Stiffened Shell Structure Research Articles

Impact behavior of a stiffened shell structure with optimized GFRP corrugated sandwich panel skins

The aim of this work is to provide a virtual testing for the three-layered shells made of the optimized corrugated core sandwich panels. Starting from the unit cells level, we define their geometry found from the preliminary optimization and ensured the requirements of strength, stability and effective thermal conductivity. Effective stiffness properties of the core are evaluated then by using asymptotic homogenization method. The mechanical performance of the non-axisymmetric stiffened shell made of the considered sandwiches is studied numerically for the low-velocity impact. Solid-shell model with homogenized properties of the core is used for the skin material of the shell. Multi-scale approach is involved in the evaluation of the global and local stress state of the panels using stress concentration tensor estimated during asymptotic homogenization. As result, we check the robustness of the design approaches for the unit cells and its validity in the case of the complex loading conditions in the three-dimensional structure.

On the study of vibrational interactions of an internal substructure with a main structure submerged in water and its acoustic radiations using admittance approach

It is important and practical to design an internal substructure for supporting machines which generates vibration sources for a main submerged stiffened shell structure. Vibration propagates from the internal supporting structure to the wetted shell structure, thus radiating acoustic energy into water. The present study can be divided into two categories: 1. the main wetted structure, including stiffeners and bulkheads, etc., radiates acoustic energy into water subject to forces which is the junction interacting forces arisen from the vibrating machine, 2. the interaction force pertains to the coupling dynamic characteristics between wetted structure and the internal structure. An admittance approach is adopted to characterize individual structures and the coupled equation is derived by continuity of displacement variables and equaling forces with opposite sign at the junction of connected structures. Admittance matrices are computed by using junction forces between the structures whereas the responded displacements at the junctions are the elements of the admittance matrices. The coupled admittance equation is complex symmetric matrix. An eigenvalue analysis is performed to investigate the interaction junction forces, accordingly, the radiation characteristics of the coupled main wetted structure under fluid loading in connection with supporting internal structures.It is important and practical to design an internal substructure for supporting machines which generates vibration sources for a main submerged stiffened shell structure. Vibration propagates from the internal supporting structure to the wetted shell structure, thus radiating acoustic energy into water. The present study can be divided into two categories: 1. the main wetted structure, including stiffeners and bulkheads, etc., radiates acoustic energy into water subject to forces which is the junction interacting forces arisen from the vibrating machine, 2. the interaction force pertains to the coupling dynamic characteristics between wetted structure and the internal structure. An admittance approach is adopted to characterize individual structures and the coupled equation is derived by continuity of displacement variables and equaling forces with opposite sign at the junction of connected structures. Admittance matrices are computed by using junction forces between the structures whereas the responded d...

Vibration isolation design for periodically stiffened shells by the wave finite element method

Periodically stiffened shell structures are widely used due to their excellent specific strength, in particular for aeronautical and astronautical components. This paper presents an improved Wave Finite Element Method (FEM) that can be employed to predict the band-gap characteristics of stiffened shell structures efficiently. An aero-engine casing, which is a typical periodically stiffened shell structure, was employed to verify the validation and efficiency of the Wave FEM. Good agreement has been found between the Wave FEM and the classical FEM for different boundary conditions. One effective wave selection method based on the Wave FEM has thus been put forward to filter the radial modes of a shell structure. Furthermore, an optimisation strategy by the combination of the Wave FEM and genetic algorithm was presented for periodically stiffened shell structures. The optimal out-of-plane band gap and the mass of the whole structure can be achieved by the optimisation strategy under an aerodynamic load. Results also indicate that geometric parameters of stiffeners can be properly selected that the out-of-plane vibration attenuates significantly in the frequency band of interest. This study can provide valuable references for designing the band gaps of vibration isolation.

