| Project Team | Institution | Role | ||
| Mr Chris Avdalis | The University of New South Wales | Undergraduate Student | ||
| Professor Matthew Barnett | Deakin University | Chief Investigator | ||
| Professor Yuri Estrin | Monash University | Chief Investigator | ||
| Professor Mark Hoffmann | The University of New South Wales | Chief Investigator | ||
| Ms Maizlinda Idris | The University of New South Wales | Postgraduate Student | ||
| Mr Kaveh Kabir | The University of New South Wales | Postgraduate Student | ||
| Dr Sreekumar Vadakke Madam | Deakin University | Research Fellow | ||
| Ms Arwa Tawfeeq | Deakin Univeristy | Postgraduate Student | ||
| Dr Tania Vodenitcharova | The University of New South Wales | Research Fellow | ||
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Executive Summary
Progress has been made towards understanding and developing hybrid foam and truss cored sandwich panels that are resistant to damage. Models of stress, strain and damage of thin foam cored panels have been developed. Preliminary design analysis and early experiments point to the prospect of exploiting optimum core-shell hybrid structures for the struts of formable truss cored sandwich panels.
Progress has been made towards understanding and developing hybrid foam and truss cored sandwich panels that are resistant to damage. Models of stress, strain and damage of thin foam cored panels have been developed. Preliminary design analysis and early experiments point to the prospect of exploiting optimum core-shell hybrid structures for the struts of formable truss cored sandwich panels.
Project Aims/Targets
The aim of this project is to propose material design strategies to increase the tolerance of foam and truss sandwich panels to deformation and damage. A major sub-aim is to develop mathematic models that are able to describe the material behavior to the extent required to permit optimal solutions to be identified.
The aim of this project is to propose material design strategies to increase the tolerance of foam and truss sandwich panels to deformation and damage. A major sub-aim is to develop mathematic models that are able to describe the material behavior to the extent required to permit optimal solutions to be identified.
The appeal of sandwich panels comprising hybrid metal-air core structures is in their weight to stiffness ratios and the potential for multifunctional (e.g. heat transfer and energy absorption) use in structural applications. Sandwich structures with a foam core are stochastic in nature – the cell and wall sizes vary statistically over the materials – and support applied tractions by localized bending. Those with truss type cores are periodic in nature. They are ‘regular’ and resist applied forces in the core by compressive or tensile reactions of the struts.
Though both have been quite intensely studied of late there is a need to understand how these materials behave in the non-perfect condition after experiencing some ‘damage’. For the case of foam cores, we are interested in increasing resistance to the sort of damage that might be encountered in service, such as a bend or an indentation. In the instance of truss cores, we are concerned with raising the extent to which moderate degrees of plastic forming can be applied, without prohibitive degradation of performance. These improvements to damage resistance promise to increase the opportunity to exploit lightweight metal sandwich structures in energy saving applications.
We have made considerable progress towards understanding the nature of the damage process of thin foam cored aluminium sandwich panels with thin skins.
First, the structural response of sandwich panels consisting of a commercial closed-cell foam core and thin aluminium sheet skins were examined under static three-point bending loading. Panels of different thicknesses and span lengths were tested, and the influence of the foam density, core thickness and skin type on the response was revealed. The failure modes in bending were greatly dependent on the span length but independent on the foam thickness. For short spans, the deformed shape at failure was asymmetric (see Figure B5 below), as opposed to a symmetric mode for long spans. Damage by shear cracking and cell collapse was observed. The density and thickness of the foam core, the presence of reinforcing face sheets and the beam span determined the failure load and bending strength of the sandwich panels.
Though both have been quite intensely studied of late there is a need to understand how these materials behave in the non-perfect condition after experiencing some ‘damage’. For the case of foam cores, we are interested in increasing resistance to the sort of damage that might be encountered in service, such as a bend or an indentation. In the instance of truss cores, we are concerned with raising the extent to which moderate degrees of plastic forming can be applied, without prohibitive degradation of performance. These improvements to damage resistance promise to increase the opportunity to exploit lightweight metal sandwich structures in energy saving applications.
We have made considerable progress towards understanding the nature of the damage process of thin foam cored aluminium sandwich panels with thin skins.
First, the structural response of sandwich panels consisting of a commercial closed-cell foam core and thin aluminium sheet skins were examined under static three-point bending loading. Panels of different thicknesses and span lengths were tested, and the influence of the foam density, core thickness and skin type on the response was revealed. The failure modes in bending were greatly dependent on the span length but independent on the foam thickness. For short spans, the deformed shape at failure was asymmetric (see Figure B5 below), as opposed to a symmetric mode for long spans. Damage by shear cracking and cell collapse was observed. The density and thickness of the foam core, the presence of reinforcing face sheets and the beam span determined the failure load and bending strength of the sandwich panels.
