsilon = 4%, sigma = 39.1 MPa) networks with a single resin vat, we demonstrate the efficient fabrication of 3D structures with locally controlled mechanical properties using a dual-wavelength 3D printer operating at 405 and 365 nm. In contrast to previous work in this field, we were able to significantly expand the range of mechanical properties by appropriate selection of the acrylic components and to drastically accelerate the build speed by changing the cationic photoinitiator and using a customized printer with high intensity LED sources.

The increasing importance of Laser Sintering (LS) for the industry brings a rising number of available materials for this process. One example is the enhanced use of alternative polyamide types like polyamide 6 (PA 6). For polyamide 12 (PA 12), the powder ageing effect is well-known and of great importance from an economical point of view. However, relatively little is known about the powder ageing behavior of newer LS materials. Hence this paper focuses on the experimental powder ageing study of PA 6 in the LS process, both on powder and part level. The powder was reused in total seven times without refreshing and constant sinter parameters were used to produce the samples. Morphological, thermal and rheological measurements of the powder showed a rising number of agglomerates, an apparent increase in the powder crystallinity, and an increase in the viscosity by a factor of 7. These increases consequently affected the density and the mechanical properties of the parts, latter were determined by tensile tests. Between the first and the last iteration, a decrease of the Young’s Modulus of 60 % and the tensile strength of 70 % was observed. Besides the decline of mechanical properties, the geometrical dimensions of the last iteration were only 60 % of the first iteration, the mass was halved and the surface quality suffered from the orange peel effect. The results prove that similar powder ageing effects known for PA 12, and here especially the polyamide post condensation, also take place in LS printing of PA 6.

It is well known that the processing conditions in polymer processing have a high impact on the resulting material morphology and consequently the component's mechanical behavior. However, especially for semicrystalline polymers, the tools available for predicting the final morphology of injection molding parts still have significant limitations. In order to investigate the potential of injection molding simulation for the prediction of the morphology, POM homopolymer specimens were injection molded. The crystallization kinetics data were measured, and simulations in 3D and 2.5D with and without crystallization analysis were conducted in Autodesk Moldflow. The simulations are found to be good accordance with the experiments. Predicted spherulite size and crystalline orientation factor reveal a good qualitative correlation with optical micrographs. Also, the evolution of these parameters along the flow path is plausible. The simulation is found to be a powerful tool for morphology prediction in polymeric parts. Its applicability, however, is still limited to 2.5D models in Autodesk Moldflow, which, of course, is insufficient for complex, thick-walled 3-dimensional parts.

Mechanical metamaterials with zero or negative Poisson’s ratio were subject to increasing research interest over the last few years. Their energy absorption capabilities make them suitable for impact and dampening applications, such as personal protection equipment or packaging materials. The variable porosity and unusual mechanical properties also make them applicable in drug delivery systems and wound management. Herein, we present an extension to common auxetic structures, including tetra-chirals and tetra-antichirals. By introducing an asymmetry in the design of their unit cell, Poisson’s ratio can be varied over a broad range. Specimens with a selected amount of asymmetry were additively manufactured with a thermoplastic polyurethane using fused filament fabrication. Compression tests were performed to investigate the influence of the asymmetry on Poisson’s ratio and the compression modulus. Two different numerical models were employed using ABAQUS to describe the mechanical properties of the structures and were verified by the experiments. The numerical models are based on three-point bending test data. Both asymmetric designs show an influence of the asymmetry onto Poisson’s ratio, resulting in variable Poisson’s ratio, porosity, and compression modulus.

Fracture mechanics are of high interest for the engineering design and structural integrity assessment of polymeric materials; however, regarding highly ductile polymers, many open questions still remain in terms of fully understanding deformation and fracture behaviors. For example, the influence of the constraint and specimen size on the fracture behavior of polymeric materials is still not clear. In this study, a polymeric material with an elastic plastic deformation behavior (ABS, acrylonitrile butadiene styrene) is investigated with regard to the influence of constraint and specimen size. Different single-edge notched bending (SENB) specimen sizes with constant geometrical ratios were tested. The material key curve was used to investigate differences in the constraint, where changes for small and large specimen sizes were found. Based on a size-independent crack resistance curve (J–R curve), two apparent initiation parameters (J0.2 and Jbl) were determined, namely, the initiation parameter Jini (based on the crack propagation kinetics curve) and the initiation parameter JI,lim (based on an ESIS TC 4 draft protocol). It was found that J0.2 and Jbl could be used as crack initiation parameters whereby Jini and JI,lim are indicative of the onset of stable crack growth.