Taking the hits without flinching

Tom Shelley reports on a new means of creating composites to absorb impacts, regardless of whether they are delivered fast or slow



Wound, hollow fibre tubes, with a winding angle of just 15 [degrees], form the basis of composites that can absorb about twice the impact per unit weight of conventional laminated fibre reinforced composites.
Based on a discovery made more than 20 years ago, interest has been re-awakened recently in light of increasing concerns about terrorist threats, although the technology is equally applicable to the construction of components for cars and aircraft.
The latest innovation is the brainchild of Professor George Jeronimidis in the School of Construction Management and Engineering at the University of Reading. And while he acknowledges the part played by colleagues in their development, the material has become known as 'George’s Wood'.
The excellent impact absorption capabilities of wood are well known. However, in the past, they have been poorly understood, because wood is such a complex material. In a paper published with JE Gordon in 1980, Jeronimidis pointed out that the principle load-bearing component in wood is the cell walls. These exist as hollow tubes, whose secondary layers are made up of helically disposed cellulose fibrillae, wrapped around the cells at angles that vary from 10 to 40 [degrees]. When tubes helically wound with elastic fibres are pulled, they can undergo what is termed ‘tension buckling’, which the paper says results in “a very large irrecoverable absorption of energy”.
The Reading researchers then built a special machine that enabled tubes to be wound, in which the fibre winding angle could be varied. After it was impregnated with a suitable resin, cured, then loaded in tension and prevented from rotating, the tubes buckled and showed an elastic-plastic behaviour very much like that of wood cells -- with maximum energy absorption occurring at a winding angle of 15 degrees.
This discovery has led to the development of composites with a specific toughness of 820 – far greater than tough steels, which weigh in at 156, aluminium at 150, oak at 10 and carbon fibre reinforced plastic (CFRP) at 0.6. However, the material only has a similar stiffness to oak (17 compared with 15), and is well below the 87.5 of CFRP.
“The smallest tubes we ever made were 1mm in diameter and, at the upper end, could be 5-6mm in diameter,” states Jeronimidis. “We have made samples for low- and high-velocity impact.”
The main potential applications are seen in absorbing impact, whether as general bullet-proof shielding -- such as is required for cockpit doors, ballistic riot shields or personal body armour -- or for armoured seat backs for use in public transport, other transport protective panels, and high-performance packaging and security boxes. On account of its good damage tolerance, retaining what is said to be 90% performance (compared with 50% for a typical CFRP), it can be joined using screws and nails, just like wood.
The need now, it seems, is to secure an industrial partner with a specific target application.
“It’s been going around, but it really needs someone to focus their mind on what they want it to do,” concedes Jeronimidis. “It’s very difficult to move forward without a focus.”

Pointers

* Reinforcing plastic with helical fibre tubes has produced materials that can absorb very large amounts of impact energy before they break

* The optimum fibre winding angle is 15 degrees

* Applications are seen in security applications, defence, aerospace and motor vehicles, but the team is now looking for industrial partners so it can focus on specific target markets

Author
Tom Shelley

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