Eduards Skukis - Academia.edu (original) (raw)

Papers by Eduards Skukis

In the Doctoral Thesis, the vibration correlation technique was developed as a non-destructive te... more In the Doctoral Thesis, the vibration correlation technique was developed as a non-destructive test method for evaluating the critical load of shells. Nondestructive methods for calculating the buckling load of imperfection sensitive thin-walled structures are one of the most important techniques for the validation of new structures and numerical models of large-scale aerospace structures. The vibration correlation technique (VCT) allows determining the buckling load for several types of structures without reaching the instability point.

Promocijas darbā izstrādāta vibrācijas korelācijas tehnika kā nesagraujoša testa metode čaulu kri... more Promocijas darbā izstrādāta vibrācijas korelācijas tehnika kā nesagraujoša testa metode čaulu kritiskās slodzes novērtēšanai. Nesagraujošās metodes pret nepilnībām jutīgu plānsienu konstrukciju kritiskās slodzes aprēķiniem ir viens no svarīgākajiem paņēmieniem jaunu konstrukciju un liela mēroga kosmosa konstrukciju skaitlisko modeļu validācijai. Vibrācijas korelācijas tehnika (VCT) ļauj noteikt kritisko slodzi dažādu veidu konstrukcijām, nesasniedzot bifurkācijas punktu.

As non-destructive method for axial buckling load determination - Vibration Correlation Technique... more As non-destructive method for axial buckling load determination - Vibration Correlation Technique (VCT) showed major advantages for a range of industrial application. Particular technique for validation of structural limit state in accordance to numerical model prediction for large (true) scale structures are getting the required momentum. The Vibration Correlation Technique (VCT) allows to correlate the ultimate load or instability point with rapid decrement of self-frequency response. Nevertheless this technique is still under development for thin-walled shells and plates. The current research discusses an experimental verification of extended approach, applying vibration correlation technique, for the prediction of actual buckling loads on unstiffened isotropic cylindrical shells with circular cut-outs, loaded in axial compression. Validation study include several aluminium cylinders which were manufactured and repeatedly loaded up to instability point. In order to characterize a...

Present paper describes the comparison of two different material properties identification techni... more Present paper describes the comparison of two different material properties identification techniques in order to investigate a possibility of combination of two techniques into one powerful and fast tool for material properties identification. Both methods are theoretically described and main options are given in this article. Some experiments wherein aluminium material properties are identified are carried out. Methods are qualified, conclusions and proposals are given

New nondestructive technique, namely an inverse technique based on vibration tests, to characteri... more New nondestructive technique, namely an inverse technique based on vibration tests, to characterize nonlinear mechanical properties of adhesive layers in sandwich composites is developed. An adhesive layer is described as a viscoelastic isotropic material with storage and loss moduli which are both frequency dependent values in wide frequency range. An optimization based on the planning of experiments and response surface technique to minimize the error functional is applied to decrease considerably the computational expenses. The developed identification technique has been tested on aluminum panels and successfully applied to characterize viscoelastic material properties of 3M damping polymer ISD-112 used as a core material in sandwich panels.

Materials, 2022

The production of hybrid layered composites allows comprehensive modification of their properties... more The production of hybrid layered composites allows comprehensive modification of their properties and adaptation to the final expectations. Different methods, such as hand lay-up, vacuum bagging, and resin infusion were applied to manufacture the hybrid composites. In turn, fabrics used for manufacturing composites were made of glass (G), aramid (A), carbon (C), basalt (B), and flax (F) fibers. Flexural, puncture impact behavior, and cone calorimetry tests were applied to establish the effect of the manufacturing method and the fabrics layout on the mechanical and fire behavior of epoxy-based laminates. The lowest flammability and smoke emission were noted for composites made by vacuum bagging (approximately 40% lower values of total smoke release compared with composites made by the hand lay-up method). It was demonstrated that multi-layer hybrid composites made by vacuum bagging might enhance the fire safety levels and simultaneously maintain high mechanical properties designed fo...

Numerical modelling and dynamic analysis of laser-welded sandwich panels are carried out using th... more Numerical modelling and dynamic analysis of laser-welded sandwich panels are carried out using the broken line sandwich finite elements. To homogenize material properties of complex sandwich core with evenly spaced vertical webs and low-density core material, two approaches have been applied. The first is connected with the rule of mixture and the second – with a calculation of equivalent stiffnesses. In order to describe the rheological behaviour of viscoelastic core materials under dynamic loading, the complex modulus representation is used. Dynamic characteristics of laser-welded sandwich panels are evaluated by the method of complex eigenvalues and from the resonant peaks of the frequency response function. To estimate correctness of the methodology developed, the ANSYS solutions for empty I-core steel sandwich beams and frequency response experimental measurements for different types of laser-welded sandwich beams with longitudinal and transverse webs and foam materials have be...

