Lattice materials get tough
The failure mechanics of steel lattice materials have been explored by a team of engineers from Cambridge University in the UK.
The work aims to quantify the materials’ resistance to crack initiation, potentially leading to improved energy absorbing structures, with applications in defence, in particular, as a material for improved body armour.
‘The potential [of lattice materials] is largely unexplored’, says Professor Norman Fleck, the principle investigator working on the project. ‘In particular, the interplay between connectivity and imperfection in dictating macroscopic structural response.’
The scientists have explored the impact on stiffness, strength, macroscopic toughness and fatigue strength of 2D and 3D lattice materials using analytical and finite element (FE) calculations.
The results show that imperfections, such as missing cell walls, randomly dispersed nodes and cell wall waviness, will degrade properties such as the tensile and fatigue strength, and that structures of low connectivity deform by cell wall bending regardless of the degree and type of imperfection.
However, the role of imperfections is unclear for highly redundant lattices of high connectivity, explains Fleck. ‘It is anticipated that lattices of high connectivity have sufficient redundancy to remain stretching-dominated in the presence of imperfections such as missing cell walls. Consequently, they may be defect tolerant’, he adds. ‘However, it is possible that unzipping modes of crack advance will occur, and this requires further investigation.’
Fleck and his team have developed a fracture mechanics framework to show that lattice materials could be reinforced using a small proportion of strong, ductile bars made from ABS plastic, which blunt advancing crack fronts.
Finite element calculations have been performed on the fracture resistance of hierarchical lattice structures to test this. Existing rapid-prototyping machines can be used to construct such reinforced materials, says Fleck.
The team are also looking at other types of filler materials for this application. ‘Multifunctional benefit for sandwich structures can be accrued by filling empty space within the core with foam, rockwool fibres, low density cement or an aggregate of polymer spheres’, says Fleck. ‘These fillers give enhanced local strength during an impact event by providing lateral support to the lattice and thereby delaying the buckling of the core struts. Furthermore, the core is expected to give multifunctional benefits by increasing the vibration and acoustic damping capacity of the sandwich panels, compared to those without the filling materials’.Materials World Magazine, 01 Jun 2010
The work aims to quantify the materials’ resistance to crack initiation, potentially leading to improved energy absorbing structures, with applications in defence, in particular, as a material for improved body armour.
‘The potential [of lattice materials] is largely unexplored’, says Professor Norman Fleck, the principle investigator working on the project. ‘In particular, the interplay between connectivity and imperfection in dictating macroscopic structural response.’
The scientists have explored the impact on stiffness, strength, macroscopic toughness and fatigue strength of 2D and 3D lattice materials using analytical and finite element (FE) calculations.
The results show that imperfections, such as missing cell walls, randomly dispersed nodes and cell wall waviness, will degrade properties such as the tensile and fatigue strength, and that structures of low connectivity deform by cell wall bending regardless of the degree and type of imperfection.
However, the role of imperfections is unclear for highly redundant lattices of high connectivity, explains Fleck. ‘It is anticipated that lattices of high connectivity have sufficient redundancy to remain stretching-dominated in the presence of imperfections such as missing cell walls. Consequently, they may be defect tolerant’, he adds. ‘However, it is possible that unzipping modes of crack advance will occur, and this requires further investigation.’
Fleck and his team have developed a fracture mechanics framework to show that lattice materials could be reinforced using a small proportion of strong, ductile bars made from ABS plastic, which blunt advancing crack fronts.
Finite element calculations have been performed on the fracture resistance of hierarchical lattice structures to test this. Existing rapid-prototyping machines can be used to construct such reinforced materials, says Fleck.
The team are also looking at other types of filler materials for this application. ‘Multifunctional benefit for sandwich structures can be accrued by filling empty space within the core with foam, rockwool fibres, low density cement or an aggregate of polymer spheres’, says Fleck. ‘These fillers give enhanced local strength during an impact event by providing lateral support to the lattice and thereby delaying the buckling of the core struts. Furthermore, the core is expected to give multifunctional benefits by increasing the vibration and acoustic damping capacity of the sandwich panels, compared to those without the filling materials’.Materials World Magazine, 01 Jun 2010
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