Browsing by Author "Jelovica, Jasmin"

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  • Abaqus UGENS subroutine for nonlinear analysis of periodic panels
    (2016) Reinaldo Goncalves, Bruno; Jelovica, Jasmin; Romanoff, Jani
     School of Engineering | D4 Julkaistu kehittämis- tai tutkimusraportti tai -selvitys
    This report describes an Abaqus UGENS subroutine for geometric and material nonlinear analysis of periodic panels using the first-order shear deformation theory. The structure is modelled with shell elements, as one layer of equivalent mechanical properties. The subroutine modifies the stiffness matrix of each shell element of the mesh separately based on its strain state. It relies on pre-computed stiffness curves that define the ABCD stiffness matrix of a unit cell. By looking at combinations of force and strain, the code interpolates the stiffness curves to calculate equivalent nonlinear stiffness. Complex stress states with different types of nonlinearity occurring simultaneously in the structure can be described. The examples show that the subroutine can deal with nonlinearities such as global buckling, local buckling and post-yield response with good accuracy and low computational cost compared to conventional FEM. The report includes the necessary information to set up the subroutine, including selection and compatibility of software and packages and input file preparation. Web-core sandwich panels are used as example throughout the report; the same principles are valid for any periodic structure. The full implementation is given in Appendix.
  • Fatigue strength modelling of high-performing welded joints
    (2020-06) Remes, Heikki; Gallo, Pasquale; Jelovica, Jasmin; Romanoff, Jani; Lehto, Pauli
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This paper investigates the fatigue strength modelling of high-performing welded steel joints. The work considers the strain-based, notch stress, averaged strain energy density approaches, and linear elastic fracture mechanics. A comparison of the methods with experimental data shows that the predictions vary significantly for different modelling assumptions. Only the microstructure-sensitive strain-based approach can predict the fatigue life of various weld geometries and plate thicknesses. The explicit modelling of the localised plasticity using the microstructure-dependent representative volume element is required for accurate prediction of the short crack initiation and growth periods, which dominate the fatigue life modelling of high-performing welds.
  • Feasibility of 3 dimensional modelling for design of ship general arrangement
    (2016-09-19) Jokinen, Markus
     Insinööritieteiden korkeakoulu | Master's thesis
    Nowadays, ships are designed in a logical, systematic and iterative way. This technique is called a design spiral that consists of the concept, contract, basic and detail design stages. One design round includes all fields of ship design including General Arrangement (GA). However, the traditional design spiral is ship design-wise inflexible. Partial reason for this is that currently GA is drawn as 2D in the concept stage. The flexibility of the design spiral has been tried to improve in the world by developing 3D GA design processes and software. This thesis focuses on state-of-the-art 3D GA design processes. The aim of the thesis was to find out whether these 3D processes give so much advantage that it is recommended to use these already in the concept design stage instead of 2D. The feasibility study was carried out by a questionnaire where ship designers were asked to compare 2D, manual and computer-assisted 3D GA design processes against each other. The decision about the best design process was made based on different criteria such as the quantity, quality and utilization of outputs obtained from GA model, malleability and perceivability of GA model, the simplicity of GA design software and time saving. Weights of the criteria were calculated using an analytical hierarchy process (AHP, a decision making tool). Weights of the criteria were calculated based on the answers of the designers. Based on the results obtained from interviews and AHP, 3D is more efficient and suitable for GA design in the concept stage than 2D process. 3D got higher weights in almost all criteria than 2D except the simplicity of GA design software criterion. Especially the malleability of GA criterion got high values, and its contribution had high impact on the fact that 3D GA was better than 2D. When considering 3D GA, the computer-assisted design process won the manual one almost in all criteria. As a summary, the thesis revealed that computer-assisted GA design process where GA is formed from modular 3D design building blocks (ship systems), would be the ideal ship design process. These blocks should contain all relevant data about ship systems so that GA model could be utilized in other fields of ship design. The design process should proceed from large blocks into smaller ones. This ideal design process maximizes the malleability of GA and enables the making of innovative and creative designs.
