12. Artikkelit / Articles

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Browsing 12. Artikkelit / Articles by Department "Department of Applied Mechanics"

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  • ‘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.
  • 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.
  • Model experiments of the transient response to flooding of the box shaped barge
    (2015) Manderbacka, Teemu; Ruponen, Pekka; Kulovesi, Jakke; Matusiak, Jerzy
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Coupling of the flooded water and ship motions was studied experimentally.Roll decay tests for one flooded compartment and transient abrupt floodingtests were performed for the box shaped barge model. The tests were conductedto obtain information on the flooding process for the developmentof numerical tools and to provide validation data. Quantitative values onthe effect of flooded water on the roll damping were obtained. Flooded waterbehaves in a different manner in undivided and divided compartments.Flooded water in divided compartment increases roll damping significantly.In undivided compartment roll damping was high at low amount of floodedwater. For higher amounts damping was of the same order as for the intactmodel. Initial flooding is a complex process where the ship and floodedwater motions are coupled. Propagation of the flooding water inside thecompartment, at a dam-break type abrupt flooding, was studied by trackingthe surface of the flooded water. Image processing algorithm was used to obtain the tracked surface. Flooded water volume and its center of gravity were estimated from the tracked surface. Different internal layouts of theflooded compartment can lead to a totally different roll response. The infloodingjet plays an important role on the response in case of the undividedcompartment. While, for a divided compartment, asymmetric flooding dueto the obstructions, causes high heel angle on the damage side.
  • Transient response of a ship to an abrupt flooding accounting for the momentum flux
    (2015) Manderbacka, Teemu; Mikkola, Tommi; Ruponen, Pekka; Matusiak, Jerzy
     School of Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Numerical non-linear time domain simulation method for damaged ship motionsis presented. Floodwater motion modelling is based on the lumpedmass method with a moving free surface. The ship and floodwater motionsare fully coupled. The variation of the floodwater mass is accounted for. Amodel to account for the flooding ingress transporting the momentum is presented.The experiments of abrupt flooding have shown that the ship mayexperience the first large roll towards the undamaged side, especially when alarge undivided compartment is flooded. The presented time domain modelis validated against the experimental data on the roll damping of the floodedship and transient flooding. Two different initial stability conditions andtwo different compartment layouts are studied. Viscous dissipation of thefloodwater motions is modelled with an equivalent friction coefficient. Theimpact of the viscous damping is studied. Transient flooding tests show thatthe inflow momentum has to be accounted for when the undivided compartment is flooded. The simulation model is capable of capturing the impact of the inflooding jet and the first roll on the opposite side of the damage is reproduced.
  • 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.