Investigation of the Structural Coating Homogeneity in Open‐Porous Nickel/Polyurethane Hybrid Foams Produced by Flow‐Controlled Electrodeposition

In today’s world, the saving of raw materials together with the reduction of emissions necessitates the development of new, customized and applicationoriented materials which fulfill various requirements at the same time. Metal foams achieve a high strength combined with low weight. The present work deals with a flow-controlled production technique for hybrid foams to deposit a nanocrystalline nickel (Ni) layer on pre-treated, open-porous polyurethane (PU) foams. The analysis of the resulting coating thickness distributions with gravimetric and microscopic methods shows the qualitative and quantitative influences on the local deposition by varying the process parameters. For the first time, the coating thickness distribution of Ni/PU hybrid foams is investigated regarding the analysis of a global and local homogeneity. The comparison of experimental data with simulated flow velocity profiles leads to an estimation of the correlation between flow velocity, anode distance, and coating thickness distribution, which represents the mass transport. The correlations show, that the coating process is strongly controlled by the electrolyte’s flow velocity as well as the distribution of the electric field

Simulation of a multimaterial model for modified auxetic structures

Auxetic structures, which is a term used to describe materials with a negative Poisson’s ratio, show beneficial properties like a low density, a high energy absorption capacity and an increased indentation resistance. This enables applications in many fields, such as aerospace and sports industries. Given their potential, many studies have already been conducted. Previously, the geometry of a selected auxetic re-entrant structure was optimized to maximize its mass-specific energy absorption capacity for ideal usage in lightweight applications. Moreover, a homogeneous material was used, whereas the combination of multiple materials could drastically increase the performance of such structures. Hence, in this study the use of two different materials combined into a modified re-entrant structure is investigated via Finite Element simulation. The Poisson’s ratio could thus be improved, which leads toa more pronounced and longer lasting auxetic effect

Limitations in Written Summative E-Assessment in Higher Education – An Analysis of a Student Survey

Written summative online examinations are usually conducted virtually from remote locations (Bloh, 2006) and ofer various advantages and challenges like high fexibility, low travelling cost and lower climate impact due to less paper consumption (Alruwais et al., 2018; Guàrdia et al.,2017). But virtual methods will not necessarily simplify the examination process at universities (Broadfoot, 2016). Observations at Technische Universität Dresden (TUD) have shown that even with a high level of efort in creating summative e-assessment online, it is hardly possible to develop a widely accepted method for implementation of written online exams mostly because it is technically complicated and leaves not enough room for various didactical approaches. Summative e-assessment has been the exception before 2020 (Riedel & Möbius, 2018) and the rush to digitize written exams due to the pandemic leaves both students and teachers dissatisfed with the outcome of the many written online exam approaches (Handke & Schäfer, 2012). Research shows how socio-demographics infuence the success of e-assessment (Bahar & Asi, 2018) or address security issues for users (Uotinen et al., 2020). But there is no research so far on specifc technical limitations that infuence students’ performance in written online exams. This paper addresses that gap with a quantitative analysis of a survey of business and economics students at TUD in the winter semester 2020/2021, who were examined exclusively virtually due to the pandemic. With these fndings, new technical and didactical methods for the implementation of summative e-assessment can be developed. [Aus: Introduction

Modelling of cellular materials by a microsphere‐based material model

Metal foams are a very interesting class of cellular materials which, due to their structure, can be used both for lightweight construction and for the absorption of kinetic energy. They have a microheterogeneous structure, which makes it difficult to simulate these materials efficiently. Although microstructure models are very precise in terms of strut size and pore geometry, they are very computationally intensive due to their high resolution and therefore do not allow the simulation of entire components. While continuum models that do not resolve the specific microstructure are very efficient, they do not allow the influence of variations in strut size, strut geometry or pore size to be modelled directly by the simulation. Therefore, simulation approaches such as microsphere models are necessary, which combine the macroscopic component scale with the microscopic microstructure

Investigation of the Electrodeposition Parameters on the Coating Process on Open Porous Media

Porous materials such as bones, sponges or cork are used in nature due to their light weight. Metal foams with stochastically distributed pores are such a porous bionic material based on nature. Their low weight and mechanical properties make them perfect for use in aerospace, automotive and building construction industry. To improve the mechanical properties, hybrid metal foams are produced consisting of a substrate foam with a coating applied by electrodeposition. An electrochemical coating cell consists of a positively charged anode and a negatively charged cathode. The metal of the anode is oxidised to a positively charged cation and electrons. The cation moves through the electrolyte to the cathode, where it is reduced back to the metal and electrons are consumed. Due to this process the cathode is coated with the anode material. The mass transport during the electroplating process can be divided into four parts: Convection, diffusion, migration and reaction. Convection is a forced flow, e.g. by pumping. Diffusion describes the movement caused by concentration gradients and the migration is the movement by an electric field. Reaction is the ion consumption at the cathode during the electrodeposition process and is therefore called sink. During the electrodeposition process, coating thickness inhomogeneities occur due to mass transport limitations [2,4] and lead to non‐uniform mechanical properties within the coated foams. These inhomogeneties motivate an investigation of the electrodeposition process and its parameters

MULTIAXIAL INVESTIGATION OF PVC FOAMS AND ANALYSIS OF THE DEFORMATION MECHANISM BY 3D-DIC

Closed-cell polyvinylchloride (PVC) foams are widely used as core for sandwich composites for applications, in which multiaxial loads are involved. In the present work a wide range of uniaxial (tension, compression and torsion) and multiaxial experiments (both simultaneous tension-torsion and compression-torsion) were conducted on a high performance PVC foam. Failure data for each experiment were collected and depicted in the invariants plane. The whole cylindrical surface of the specimen was monitored by means of an 8-camera-system, strain fields were obtained by 3D-DIC. Hence, the occurrence and the evolution of deformation bands were inspected. The usage of an 8-camera system was essential for the observation of the deformation mechanism, especially for pure compression, pure torsion and combined axial load-torsion, in which the arising of deformation bands is affected by the occurrence of buckling and the orthotropy of the foam

Neural Networks for Structural Optimisation of Mechanical Metamaterials

Mechanical metamaterials are man‐made designer materials with unusual properties, which are derived from the micro‐structure rather than the base material. Thus, metamaterials are suitable for tailoring and structural optimisation to enhance certain properties. A widely known example for this class of materials are auxetics with a negative Poisson's ratio. In this work an auxetic unit cell is modified with an additional half strut.During the deformation this half strut will get into contact with the unit cell and provide additional stability. This leads to a higher plateau stress and consequently to a higher energy absorption capacity. To achieve the maximum energy absorption capacity, a structural optimisation is carried out. But an optimisation exclusively based on finite element simulations is computationally costly and takes a lot of time. Therefore, in this contribution neural networks are used as a tool to speed up the optimisation. Neural networks are one of many machine learning methods and are able to approximate any arbitrary function on a highly abstract level. So the stress‐strain behaviour and its dependency from the geometry parameters of a type of microstructure can be learned by the neural network with only a few finite element simulations of varying geometry parameters. The modified auxetic structure is optimised with respect to the mass specific energy absorption capacity. As a result a qualitative trend for the optimal geometry parameters is obtained. However, the Poisson's ratio for this optimisation is close to zero