Design of steel sandwich panel joints

Transcription

1 Sören Ehlers, Helsinki University of Technology, Ship Laboratory PL 5300, TKK, tel: , fax: , soeren.ehlers@hut.fi Design of steel sandwich panel joints Abstract With respect to the crowing demand for bigger and lighter ships the development of efficient structures, such as steel sandwich panels, is in exist. They consist of a top and bottom faceplate and the core. As a result of the relative height of a sandwich panel, respectively the distance between the top and the bottom plate, joining would require welding from both sides. Hence there is the necessity to design a joint element, which enables a simple welded connection. As the fatigue life of such structures becomes an important design limit, a finite element based approach is chosen to consider the fatigue life of the joint. Different designs and their calculated fatigue notch factor K f will be given. The result are in reasonable correspondence with testing results. The calculation method of the fatigue life achieves good fatigue life results, specially in the stage of design, where it gives fast and simple expressive solutions by 2D FEM. Key words: Design, Steel sandwich panels, Fatigue life approach, FEM 1 Introduction The increasing application of thin walled structures, such as steel sandwich panels, count for the development of joining elements. The reason for such elements is the resulting height of a sandwich panel and its thin faces. The joining element needs to enable a simple connection by welding the panels from a single side with conventional welding. Hence, the sandwich structure can be connected to other structures similar than conventionally stiffened plates. The objective of this paper is to design practical connections. As a measure to evaluate the applicability of the joining element a fatigue life approach is chosen. The fatigue life is mainly influenced by local effects at the welds. 2 Radaj based fatigue life approach According to the hypothesis of Neuber [1], Radaj derives the fatigue notch factor of welded joints as a subset method of notch stress approaches. The substitute microstructural support length, ρ*, is the material parameter to describe this effect, and where the stresses are averaged. The fatigue notch factor K f at weld root or toe is derived from the stress concentration factor K t of the elliptical hole according to the following formula: Kt 1 Kf = 1+ s ρ * (1) 1+ ρ where s is the multiaxiality factor and ρ is the notch radius. From the fatigue effective notch stress σ c which is determined by considering the notch with an actual radius ρ enlarged to the fictitious radius ρ f, the notch stress analysis is performed:

2 σc Kf = (2) σn ρf = ρ + s ρ * (3) where σ n is the nominal stress to be defined. The material constant ρ* is considered to be dependent on the yield stress σ Y0.2. A uniform value of s = 2.5 has is commonly used in connection with welded joints in lower strength steels. With ρ = 0.4 mm the fictitious notch radius, ρ f equals 1 mm. Kf max = Kt ρf = 1mm (4) where K t (ρ f = 1mm) is the stress concentration factor. 3 Design studies [2]. The following chapter represents different joining designs that have been developed 3.1 Description of the structure The studied structure consist of a sandwich panel with longitudinal I - core and a steel joint. Due to the asymmetric shape of the joint, an even surface is received (see Figure 1). The support cases are a fixed displacement at the edge of the panel (black) and a fixed displacement at the bottom plate (gray). The applied displacement equals 0.1 mm. Figure 1: Overview of the panel connected to the asymmetric joint 3.2 Joint designs Type A: This joint is extruded from steel. The core is modified with different cut out shapes to find a optimum stress distribution.

3 a) with straight core b) half circle cut - out c) asymmetrical cut out d) straight core and perpendicular plate Figure 2: Type A joint Type B: Here the even surface is enabled due to a bended bottom plate, therefore the core needs to be modified. Different versions are studied. The first models study the influence of the bending angel α. It was found that a bending angle of 35.5 gives good results. Different connection possibilities, by laser and conventional welding, are studied.

4 a) Laser welded plate with core cut - out Figure 3: Type B joint b) conventionally welded thick plate Type C: This curved joint is modelled by reducing low stress elements from the FE model. Figure 4: Type C joint 4 Discussion All presented joints have a plane surface and can be conventionally welded from as single side. The joints can be connected to another panel or to another structure. In case of Type A joints, the critical laser weld is where the core ends at the panel (K f =2..2.2), the critical conventional weld is where the panel and the joint is connected (K f =2.9) (see Figure 2). The perpendicular plate gives better results, but it is necessary to flip the panel over and to weld it from both sides were it needs high fitting accuracy. Type B joint with asymmetrical core cut out shows an overall high stresses (K fmax =11.5) (see Figure 3a). The thick plate (see Figure 3b) enables the connection to have a plane surface. The

5 modelled full circle at the root of butt weld delivers high stresses, indicating the crack initiation position (K f =4.6). The Type C joint gives overall good results due to the manually optimised shape, K fmax = 2 (see Figure 4). 5 Summary The various joint design presented allow a connection to another panel or to another structure. As a result of specimen testing the presented fatigue life approach has been found to deliver reasonable results, especially under consideration of fatigue life accuracy and at the stage of design. However, the method and the resulting FE models are very sensitive, respectively the specimens, in order to calculate the same result as tested. The specimens welding should contain constant and equal properties. The designs for sandwich panel joining are not yet sufficient to be applied at critical ship positions, therefore further studies are needed. Acknowledgements This paper is based mainly on results obtained in the EU project: Advances Composite Steel Structures (Project No ). The financial support from the Helsinki University of Technology/Ship Laboratory is gratefully acknowledged here. The author is also grateful for the help he received at the Helsinki University of Technology/Ship Laboratory and would like to thank the people who have made this work possible, especially, D.Sc. Pentti Kujala, M.Sc. Heikki Remes, M.Sc. Alan Klanac and M.Sc. Jani Romanoff. References 1 Radaj, D.; Sonsino C.M.; Fatigue assessment of welded joints by local approaches; Abington Publishing, Cambrige England, Kujala, P., Ehlers, S., Deliverable 21c, Sandwich joint design principles and design catalogue. Helsinki University of Technology, Ship Laboratory. D 21C, internal/confidential. Version 1, Ehlers, S.; Ranta, J.; Kujala, P.; Klanac, A, Deliverable 21d, Fatigue testing of joints and comparison with FE-calculations,. Helsinki University of Technology, Ship Laboratory. D 21D, internal/confidential. Version 1,