Browsing by Organization "Department Maschinenbau"

Traditionally, modal analysis and the extraction of modal parameters from vibration data is a process that requires a more or less extensive amount of manual interaction from setting input parameters up until finding the eigenfrequencies. The growing interest in continuously monitoring mechanical structures e.g. for automated damage detection methods has led to the development of many approaches to automate different aspects of modal analysis. In this context, the Covariance-driven Stochastic subspace identification (Cov-SSI) is a widely used method. The present paper provides an automated Cov-SSI algorithm combined with a peak-picking approach for the automatic determination of input parameters. In this regard, using the Prominence-parameter allows to examine the PSD by finding the most relevant peaks. The herein shown algorithm is currently suitable for systems with a limited number of sensors. Cov-SSI results are arranged in stability plots and interpreted using the hierarchical clustering method. By creating stability plots for a wide range of block rows a sensitivity analysis is used to find the optimal result based on the averaged standard deviation of damping of the clusters in every stability plot. A second aspect of this paper is comparing the common method for order reduction with a modified method described in [1], which preserves the orthogonality of the , and matrix of the singular value decomposition. Exemplary results on both methods are provided using simulated data (state-space, 3 DoF)

A loss of preload occurs in every bolted joint after tightening, which is referred to as “preload relaxation”. The magnitude of this loss is mainly influenced by the material of the screw and the clamped parts, which are in the flow of forces, as well as the prevailing temperature. As the sufficient height of preload is of great importance for the load bearing capacity of a bolted joint, a method of predicting the preload relaxation has been searched for a long time. Due to dissatisfactory predictive accuracy, missing transferability to different designs or the high effort for calculation and parameter determination, previous approaches have mostly turned out to be impractical and have thus not been established. In this thesis an analytical (equation-based) calculation model was therefore developed, for which all parameters, besides general material properties, can be derived from only one simple preload measurement, which are necessary to describe the loss of preload. At the same time all individual contributions of preload relaxation as well as the real resiliencies are considered. The calculation is based on a joint, where lightweight materials are clamped with steel bolts and nuts. Thus, the loss of preload mainly occurs from the time- and load-dependent plastic deformation of the clamped parts. It is shown that this method is suitable to predict continuously measured preload developments very well. The prediction of relative residual preloads of other test setups with different relative resiliencies, changed fasteners and different ambient temperatures is also practically sufficient. Moreover, the underlying material law can be varied, if fiber-reinforced plastics are used instead of lightweight metals. Furthermore, general statements on the influencing parameters on preload relaxation are given, based on the large experimental basis.

Since additive manufacturing (AM, i.e., 3D printing) has been widely used for production of end-used products, the mechanical strength of parts fabricated by this technology have gained considerable significance. The current study presents fracture load assessment of 3D-printed components fabricated by the stereolithography (SLA) technique using UV sensitive resin material. In this context, dumbbell-shaped specimens were printed and tested to determine basic mechanical properties. Moreover, V-notched semi-circular bending test coupons with various notch opening angles (i.e., 15°, 30°, 45°, 60°, 75°, and 90°) were printed and examined. Parallel to the experimental tests, we developed a finite element model to simulate the load carrying performance of 3D-printed parts. Moreover, we used the digital image correlation technique to determine displacement and strain field on the surface of the examined specimens. Since the mechanical strength and fracture behavior of 3D-printed parts are investigated in the current study, the presented outcomes can be utilized for innovative designs of parts fabricated by SLA with a higher mechanical strength, and improved load-carrying capacity.

Fabrication based on additive manufacturing (AM) process from a three-dimensional (3D) model has received significant attention in the past few years. Although 3D printing was introduced for production of prototypes, it has been currently used for fabrication of end-use products. Therefore, the mechanical behavior and strength of additively manufactured parts has become of significant importance. 3D printing has been affected by different parameters during preparation, printing, and post-printing processes, which have influence on quality and behavior of the additively manufactured components. This paper discusses the effects of two printing parameters on the mechanical behavior of additively manufactured components. In detail, polylactic acid material was used to print test coupons based on fused deposition modeling process. The specimens with five different raster orientations were printed with different printing speeds. Later, a series of tensile tests was performed under static loading conditions. Based on the results, strength and stiffness of the examined specimens have been determined. Moreover, dependency of the strength and elastic modulus of 3D-printed parts on the raster orientation has been documented. In the current study, fractured specimens were visually investigated by a free-angle observation system. The experimental findings can be used for the development of computational models and next design of structural components.

