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One of the ongoing tasks in space structure testing is the vibration test, in which a given structure is mounted onto a shaker and excited by a certain input load on a given frequency range, in order to reproduce the rigor of launch. These vibration tests need to be conducted in order to ensure that the devised structure meets the expected loads of its future application. However, the structure must not be overtested to avoid any risk of damage. For this, the system’s response to the testing loads, i.e., stresses and forces in the structure, must be monitored and predicted live during the test. In order to solve the issues associated with existing methods of live monitoring of the structure’s response, this paper investigated the use of artificial neural networks (ANNs) to predict the system’s responses during the test. Hence, a framework was developed with different use cases to compare various kinds of artificial neural networks and eventually identify the most promising one. Thus, the conducted research accounts for a novel method for live prediction of stresses, allowing failure to be evaluated for different types of material via yield criteria
The interest in micro applications increases in recent years due to new methods of fabrication. One fabrication process is direct laser writing, which can fabricate high-precision structures in the micrometer range. The material properties of the micro structures are related to the writing parameters, such as laser power, scan speed, distance between written lines and writing direction. This work presents investigations of the thermal length expansion coefficients of a laser-written polymer in regard to laser power. To this end cantilever structures are fabricated. The small cantilevers are heated and their length expansions observed using a microscope. Images of the cantilevers at different temperatures are taken and by image post processing, the change in length and their coefficients of thermal expansion is determined.
In grinding, the crystal grain size of the workpiece material is relatively same range compared to the removal depth. This raises a question if an anisotropic material model, which considers the effect of the crystal grain size and orientations, would better predict the process forces when compared to an isotropic material model. Initially, a simple micro-indentation process is chosen to compare the two models. In this work, a crystal plasticity model and an isotropic Johnson-Cooke plasticity model are employed to simulate micro-identation of a twinning induced plasticity (TWIP) steel. The results of the two models are compared using the force-displacement curves from the micro-indentation experiments. In the future, the study will be extended to describe the material removal process during a single grit scratch test.
In this work, we investigate the locomotion of a snake-like soft robot in terms of its design. Therefore the backbone of the robot is represented by a curve in plane which is actuated by a given curvature. By adding anisotropic friction between robot and surface the robot “moves” on the surface.
With this simple model we are able to predict the locomotion of the robot for certain sets of parameters. This allows to evaluate the influence of design changes and hence to facilitate the design process. As an example, we discuss results concerning the precision of actuation, the bending radius of the robot and the influence of friction between robot and surface.
Design improvement by a simulative investigation of the locomotion of a snake-like soft robot
(2021)
This work aims to improve the design of a snake-like soft robot in terms of its velocity of locomotion by a geometric model. Therefore, we determine the locomotion of the snake-like soft robot as the result of a given excitation curvature and a given friction anisotropy between the robot and the ground.Varying the design parameters of the robot in the model allows to identify important parameters to increase the velocity of locomotion of the snake-like soft robot. Whereas its body design is sufficient, the transverse friction of its artificial skin is the main parameter to be improved. The transverse friction can be adjusted by turning the scales of the artificial skin. The velocity of locomotion of the robot increases significantly by this simple trick.
An FEM-based physical force model is an important step to obtain a full understanding of the grinding process itself. Such a physical force model is already under development and is based on Abaqus-FEM. In order to examine basic material behavior and material parameters for such a physical force model and to validate it, scratch tests have been carried out with single grains. However, the current physical force model is only designed for grinding processes that do not require cooling lubricants. Therefore, the aim of this work is to extend this physical force model in such a way that grinding processes with cooling lubricants can also be considered. In order to include the cooling lubricants in the FEM model, it is essential to carry out scratch tests with cooling lubricants in addition to the scratch tests in a dry environment. The aim is to identify basic mechanisms in connection with cooling lubricants, which are needed to expand the FEM model and to create a data basis for subsequent validation.
With direct laser writing micro structures can be manufactured by solidifying a photo resist when the laser beam triggers a photochemical reaction in the focal voxel. We have used direct laser writing to fabricate a thermally actuated microgripper, which can move its two cantilever like arms to grip micro-objects. One cantilever consists thereby of two strips with different coefficients of thermal expansion such that both cantilevers bends towards each other for an increasing temperature like a welded bimetal.This work investigates the impact of each cantilever's geometry on the gripping performance of the micro gripper theoretically. The tip deflection of the gripper is calculated by the analytical model of Timoshenko's theory of elasticity. After fabricaiton of the microgripper, its gripping performance is observed under the microscope while heated by a heating element.
Tribological systems are often characterized based on time-averaged quantities such as wear rates, friction coefficients and material properties. It is well known that some tribological metrics show variations depending on the laboratory conducting the study and the reproduction method selected. Perhaps the key to overcome this problem is to avoid a strong compression of the information generated. In this context, the arising forces and the coefficient of friction in three-body wear systems are investigated in more detail. The mean value of a time series of these physical quantities is only a single property and by no means an exhaustive description. A more detailed consideration of the variances could be a necessary condition to allow an appropriate comparison of tribological parameters and a correct interpretation of the properties of tribological systems. For this purpose, we examine two very simple tribological systems exemplarily and take a closer look at the properties of some characteristic process quantities.
Development of a simple substitute model to describe the normal force of fluids in narrow gaps
(2023)
Fluids in narrow gaps are employed frequently in many applications. The motivation for their use is diverse and ranges from hydrodynamic lubrication in plain bearings to the transport of hard particles into the working gap for the purpose of machining workpiece surfaces in lapping processes. Depending on the focus of the analysis, it may be useful to investigate the entire pressure field or to calculate only individual quantities. For example, in sophisticated simulations it may be of interest to know the resulting force of a fluid as a function of the external system state in order to describe its damping characteristics. Especially for the simulation of flows in narrow gaps, the Reynolds equation is a convenient choice, which, in contrast to the more general Navier-Stokes equations, can lead to considerable savings in computational time because no three-dimensional discretization is required, but only a two-dimensional discretization. However, if not a highly detailed pressure field is of interest, but only simple relations such as the resulting force as a function of distance and velocity, and if this relation to be evaluated many times for different parameter combinations over a wide range of values, the use of a robust substitute model is a good choice. This article deals with the creation of such a substitute model based on the Reynolds equation taking cavitation into account.
Model-based prediction is becoming increasingly important to meet the ever-increasing demands on manufacturing. In grinding, the prediction of the process forces and the generated surface by physical models are particularly important.Since cooling lubricants are almost always used on an industrial scale, the grinding model, developed at our institut, must be extended to include this component. Therefore, in order to implement cooling lubricants into the FEM-based model, it is first necessary to investigate the behaviors and effects of cooling lubricants in real experiments. Various influencing factors such as the scratching speed of individual abrasive grains in interaction with cooling lubricants need to be investigated. However, the existing physical grinding model is not limited exclusively to the prediction of the resulting forces. It is also supposed to be able to qualitatively predict the expected resulting surface of the workpiece. Hence, this paper will focus on the topographic characteristics that can occur in the scratch test due to different cooling lubricants and scratching speeds.Based on real experiments on a test rig for such scratch tests, it has been shown that different scratch speeds have a negligible influence on the topographical nature and expression of a scratch. In contrast, however, there is a direct influence of cooling lubricants on the topographic properties. This effect is additionally influenced by the viscosity of the cooling lubricant used.