Scientists Tsu-Wei Chou and Erik Thostenson say that the network acts "much like the nerves in a human body".

"Nanotubes are so small that they can penetrate the areas between the bundles of fibre and between the layers of the composite, in the matrix-rich areas," said Thostenson. "If there is a microcrack, it breaks the pathway of the sensors and we can measure the response."

The researchers incorporated the nanotubes in glass fibre-epoxy composites before carrying out a variety of structural tests. Typically the nanotubes made up just 0.15% of the total volume of the composite.

The resistance of the nanotube network changed in the presence of matrix microcracks between fibres, interlaminar delamination in unidirectional composites or transverse microcracking in cross-ply laminates. Microcracking of a composite's polymer matrix can affect factors such as fatigue life and damage tolerance.

The technique could have applications both in laboratory testing of the effectiveness of self-healing systems for polymer matrices and in monitoring composite damage in situ, such as in aeroplane parts. According to the researchers the method can detect the "onset, nature and progression of damage".

Testing for self-healing systems currently involves measuring residual stiffness, but this may still be high despite considerable cracking in the composite matrix. The change in resistance of a carbon nanotube network should provide a more sensitive test.

The researchers reported their work in Advanced Materials.