FG Technische Mechanik und Maschinendynamik
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Characterization and Simulation of Shear-Induced Damage in Selective-Laser-Sintered Polyamide 12
(2024)
This paper presents the characterisation of selective-laser-sintered (SLS) samples of polyamide 12 (PA12) under shear loading. PA12 is a semi-crystalline thermoplastic and is used in various industries. Its behaviour under shear stress, which is particularly important for product reliability, has not yet been sufficiently investigated. This research focuses on understanding the material and damage behaviour of PA12 under shear-induced stress conditions. The study included quasi-static experiments and numerical simulations. Samples were prepared via SLS and tested according to ASTM standards. Digital image correlation (DIC) was used for precise deformation measurements. The Chaboche material model was used for the viscoplastic behaviour in the numerical simulations. Due to existing material discontinuities in the form of voids, the material model was coupled with the Gurson–Tvergaard–Needleman (GTN) damage model. A modified approach of the GTN model was used to account for low stress triaxiality under shear loading. These models were implemented in MATLAB and integrated into Abaqus via a User Material (UMAT) subroutine. The results of the experiments and simulations showed a high degree of accuracy. An important finding was the significant influence of the shear factor kw on the damage behaviour, especially during failure. This factor proved to be essential for the accurate prediction of material behaviour under shear-induced stress conditions. The integration of the modified GTN model with the Chaboche material model in UMAT enables an accurate prediction of the material and damage behaviour and thus makes an important contribution to the understanding of the mechanical material behaviour of SLS PA12 specimens.
This contribution presents an investigation of strain rate dependent behaviour for selective laser sintered polyamide 12. Two different cases are considered: a strain rate change within tensile loading and a relaxation test incorporating a strain rate change after each holding time section. For the simulation of material behaviour the material model of Bodner-Partom and Chaboche were used. The damage behaviour was considered by the Lemaitre crack model. Thereby it was determined which numerical model is more suitable to reproduce the strain rate-dependent behaviour. Tensile and relaxation tests at constant speeds served as the basis for these investigations.
The material and damage behaviour of additively manufactured polyamide 12 under cyclic loading was characterized by cyclic tests and microstructure analysis by using microscopy, X-ray refraction, and computed tomography. The results were used to determine parameters for the viscoplastic material model by Chaboche and a damage model by Gurson-Tvergaard-Needleman. The temperature was monitored during the experiments and the self-heating effect was observed. By including this effect, a higher accuracy could be achieved with the results of mechanical experiments.
It is generally known that significant improvements in the properties of nanocomposites can be achieved with graphene types currently commercially available. However, so far this is only possible on a laboratory scale. Thus, the aim of this study was to transfer results from laboratory scale experiments to industrial processes. Therefore, nanocomposites based on polyamide (PA) and graphene nanoplatelets (GnP) were prepared in order to produce membranes with improved gas barrier properties, which are characterized by reduced permeation rates of helium. First, nanocomposites were prepared with different amounts of commercial availably graphene nanoplatelets using a semi-industrial-scale compounder. Subsequently, films were produced by compression molding at different temperatures, as well as by flat film extrusion. The extruded films were annealed at different temperatures and durations. In order to investigate the effect of thermal treatment on barrier properties in correlation to thermal, structural, and morphological properties, the films were characterized by differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS), optical microscopy (OM), transmission electron microscopy (TEM), melt rheology measurements, and permeation measurements. In addition to structural characterization, mechanical properties were investigated. The results demonstrate that the permeation rate is strongly influenced by the processing conditions and the filler content. If the filler content is increased, the permeation rate is reduced. The annealing process can further enhance this effect.
Thermo-viscoplastic material modelling for self-heating loads and its experimental verification
(2020)
The paper examines a modelling approach for thermomechanically coupled problems and an experimental concept for a material law validation and verification for self-heating with small to moderate temperature ranges. The study compares two different model formulations and is generally applicable to a variety of material classes. One model is based on a rheological network with an extension for dissipative deformation below the elastic limit. The other model operates without a yield condition. Both models are applied to published experimental data in terms of rate-independent behaviour and the evaluation is carried out on stress-strain-level, temperature evolution and the energy transformation ratio. Furthermore the two models are applied to a strain rate-dependent load case conducted at our institute discussing the same entities. It is pointed out, that the approach of a thermomechanical analysis is valuable and informative to assess the observed deformation processes and to describe the material behaviour with a thermodynamically valid parameter set.