Minimum stiffness criteria for ring frame stiffeners of space launch vehicles

Frame stringer-stiffened shell structures show high load carrying capacity in conjunction with low structural mass and are for this reason frequently used as primary structures of aerospace applications. Due to the great number of design variables, deriving suitable stiffening configurations is a demanding task and needs to be realized using efficient analysis methods. The structural design of ring frame stringer-stiffened shells can be subdivided into two steps. One, the design of a shell section between two ring frames. Two, the structural design of the ring frames such that a general instability mode is avoided. For sizing stringer-stiffened shell sections, several methods were recently developed, but existing ring frame sizing methods are mainly based on empirical relations or on smeared models. These methods do not mandatorily lead to reliable designs and in some cases the lightweight design potential of stiffened shell structures can thus not be exploited. In this paper, the explicit physical behaviour of ring frame stiffeners of space launch vehicles at the onset of panel instability is described using mechanical substitute models. Ring frame stiffeners of a stiffened shell structure are sized applying existing methods and the method suggested in this paper. To verify the suggested method and to demonstrate its potential, geometrically non-linear finite element analyses are performed using detailed finite element models.

Symmetric Stiffened Shell Structures

In this paper the analysis of the symmetric stiffened shell structure are introduced. The main purpose of this paper is to reduce the number of the unknowns in the compatibility equation to ease vehicle preliminary design process. One shell field contains a square-shaped shell and four rod with joints in the corner. Different cases are examined depending on the number of the shells.

Review on Integral Stiffened Shell Structure Technology of Large Space Station

Review on Integral Stiffened Shell Structure Technology of Large Space Station

Postbuckling of shear deformable stiffened anisotropic laminated cylindrical shell under axial compression

A postbuckling analysis is presented for a shear deformable anisotropic laminated cylindrical shell with stiffener of finite length subjected to axial compression. The material of each layer of the shell is assumed to be linearly elastic, anisotropic and fiber-reinforced. The governing equations are based on a higher order shear deformation shell theory with von Kármán-Donnell-type of kinematic nonlinearity and including the extension/twist, extension/flexural and flexural/twist couplings. The ‘smeared stiffener’ approach is adopted for the beam stiffeners. This arrangement allows the beam stiffeners to be assembled directly into the global stiffness matrix. The nonlinear prebuckling deformations and initial geometric imperfections of the shell are both taken into account. A singular perturbation technique is employed to determine the buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling response of perfect and imperfect, grid, axial, ring stiffened, and unstiffened shells. The results confirm that there exists a compressive stress along with an associate shear stress and twisting when the anisotropic shell is subjected to axial compression. The postbuckling equilibrium path is unstable for the moderately thick cylindrical shell under axial compression and the stiffened shell structure is imperfection-sensitive.

Spectral element modelling of wave propagation in isotropic and anisotropic shell-structures including different types of damage

During the last decades, guided waves have shown great potential for Structural Health Monitoring (SHM) applications. These waves can be excited and sensed by piezoelectric elements that can be permanently attached onto a structure offering online monitoring capability. However, the setup of wave based SHM systems for complex structures may be very difficult and time consuming. For that reason there is a growing demand for efficient simulation tools providing the opportunity to design wave based SHM systems in a virtual environment. As usually high frequency waves are used, the associated short wavelength leads to the necessity of a very dense mesh, which makes conventional finite elements not well suited for this purpose. Therefore in this contribution a flat shell spectral element approach is presented. By including electromechanical coupling a SHM system can be simulated entirely from actuator voltage to sensor voltage. Besides a comparison to measured data for anisotropic materials including delamination, a numerical example of a more complex, stiffened shell structure with debonding is presented.

