Forces at work


“One reviewer went as far as to call it ‘black magic’.”

Professor Sinniah Ilanko

Engineering Professor Sinniah Ilanko is interested in the stability and vibration behaviour of mechanical and structural systems.

He says structures, like people, undergo more stress when they are less flexible. "So sometimes it is useful to introduce some flexibility in structures to reduce stress levels to ensure they don't fail. Modelling structural supports is often a challenge. In what is called the 'penalty method', a support is modelled as a very stiff spring, but until recently there remained an uncertainty in choosing the right stiffness.

Values that are too high cause problems with rounding off errors and lower values may permit support movements.

For about 60 years researchers have been grappling with this problem. But, through an error in his PhD, picked up years later by one of his students, Professor Ilanko has found a way to refine the Penalty Method. 

His mistake was to put a negative symbol instead of a positive one in front of a 'mass' in a frequency calculation, which led to his discovery that negative stiffness and negative mass produced similar results if their magnitudes are large enough to make the supports effectively rigid. 

"One reviewer went as far as to call it 'black magic', so in engineering circles 'negative mass' is probably still quite controversial but it is heartening to see it being cited and used in a biology journal," says Professor Ilanko.

The idea of negative stiffness and mass led to a new concept of negative structures.

Professor Ilanko has secured a three-year Marsden Fund grant to study the use of negative structures in modelling voids, cut-outs and holes that are commonly encountered in machine components.