  • Geometrically nonlinear bending response of steel sandwich box girder using equivalent single layer theory
    (2016-12-12) Metsälä, Mika
     Insinööritieteiden korkeakoulu | Master's thesis
  • Geometrically nonlinear stiffness of web-core sandwich plate
    (2013) Tavakoli, Hamidreza
     School of Engineering | Master's thesis
    Stiffness of the unit cell of a web-core sandwich plate is studied in this thesis. The effect of geometrical imperfections is assessed. The unit cell is presented by the stiffness matrices based on the equivalent single layer theory. The geometrically nonlinear behaviour of stiffness components are obtained numerically by exposing the unit cell to one particular loading condition in each analysis. The first buckling Eigen-mode of the unit cell in web-plate direction is considered as the initial imperfection shape since it resembles to production imperfections. Results are in good agreement with theoretical values with an error less than 5% before the imperfections effect become dominant after that the nonlinear behaviour of stiffness coefficients starts to deviate from the theoretical values. The study has shown that geometrical imperfections significantly affect the plate stiffness coefficients. The method is potentially applicable to large complex structures in order to achieve the response of such structures with a realistic model due to consideration of the effect of geometrical imperfections. This new method can be determined as the first step for coupling the local and global buckling of structures via a numerical approach. Since the presented approach uses the equivalent single layer theory, the time and cost efficiency of the method is high.
  • Global buckling and post-buckling of web-core sandwich and stiffened panels: sensitivity to general corrosion
    (2014) Jelovica, Jasmin; Romanoff, Jani
     School of Engineering | C2 Toimitettu kirja, kokoomateos, konferenssijulkaisu tai lehden erikoisnumero
    Corrosion can lead to reduction of structural stiffness and strength. This paper investigates the influence of a reduction in the thickness of the plates as a result of general corrosion on sandwich panel buckling load and onset of plasticity. The results are compared to the stiffened panel of the same in-plane and bending stiffness. Current guidelines for corrosion protection threat these two structures equally. Load-shortening curves are obtained with the finite element method, with the kinematics being represented using two approaches: (1) equivalent single-layer with first-order shear deformation theory, and (2) a three-dimensional model of the actual geometry of the structure, modeled using shell and connector elements. The former is also used to identify the influence of corrosion on the stiffness coefficients and, consequently, the buckling load, also via analytical equation. The decrease of the buckling load is found higher in sandwich panel than in stiffened panel. The reduction is especially high in the case of the diffusion of moisture (water) into the core. The reason for the higher sensitivity of sandwich panel is a larger reduction of transverse shear stiffness opposite to the stiffener direction due to corrosion.
  • Global buckling response of web-core steel sandwich plates influenced by general corrosion
    (2014) Jelovica, Jasmin
     School of Engineering | Doctoral dissertation (article-based)
    A web-core steel sandwich plate is a lightweight, orthotropic structure. The constituent thin plates (2-4 mm) are joined by laser-welding. This thesis investigates the buckling and post-buckling behaviour of slender web-core sandwich plates loaded in the direction of the web plates. The influence of corrosion on the plate buckling is studied via finite element method (FEM). The corrosion scenario used is based on experimental observations from specimens submerged into the sea for 1 and 2 years. The plate strength analyses are performed with two methods: FEM having shell element mesh of the three-dimensional topology and the equivalent single-layer theory (ESL). In the later, the sandwich plate is represented with constant, homogenised stiffness coefficients, which are related to physical properties of the structure. The first buckling mode of slender web-core sandwich plates is characterised with global deformation between the edge supports. The buckling strength depends on the bending and transverse shear stiffnesses. This thesis revealed that the buckling strength is very sensitive to the variation in transverse shear stiffness opposite to the web plate direction, DQy, especially in sandwich plates with high bending stiffness. Furthermore, the stiffness of the sandwich plate as a whole in the post-buckling is controlled by that of the in-plane stiffness. The web plates impose high, shear-induced, secondary bending stresses on the face plates and these were found to be important for accurate estimation of the onset of yielding. The deformation resulting from the secondary bending of the face plates makes the unloaded edge stiffer. Although membrane stress can be higher there, local buckling during global post-buckling occurs further away where the secondary deformations are smaller, primarily in the centre of the face plate (x=a/2, y=b/2). Furthermore, the corrosion tests revealed that the cross-section is primarily affected by general corrosion. Under this circumstance, the reduction of the thickness of the face and web plates reduces the stiffness coefficients and also the buckling strength linearly. The buckling strength reduces rapidly, especially because of the reduction in the transverse shear stiffness DQy. The reduction of buckling strength doubles if, in addition to the outer faces, corrosion also occurs inside the sandwich plate. Beam bending tests also showed rapid reduction of the ultimate strength but, in addition, that it can be maintained using different protection methods. The results thus indicate that the protection against corrosion should be carefully performed. The future work will involve improving the accuracy of the ESL theory in the presence of local buckling.
