Design of structures by a splitting method

A simplified method for the design of multi-spar wing boxes is presented. In typical multi-spar wing boxes the spars divide the boxes into cells. In the method presented these are analyzed individually, with adjacent cells taking their share of the stiffnesses of the common spar wall. This splitting method yields a design method that is computationally much quicker than designing a complete wing box, because each cell is considered separately from the others, except for linking between their design variables. The critical buckling load factor of the assembled structure when designed in this way will usually exceed the design load factor and otherwise will be equal to it, i.e. the design is guaranteed to be conservative

Buckling load reduction for stiffened panels due to cutouts in ribs

In aerospace structures it is common to find stiffened panels with transverse supporting structures, e.g. wing ribs or fuselage frames. Incorporating cutouts into these supporting structures to allow the stringers to pass through freely considerably reduces the buckling load of the panels. It is shown that a minor modification in the fabrication of the stiffened panel gives most of the advantages of cutouts while still giving a buckling load close to that of a panel with no cutouts

Optimization of postbuckled stiffened panels with multiple stiffener sizes

The panel analysis and optimization code VICONOPT, based on exact strip theory, is utilized to investigate the optimum design of stiffened panels with multiple stiffener sizes or substiffeners. The optimization ensures that the buckling stability of the panel includes an allowance for postbuckling reserve of strength. The adoption of this approach necessarily results in the local buckling stress being lower than the overall buckling stress and with the introduction of substiffeners introduces extra buckling modes. This complicates the post buckling behavior of the panel which is investigated by examining the case when the smaller stiffeners lose stiffness, i.e. there is a change from a local to a torsional mode. The panels are loaded in axial compression with a sinusoidal imperfection. It is found that small mass savings are achieved by using stiffeners of more than one size and there is an increase in the spacing of the major stiffeners and transverse supports. The optimum panel designs obtained by VICONOPT are evaluated by comparison with the optimum designs produced with one size of stiffener

Postbuckled stability of panels with torsional buckling

The panel analysis and optimization code VICONOPT, based on exact strip theory, is utilized to investigate the postbuckling stability of a stiffened aerospace panel in a torsional buckled state. The paper shows that the postbuckling characteristics of a panel buckling in a torsional mode has similarity to the postbuckling behavior of a panel with a skin initiated mode and a panel initiated mode. The postbuckled stiffness of the torsional mode is similar to the skin mode in terms of load versus end shortening and is similar to the panel postbuckling behavior in terms of load versus out-of-plane deflection. If the panel has stiffeners of more than one size then there are multiple torsional modes. For panel design it is suggested that small stiffener buckling, i.e., in a torsional mode, can have postbuckling stability with regard to the growth of the out-of-plane deflection. If the large stiffeners initiate the buckling then there is no postbuckling reserve of strength. This has implications for design of such panels as mass could be saved if allowance is made for small stiffener buckling in the optimization process

Buckling and vibration of stiffened panels or single plates with clamped ends

An efficient method for the buckling and vibration analysis of plates or stiffened panels with clamped ends is presented. The method uses Lagrangian multipliers to couple sinusoidal modes with appropriate half-wavelengths of response, thereby enforcing the end conditions at discrete point supports. Clamped ends can usually be modelled accurately using only a few point supports, while arguments from symmetry often enable some of the required end conditions to be satisfied without explicitly applying constraints. In such cases few half-wavelengths are needed to obtain excellent accuracy. Solutions obtained for the simple limiting case of single plates are exact or within 1% of the classical or other reported solutions. Solutions obtained for stiffened panels are in close agreement with those obtained using finite element analysis

Optimum design and testing of a postbuckled stiffened panel

The efficient, industrially used, linear elastic preliminary design software VICONOPT is employed to design a stiffened panel with a post-buckled reserve of strength. The initial buckling mode is a local skin mode in longitudinal compression with allowance being made for the effects of an initial overall imperfection. The resulting panel has been analyzed using the non-linear FE package ABAQUS and four laboratory specimens have been tested to failure. The similarity of the experimental failure with the VICONOPT and ABAQUS predictions suggests that VICONOPT can give a satisfactory preliminary design. While neither model matches completely the boundary conditions found in a real aircraft compression panel, it is suggested that the VICONOPT model may be a better representation than either the ABAQUS model or the experimental tests

Postbuckling of stiffened panels using strut, strip, and finite element methods

Postbuckling results are presented for isotropic stiffened panels loaded in compression. Comparisons are made between single-bay and double-bay nite element (FE) models (where “bay” denotes a repeating portion, between supports, in the load/length direction) and a new strut model, following a Shanley-type approach, for single-bay and multibay panels. The strut model has been incorporated within the strip programVIPASA with CONstraints and OPTimization (VICONOPT) to design a multibay example panel with postbuckling reserve of strength in its skins, assuming linear elastic material properties. The panel has been shown by VICONOPT to have a stiffener buckling failuremode when an overall sinusoidal imperfection causing increased stiffener compression is present. The failure is con rmed by the double-bay FE model, which is shown to be an imperfect representation of the multibay case. Single-bay analysis using the strut model shows good agreement with the single-bay FE results. The VICONOPT code is able to design a metallic panel of realistic dimensions and loading using 50 strip elements (compared with the 9600 shell elements required by the nite element model) but cannot correctly account for material nonlinearity. The important phenomenological difference between postbuckling of single-, double-, and multibay panel models are indicated

Analysis and testing of a postbuckled stiffened panel

The suitability of using the ef cient, linear elastic design softwareVICONOPT for the analysisof a stiffened panel with a postbuckling reserve of strength is investigated. A longitudinallycompressed panel, which initially buckled in a local skin mode, was analyzed with allowance being made for the effects of an initial overall imperfection. The panel was also analyzed using the nonlinear nite element package ABAQUS, and four laboratory specimens that represent the panel were tested to failure. The similarity of the experimental failure with the VICONOPT and ABAQUS predictions indicates that VICONOPT can give satisfactory analysis results for use in preliminary design

The 2017 OHBM Replication Award

Protocol, submissions and scores of the 2017 OHBM Replication Award

Optimum design and testing of a post-buckled stiffened panel

The efficient, industrially used, linear elastic preliminary design software VICONOPT is employed to design a stiffened panel with a post-buckled reserve of strength. The initial buckling mode is a local skin mode in longitudinal compression with allowance being made for the effects of an initial overall imperfection. The resulting panel has been analyzed using the non-linear FE package ABAQUS and four laboratory specimens have been tested to failure. The similarity of the experimental failure with the VICONOPT and ABAQUS predictions suggests that VICONOPT can give a satisfactory preliminary design. While neither model matches completely the boundary conditions found in a real aircraft compression panel, it is suggested that the VICONOPT model may be a better representation than either the ABAQUS model or the experimental tests