Nondestructive methods, to calculate the buckling load of imperfection sensitive thin-walled stru... more Nondestructive methods, to calculate the buckling load of imperfection sensitive thin-walled structures, are one of the most important techniques for the validation of new structures and numerical models of large scale aerospace structures. The vibration correlation technique (VCT) allows determining the buckling load for several types of structures without reaching the instability point, but this technique is still under development for thin-walled plates and shells. This paper presents and discusses an experimental and numerical validation of a novel approach, using the vibration correlation technique, for the prediction of realistic buckling loads on unstiffened cylindrical shells loaded in compression. From the experimental point of view, a batch of three composite laminated cylindrical shells are fabricated and loaded in compression up to buckling. An unsymmetric laminate is adopted in order to increase the sensitivity of the test structure to initial geometric imperfections. I...

The vibration method for the identification of elastic parameters of plate-type structural elemen... more The vibration method for the identification of elastic parameters of plate-type structural elements consists of finding parameter values that minimize the difference between physically measured and numerically calculated (mostly using the finite element method (FEM)) natural frequencies. The main problem here is the estimation of the accuracy of results and minimizing the effects of different sources of uncertainty. The influence of plate thickness and material density variance can be reduced using the first L nondimensional frequencies Ψi = fi f̅ ⁄ (i=1, 2,..., L), where fi is the i-th natural frequency and f̅ = ∑ fj L j=1 L ⁄ . This scaling can be done with measured frequencies fi measured as well as with frequencies fi num calculated by FEM. The classical nondimensionalization approach uses coefficients that depend on unknown elastic parameters, thereby leading to an iterative minimization process. Thus the identification of component values of the elastic constants vector E* (in...

Materials, 2020

Sandwich beams that are composed of laminated face sheets and aluminum pyramidal truss cores are ... more Sandwich beams that are composed of laminated face sheets and aluminum pyramidal truss cores are considered to be essential elements of building and aerospace structures. In this paper, a methodology for the experimental and numerical analysis of such structures is presented in order to support their industrial application. The scope of the present research covers both the experimental and numerical extraction of the dynamic parameters of the sandwich beams. Vibration tests are performed while using an optical system for three-dimensional vibrations sensing. The in-plane and out-of-plane vibration modes can thus be examined. A detailed numerical model of the sandwich beam is developed, including an adhesive joint (an additional layer of material) between the parent components of the beam. The numerically predicted modal parameters (eigenfrequencies, mode shapes, modal loss factors) are comported with their corresponding experimentally-obtained values. The modal loss factors are pred...

Shock and Vibration, 2017

Applicability of the vibration correlation technique (VCT) for nondestructive evaluation of the a... more Applicability of the vibration correlation technique (VCT) for nondestructive evaluation of the axial buckling load is considered. Thin-walled cylindrical shells with and without circular cutouts have been produced by adhesive overlap bonding from a sheet of aluminium alloy. Both mid-surface and bond-line imperfections of initial shell geometry have been characterized by a laser scanner. Vibration response of shells under axial compression has been monitored to experimentally determine the variation of the first eigenfrequency as a function of applied load. It is demonstrated that VCT provides reliable estimate of buckling load when structure has been loaded up to at least 60% of the critical load. This applies to uncut structures where global failure mode is governing collapse of the structure. By contrast, a local buckling in the vicinity of a cutout could not be predicted by VCT means. Nevertheless, it has been demonstrated that certain reinforcement around cutout may enable the ...

IOP Conference Series: Materials Science and Engineering, 2019

The present study examines sandwich panels, which are made of carbon fiber facesheets with alumin... more The present study examines sandwich panels, which are made of carbon fiber facesheets with aluminium honeycomb core and focuses on how varying factors (skin thickness, layup, core density and diameter of hemispherical indenter) are affecting collapse of artificially damaged panels in edgewise compression. Damage in the middle of sandwich specimens was introduced using quasi-static indentation. Hemispherical indenters of two diameters, namely, 20 mm and 150 mm, were applied. The residual strength was estimated through the edgewise compression test of damaged specimens. The study demonstrated that increase of the honeycomb density led to decrease in the safety margin in structure. Thicker skins may also lead to a catastrophic propagation of indent.

