Explaining the Difference Between Real Part and Virtual Design of 3D Printed Porous Polymer at the Microstructural Level

The combination of experimental and numerical approaches is attempted to shed more light on 3D microstructural imperfections and mechanical performance of 3D printed acrylonitrile butadiene styrene parts. The starting point is the virtual building of airy structures using a reverse engineering approach. This approach combines microstructure generator, finite element model, and optimization strategy to propose virtual airy structures satisfying structural and mechanical criteria up to a desired porosity content of 60%. Optimal structures are printed using fused deposition modeling and X‐ray microtomography is used to assess all microstructural defects. Compression testing is performed for load levels above 50% of reduction in sample height. The main outcome of this work is the demonstration of small amount of process induced porosity inducing high pore connectivity. The interdependence of process induced and desired porosity reveals genuine microstructural effects that are only characteristics of 3D printed materials.

     

On channel estimation in cell-free massive MIMO for spatially correlated channels with correlated shadowing under Rician fading

This work addresses channel estimation (CE) in the uplink phase for a cell-free massive multiple-input multiple-output system operating under the time division duplex protocol. Considering that, channels are spatially correlated under the Rician fading model, where the investigated model is composed of two components: deterministic and nondeterministic, with the deterministic component describing the line-of-sight paths and the nondeterministic component describing the non-line-of-sight paths. Additionally, we dealt with correlated shadow fading that represents the most realistic situation. On the other hand, this work introduces a dynamic cooperation cluster framework in which the user is not served with the whole network ( i.e., all access points [APs]) but only the APs that present the best channel conditions regarding that user. In other words, this work proposes partial CE for each user because only APs with the best channel conditions are allowed to compute channel estimates. Consequently, we proposed partial channel estimators that perform the CE process with low complexity, namely, a partial minimum mean square error estimator and a partial element-wise minimum mean square error estimator. In addition, a simple pilot assignment technique is proposed in order to reduce interference signals so that each user experiences low interference from other users. Furthermore, the computational complexity required by each estimator is derived, where it is represented by the number of complex multiplications that each estimator requires in each consistency block. Theoretical and simulated results are provided, where the performance of each estimator is evaluated and analyzed using the normalized mean-square error metric.     

Experimental investigation and numerical modeling of the thermal behavior of a micro-heater

In micro-heater, heat flux is generated by Joule effect thanks to short electric pulses. This leads to a rapid increase of the micro-heater temperature that reaches a few hundreds degree Celsius in a few microseconds. In addition to this, the cyclic nature of the energizing signal may cause an excessive heat accumulation and hence a reduction of the device life expectancy. It is thus of utmost importance to accurately model heat transfer in the whole device. This work focuses on a 200 dots per inch printing head system which consists of a row of micro-heaters. Structure and chemical composition of a single micro-heater are determined by scanning electron microscopy coupled to an EDX analyzer (energy dispersive X-ray spectrometry). These data are used to build a two dimensional numerical model which represents a micro-heater cross-section. This model gives the spatiotemporal evolution of the temperature field which highlights clearly the thermal loading phenomenon in the micro-heaters. In parallel, electric measurements are performed during the printing process to access to the actual power supplied to the micro-heaters. Infrared thermography was used to measure the thermal response of the micro-heaters to the electrical solicitation. The comparison of experimental and numerical results shows that the numerical model correctly predicts the thermal behavior of micro-heaters.     

Heterogeneous tensile test on elastoplastic metallic sheets: Comparison between FEM simulations and full-field strain measurements

The aim of this work is to propose a non-standard tensile test suitable for the identification of material parameters using full-field strain measurements and finite element analysis. The shape of the sample to be used in this new test must verify three criteria: (i) large heterogeneity of the strain in the gauge area, (ii) large strain-paths diversity and (iii) a good sensitivity of the strain field to the material parameters. After identifying the mechanical parameters of a dual-phase steel sheet using σ–ε and r–α curves, samples of different shapes were studied in order to choose the one that presents the best compromise between the three criteria. The comparison between simulated and measured fields shows a qualitative accordance. Taking into account the difference between these fields in the expression of the cost-function to minimize is expected to improve the quality of the identified material parameters.     

Effect of printing temperature on microstructure,thermal behavior and tensile properties of 3D printed nylon using fused deposition modeling

The present investigation aims at the thermal conditions for the printability of nylon using fused deposition modeling (FDM). Dog-bone like specimens are manufactured under two printing temperatures to measure the tensile performance of 3D printed nylon with respect to the feedstock material properties. Both Scanning Electron Microscopy (SEM) and X-ray micro-tomography analysis are conducted to shed more light on the microstructural arrangement of nylon filaments. Finite element computation based on microstructural implementation is considered to study the main deformation mechanisms associated with the nylon filament arrangement and the process-induced porosity. The results show a narrow temperature range for printability of nylon, and a significant influence of the printing temperature on the thermal cycling, porosity content and mechanical performance. With the support of both numerical and experimental results, complex deformation mechanisms are revealed involving shearing related to the filament sequencing, compression at the junction points and tension within the raster and the frame. All these mechanisms are associated with the particular and regular arrangement of nylon filaments.     

Extension of Einstein's Law for Power‐Law Fluid to Describe a Suspension of Spherical Particles: Application to Recycled Polymer Flow

In this work, Einstein's equation is extended considering a power‐law suspending fluid without any Newtonian approximation. To validate the developed equation, an experimental setup is carried out. Polypropylene (PP) and polyethylene (PE) are injected at different volume fractions. The pressure drops measured in a cylindrical die are analyzed. The results show that the developed relationship allows better prediction of the viscosity of PP/PE blends compared to existing laws. During the recycling of PP, some pollutants are likely to be present in the polymer, mostly PE which tends to form a heterogeneous melt with PP. At low volume fractions, PE disperses mostly as solid spheres in PP due to its higher viscosity, but the viscosity of the PP/PE mixtures is hard to predict. Several studies have derived equivalent viscosity equations for dispersed spherical suspensions in shear‐thinning polymers. Nevertheless, these equations mainly refer to Einstein's equation for suspended spheres in Newtonian fluids. POLYM. ENG. SCI., 59:E387–E396, 2019. © 2019 Society of Plastics Engineers     

Microstructure,Thermal and Mechanical Behavior of 3D Printed Acrylonitrile Styrene Acrylate

Fused deposition modeling (FDM) is one of the most popular additive manufacturing techniques for polymers. Despite the numerous works on the printability of various types of polymers, there is a lack in understanding the role of the microstructure on the mechanical performance of printed parts. This work aims at addressing this particular point for the case of a polymer that did not receive much attention, namely acrylonitrile styrene acrylate or ASA. This study emphasizes on the effect of the printing temperature on thermal and mechanical performance of printed ASA using differential scanning calorimetry, infra‐red measurements, mechanical testing, X‐ray micro‐tomography, and finite element computation. The experimental results demonstrate a narrow window of printability of ASA based on the thermal response of this polymer during the laying down process. In addition, both experimental and numerical results show an evident loss in the performance that represents one third of the performance of the raw material. Despite this loss, the limited amount of generated porosity and the level of tensile strength of ASA make it a good choice as a feedstock material for FDM compared to other polymers such as acrylonitrile butadiene styrene.