Aim of this work was the investigation of the high temperature fatigue behavior of the gamma-TiAl-alloy TNB-V2. Total strain-controlled fatigue experiments under isothermal (LCF) and thermo-mechanical (TMF) conditions were performed at a strain ratio of R=-1. The alloy shows under LCF conditions a clear dependency of the fatigue life on the strain amplitude. At low temperatures and high strain amplitudes strain aging leads to a cyclic hardening until fracture. The microstructure shows mainly formation of twining and high dislocation density. Above 650°C cyclic softening takes place at high strain amplitudes. The microstructure shows above the BDTT a degradation of the lamellar morphology, where a transformation of the excess alpha2 phase into the gamma phase takes place. Under in-phase (IP)-condition the compressive mean stresses exhibit a beneficial effect on the fatigue life. This leads at low strain amplitudes to higher fatigue lives than under LCF conditions. The TMF experiments show also that a low minimum temperature of 350°C enhances the dynamic strain aging and causes together with the environment a considerable decrease of the fatigue life under out-of-phase (OP) condition. In contrast to LCF microstructure the TMF microstructure shows no dynamic recrystallisation in the gamma-grains. Under TMF condition dislocations are generated during the minimum temperature phase. The fatigue lives under IP, OP, and LCF conditions were described by means of a single model, which consists of two parts. In the first, part the TMF hysteresis loop was simulated by means of a modified multi-component model. In the second part, a damage parameter for the fatigue life description was defined. The parameters of the stress-strain response needed for this model were calculated by means of the multi-component model mentioned above. Due to the combination of these two models a microstructure reference was assured during fatigue life description.

Due to the exponential increase of failure cost during the product development process, problems have to be effectively remedied as early as possible and with shortened innovation cycles, increasingly efficient. For the manufacturing of complex products at low maturity levels (referred to as physical product development), nonconformance problem solving constitutes a major difficulty in this regard (Camarillo et al., 2017; Walter et al., 2010). The data presented in this article was collected from German companies, differing in size and industry sector, manufacturing highly complex products at low maturity. Selected and consulted companies therefore operate in (or comparable to) the automotive prototyping, air and space, shipbuilding, special machinery or electronics domain. The survey comprises the answers of 46 participants, gathered via online questionnaire. It subdivides into 18 questions covering the companies’ characteristics, knowledge management and documentation systems within the product development process, as well as the appraisal of technological potentials. The obtained data gives an insight into the industrial status quo of nonconformance problem solving. The data allows to derive existing deficits and dedicated research on solutions.

Damage in adhesively bonded joints typically initiates in the overlap area due to high level of bonding (peel) stress. Different approaches are being considered to decrease the peel stress and improve the overall strength of the joint. One possible approach is to shape the over lap area into a stepped form configuration and enhance the performance of the joint. In the current study, we investigate effects of stepped-shape overlap area on the load bearing capacity of additively manufactured single-lap joints. To this aim, stepped-lap adhesively bonded joints with different designs and geometries in the overlap (bonding) area are considered with 3D-printed polylactic acid (PLA) adherends using the fused deposition modeling (FDM) process. Three configurations with different step sizes are considered to manufactured a set of adhesively bonded single-lap joints and to investigate the optimum length of the steps. The results are compared with our previous experimental findings on 3D-printed conventional single-lap joints. The obtained outcomes reveal that creating steps in the overlap area has a significant influence on the structural integrity and fracture load of 3D-printed adhesive-bonded joints and the bonded structures with identical step size in boding area reveal a better performance in load carrying capacity and shows a higher fracture load. Parallel to the experimental practices, a finite element model also developed to simulate the load carrying performance of the adhesively bonded singlelap joints with equal step size and 3D-printed PLA adherends. The FE model confirms the experimental outcomes and reveals the details of the cohesive failure and damage evolution mechanism in this bonded structures with PLA printed adherends. The proposed technique has a great potential to be a competitive alternative to conventional single-lap joints made by 3D printing. The presented results can be used for further fabrication of 3D-printed joints with a better structural performance.

Multi-scale problems like the generation of sound in a flow field and its sound wave propagation in the far field have become increasingly important in the design phase of industrial devices: One example is noise reduction of aircrafts or wind turbines. Although the generation of sound as well as its propagation can both be described by the same governing equations, wave propagation is a linear phenomenon and its equations can be simplified, which results in less computational effort. Additionally, the generation of sound in a flow field occurs at small spatial scales, while its propagation in the far field has to be observed on a large spatial scale. These large differences in scales are particularly challenging for numerical simulations. Resolving the entire domain with the high resolution that is required for the small scales of the flow domain is impossible due to the vast computational demand. In this thesis, we propose a partitioned coupling approach for the efficient simulation of such problems on massively parallel supercomputers. In partitioned coupling the physical domain is split into smaller subdomains, each covering a different physical phenomenon. Their interaction is realized by exchanging data at the joint coupling interface. Subsequently, these subdomains can be solved with numerical methods, resolutions, and equations tailored to the local physical requirements. Even different solvers can be used. However, this approach also holds numerical challenges at the coupling interface: E.g. for a consistent data exchange in case of different spatial resolutions, direct data evaluation, or efficient interpolation methods are necessary. Within this work, two different approaches of partitioned coupling are implemented and compared: A blackbox and a white-box approach. The black-box approach is characterized by a flexible choice of numerical solvers which allows for a wide range of different applications. Its generality comes with limited access to information inside each solver and, therefore, with a potential loss of performance. However, a black-box approach only acts on point data at the coupling interface and therefore requires external interpolation methods for a consistent coupling in space which is expected to be less efficient than solver-internal data mapping. In contrast, the white-box approach is fully integrated within one numerical framework. Accordingly, it can access solver-internal data mapping methods which promises better numerical results. This tight integration allows for the exploitation of knowledge about internal data structures and, therefore, yields performance benefits. On the other hand, it comes with less flexibility. Both strategies will be compared with respect to quality of data mapping at the coupling interface as well as performance on modern supercomputers. In order to achieve the best performance, the optimal load balancing strategy for a coupled setup is investigated. The benefits of the partitioned coupling approach are demonstrated on an industrial application of a 3D free-stream jet with a high Reynolds number showing that a multi-scale problem can be simulated using today’s compute resources.

