Thin-walled Structural Components Research Articles

Amos Henry Chilver FREng, Baron Chilver of Cranfield. 30 October 1926 — 8 July 2012

Henry Chilver was a structural engineer and educationalist. During his early career at Cambridge and University College London, he and his team of researchers made significant contributions, both theoretically and experimentally, to the understanding of imperfection sensitivity in the elastic post-buckling of thin-walled structural components. Awareness of this sensitivity was essential for the safe design of load-carrying shells that were increasingly being used in nuclear reactors and aerospace structures. As Vice-Chancellor of Cranfield Institute of Technology he focused on the application of knowledge rather than merely on its acquisition and led the way in harnessing research relevant to the demands of industry. With his incisive intellect, innovative thinking and farsighted vision he was also in great demand as an adviser to the government, overseeing advisory committees, and holding chairmanships and directorships of numerous companies.

Press Forging of Thin-Walled AZ31 Magnesium-Alloy Components

The press forging of AZ31 magnesium-alloy sheets for producing thin-walled structural components used in the electronics industry was examined by both the finite element analysis and the experimental approach in the present study. The experimental results obtained from the compression tests and ring compression tests were employed in the finite element simulations to investigate the effects of process parameters, such as forming temperature, friction condition, embossment location, and sheet thickness on the formation of embossments in a press forging process. The finite element simulation results reveal that a cost-effective press forging process of AZ31 magnesium-alloy requires an optimum combination of the above parameters. The detailed examination of the effects of the process parameters on the formation of embossments made in the present study could provide a design guideline for a press forging process of AZ31 magnesium-alloy sheets.

Buckling and fracture behaviour of cracked thin plates under shear loading

Thin-walled structural components are widely used in several engineering applications such as in aerospace, naval, nuclear power plant, pressure vessel, mechanical and civil fields. Since they are frequently characterised by a high slenderness, the safety assessment of such structural components requires to carefully consider the buckling collapse which can heavily limit their allowable bearing capacity. For very thin plates, buckling collapse can occur under compression, shear, or even under tension. In the present paper, the buckling and fracture collapse mechanisms in an elastic rectangular thin-plate with a central straight crack under shear loading are analysed. Different boundary conditions, crack length and orientation are considered. Through a parametric finite elements (FE) numerical analysis, the crack sensitivity of the collapse load of such a structural component is examined. The obtained results are discussed, and some interesting and useful conclusions are drawn. The collapse mechanism occurring earlier (buckling or fracture) is found by varying the fracture toughness of the material, and some failure-type maps depending on the geometrical parameters of the crack are determined.

Influence of a central straight crack on the buckling behaviour of thin plates under tension, compression or shear loading

Thin-walled structural components, such as plates and shells, are used in several aerospace, naval, nuclear power plant, pressure vessels, mechanical and civil structures. Due to their high slenderness, the safety assessment of such structural components requires to carefully assess the buckling collapse which can strongly limit their bearing capacity. For very thin plate, buckling collapse can occur under shear, compression or even under tension. In the latter case, fracture or plastic failure can also take place instead of elastic instability. In the present paper, the effects of a central straight crack on the buckling collapse of rectangular elastic thin-plates—characterized by different boundary conditions, crack length and orientation—under compression, tension or shear loading are analysed. Accurate FE numerical parametric analyses have been performed to get the critical load multipliers in such loading cases. Moreover the effect of crack faces contact is examined and discussed. Some useful conclusions related to the sensitivity to cracks of the buckling loads for thin plates, especially in the case of shear stresses, are drawn. Cracked plates under tension are finally considered in order to determine the most probable collapse mechanism among fracture, plastic flow or buckling and some failure-type maps are determined.

