45 research outputs found
Quantifying Epistemic and Aleatoric Uncertainty in the Ampair 600 Wind Turbine
No full textDetermining the uncertainty in a mechanical joint is very important and very difficult. This paper presents two methods of determining the uncertainty in the joint: maximum entropy approach and sampling methods. Maximum entropy is an approach that can quantify the aleatoric and epistemic uncertainty independently. This approach is applied on a rigid connection of the Ampair 600 Wind Turbine and shows that the epistemic uncertainty of the system is very high. Sampling methods are used on an simplified representation of the wind turbine as a lumped mass approximation. The sampling methods are able to treat the joint in a nonlinear sense by using a discrete four-parameter Iwan model as the joint model. This is able to predict accurately the data within the uncertainty bounds when considering epistemic uncertainty. The Iwan joint model is then implemented on the high fidelity model and preliminary results are presente
Experimental Parameter Identification of Nonlinear Mechanical Systems via Meta-heuristic Optimisation Methods
No full textMeta-heuristic optimisation algorithms are high-level procedures designed to discover near-optimal solutions to optimisation problems. These strategies can efficiently explore the design space of the problems; therefore, they perform well even when incomplete and scarce information is available. Such characteristics make them the ideal approach for solving nonlinear parameter identification problems from experimental data. Nonetheless, selecting the meta-heuristic optimisation algorithm remains a challenging task that can dramatically affect the required time, accuracy, and computational burden to solve such identification problems. To this end, we propose investigating how different meta-heuristic optimisation algorithms can influence the identification process of nonlinear parameters in mechanical systems. Two mature meta-heuristic optimisation methods, i.e. particle swarm optimisation (PSO) method and genetic algorithm (GA), are used to identify the nonlinear parameters of an experimental two-degrees-of-freedom system with cubic stiffness. These naturally inspired algorithms are based on the definition of an initial population: this advantageously increases the chances of identifying the global minimum of the optimisation problem as the design space is searched simultaneously in multiple locations. The results show that the PSO method drastically increases the accuracy and robustness of the solution, but it requires a quite expensive computational burden. On the contrary, the GA requires similar computational effort but does not provide accurate solutions.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Ship Design, Production and Operation
Determining the Influence of Experimental Setup and Inputs on Nonlinear Systems
No full textIn the automotive and aerospace industries, every component experiences dynamic loading. The complex, multi-axial loads and surrounding system are the environmental conditions. Due to the lack of understanding of nonlinear system dynamics, the components and structures have been over-designed to handle these environmental conditions and reduce nonlinear influences. Over-designing leads to an increase in weight, which in turn reduces fuel efficiency.
In recent years, there has been a drive to improve fuel economy resulting in the redesign of these over-designed systems. Reducing the component conservativeness increases nonlinear influences on the dynamic response of the system. Research is ongoing to characterize these new systems in laboratory settings. However, there is a disconnect between the environmental conditions and laboratory experiments. These disconnects are a result of improper model simplifications and input forces, costing billions of dollars in inappropriate/ additional qualification tests. Currently, researchers are using varying levels of fidelity to model the salient physics of the nonlinear systems. Unfortunately, they are still unable to match the response of systems with strong nonlinearities fully.
This thesis is motivated by the need to improve and develop experimental methods to more accurately update models for environmental conditions. The specific issues addressed and significant contributions include:
1. A refinement of a recently formalized method proposed by D.J. Ewins, which consists of ten steps to perform model validation of nonlinear structures. This work details through a series of experimental studies that many standard test setup assumptions and properties of nonlinearities are invalid. This invalidation is due to both known and unrecognized nonlinear properties. A review of current methods for characterizing nonlinear and gaps in the approaches.
2. A series of tests to determine the influence of multi-axial excitation on system responses. Due to the complex environmental loads, a component will not experience excitation from only one direction. Current laboratory tests qualify the system one axis at a time, under the assumption that a nonlinearity has no dependence on the load direction.
3. A design methodology to create a test fixture to emulate the environmental boundary condition. The methodology truncates boundaries to a more reasonable size for laboratory testing by utilizing spring-mass systems to account for the loss of mass and stiffness. The method is validated by comparing stress states of an attached component of the full system and truncated one
The Variability of Strains in Bolts and the Effect on Preload in Jointed Structure
No full textTorque wrenches are the most common method used for tightening bolts; however, this method can cause a large amount of uncertainty in the resulting bolt preload. With the inability to determine the preload in bolted structures precisely, overdesign is necessary for bolts to meet safety requirements and prevent jointed structures from looseness failure. A series of comparative experiments are completed to quantify the influence of bolt strain variation based on strain gauges, force transducers, and accelerators. Moreover, this research uses the Peak Finding and Fitting (PFF) PFF algorithm to analyze nonlinear dynamics of the experimental system, which demonstrates the ramifications of the experimental results. Measured strains in bolts showed significant variabilities, which indicates larger the expected uncertainty in preload. Impact test results also indicate the advantages of pre-strain method for tightening bolts compared to torque wrenches
Measurement of Bolt Dynamics in Jointed Structures Undergoing Dynamic Loading
No full textIt is known in the joints community that relying solely on torque wrenches to ap- ply bolt preload results in varied and incredibly inaccurate bolt preloads; there is a large discrepancy between bolt theory and reality. The industry proposed solution to this issue is to use instrumented strain gauge bolts, which are capable of providing an accurate bolt preload measurement. However, the available instrumented strain gauge bolts are often priced at inaccessible prices for individuals and labs. This unfortunately prevents labs from acquiring proper instrumentation.