A new stiffened shell element for geometrically nonlinear analysis of composite laminates

A new stiffened shell element combining shallow beam and shallow shell elements is developed for geometrically nonlinear analysis of stiffened composite laminates under mechanical loading. The formulation of this element is based on the principle of virtual displacements in conjunction with the co-rotational form of the total Lagrangian description of motion. In the finite element formulation, both the shell and the beam (stiffener) elements account for transverse shear deformations and material anisotropy. The cross section of the stiffener (beam) can be arbitrary in geometry and lamination. In order to combine the stiffener with the shell element, constraint conditions are applied to the displacement and rotation fields of the stiffener. These constraint conditions ensure that the cross section of the stiffener remains co-planar with the shell section after deformation. The resulting expressions for the displacement and rotation fields of the stiffener involve only the nodal unknowns of the shell element, thus reducing the total number of degrees-of-freedom. Also, the discretization of the entire stiffened shell structure becomes more flexible. The robustness of the stiffened shell element has been proven by comparison against other shell elements considered previously.

Postbuckled behavior of composite isogrid stiffened shell structure

This paper investigated reliability of a postbuckled composite isogrid stiffened shell structure under a compression load. The outside diameter of the isogrid cylinder was 624.8 mm, the length was 368.3 mm and the total weight of the cylinder was 3.24 kg. A finite element buckling analysis result was compared with the axial compression experimental data. The average critical buckling load of 186.56 kN was obtained by using the Patran finite element analysis (FEA). The isogrid shell was modeled using 3-node triangular elements and the stiffeners were modeled using 4-node quad elements with a total of 504 nodes and 900 elements. The postbuckled isogrid cylinder was compression tested to the failure load of 177.35 kN. The postbuckled cylinder continued to resist compression loading even after one or more stiffeners had fractured. The testing evaluation revealed that the stiffener buckling was the critical failure mode and it has been demonstrated to be tolerant to structural damage due to the multiplicity of load paths. The lower experimental bucking load was due to the small imperfections in the cylinder but this problem can be overcome by advancement in the manufacturing methods.

Strength Analysis of Shell Structure with Stiffenersby Finite Strip Method (Second report)

The strength analysis of stiffened shell structure in ship hull involve to clarify strength characteristics of shell itself and the effect of shapes of the objects in addition to the effect of various reinforced ways. The first report presented a method of analyzing stiffened shells with simply supported edges and of rectangular form by means of finite strip method. And it is found that fairly high accurate solutions were obtained by this method in spite of lightening the volume of calculations.Boundary conditions for stiffened panels of actual ship-hull-structure are considered as free or elastic supported besides simply supported depending on adjacent structural conditions. Moreover, the boundary shapes of curved hull panels in ship bow or stern structure are non-rectangular occasionally and the directions of fitted stiffeners are oblique to shell edges.To increase the applicability for strength analysis of ship construction, this paper presents extensions of the analytical method described in the first report by treating following items;a) various boundary conditions for applying practical cases b) curved shell panels of non-rectangular shapes c) shell with obliquely fitted stiffeners.For the item a), it is settled by using a buckling modes of beam as displacement functions and for the item b) and c), the method of linear transformations of variables has been applied.Some examples are shown for the cases of analyzing bending of shallow shells without and with stiffeners which are of various shapes and various edge conditions. The results prove that proposed method is one of effective means for analyzing the curved parts of ship construction.

Random polar sampling technique for the reliability analysis of submarine pressure hull

Reliability analysis is essential to reduce the structural system failures to an acceptance level. In this paper, a quasi-Monte Carlo Simulation method called ‘Random Polar Sampling Technique' (RPST) has been described for the reliability analysis of ring stiffened shell structure, in which the combinations of variates are obtained using a polar sampling of Latin hypercube sampled values. An efficient computer program coded in C++ has been developed for use with the RPST. Reliability analysis of internally ring stiffened submarine pressure hull has been described as an illustration and the results are compared with those given by First Order Second Moment method (FOSM), Advanced First Order Second Moment method (AFOSM) and Monte-Carlo Simulation methods. Keywords: Random Polar Sampling Technique, Reliability, Ring stiffened shells, Submarine pressure hull, Latin Hypercube Samples.  doi: 10.3329/jname.v1i1.2036 Journal of Naval Architecture and Marine Engineering 1(2004) 26-34