Buckling sensitivity analysis of cracked thin plates under membrane tension or compression loading

Thin-walled structural components such as plates and shells are commonly used in practical applications such as aerospace, naval, nuclear power plant, pressure vessels, mechanical and civil engineering structures and so on, and the safety assessment of such structures must carefully consider all the phenomena which can decrease the bearing capacity of such elements. Among them, the presence of cracks in thin-walled structures can heavily affects their safety factor with respect to the more common modes of failure such as buckling or fracture. For very thin plate, buckling collapse under compression or even under tension, apart fracture or plastic failure in this last case, can easily take place: the presence of flaws such as through-the-thickness cracks can sensibly modify such ultimate loads. In the paper the effects of cracks’ length and orientation on the buckling loads of rectangular elastic thin-plates – characterised by different boundary conditions and by various Poisson's ratio – under tension and compression, is considered. For tensioned flawed plates a fracture-buckling and a plastic-buckling collapse maps are obtained. After a short explanation of the buckling phenomena in plates, several FE numerical parametric analyses results are presented in terms of critical load multiplier in compression or in tension in cracked plates. The obtained results are discussed and some interesting and useful conclusions regarding the sensitivity to cracks’ presence of buckling loads of thin plates under compression or tension (or fracture in this last case) are explained. The interesting case of tensioned cracked plates is considered by studying the easiest collapse between fracture, plastic flow and buckling: in such cases some failure-type maps are finally determined.

Towards a new generation of 2D mathematical models in the mechanics of thin-walled fibre-reinforced structural components

Two-dimensional mathematical modelling of fibre-reinforced thin-walled structures is based on the simplifying assumption that fibres have negligible thickness and they are perfectly flexible. Micromechanics considerations reveal that, although this assumption is a valid approximation in many cases of interest, resistance of fibres in bending gives rise to asymmetric elasticity theory effects. A relevant finite elasticity theory developed recently [A.J.M. Spencer, K.P. Soldatos, Int. J. Non-lin. Mech. 42 (2007) 355–368] has produced, as a particular case, a version of asymmetric linearised elasticity theory that takes into consideration the effects of fibres bending stiffness. The latter, asymmetric theory of linear elasticity is currently available in a form appropriate to the material symmetries of transverse isotropy only but it can serve as a means for the incorporation of the aforementioned micromechanics effects into accurate two-dimensional thin-walled structures modelling. In this context, this study is the first attempt towards two-dimensional mathematical modelling of plate-type structural components, the fibre-reinforced material of which contains fibres that resist bending.

Some comments on the numerical analysis of plates and thin-walled structures

The paper briefly reviews the theoretical analysis of plates structures that might exhibit multiple ‘loading paths’ and highlights the need for engineers using non-linear numerical modelling to be aware of the multi-mode phenomenon and to ensure that the modelling is set up in such a manner that the various ‘loading paths’ and possible changes of path would be incorporated in the modelling response. The paper presents a simple example of numerical analysis of thin-plate buckling that involves ‘coupled buckling modes’ and provides comments on suitable methods for defining in a simple and straightforward way the numerical modelling that could ensure that results from computer analysis describe the physically correct relationship between applied loadings and deformations of thin-walled structural components.

Solving the problems of forecasting long-term strength under plane stress

This work is carried out in view of development of analytical approach to the time to fracture calculation of thin-walled structural components under long-term biaxial loading. The approach is based on the equivalent stresses concept. The calculation of the time to fracture of thin-walled tubular specimens under internal pressure, internal pressure with tension, pure torsion, torsion with tension is presented. A satisfactory agreement of the calculations with experimental data is obtained. The assessment of the agreement is performed by means of regression analysis and drawing of the confidence boundaries.

