Advanced Grid Stiffened Structures Research Articles

Experimental investigation on buckling of GFRP cylindrical shells subjected to axial compression

Composite cylindrical shells are being used in submarine, underground mines, aerospace applications and other civil engineering applications. Thin cylindrical shells are more prone to fail in buckling rather than material failure. An experimental study on buckling of glass fiber reinforced plastics layered composite cylindrical shells under displacement and load controlled static axial compression are reported The experimental results are compared with general purpose finite element program (ANSYS). Limit point loads evaluated for geometric imperfection magnitudes shows an excellent agreement with experimental results which clearly indicates the confidence gained on the numerical results presented. Present study finds direct application to qualitatively investigate the influence of geometric imperfection on other advanced grid-stiffened structures.

Three-dimensional modeling of the advanced grid stiffened structures in the co-curing process

Advanced grid stiffened (AGS) structures are made of carbon fiber reinforced polymer composites, which consists of a main panel with isogrid stiffeners. During the co-curing process, the autoclave pressure will compress the surface of the AGS structure to repel excess resin and voids. Because of the thermal expansion of the rubber, the rubber mold can provide the necessary lateral compacting forces on the rib stiffeners. This study constructs a three dimensional numerical model to simulate the consolidation behavior between the skin prepreg and the silicone rubber under a preset temperature profile. The model could provide the thickness distribution and fiber volume fraction of the AGS after the co-curing process. The consolidation pressure of the silicone rubber is proportional to its CTE and the curing temperature. With the numerical model, a suitable type of silicone rubber can be selected for the tooling.

A Stiffened Plate Element Model for Advanced Grid Stiffened Composite Plates/Shells

A triangular composite stiffened plate/shell element is developed to analyze the advanced grid stiffened (AGS) structures based on Mindlin shear deformation theory. The rotations of ribs and skin are interpolated, respectively, while the conditions of displacement compatibility are satisfied along the interface of rib and skin. There are no restrictions on the number, positions, and orientation of the ribs in establishing element mesh. The numerical results indicate its effectiveness, especially for the case of the AGS structures of higher ribs. Stability analyses for an ortho-grid plate and iso-grid cylindrical shell are also carried out.

A Generalized Analytical Modeling of Grid Stiffened Composite Structures

With the advancement of manufacturing technology, advanced grid stiffened (AGS) composite structures have been extensively studied recently due to their superior structural performance at a lower cost. Currently, the theoretical analysis involving AGS structures without fillers (empty bay) is dominated by the smeared method and finite element analysis (FEA). Obviously, the smeared method averages the local information and fails to predict structural behavior when the length scale (contact area) of the external load is smaller than the length scale (size) of the unit cell. This is because the actual structural behavior heavily depends on the spatial location of the applied load (at the rib, node, or cell). The limitation of FEA persists in its difficulty in conducting structural optimization. For AGS structures with fillers (filled bay), the interaction between the filler and the grid skeleton offers a higher level of challenge to theoretical modeling. In this study, a generalized analytical model was developed to predict the static structural behavior without smearing. This model was generalized in such a way that it unified the analysis of both empty bay and filled bay and could be reduced to conventional laminated composite or sandwich composite. This model was developed based on the classical lamination theory by treating the bay area as inclusion and the physical discontinuity between the rib and the bay was considered by introducing a stiffness distribution function. Using Hamilton's variational principle, the governing equation and boundary condition were obtained. The variable coefficient differential equation was solved by Fourier series expansion technique. The model was first validated by a numerical result found in the literature. Parametric studies were then conducted to evaluate the effect of various design parameters on the structural behavior subjected to both distributed load and concentrated load.

Numerical Investigation of Post Buckling Strength and Failure Modes in Advanced Grid Stiffened Structure under Thermal-Mechanical Loads

This work investigated the post buckling strength and failure behavior of advanced grid stiffened structures (AGS) under thermal-mechanical load using a finite element method. Based on the first order shear deformation theory (FSDT), Von Karman non-linear deformation assumption, and a progressive failure criterion, the buckling, large deformation，local failure modes in the AGS were studied. The thermal effect was also analyzed. By some numerical examples, the failure characteristics of the AGS were discussed.

Investigation of the Sound Transmission into an Advanced Grid-Stiffened Structure

The noise transmission behavior of an advanced grid-stiffened (AGS) composite structure has been investigated by combining numerical and experimental methods. Structural-acoustic coupling was found to be light, permitting separate analysis of the structure and acoustic cavity. Finite element analysis permitted the resonant frequencies of acoustic cavity and structure to be calculated, which play an important role for noise transmission through the structure. Acoustic mode shapes permitted internal coincidence frequencies to be estimated and provided insight into modal pressure distributions, when considering payload location. Experimental structural and acoustic modal analysis permitted the resonant frequencies and damping ratios for the structure and cavity to be determined, which in turn were used to corroborate the FEA model. Finally, direct measurement of the noise transmission was performed based on noise reduction spectrum (NRS), which is calculated from spatial averages of the RMS acoustic pressures inside and outside of the shell. It was found that the NRS was dominated by acoustic resonances, which were marked by sharp dips in the NRS curve. Internal coincidence of the axial wavenumbers was also found to be a significant mechanism for noise transmission. External coincidence and ring frequencies were found to provide less of an impact on the overall NRS for the structure.

Manufacturing theory for advanced grid stiffened structures

Abstract Lattices of rigidly connected ribs, known as advanced grid stiffened (AGS) structures, have many advantages over traditional construction methods, which use panels, sandwich cores and/or expensive frameworks. The technology behind these structures has progressed significantly during the past five years to the point where these structures are being integrated into operational systems. Two tooling methods for fabricating these structures using composite materials have proven to be highly effective at achieving high quality, low cost AGS structures: the hybrid tooling method and the expansion block method. Both methods rely on a precise understanding of tooling behavior during cure to achieve proper consolidation, often determined through trial and error. This paper proposes a theory governing the behavior of both tooling types during the cure cycle in order to minimize the trial and error required to understand tooling expansion during cure. The theory is validated by experimental data.

The Effect of Varying Thickness on the Buckling of Orthotropic Plates

Although stability failure of constant thickness plates is a fairly well understood problem, buckling of plates with varying thickness has seen little research. This is not a common problem, but buckling of varying thickness plates does occur, as in the case of Advanced Grid Stiffened structures. These structures are characterized by lattices of rigidly connected rib stiffeners. While real-world Advanced Grid Stiffened structure ribs are modeled as orthotropic plates, they often have complex cross-sections (i.e., varying thickness). Unfortunately, failure analysis techniques for these ribs have been limited to rectangular cross-sections. To address this shortcoming, a buckling theory is presented for orthotropic plates of varying thickness. Orthotropic plates with both linear and hourglass thickness variations are considered. These are common grid structure cross-section geometries resulting from existing manufacturing processes. For both geometries, results are given that allow designers to predict how these plates will behave, relative to a constant thickness plate, for a variety of material properties and plate aspect ratios.