  • ‘Healing’ and ‘repairing’ techniques for faster optimization with genetic algorithm
    (2009) Jelovica, Jasmin; Klanac, Alan
     School of Engineering | A4 Artikkeli konferenssijulkaisussa
    This paper presents two techniques, the ‘healing’ and ‘repairing’ that can reduce optimization time when using genetic algorithm for structural optimization. The techniques can be applied to: (a) quickly find feasible designs from completely infeasible set of alternatives, and (b) to make the best infeasible designs feasible. These procedures are implemented into a genetic algorithm ‘VOP’. The performance of the original and the modified version of the algorithm are compared with the widespread genetic algorithm ‘NSGA-II’ for the weight optimization of a 40 000 DWT chemical tanker midship section. The results show that these procedures can decrease the optimization time by approximately half.
  • Influence of general corrosion on buckling strength of laser-welded web-core sandwich plates
    (2014) Jelovica, Jasmin; Romanoff, Jani; Remes, Heikki
     School of Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The strength of a web-core steel sandwich plate is potentially reduced in a corrosive environment. This study is dedicated to the influence of a reduction in the thickness of the plates as a result of general corrosion on sandwich plate buckling strength and first-fibre failure. Two scenarios are investigated in which corrosion reduces the thickness of (a) the outer sides of the face plates and (b) all surfaces, including the core. The laser weld between the face sheets and the core is assumed to be intact. The assumptions are made on the basis of earlier experimental findings. Critical buckling and geometric non-linear analysis are carried out with the finite element method, with the kinematics being represented using two approaches: (1) equivalent single-layer with first-order shear deformation theory, and (2) a three-dimensional model of the actual geometry of the structure, modelled using shell and connector elements. The former is used to identify the effect of corrosion on the stiffness coefficients and, consequently, the buckling strength. The later is used for verification and for stress prediction during post-buckling. A rapid decrease in the buckling strength was found for corrosion affecting the outer sides of the sandwich plate. The decrease in the buckling strength doubled in the case of the diffusion of moisture (water) into the core. The shear-induced secondary bending of the faces was found to affect the first-fibre yield.
  • Influence of weld stiffness on buckling strength of laser-welded web-core sandwich plates
    (2012) Jelovica, Jasmin; Romanoff, Jani; Ehlers, Sören; Varsta, Petri
     School of Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This paper investigates the influence of weld rotation stiffness on the global bifurcation buckling strength of laser-welded web-core sandwich plates. The study is carried out using two methods, the first is the equivalent single-layer theory approach solved analytically for simply supported plates and numerically for clamped plates. First-order shear deformation theory is used. The second method is the three-dimensional model of a sandwich plate solved with finite element method. Both approaches consider the weld through its rotation stiffness. The weld rotation stiffness affects the transverse shear stiffness. Plates are loaded in the web plate direction. Four different cross-sections are considered. Weld stiffness is taken from experimental results presented in the literature. The results show a maximum of 24% decrease in buckling strength. The strength was affected more in plates with high reduction of transverse shear stiffness and high bending stiffness. Furthermore, clamped plates were influenced more than simply supported. The intersection between buckling modes shifted towards higher aspect ratios, in the maximum case by 24%. The results show the importance of considering the deforming weld in buckling analysis.
  • Limit state analyses in design of thin-walled marine structures - Some aspects on length-scales
    (2020-06-01) Romanoff, Jani; Remes, Heikki; Varsta, Petri; Reinaldo Goncalves, Bruno; Körgesaar, Mihkel; Lillemäe-Avi, Ingrit; Jelovica, Jasmin; Liinalampi, Sami
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This paper gives an overview of the factors that affect the strength and structural design of advanced thin-walled marine structures with reduced plate thickness or alternative topologies to those used today in the marine industry. Due to production-induced initial deformations and resulting geometrical non-linearity, the classical division between primary, secondary, and tertiary responses becomes strongly coupled. Volume-averaged, non-linear response of structural element can be used to define the structural stress-strain relation that enables analysis at the next, larger, length scale. This, today's standard homogenization process, needs to be complemented with localization, where the stresses are assessed at the details, such as welds for fatigue analysis. Due to this, the production-induced initial distortions need to be considered with high accuracy. Another key question is the length-scale interaction in terms of continuum description. Non-classical continuum mechanics are needed when consecutive scales are close. Strain-gradients are used to increase the accuracy of the kinematical description of beams, plates, and shells. The paper presents examples of stiffened and sandwich panels covering limit states such as fatigue, non-linear buckling, and fracture.