Experimental investigation of plastic collapse of aluminium extrusions in biaxial bending

In this work results of an experimental study on the bending collapse of thin-walled beams subjected to biaxial bending (i.e. moment with components with respect to the section principal axes) are reported. The objective of the present work is to get a fundamental understanding of the phenomenon with particular enphasis to the test loading conditions and to the identification of the most suitable parameters for the description of such a large rotation collapse. The test equipment, specifically designed and build to execute biaxial bending tests, is briefly described. A series of tests was made on thin-walled aluminium structural components, with square and rectangular section. The study is focused on the description of the collapse and the post-collapse behaviour under in-plane and biaxial loading conditions. The adopted loading procedures, of the constrained bending plane type allows to study the evolution of the resisting moment (and of its components) developped during the collapsing process. The behaviour was analyzed under two points of view: a global one, with the determination of the structural response as per generalized loads and displacements, and a local one, through the study of the kinematic of the plastic hinge mechanism.

Embossment formation in press forging of AZ31 magnesium-alloy sheets

Embossment formation in press forging of AZ31 magnesium-alloy thin-walled structural components used in the electronics industry was examined by both the finite element analysis and the experimental approach in the present study. The mechanical properties of the AZ31 magnesium-alloy at elevated temperatures were first obtained from the compression tests under various strain-rates. The compression test results suggest that a feasible press forging process of AZ31 magnesium-alloy needs to be conducted at a forming temperature higher than 300°C. The ring compression tests were also performed at various temperatures to obtain the friction factors at the die-blank interface under different lubrication conditions. The experimental results were then employed in the finite element simulations to investigate the effects of process parameters, such as punch shape and sheet thickness on the formation of embossments in a press forging process, and a modified punch shape was proposed to improve the efficiency of embossment formation according to the finite element analysis. Experiments of press forging of AZ31 magnesium-alloy sheets were conducted to validate the finite element analysis. Good correlations were observed between the finite element simulation results and the experimental data in all respects. The experiments conducted in the present study not only validated the finite element analysis but also confirmed the efficiency of the modified die design in helping the embossment formation. The mechanical properties of AZ31 magnesium-alloy at elevated temperatures and the detailed examination of the material flow on the formation of embossments made in the present study could provide a design guideline for a press forging process of AZ31 magnesium-alloy sheets.

THIXOMOULDING INJECTION MOULDING OF SEMI-SOLID MAGNESIUM ALLOYS

The thixomoulding technology, i.e. the injection moulding of thixotropic, semi-solid magnesium, represents a new, innovative and fully automated processing technique for magnesium alloys, which combines substantial processing elements of the magnesium die casting and the conventional plastics injection moulding. The process technology is very similar to the injection moulding of thermoplastics, except magnesium is being moulded. The two objectives pursued by thixomoulding are the process substitution of die casting of certain component parts on the one hand and the material substitution in special fields of plastics injection moulding on the other hand. Magnesium offers a considerable potential for material and structural lightweight construction. Thus, by means of the thixomoulding technology, a multitude of lightweight and mechanically high-grade net or near net shape parts can be manufactured, requiring no or very little finishing. Thixomoulded parts can be found in the areas of consumer electronics (for example housings for cell phones, laptops, cameras etc.) and in the fields of the automotive industry (in the form of thin-walled structural components). The thixomoulding process is environmentally friendly, since small amounts of argon can be used for an inert gas atmosphere in the mostly encapsulated plasticising unit, instead of ozone depleting gases, such as SF 6 , generally used in die-casting. At the same time, the process is operator friendly as well, since the danger of exposure to molten magnesium is minimised due to the closed machine.

Method of solving problems of the longitudinal dynamic bending of viscoelastic thin-walled structural components

Method of solving problems of the longitudinal dynamic bending of viscoelastic thin-walled structural components

Explicit Solutions for Plate Buckling Analysis

Solutions are presented for the analysis of the nonlinear buckling behavior of initially deformed plates subject to in-plane compressive and shear loading. It is shown that perturbation techniques can be used in conjunction with computer-oriented numerical methods to produce simple explicit polynomial series solutions for displacements and forces. The numerical method used is a finite difference approximation to the nonlinear equilibrium equations together with dynamic relaxation. Examples are given which illustrate the potential of the expressions for design of initially deformed thin-walled structural components.