Local Stiffener Research Articles

Structural Analysis of Deck Reinforcement on Composite Yacht for Crane Installation

The crane installation on the deck of a yacht redistributes the stress field and affects the local structural integrity and performance. The safe operation of the yacht is associated with the optimal placement of the crane on the deck and the proper local structural reinforcement. Here, the structural analysis of the bow part of a yacht made of composite materials is studied, considering the retrofit installation of a crane, in three different cases of reinforcing the deck: (a) without any reinforcement, (b) with a T-type reinforcement, and finally, (c) with a longitudinal beam. The T-type connects the longitudinal bulkhead and the deck, reinforced locally with overlamination skin and adhesive-filler. The longitudinal beam works as a local longitudinal stiffener attached to the deck and connects the second, third, and fourth transverse frames. The structural analysis is performed using the finite element method following the classification societies’ rules. The local reinforcements are made from the same composite materials as the unreinforced deck. The maximum deformations, the principal stresses, and the safety factors following Tsai-Wu and Hashin criteria are calculated and compared for the three different cases. The T-type and longitudinal reinforcements reduce deck stresses by 33%, with longitudinal reinforcement reducing deck deformation by 17%. Composite failure analysis shows the structure was near failure, and the reinforcements enhance safety; T-type is better for multiaxial loads (Tsai-Wu), and longitudinal is superior for micromechanical failure (Hashin). By considering the structural performance and safety aspects, designers and engineers can make optimal decisions regarding yacht crane installation and proper reinforcement, leading to safer and more efficient structures.

Progressive collapse analysis and ultimate strength estimation of continuous stiffened panel under longitudinal extreme cyclic load and lateral pressure

Stiffened panel is a fundamental member of hull structures which is designed to withstand multiple loads such as revered cyclic load and lateral pressure. At present the ultimate strength of stiffened panel is conventionally determined by the monotonic compression criterion, ignoring the real progressive collapse of ship in rough sea conditions. To provide useful insights for improving the ship design, the present study focuses on dealing with the structural collapse characteristics and ultimate strength estimation of continuous stiffened panel under combined longitudinal extreme cyclic load and lateral pressure. A large number of nonlinear finite element analyses (FEA) are conducted to identify the influences of various factors on the failure mode and ultimate strength including the slenderness ratio of local panel, stiffener type, size and number, sequence and amplitude of the cyclic load, cycle number and lateral pressure. It is found that the ultimate limit state behaviours are governed by the coupling effects of several influential parameters. Based on artificial neural networks (ANN), a prediction model with high accuracy is proposed to evaluate the ultimate strength in different cycles.

Numerical Investigation of the Ultimate Strength of D-Ring Devices and Deck Structures

An accurate prediction of the ultimate strength of lashing devices and deck structures is important to ensure the safety of the crews and carrying ships. In this study, finite element analysis using ABAQUS/implicit was performed to investigate the ultimate strength of D-ring devices subjected to various external loads. The resistance of deck plates to which the D-ring devices were clamped was also analyzed numerically, considering the effects of the plate thickness and corrosion wastage. The resultant force-displacement relationship of the devices and the deck plate was investigated from the simulations and the threshold was determined by means of the tangent interaction method. The numerical results were compared with the Cargo Stowage and Securing Code proposed by the International Marine Organization and the results showed that the code predicts conservative ultimate strength of the D-ring devices in most cases. The deck plate with a thickness of 6 mm should have a local stiffener to increase structural strength whereas corrosion wastage has negligible effect on the deck strength. The numerical analysis verified the feasibility of predicting the ultimate strength of D-ring devices and deck structures. Nevertheless, the need for further experimental study is acknowledged to validate the feasibility of the numerical results.