Thin-Walled Structures, 2019

Traditional buckling experiments of imperfection-sensitive structures like cylindrical shells can... more Traditional buckling experiments of imperfection-sensitive structures like cylindrical shells can cause the permanent failure of the specimen. Nevertheless, an experimental campaign is crucial for validation of the design and numerical models. There is, therefore, interest in nondestructive methods to estimate the buckling load of such structures from the prebuckling stage. The vibration correlation technique allows determining the buckling load without reaching the instability point. Recently, a novel empirical vibration correlation technique based on the effects of initial imperfections on the first vibration mode demonstrated interesting results when applied to composite and metallic unstiffened cylindrical shells. In this context, this paper explores this novel approach for determining the axial buckling load of a metallic orthotropic skin-dominated cylindrical shell under internal pressure, which represents a simplified downscaled model of a launcher propellant tank. An experimental campaign consisting of buckling tests and noncontact vibration measurements for different axial load levels is conducted considering the specimen without and with three different internal pressure levels. The experimental results validate the above-mentioned vibration correlation technique for determining the axial buckling load of pressurized cylindrical shells. Moreover, finite element models are calibrated in order to evaluate the frequency variation within a broader and dense range of the axial loading leading to an assessment of the considered maximum load level and number of load steps as related to the deviation of the estimation. The results corroborate the applicability of the vibration correlation technique as a nondestructive experimental procedure to assess the axial buckling load of imperfection-sensitive orthotropic skin-dominated cylindrical shells under internal pressure.

Procedia Engineering, 2017

Non-destructive methods to estimate the actual buckling load in particularly for imperfection sen... more Non-destructive methods to estimate the actual buckling load in particularly for imperfection sensitive thin-walled structures, are of severe interest among many fields. Particular techniques for validation of structural limit state and numerical model predictions for large scale structures are getting momentum. The vibration correlation technique (VCT) allows to correlate the ultimate load our instability point with rapid decrement of self-frequency response. Nevertheless this technique is still under development for thin-walled shells and plates. The current research discusses an experimental verification of extended approach, using vibration correlation technique, for the prediction of actual buckling loads on unstiffened cylindrical shells loaded in axial compression. Validation study include two laminated composite cylinders which were manufactured and repeatedly loaded up to instability point. In order to characterize a correlation with the applied load, several initial natural frequencies and mode shapes were measured during tests by 3D laser scanner. Results demonstrate that proposed vibration correlation technique allows one to predict the experimental buckling load with high reliability, without actually reaching the instability point. Additional experimental tests and numerical models are currently under development to further validate the proposed approach to extended composite and metallic structures.

Shell Structures: Theory and Application, 2013

ZAMM - Journal of Applied Mathematics and Mechanics / Zeitschrift für Angewandte Mathematik und Mechanik, 2015

This paper describes a method for determination of elastic parameters (elastic moduli and Poisson... more This paper describes a method for determination of elastic parameters (elastic moduli and Poisson's ratio) of orthotropic composite plate‐type structural elements using the results of natural frequency measurements. The identification of parameter values is provided by minimization of weighted squared difference (discrepancy) between physically measured frequencies and natural frequencies calculated by Finite Element Method. The metamodels for the frequency dependence on the elastic parameters and other geometrical and physical parameters of test specimens, including parameters with uncertainty (“noisy constants”) are built using experimental designs optimized according to the Mean Squared Error space filling criterion and third‐order polynomial approximations. The minimum of weighted squared difference between measured and calculated frequencies is found using the multistart random search method. The expressions for standard deviations of identified parameters depending on deviations of “noisy constants” are derived using linearized metamodels. The expressions for identification errors allow the statement of the identification task as a robust minimization problem by simultaneous minimization of the discrepancy function and standard deviations of the identified values by varying the values of unknown elastic parameters and weighting coefficients for different frequencies. The partial scaling of natural frequencies is used for the reduction of the uncertainty impact on the identification error. This allows reducing the identification error of elastic moduli about two times and Poisson's ratio about 20 times in comparison with the results obtained by using dimensioned frequencies.

Key Engineering Materials, 2007

An inverse technique based on vibration tests to characterise isotropic, orthotropic and viscoela... more An inverse technique based on vibration tests to characterise isotropic, orthotropic and viscoelastic material properties of advanced composites is developed. An optimisation using the planning of experiments and response surface technique to minimise the error functional is applied to decrease considerably computational expenses. The inverse technique developed is tested on aluminium plates and applied to characterise orthotropic material properties of laminated composites and viscoelastic core material properties of sandwich composites.

Journal of Sound and Vibration, 2009

A new inverse technique to characterise the nonlinear mechanical properties of viscoelastic core ... more A new inverse technique to characterise the nonlinear mechanical properties of viscoelastic core layers in sandwich panels has been developed based on simple vibration tests. The present methodology allows one to preserve the frequency and temperature dependences of the storage and loss moduli of viscoelastic materials for a wide range of frequencies and to perform a structural analysis using high damping tests. The computational effort has been substantially reduced by using an optimisation based on the planning of the experiments and the response surface technique in order to minimise the error functional. This new inverse technique has been tested and applied to characterise the viscoelastic properties of a 3M damping polymer (ISD-112) used as a core material in sandwich panels. The identified material properties have been verified successfully by comparing structural dynamic parameters obtained from the numerical analysis with experiments.