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Aalto

Bending Response of Laser-Welded Web-Core Sandwich Plates

Jani Romanoff

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Mechanical Engineering for public examination and debate in Auditorium K213 at Helsinki University of Technology (Espoo, Finland) on the 8th of November, 2007, at 12 noon.

Overview in PDF format (ISBN 978-951-22-8977-6)   [417 KB]
Dissertation is also available in print (ISBN 978-951-22-8976-9)

Abstract

The thesis presents a theory for the bending of laser-welded web-core sandwich plates. The sandwich plate theory and homogenization are utilised, enabling a similar, fairly coarse, FE mesh to be used for design alternatives with different cross-sectional dimensions. This is considered to be beneficial during the design, when different alternatives should be evaluated. The thesis also studies the main factors that contribute to the total bending response of laser-welded web-core sandwich plates.

The actual periodic structure is homogenized, giving equivalent stiffness properties for the sandwich plate. The differential equations for this homogenous sandwich plate, with thick face plates, consist of those for displacements of the well-known Reissner-Mindlin and Kirchoff plate theories. Therefore, the solution of these differential equations can be carried out using commercial Finite Element software. The periodic structure is reconsidered when the stress formulations are derived. The approach is validated with Finite Element calculations based on actual 3D topology.

The rotation stiffness of the laser stake welded T-joint between the web and face plate is derived experimentally. This is considered to be important since the rotation stiffness affects the shear stiffness in the opposite direction to that of the web plate. An experimental procedure for the determination of the T-joint rotation stiffness is developed and validated. The mechanics of the T-joint are investigated with Finite Element analyses.

The moment introduced by the web plate to the face plate is very important when deflections and normal stresses are considered. This moment is affected by the stiffnesses of the web plate and the T-joint. The deflections are significantly increased by a decrease in the web plate or T-joint rotation stiffness. The influence is greatest with cross-sections where the face plate thickness is large and the web plate spacing is small. Plates with a low aspect ratio under a uniform pressure load have the same maximum deflection, regardless of the T-joint rotation stiffness. Contrary to this, plates with a high aspect ratio or a patch load on a small area are very sensitive to rotation stiffness. The periodicity of the structure is found to have a significant influence on normal stresses in the face and web plates. Because of homogenization shear-induced, periodic, normal stresses vanish. In actual structures these stresses can be an order of magnitude higher than those induced by the bending moments of the sandwich plate. Therefore, the periodic structure as presented in this thesis should be taken into consideration when the normal stresses are calculated. Patch loads on a very small area can cause normal stresses even higher than those induced by shear. The web plate thickness is found to have a significant influence on stresses, but in the measured range of T-joint rotation stiffness values, the stresses are found to be unaffected.

This thesis consists of an overview and of the following 5 publications:

  1. Romanoff, J. and Varsta, P., Bending response of web-core sandwich beams, Composite Structures, Vol. 73, No. 4, 2006, pp. 478-487.
  2. Romanoff, J., Varsta, P. and Klanac, A., Stress analysis of homogenized web-core sandwich beams, Composite Structures, Vol. 79, No. 3, 2007, pp. 411-422.
  3. Romanoff, J., Remes, H., Socha, G., Jutila, M. and Varsta, P., The stiffness of laser stake welded T-joints in web-core sandwich structures, Thin-Walled Structures, Vol. 45, No. 4, 2007, pp. 453-462.
  4. Romanoff, J. and Varsta, P., Bending response of web-core sandwich plates, Composite Structures, Vol. 81, No. 2, 2007, pp. 292-302.
  5. Romanoff, J., Varsta, P. and Remes, H., Laser-welded web-core sandwich plates under patch loading, Marine Structures, Vol. 20, No. 1-2, 2007, pp. 25-48.

Errata of publication 4

Keywords: web-core, sandwich plate theory, stress analysis, laser weld, stake weld, rotation stiffness

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© 2007 Helsinki University of Technology

Last update 2011-05-26