Global–Local Collaborative Optimization Method for Stiffened Cylindrical Shells with Cutout

In the cutout reinforcement for a stiffened cylindrical shell, the layout optimization of global stiffeners and the stiffeners around the cutout are generally separated. However, considering the two optimizations separately is noncollaborative. To solve this problem, a global–local collaborative optimization method is proposed. Based on the sequential sampling method, a set of sampling points is obtained in the design space. Local optimization is carried out to optimize the stiffener layout around the cutout for each sampling point along with its strain energy and stress. The radial basis function surrogate model is constructed based on the aforementioned sampling points. Based on the surrogate model, global optimization is carried out to obtain the optimal global stiffener layout and the stiffener layout around the cutout. In this way, the layout of the global stiffener and the layout of the local stiffener can be designed collaboratively. An illustrative example of the stiffened cylindrical shell with the cutout is carried out to verify the effectiveness of the proposed method. An innovative stiffened cylindrical shell with the cutout is obtained by a collaborative optimization method. The weight of the innovative design is reduced by 8.70%, and the maximum von Mises stress is reduced by 14.47% when compared with the optimal design by a noncollaborative optimization method.

Evaluating the influence of geometric distortions to the buckling capacity of stiffened panels

This study presents an effort to clarify the effects of initial geometric distortions to the buckling capacity of stiffened panels through numerical experimentation. Four types of imperfections have been considered, that is local (plate, web and stiffener) and global distortion types (column-like). These discrete types were considered as the factors that tend to affect the buckling capacity of the adopted finite element model. Statistical techniques of Design Of Experiments have been applied. A One Factor At a Time analysis along with a full factorial analysis have been performed for screening purposes, and the significant main and interaction effects have been evaluated. The Response Surface Method was finally employed so as to verify and narrow down the number of significant factors. From the four distortion types examined and for the particular involved collapse modes in the evaluated geometries, it was concluded that when the stiffened panel involves global distortion such that maximum global bending stress develops at the plate, then only local plate and web distortions are significant, while stiffener and global distortion may be neglected from the analysis. On the other hand, when global distortion maximizes the global bending stress at the stiffener top, then local web and this particular global distortion type have to be included.

Vibration Characteristics of Composite Plate Embedded with Shape Memory Alloy at Elevated Temperature

Unique functional material of shape memory alloy has attracted tremendous interest from researches, thus has been broadly investigated for a wide range application. Current research effort extends the use of SMA for the design of smart composite structures due to its shape memory effect, pseudo-elasticity and high damping capability. This paper presents an assessment of applications of the SMA materials for structural vibration controls, where the influences of SMA as reinforcement in the composite plate at different temperature are investigated. Four cases of composite plate are studied, which two of them are SMA-based composite fabricated at 0° and 45° angles, and the other two plates are neat (without SMA wires) and built with local stiffener. By using modal testing, the free vibration analysis is carried out to determine the vibration characteristics of composite plates. The results show that infusing SMA wires into composites increased the natural frequencies of the plate considerably, while decreased slightly for damping percentage. However, when SMA wires are heated, the damping percentage improved tremendously due to the phase transformation temperature of SMA from martensite to austenite. The outcome of this study reveals the potential of SMA materials in active vibration control.

Design, Optimization, and Evaluation of Al–2139 Compression Panel with Integral T-Stiffeners

A T-stiffened panel was designed and optimized for minimum mass subjected to constraints on buckling load, yielding, and crippling or local stiffener failure using a new analysis and design tool named EBF3PanelOpt. The panel was designed for a compression loading configuration, a realistic load case for a typical aircraft skin-stiffened panel. The panel was integrally machined from a 2139 aluminum alloy plate and was tested in compression. The panel was loaded beyond buckling and strains, and out-of-plane displacements were extracted from 36 strain gages and one linear variable displacement transducer. A digital photogrammetric system was used to obtain full-field displacements and strains on the smooth (unstiffened) side of the panel. The experimental data were compared with the strains and out-of-plane deflections from a high-fidelity nonlinear finite element analysis. The test data indicated that the panel buckled at the linear elastic buckling eigenvalue predicted for the panel. The out-of-plane displacement measured by the digital photogrammetric system compared well both qualitatively and quantitatively with the nonlinear finite element solution in the postbuckling regime. Furthermore, the experimental strains compared well with both the linear and nonlinear finite element models before buckling. The weight of the optimized panel was 20% less than that of a T-stiffened panel optimized using conventional design techniques.

