NON-LINE ANALYSIS OF STIFFNESS IN COMPRESSION CONDITIONS

The analyzes were aimed at demonstrating the influence of parameters describing the deformation of the structure on the uncertainty of critical force, and the impact of technological imperfections on stress uncertainty in compression conditions. In a linear buckling analysis, the problem is considered only for the initial, permanent state of the stiffness matrix. In the case of demonstrating the influence of initial deformations on the behavior of the structure under load, it is necessary to visualize changes in stiffness over time. To this end, a non-linear MES analysis was carried out, which will take into account local changes in the stiffness of the model through a gradual increase in the load. Thus, the difference in stiffness is taken into account, which in the linear problem is infinite. The analysis was used to examine the local and global sensitivity of the parameters describing: plating thickness as well as deformation caused by the technological process on the stress value reduced by Huber hypothesis, and the value of normal stress. To take into account the influence of non-specified values of the magnitude of geometric deviations, and their simultaneous influence on the range of obtained results, the Experimental Planning Method and the Surface Method of Answers were used.

Numerical assessment of ultimate strength of severe corroded stiffened plates

K. Wołoszyk , M. Kahsin , Y. Garbatov – ENGINEERING STRUCTURES – 2018
The objective of this work is to investigate numerically (using the non-linear FEM and the approach stipulated by the Common Structural Rules) the severe nonuniform corrosion degradation effect on the ultimate strength of stiffened plates and compare the results to the already published experimental works. Different factors governing structural behavior of corroded stiffened plates are investigated, such as corrosion degradation level, material properties, initial imperfections and boundary conditions. The numerically estimated ultimate strength demonstrated to be very close to those observed during the experimental test. A sensitivity analysis with respect to the most important governing parameters of the numerical estimation of the ultimate strength is also performed and several conclusions are derived. The applied calculation procedure avoids using of a pitted surface of the corroded plates and instead of that an equivalent thickness is applying leading to a relatively fast and practical approach for ultimate trength assessment of corroded stiffened plates.

Numerical Modelling of Structures with Uncertainties

The nature of environmental interactions, as well as large dimensions and complex structure of marine offshore objects, make designing, building and operation of these objects a great challenge. This is the reason why a vast majority of investment cases of this type include structural analysis, performed using scaled laboratory models and complemented by extended computer simulations. The present paper focuses on FEM modelling of the offshore wind turbine supporting structure. Then problem is studied using the modal analysis, sensitivity analysis, as well as the design of experiment (DOE) and response surface model (RSM) methods. The results of modal analysis based simulations were used for assessing the quality of the FEM model against the data measured during the experimental modal analysis of the scaled laboratory model for different support conditions. The sensitivity analysis, in turn, has provided opportunities for assessing the effect of individual FEM model parameters on the dynamic response of the examined supporting structure. The DOE and RSM methods allowed to determine the effect of model parameter changes on the supporting structure response.

NUMERICAL MODEL QUALITY ASSESSMENT OF OFFSHORE WIND TURBINE SUPPORTING STRUCTURE BASED ON EXPERIMENTAL DATA

M. Kahsin , M. Łuczak – 2015
As a structure degrades some changes in its dynamical behavior can be observed, and inversely, observation and evaluation of these dynamical changes of the structure can provide information of structural state of the object. Testing of the real structure, besides of being costly, can cover only limited working states. It is particularly considerable in case of hardly accessible, and randomly/severely dynamically loaded offshore structures. As a testing instrument, numerical simulations are not limited as much, however a quality of answers depends significantly on an excellence of numerical model, and how in reality it reassembles the actual structure and the test results. One of the strategies is to correlate and update numerical models basing on the experimental data. Presented outcomes were obtained in frame of “AQUILO” project that aims to create a knowledge base, from which the investor will be able to decide on the best type of support structure for offshore wind farm specific location in Polish maritime areas. The examined object is laboratory tripod type support model (scaled) of the offshore wind turbine supporting structure, with appended flange on the one of the branches, allowing simulation of a fatigue cracking process. For the assessment and comparison with numerical model calculation results of the dynamical state of the structure the Experimental Modal Analysis (EMA) approach was selected. After several measuring campaigns a database of results including varying type of supporting condition, and crack opening stages, was obtained. The numerical model was constructed with use of Finite Element Method (FEM) approach. The quality of FE model was assessed using Modal Assurance Criterion (MAC) that compares both models modal vectors, that is modal deformation shapes. Also the differences in frequencies of modes was assessed and taken as quantification of an compatibility. The results (EMA) show importance of applying and modelling (FEM) of supporting condition. An additional senility analysis directs indicated best fit parameters for a start of FE optimization process.

Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014

M. Kahsin – 2014
Since offshore wind turbine supporting structures are subjected to dynamic environments with time-varying loading conditions, it is important to model their dynamic behavior and validate these models by means of vibrational experiments. In this paper assessment of dynamical state of the structure is investigated by means of both: numerical modeling, and experimental modal analysis. In experimental modal analysis, capturing the real dynamic behavior of tested structure requires a proper sensors and exciters localization. It is often useful to know probable dynamical behavior of the structure before experimental campaign planning. Therefore, the initial FE model results are exploited in order to predict the best configuration for a measuring equipment placement. Acquired test results are compared with FE solutions subsequently. Such a routine allows to asses quality of the preliminary numerical model on the global level, and along with a sensitivity analysis assemble a good starting point for fine-tuning of FE model.

Uncertainty Quantification in Experimental Structural Dynamics Identification of Composite Material Structures

M. Łuczak, B. Peeters, M. Kahsin , S. Manzato, K. Branner – 2014
Composite materials are widely used in manufacture of aerospace and wind energy structural components. These load carrying structures are subjected to dynamic time-varying loading conditions. Robust structural dynamics identification procedure impose tight constraints on the quality of modal models estimates obtained from vibration experiments. Modal testing results are influenced by numerous factors introducing uncertainty to the measurement results. Different experimental techniques applied to the same test item or testing numerous nominally identical specimens yields different test results. This paper aims at a systematic approach for uncertainty quantification of the parameters of the modal models estimated from experimentally obtained data. Statistical analysis of modal parameters is implemented to derive an assessment of the entire modal model uncertainty measure. Investigated structures represent different complexity levels ranging from coupon, through sub-component up to fully assembled aerospace and wind energy structural components made of composite materials. The proposed method is demonstrated on two application cases of a small and large wind turbine blade.

Updating Finite Element Model of a Wind Turbine Blade Section Using Experimental Modal Analysis Results

M. Łuczak, S. Manzato, B. Peeters, K. Branner, P. Berring, M. Kahsin SHOCK AND VIBRATION – 2014
This paper presents selected results and aspects of themultidisciplinary and interdisciplinary research oriented for the experimental and numerical study of the structural dynamics of a bend-twist coupled full scale section of awind turbine blade structure.Themain goal of the conducted research is to validate finite elementmodel of themodified wind turbine blade section mounted in the flexible support structure accordingly to the experimental results. Bend-twist coupling was implemented by adding angled unidirectional layers on the suction and pressure side of the blade. Dynamic test and simulations were performed on a section of a full scale wind turbine blade provided byVestasWind Systems A/S.Thenumerical results are compared to the experimental measurements and the discrepancies are assessed by natural frequency difference andmodal assurance criterion. Based on sensitivity analysis, set ofmodel parameters was selected for the model updating process. Design of experiment and response surface method was implemented to find values of model parameters yielding results closest to the experimental.The updated finite element model is producing results more consistent with the measurement outcomes.

In-service measurement of the small wind turbine test stand for structural health monitoring

M. Kahsin – 2013
This paper presents the research activity performed on a Small Wind Turbine (SWT) test stand. Commercially available turbine was modified towards incorporation of the sensors system for condition monitoring. Installed sensors measure angular shaft position, torque applied from the wind loads, vibration accelerations and last but not least rotational speed. All gathered data are then transferred and processed in Test.Lab by means of automatic in house developed Visual Basic application which afterwards converts TDF files to text files and stores them in a desired directory. The numerical simulation is being run in parallel to installed sensors measurements and is as well controlled by the same Visual Basic application. By having actual measurements and numerical simulation results one will be able to compare those two outcomes. In particular, if the numerical model is tailored to the physical one, data comparison will allow identifying malfunctioning due to component damage or extreme working condition.

Application of the finite element methods in long-term simulation of the multi-physics systems with large transient response differences

Application of the Finite Element Method (FEM) and the Multibody Dynamics Method allows analyzing of complex physical systems. Complexity of the system could be related both to the geometry and the physical description of phenomenon. The metod is the excellent tool for analyzing statics or dynamics of the mechanical systems, and permits tracking of Multi Body System (MBS) transient response for the long-term simulations and application of any arbitrary set of mechanical forcing functions. In case of FEM most of algorithms encounter continuity conditions across the element boundaries, and thanks to this FEM is one of the most suitable calculation method for continua multi-physics systems (i.e. thermo-structural, electro-thermo-structural, magneto-thermo-structural, MEMS, etc.). Common problem with FEM is that there are major calculation difficulties when tong-term simulation results are required and/or large relative motions are present in the system. Drawbacks of FEM and MBS could be overcome with use of algorithm based on the modified Hybrid Finite Element Method presented further in this paper. Traditional Hybrid Finite Element Method model consists of rigid end deformable elements, system matrices derived for all compound elements are calculated concurrently. In this approach both advantages and disadvantages of FEM and MBS are transferred to the model. Proposed modified Hybrid Finite Element Method algorithm exploits two corresponding coupled discrete models, one containing FEM elements and the other MBS only. Both models are coupled by means of forcing functions. Such approach is applicable for the multi-physics systems with large transient response differences.