Paper presents results of dynamic FSI simulation of unmanned aerial
vehicle, where for first case, a typical linear material was used, and for second case, a composite material was used. The simulations were performed with the objective of studying model reaction with forces indicated by flow which were defined by initial conditions. Secondary objective was to evaluate critical flutter speed. Simulations provided interesting results: for linear case, applied flow speed resulted with occurrence of flutter phenomena, while composite case exhibited damping of vibration in designated points along the wing. Presented
simulations were performed within an environment using loosely-coupled
approach. CFD simulations were carried out with use of parallel code DLR-Tau.
FSI computations were executed with applied modal approach for structural model and full data exchange between fluid and structure as well as mesh deformation.
The paper presents a modelling approach for thermo-mechanically coupled problems and an experimental concept for a material law validation. The approach is generally applicable to a variety of material classes. The main objective is to integrate viscoplasticity as one major aspect in cyclic loading of high-performance polymers at small deformation gradients. After a classification of the subject a brief outline of the theoretical framework is given, which is then followed by the description of the experimental setup and some specifics to be accounted for. The Paper continues with some results of the numerical analysis on how to validate the applied material law.
In order to characterise the material and damage behaviour of additively manufactured polyamide 12 (PA12) under quasi-static load and to implement it in a numerical model, experiments under quasi-static load as well as microstructural investigations were carried out. Selective laser sintering (SLS) was used as the manufacturing process. For the classification of the material behaviour, quasi-static cyclic tests with holding times as well as tensile tests were performed. X-ray refraction and computed tomography (CT) were used to investigate the damage behaviour. The Chaboche model, which has already been applied for metallic materials under thermomechanical loading, served as the basis for the selection of the numerical material model. The same procedure was used for the selection of the damage model, where the Gurson–Tvergaard–Needleman (GTN) model was chosen, which was already used for porous metallic materials. The Chaboche model shows very good agreement with experimental results. Furthermore, the coupling with the GTN model allows a very good modelling of the damage behaviour. Finally, it could be shown that the selected models are suitable to simulate the material and damage behaviour of 3D printed PA12.
In this paper we want to highlight the chain of linked topics, which are necessary to cover when implementing a fatigue analysis for polymeric materials and/or fibre-reinforced plastics. Starting from experimental observation the time dependency in cyclic loading responses are emphasized by viscoelastic and viscoplastic phenomena in applied material models. Incorporating an efficient failure criteria in this framework is pointed out. Finally, a concept for a thermo-mechanical model is discussed to correlate the temperature field to the fatigue status with the Chaboche model as the core for dissipative quantification.
Die 3D-Druck-Technologie als additives Fertigungsverfahren verändert die industrielle Fertigung mit einer exponentiellen Dynamik. Der Markt für Produkte und 3D-Druck-Dienstleistungen wird sich in den nächsten Jahren stark ausweiten und einen immer höheren Stellenwert in der Industrie einnehmen. Auch wenn die Technologie bereits in vielen Branchen Einzug gehalten hat, so gibt es technologisch noch ein enormes Entwicklungspotential, um Bauteile, aber auch das Verfahren selbst weiter zu optimieren. Gegenwärtig sind noch keine allgemein verbindlichen, abgestimmten Standards für die Herstellung additiv erzeugter Materialien sowie für die Bestimmung der Materialeigenschaften verfügbar. Die Erkenntnisse aus den vorliegenden Forschungsvorhaben sollen deshalb für ein allgemeines Prüfverfahren für 3D-gedruckte thermoplastische Kunststoffe genutzt werden. Langfristiges Ziel ist es, einen Standard der Prüfverfahren in diesem Bereich zu definieren.