Design, Optimization, and Evaluation of Integrally Stiffened Al-7050 Panel with Curved Stiffeners

A curvilinear stiffened panel was designed, manufactured, and tested in the Combined Load Test Fixture at NASA Langley Research Center. The panel was optimized for minimum mass subjected to constraints on buckling load, yielding, and crippling or local stiffener failure using a new analysis tool named EBF3PanelOpt. The panel was designed for a combined compression-shear loading configuration that is a realistic load case for a typical aircraft wing panel. The panel was loaded beyond buckling and strains and out-of-plane displacements were measured. The experimental data were compared with the strains and out-of-plane deflections from a high fidelity nonlinear finite element analysis and linear elastic finite element analysis of the panel/test-fixture assembly. The numerical results indicated that the panel buckled at the linearly elastic buckling eigenvalue predicted for the panel/test-fixture assembly. The experimental strains prior to buckling compared well with both the linear and nonlinear finite element model.

The effect of a local stiffener in the structural—acoustic coupled system

This article proposes an analytical method that can be used to obtain the vibro-acoustic modal and forced response characteristics of a three-dimensional structural—acoustic coupled system, where the coupled system is composed of an acoustic cavity with a rectangular plate that is stiffened by a local stiffener. Through the proposed method, the effect of the local stiffener on the natural frequencies, modes, and forced responses is identified for the plate—cavity coupled system with the local stiffener. The relationship between the thickness and the location of the local stiffener on the vibro-acoustic modal characteristics and forced responses is investigated through the structural—acoustic modal coupling coefficient. The high peak-noise due to the strong modal coupling between the plate and acoustic modes can be effectively reduced by properly designing the local stiffener in the coupled system. Finally, the modal characteristics and frequency responses obtained by the proposed method are compared with those of finite-element analysis. The results of this article provide useful information for suppressing the booming noise generated by the strong modal coupling between the plate and acoustic modes in the coupled system.

Response of stiffened and unstiffened plates subjected to blast loading

This paper describes the results of dynamic analyses carried out on both stiffened and unstiffened panels using both simplified and advanced analytical techniques. For unstiffened panels with in-plane restraint along their edges, the dynamic response of an imperfect panel was predicted using a large displacement elastic analysis based on Lagrange's equation, with the panel being treated as a shallow shell. For stiffened panels, the finite element (FE) technique was used to establish the validity of using the simplified technique to predict the inter-stiffener panel displacements for a simply supported panel. A parametric study has been carried out to analyse the effects of in-plane boundary conditions, local stiffener buckling and initial imperfections on the overall response. The significant effect of boundary conditions is demonstrated by including the actual boundary conditions of a test frame in the finite element modelling of a large-scale stiffened floorplate panel used in an experimental test series.

Acoustic design for noise reduction (ADNR), a method and computer code based on the modelization of structural acoustic and vibration behavior for semicomplex structure

Challenge of noise control in the nineties will be to introduce new tools in addition to classical ones (absorbing materials, enclosures, and mufflers) which have been used for several decades. For noise reduction at its source there are mainly two approaches: active noise control or passive noise control, based on a new mechanical design. This paper addresses the second possibility. To design and create a quieter product, an analytical model has been developed predicting the vibration and acoustic responses of a structure. The model is based on a variational method and the calculation of the Green’s function. Specific developments have been introduced to decrease the total computation time. The main design indicators are the quadratic velocity, the modal and global structural shape, the radiation factor, and the sound power. These indicators serve to illustrate how the various design specifications (geometry, thickness, added extended of local mass, added stiffness, stiffener, damping, excitation type, and location) can or cannot decrease the radiated noise. The implemented code appears to be a unique, powerful, and helpful tool for taking the right decision, as far as radiated noise is concerned, at the very beginning of the design stage.

Elastic buckling and postbuckling of eccentrically stiffened plates

The elastic buckling and postbuckling behaviour of eccentrically stiffened plates are evaluated analytically. The effects of lateral boundary conditions and of stiffener eccentricity relative to the plate plane are emphasized. Attention is confined to global buckling; local plate and local stiffener buckling effects are neglected. A simplified direct energy approach is used together with Marguerre's plate theory. Critical buckling loads are found which are generally higher than those obtained with a simple Euler column model. A simple closed-form solution is found for the postbuckling curve which for small imperfection levels coincides with the classical Koiter solution. The buckling behaviour is found to be asymmetric with loads generally in excess of the critical load in the advanced postbuckling region.