The numerical simulation of physical problems involves capturing various phenomena occurring simultaneously at different scales in a single simulation. For example, considering aeroacoustic problems, the noise generating flow and the propagation of the sound waves in the far-field need to be taken into account. The increasing computational capacities and the development of modern supercomputers allow for more detailed studies of complex multi-physics and multi-scale problems. In this work, the sound generation by moving obstacles and its propagation up to the far-field is of particular interest. For this purpose, a high-order Discontinuous Galerkin method is utilized to discretize the fluid dynamic equations. These high-order methods are exceptionally efficient as they only require a few degrees of freedom to represent smooth solutions. Therefore, they are often deployed for, e.g., the acoustics far-field, where a homogenous flow field can be found. From the computational perspective on modern high-performing architectures, few degrees of freedom are an exceptional advantage, with memory bandwidth being a bottleneck on modern systems. Additionally, the ratio between communication and computation is minimal due to the loose connection of computational elements at their respective interfaces, which is an additional advantage of these methods, when considering distributed and massively parallel computing systems. However, the high-order representation of complex geometries has been a critical limitation for their application in various fields. The representation of geometrical shapes has to be appropriate to preserve the quality of the numerical solution, which has been discretized with high-order. Incorporating meshing techniques such as body-fitted meshes might not be robust in the workflow for the simulation. They are required to withstand different scenarios that are common, such as scenarios with general complex geometries inside the simulation domain. They can become expensive in computation when involving simulations with multiple geometries and even more when geometries can move. In these cases, the embedded method, also known as immersed boundary method, provides a promising prospect. In this work, the Brinkman penalization technique is applied to model multiple complex and moving geometries. Moving rigid bodies are common in engineering applications. The sound emitted due to the motion and the flow disturbance by geometries is of particular interest, as awareness of environmental impact in society has grown in recent years. Therefore, predicting the produced noise is a common responsibility in different fields, such as the design of wind turbines. These simulations have a complex nature and require an efficient strategy to facilitate them feasibly. Therefore we deploy the partitioned coupling approach, where the complex and large simulation domain is decomposed into smaller subdomains. Each subdomain is configured such that the occurring phenomena can be precisely captured. It results in an efficient strategy allowing for the simulation of various scales and physics, such as the large-scale simulation in this work. The simulation includes the motion of an airfoil and the induced noise that spreads over a large domain.

The thesis presented here was produced as part of a research project metabolon IIb (EFRE-0500033). In terms of the circular economy, the focus was on closing energy cycles and material circulations. The goal was either to generate regenerative energy or to return valuable materials back to the anthropogenic material cycle, with the focus on biogenic or waste residues as input material. The thesis arose while dealing with the question of the importance of hydrothermal carbonization (HTC) regarding future regenerative energy supply. The overall goal of the thesis was then to integrate the HTC process into holistic process chains and to evaluate its energetic use considering the global warming potential (GWP). Due to the lack of HTC plants on an industrial scale and standardized model implementations, a feasible reactor model based on an empirical kinetic approach for HTC was developed. The model was used to calculate the mass and energy distribution in the three production phases (solid, liquid, and gaseous) and evaluated with experimental analyses. The HTC offers a potential application from the organic fraction of municipal solid waste (OFMSW) and its digestate, which were used as feedstock. In addition, the process chains were assessed based on their efficiencies and compared with conventional process chains. The representative conventional process chains in this case were the incineration of OFMSW in a waste incineration plant (“I“) and the treatment in an anaerobe digestion plant followed by composting (“AD+comp“). Here, the exergetic net efficiency was 13,7 % for “I“ and 12,1 % for “AD+comp“. The implementation of an HTC-process increased the exergetic efficiencies by 70 % compared with “I“ and by 93 % compared with “AD+comp“. The GWP was ∼500 g CO2,Eq kW−1 h−1 in the reference cases. The integration of an HTC unit reduced the GWP by 30 % compared to the conventional pathway.