Traditional composites consist of many small fibres embedded in a matrix. Such composites are stiff or strong because they contain fibres that have these inherent qualities.
Taking a somewhat different tack, researchers at the Aerospace Corporation in the US are embedding tiny ‘machines’ into the matrix with the result that the ‘machine augmented composites’ take on the characteristics of the ‘machines’ themselves.
To demonstrate the concept, the researchers created a composite matrix that is composed of a very large number of simple ‘stress-conversion machines’ that convert shear forces into tensile/compressive forces and vice versa.
An optical micrograph taken from a thin slice of the material placed between crossed polarisers highlights the difference between the ‘machines’ and the matrix. The machines are made out of nylon and are hollow (filled with air). A polyester scrim cloth that was placed between the plies of the machines as a manufacturing aid can also be seen.
Because of the shape of the machines and their mechanical properties, when a compressive displacement is applied to the material, the material distorts in shear. Conversely, when a shear force is applied, the material modifies the force to have a compressive component.
The Aerospace researchers say that embedding simple machines like these in a matrix will allows material scientists to design materials with specific properties.
For example, when these so called ‘z-type machines’ are employed, the material distorts in shear when compressed. Converting compressive-to-shear displacements, and vice versa, can be useful in applications where it is advantageous to change the size of a part when it is under load. Gaskets and seals, for example, may be made to increase in thickness when under shear forces caused by thermal expansion of the sealing surfaces.