This thesis is motivated by the need to provide high fidelity and repeatability in experimentation as well as providing accurate baseline data for computational mechanics of structures. The following topics have been examined as an attempt to provide the aforementioned traits.
1. An understanding of uncertainties in joints, the current methods of preload mea- surement, and some joint community research questions, which can be answered with accurate bolt preload readings.
2. The history of the strain gauge bolts used in the Tribomechadynamics lab and the design, manufacturing, and calibration processes of the final instrumented strain gauge bolt.
3. Instrumented strain gauge experimentation: Bolt dynamics under varied bolt preload and excitation amplitude.
While the current instrumented strain gauge bolt iteration succeed in accomplish- ing the desired goals for the joint community, future plans for this laboratory tool are exciting and promising for jointed structures, especially in the area of system health monitoring
Constitutive Modeling of Friction in Bolted Connections
No full textBolted joints are ubiquitous in mechanical engineering, requiring accurate models to optimize designs. However, the exact nature of frictional contact between components is unknown and poses a significant challenge to modeling the nonlinear vibration of assemblies. This thesis applies empirical and physics-based modeling approaches to identify improvements to current models and a potential path towards predictive models of friction in bolted joints. The empirical modeling approach solves a multi-objective optimization to fit 26 friction model/interface representation combinations to experimental data and quantify the model form error. While the empirical models are not physical, the optimized results highlight the benefits of using smooth friction models and the limitations of a common physically motivated model. The physics-based model formulates the frictional force based on contact interactions of surface features and derives parameters from surface scans. While the physics-based model is not completely predictive, results show promising agreement with experiments
Nonlinear vibration analysis of uniform and functionally graded beams with spectral Chebyshev technique and harmonic balance method
No full textIn this paper, nonlinear forced vibrations of uniform and functionally graded Euler-Bernoulli Beams with large deformation are studied. Spectral and temporal boundary value problems of beam vibrations do not always have closed-form analytical solutions. As a result, many approximate methods are used to obtain the solution by discretizing the spatial problem. Spectral Chebyshev Technique (SCT) utilizes the Chebyshev Polynomials for spatial discretization and applies Galerkin’s method to obtain boundary conditions and spatially discretized equations of motions. Boundary conditions are imposed using basis recombination into the problem and as a result of this, the solution can be obtained to any linear boundary condition without the need for re-derivation. System matrices are generated with the SCT, and natural frequencies and mode shapes are obtained by eigenvalue problem solution. Harmonic Balance Method (HBM) is used to solve nonlinear equation of motion in frequency domain, with large deformation nonlinearity. As a result, a generic method is constructed to solve nonlinear vibrations of uniform and functionally graded beams in frequency domain, subjected to different boundary conditions
Identification of nonlinear characteristics of an additive manufactured vibration absorber
No full textAdditive manufacturing has become increasingly popular in the last decades and has shown great potential for designing and manufacturing innovative design solutions. Recently it has been demonstrated that additive manufacturing can be used to create monolithic compliant mechanisms that can avoid assembly and relative movement between components, showing considerable advantages in their use in harsh environments (i.e. space applications). In this paper, we explore the possibility of adopting 3D-printed compliant mechanisms as tuned-mass vibration absorbers: the challenge is to identify the characteristics of an equivalent nonlinear oscillator that can be used to assess the performance of the absorber. The experimental and numerical results show that the proposed compliant mechanism offers a complex nonlinear dynamic behaviour and it can effectively act as a vibration absorber for a simple cantilever beam
Challenges in Simulating the Forced Vibration Response of Assembled Space Structures
No full textThis chapter presents an example of how challenging can be the validation of system responses when a product must be certified before being commissioned for a launch or flight. Several divisions of industry dealing with dynamic models and simulations of forced vibrations are discovering how the nonlinear dynamics of joints can lead to inaccuracies in the simulated vibration responses, mainly due to the nonlinear relationships of frequency and damping with the vibration amplitude (which is high for qualification tests). Hence, the example of this chapter refers to analysis of test data gathered from a solar array in qualification tests. It is shown how dissimilar simulated and measured vibration data can be, and how the qualification levels enhance the nonlinear dynamics, often not predicted by dynamic models. Finally, it presents an effort to analyse a cut-out of the solar array in terms of frequency and damping nonlinearity.</p
An Investigation of Complex Mode Shapes
No full textThis paper presents an investigation of complex mode shape analysis caused by non-linear damping. Nowadays, most academics are accustomed to complex mode shapes, which are a characteristic of most axisymmetric structures. The topic was deeply investigated during the 1980s, sparking the sharpest debates about their physical existence or not. However, after nearly three decades, one question still stands, do we know all about complex mode shapes? This paper takes the dust off this topic again and explores how complex eigenvectors arise when the percentage frequency separation between two mode shapes is the same order of magnitude as the percentage damping. The difference between the past and present investigations relates to the non-linear damping that might arise from joint dynamics under various vibration amplitudes. Hence, the new research question is about the investigation of amplitude-dependent damping on the modal complexity. Why bother? There are several engineering applications in both space and aerospace where axisymmetric structures and joint dynamics can impair the numerical analysis that is currently performed. This paper does not offer any solutions but does expand the research on an unsolved challenge by identifying the questions posed.</p