  • Load-carrying behaviour of web-core sandwich plates in compression
    (2013) Jelovica, Jasmin; Romanoff, Jani
     School of Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This paper investigates theoretically the compressive load-carrying behaviour of geometrically imperfect web-core sandwich plates. Slender plates, which first buckle globally, are considered. The study is carried out using two approaches, both solved with the finite element method. The first is the equivalent single-layer theory approach. First-order shear deformation theory is used. The second approach is a three-dimensional shell model of a sandwich plate. Plates are loaded in the web plate direction. Simply supported and clamped boundary conditions are considered with a different level of in-plane restraint on the unloaded edge. The results show that the behaviour of the sandwich plate is qualitatively equal to the isotropic plate of the same bending stiffness for deflections lower than the plate thickness. As the deflections increase, the lower in-plane stiffness of the sandwich plate results in lower post-buckling stiffness. Local buckling of face plates in the post-buckling range of the sandwich plate further reduces the structural stiffness.
  • Post-buckling of web-core sandwich plates based on classical continuum mechanics: success and needs for non-classical formulations
    (2021-06) Romanoff, Jani; Jelovica, Jasmin; Reddy, J. N.; Remes, Heikki
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The paper investigates the post-buckling response of web-core sandwich plates through classical continuum mechanics assumptions. The compressive loading is assumed to be in the direction of the web plates. Equivalent Single Layer (ESL) plate formulation is used with the kinematics of the First order Shear Deformation Theory (FSDT). During the initial, membrane-dominated loading stages, it is observed that the effect of finite size of the periodic microstructure is barely influences the plate responses. At the higher loads, when bending is activated, the finite size of the microstructure activates secondary shear-induced bending moments at the unit cells of the plate. A method to capture the envelope of the maximum values of these bending moments is presented. The findings are validated with the shell element models of the actual 3D-geometry. Finally, the physical limits of the classical continuum mechanics are discussed in the present context.
  • Ship response estimation in early design stage
    (2016-06-13) Cai, Xiaochi
     Insinööritieteiden korkeakoulu | Master's thesis
    A practical way to estimate the ship response in early design stage is investigated in this thesis. Focus has been put on the ship vertical bending moment and shear force in operation area. ISSC spectrum is used to indicate the sea state. Napa strip method is employed to derive the transfer function. The ship response is thus generated in frequency domain. The vertical bending moment and shear force along the ship are then calculated according to the critical wave case indicated from the response function. Based on the results, the validation of DNV-GL rule and IACS rule is discussed. In this case, the overestimation is discovered for the still water vertical bending moment and shear force. On the other hand, there is underestimation in wave vertical bending moment and shear force. The dynamic vertical bending moment and shear force is reasonable. Since only static loads and dynamic loads are required in the rules, the rules are judged as valid in the early design stage. The feasibility of Napa strip method has been commented and the Napa strip method is judged practical according to its accuracy and time consumption. For ship design, the wavelength and the wave steepness are the main parameters affecting the loads on hull. The block coefficient is crucial for the nonlinearity in hogging and sagging condition. More models, especially other types of ships are expected to be analysed for this topic in future study. Other methods, such as panel method could take into use in the future work. The probability of operation can be further developed based on this study.
  • Ultimate strength of corroded web-core sandwich beams
    (2013) Jelovica, Jasmin; Romanoff, Jani; Ehlers, Sören; Aromaa, Jari
     School of Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The corrosive marine environment is a threat to the ultimate strength of steel sandwich structures. Therefore, ultimate strength experiments were carried out in three-point bending for beams with different corrosion exposure times, i.e. one and two years. Standard laser-welded web-core sandwich beams are studied and different corrosion protection systems considered. The beams experienced general corrosion. The thickness reduction in unprotected plates and laser welds is around the typical 0.1 mm/year. This led to an ultimate strength reduction of 10% and 17% for beams with exposure times of one and two years, respectively. The experimental ultimate strength is in agreement with finite element simulations. The ultimate strength was maintained for the beams protected with coating or closed-cell polyurethane (PU) foam.
  • Ultimate strength tests of corroded web-core and corrugated-core sandwich beams
    (2011) Jelovica, Jasmin; Romanoff, Jani; Ehlers, Sören; Remes, Heikki
     Insinööritieteiden korkeakoulu | D4 Julkaistu kehittämis- tai tutkimusraportti tai -selvitys