International Research Journal of Engineering and Technology (IRJET)
e-ISSN: 2395 -0056
Volume: 04 Issue: 05 | May -2017
p-ISSN: 2395-0072
www.irjet.net
Design of Integrally Stiffened Shell Structures Archa S Lal1, Jiss K Abraham2 1M.Tech
student, Dept. of Civil Engineering, Saintgits College of Engineering, Kottayam, Kerala, India Dept. of Civil Engineering, Saintgits College of Engineering, Kottayam, Kerala, India
2Assist.Professor,
---------------------------------------------------------------------***--------------------------------------------------------------------important for modeling, analysis and fabrication. This result compared with the result of an already existing buckling failures due to the compressive loads and external pressure, integrally stiffened shell structures (waffle and waffle structure having optimum design parameters.
Abstract - In launch vehicle structures, for withstanding
isogrid shell structures) are employed. This paper discusses about the design of cylindrical and conical shaped isogrid structures subjected to axial compression and the external pressure and the effect of variation of design parameters like height of the triangle, skin thickness, rib width, rib depth etc. on the buckling load capacity. First, theoretical computation of the buckling load factor has been carried out using design equations for isogrid structure. A near optimum configuration is arrived at, which results in minimum equivalent weight thickness and positive margins of safety. By using inhouse software FEAST, a parametric modeling tool has been developed for modeling isogrid and waffle shell structures. Static analysis, buckling analysis and free vibration analysis have been carried out for both structures using FEAST software. By comparing the results, isogrid structures have less static displacement and more buckling load factor when compared with waffle structure.
1.1 Waffle Shell Structures Waffle structure is also called orthogonally stiffened shell structure, where the shell is stiffened by the use of longitudinal stiffeners and circumferential rings. The stiffeners can be oriented either in 45degree or 90 degree. This wall stiffening helps in increasing the buckling strength without increasing the weight, when compared to monocoque structures. These are ideal for structures subjected to external pressure.
1.2 Isogrid Shell Structures Isogrid is a type of partially hollowed- out structure formed usually from a single metal plate with triangular integral stiffening ribs. It is extremely light and stiff. Isogrids are manufactured from single sheets of material and with large-scale triangular openings and an open pattern to the flanges. The triangular pattern is very efficient because it retains rigidity and also it is saving material content of the structure. Hence we can reduce the structural weight. The term isogrid is used because the structure acts like an isotropic material, with equal properties measured in any direction, and grid, referring to the sheet and stiffeners structure.
Key Words: Integrally Stiffened Shell Structure, Waffle and Isogrid Shell Structures, Finite Element Analysis, Static and Buckling Analysis, FEASTSMT Software. 1. INTRODUCTION
Structures form the backbone of any launch vehicle. In order to make launch vehicle low weight and effective, thin shell structures are used for the manufacturing of launch vehicle structures. For withstanding buckling failures like general instability, skin buckling and rib crippling due to the axial compressive load and external pressure, integrally stiffened shell structures are employed. Recently, isogrid structures due to its high structural efficiency because of triangular rib pattern and low weight are used. The designing of isogrid shell structure is the tedious and most important problem aroused in the field of aeronautics. In waffle structures, since lattices are made separate from the skin and assembled to the skin, it will result in tension, compression, bending, shear etc. Hence, isogrid structures developed by neglecting these problems since skin and lattice are made out of one part. Parametric study for the determination of optimum design for isogrid structures are hence © 2017, IRJET
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Isogrid design aspects An isogrid structure is characterized by face- sheet thickness(t), rib thickness(b), rib depth(d), and height of triangle(h).These parameters ensure simultaneous failure of isogrid structure by local skin buckling, rib crippling and general instability. It will provide minimum weight to structures.
2. Scope and Methodology Main scope of the thesis is to obtain an efficient design of aerospace structures by adopting isogrid pattern in cylindrical and conical shells. According to NASA-CR-124075, “ISOGRID DESIGN HANDBOOK”, to get an optimum design parameter, minimum equivalent weight thickness, t’ and positive margin of safety corresponding to buckling failure are considered
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