A study on the indentation of foam-only panels using numerical methods (FEA software ABAQUS) was also commenced. The effects of the material constants (found through simulation of uniaxial compression and verification against the experiments) and friction on the indentation results were carefully investigated; also the effect of the type of hardening law (isotropic or volumetric). It was found that those variables had no significant effect on the predictions. For all possible combinations of material constants, the indentation load was found to be underestimated, although qualitatively in agreement with the experimental values.
An investigation was undertaken on sandwich panels locally dented by quasi-static indentation with hemispherical indenters and subsequently subjected to four-point bending loading. The aim was to investigate the remnant bending strength of the damaged panels after inducing a controlled localised damage. The indentation damage was located on either the compressive or on the tensile side of the panel. Both hard and soft skin types were considered. It was found that the panels deformed and failed in a similar way to the pristine sandwich panels – 8 plastic hinges formed in the skins about which the panels rotated, and the foam core between the inner and the outer spans were sheared (Figure B7). An interesting result was attained – local damage that had not induced skin failure, did not show a considerable effect on the load bearing capacity of the sandwich panels in bending. The explanation is in the failure modes of the damaged panels, which remain the same as for the undamaged panels.
(c) Tensile forces in the skin and the resistance of the foam are shown schematically.
Work on truss cored panels has commenced with a review of the relevant literature. Studies to date reveal that pyramidal truss structures lend themselves to ready manufacture through sheet punching, bending and brazing operations. It is also evident that the failure of these materials, though compassing a range of modes, often involves the buckling of struts. For very low density cores (ie. those with long slender struts) elastic buckling is important. For squatter struts, plastic buckling controls the maximum load prior to failure.
Results of our analysis to date are as follows. To permit the commonly flat truss sandwich panels to be subjected to some degree of plastic forming it is required that the struts be as resistant to plastic buckling as possible. There are two obvious ways to achieve this; one, make the struts squatter and sacrifice weight saving, two, design/select the strut material for high resistance to plastic buckling. Work on the latter has commenced work.
Resistance to plastic buckling – or the maximum plastic strain that can be imparted without causing plastic buckling – is controlled, for all else constant, by work hardening. Thus high work hardening metals such as brasses and copper can resist buckling as a taller more slender column (strut) than can aluminium. A simple analysis based on the Euler buckling equations, as applied to plastic buckling, was performed and the maximum column height to diameter ratios for aluminim copper brass and steel are shown in Figure B8. In the course of this analysis it was discovered that if a simple core-shell metal hybrid column were constructed, considerable gains in plastic buckling resistance can be achieved. This is illustrates in Figure 32 for a hypothetical aluminium-brass hybrid. It is even possible to attain plastic buckling resistance in solid columns greater than either of the two component metals. Preliminary experiments support the reality of this effect and further analysis of the data is continuing.
Results of our analysis to date are as follows. To permit the commonly flat truss sandwich panels to be subjected to some degree of plastic forming it is required that the struts be as resistant to plastic buckling as possible. There are two obvious ways to achieve this; one, make the struts squatter and sacrifice weight saving, two, design/select the strut material for high resistance to plastic buckling. Work on the latter has commenced work.
Resistance to plastic buckling – or the maximum plastic strain that can be imparted without causing plastic buckling – is controlled, for all else constant, by work hardening. Thus high work hardening metals such as brasses and copper can resist buckling as a taller more slender column (strut) than can aluminium. A simple analysis based on the Euler buckling equations, as applied to plastic buckling, was performed and the maximum column height to diameter ratios for aluminim copper brass and steel are shown in Figure B8. In the course of this analysis it was discovered that if a simple core-shell metal hybrid column were constructed, considerable gains in plastic buckling resistance can be achieved. This is illustrates in Figure 32 for a hypothetical aluminium-brass hybrid. It is even possible to attain plastic buckling resistance in solid columns greater than either of the two component metals. Preliminary experiments support the reality of this effect and further analysis of the data is continuing.
A core-rim hybrid of aluminium and brass is also shown.
Future Activity Plan
Foam core modeling will be extended to link damage with subsequent performance and truss core design will continue to explore the potential to create work hardening structures by exploiting internal architectures and hybrid solutions.
Foam core modeling will be extended to link damage with subsequent performance and truss core design will continue to explore the potential to create work hardening structures by exploiting internal architectures and hybrid solutions.