Smart materials and structures : facts or dream ?

Institut National des Sciences Appliquées (INSA), Lyon

Abstract

The multifunctional, heterogeneous or composite materials combine, given this heterogeneity, properties which appeal- to be contradictory such as high mechanical stength, low density, chemical innocuousness and relative toughness...

This trend toward controlled complexity, to a “custom-designed” synthesised material system, was able to provide an quasi-exact answer to the requirements of the designer, but is nowadays no longer considered to be sufficient, and is not achieved in any case.

In fact, the requirements set out by the user and formulated by the designer are “mean” requirements which correspond to standard conditions of use.

It is therefore clear that the barrier to be crossed henceforth is that to the “adaptive” material, which is capable of adjusting its behavioural law relative to external influences, and in real time if possible. This adjustment assumes an intake of information from the environment and/or the material itself, subsequent processing of this data to produce a suitable response, and finally implementation of the response.

The analogy with living organisms is obvious, and it was this which led, during the 1 980's, to the notion of “intelligent” materials.

Though the appelation “smart materials” is employed quite widely, it is evident that this term does not have the same significance for everyone.

In western countries, the term “intelligent materials” is used mainly to refer to intelligent structures which are formed by the assembly of macroscopically distinct materials, such as a variety of sensors for data-acquisition, microprocessors to process these data, and actuators for implementing the response.

The “material” notion appears again if the components are incorporated into a composite material, and if the sensor and actuator functions are no longer separate but rather distribued, as is the case where optical fibre sensors are employed and shape-memory alloys or piezo-electric materials are used as actuators.

In this type of “materials system”, intelligent functions are created by the assembly of components on the mesoscopic scale (tens of um to hundreds of um), but it is possible to imagine a true material where the sensor and actuator functions exist on the microstructure scale (a few um 10 4 to a few um).

From a rapid analysis, it can be seen that the innovative aspect of the concept of intelligent materials concerns essentially the combination of primitive functions, aimed at endowing the material with a semi-biological behaviour.

This approach fits well into the current logic of the multiple-function “customised” material, but an additional threshold has been crossed, where the question no longer concerns the addition of primitive functions, but rather their combination in order to achieve a more elaborate function which can be described as “quasi-intelligence”, that is one which ensures the survival or the optimum use of the material and the system.