Nature struts its stuff

Tom Shelley reports on some of the latest material fabrication ideas originally prototyped by nature



Pultruding composite hollow tubular struts – with unfilled passages in the walls – are enabling the fabrication of extremely light, but stiff, struts.
While many biological structures are almost impossibly complicated to fabricate, advanced manufacturing techniques such as pultrusion and additive methods now permit manufacture at reasonable cost.
Formula 1 and aerospace are obvious applications for the struts, but there are many other applications, such as wind-resisting wind turbines, turbine supports and radar antennae that might well benefit from adapting some of the ideas found in nature.
Biologically inspired constructions were just one of the themes at the recent Biological Approaches for Engineering conference, organised by the Institute of Sound and Vibration Research at the University of Southampton.
It was here that Dr Markus Milwich, from the Institute of Textile Technology and Process Engineering in Denkendorf, unveiled a plant-inspired strut that he had actually made out of fibre-reinforced epoxy.
Horsetails and giant reeds inspire his ideas. Horsetail stems have internal and external cylinders, with spacers between them, and show a very high, specific bending stiffness and buckling stability at minimal weight. They might be regarded as a series of double ‘T’ beams, welded together. The giant reed arundo donax, on the other hand, which is also hollow-stemmed, shows a gradual stiffness transition between fibres and matrix tissue that results in both excellent damping and a gradual breaking behaviour that includes a series of what Milwich describes as “pre-failure incidents”.
Composite struts showing both kinds of behaviour have been experimentally manufactured at Denkendorf by a patented version of a process called ‘Braid pultrusion’. In this method, fibre yarns are drawn through a matrix impregnation bath and a heated pultrusion tool. The matrix of the impregnated fibres is cured as they are pulled through the tool. Spiral-shaped fibres can be produced in the profile by incorporating a braiding machine into the pultrusion line to provide torsional stiffness, high vibration damping and favourable structural integrity. The team has investigated structures based on thermoplastic and polyurethane foam matrices, as well as epoxy. They see applications in the aerospace, automotive, construction, medical prosthetic and sports equipment industries.
Dr Carlo Santulli, formerly a member of Reading University’s Biomimetics Group, but now at the University of Rome, has devised a hollow stem structure, with two types of plant cells and fibres running down the outside. Professor Julian Vincent, also once at Reading but now at the University of Bath, says that, while softwoods have one type of cell, hardwoods generally use two – large and small – and that wood structures are often very complicated, with many layers of hierarchy.
However, whereas as one time such structures would have been impracticable to manufacture, he observes: “You can now make complex materials by additive methods.”
It has also been pointed out that it is therefore possible to come up with naturally inspired structures that are stiff on some directions, but compliant in others – so they could, for example, bend with the wind.

Pointers

* Plant stems may be hollow and of complex construction, in order to achieve exceptional stiffness to unit weight ratios along with benign failure modes

* Such structures can now be produced artificially as composites, by using a type of braid pultrusion

* Even more complex structures, inspired by nature, can now be manufactured using additive methods

Author
Tom Shelley

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