Earthquake protection in four dimensions

Who is not familiar with the tedious task of bending bent spectacle frames back into shape? If the frame is made of a shape-memory alloy though, the wearer only has to put the bent glasses in warm water – and they quickly revert to their old shape. Is it possible to transfer this material property to architectural structures too?

Image: Ali Jafarabadi

Tensioning bridges with heat

For spectacle frames, alloys of nickel and titanium are used. These are highly elastic, but their shape alters at body temperature. This is too low for architectural structures, as mere sunlight would then be enough to trigger a change in shape. Moreover, nickel-titanium alloys are expensive. Thus, iron-based products are of interest – reinforcement bars with shape-memory properties, so to speak. To develop a memory effect, however, conventional iron alloys have to be heated to over 400 degrees Celsius, which is harmful for concrete. Christian Leinenbach, Head of Advanced Processing and Additive Manufacturing of Metals at Empa in Dübendorf, has developed a new type of alloy based on iron, manganese and silicon, which develops the memory effect between 100 and 200 degrees Celsius. This temperature is ideal for concrete structures, as it cannot even be reached in bright sunshine, whereas it is still not high enough to damage the concrete.

Reinforcement bars made from this new alloy can be used to pre-tension bridges: The metal bars are installed in the bridge while in a state that does not correspond to their memory and thus not to the ideal state. Once the bars are heated with electricity or a radiant heater, they spring back into their preferred shape. The bridge thus lifts slightly and is then stabilised – or pre-tensioned. Laborious mechanical pre-tensioning is no longer necessary.

Playing with dimensions

Even better results are achieved if the reinforcement bars made from this new kind of shape-memory steel are printed: This process combines layer-by-layer assembly, or 3D printing, with a fourth dimension – the memory property. Reversion to the favourite or ideal state is triggered by heating. This is printing in four dimensions, or 4D printing.

The full potential of the procedure is achieved for architecture if lattice structures are printed using shape-memory steel. 4D printing allows unique combinations of complex geometries and shape alteration. Or in the words of Ali Jafarabadi, doctoral student at Empa: “4D printing makes dreams come true.” Irene Ferretto, a doctoral student in Christian Leinenbach’s team who is significantly involved in development, proudly presents a small sample lattice. If scaling up is successful, such 4D-printed lattices could be used, for example, to secure architectural structures against earthquakes: The lattice absorbs the shock waves and can be restored to its original state by applying heat. And not just once, but again and again. Irene Ferretto is already thinking one step further and researching ways of making the mechanical properties differ within the lattice. Some day, areas of high flexibility are set to alternate with areas of maximum stability – and everything will be printed in a single step, using memory alloys. In such visionary projects, setbacks are inevitable. Irene Ferretto knows how to deal with them: “There is no sense in being afraid of mistakes. The important thing is to learn from mistakes and draw the right conclusions.”

Comprehensive and further information on the subject can be found in the article 4D printing.

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