COMPOSITE MATERIALS WITH PEAK STRAIN MEMORYCAPACITY

摘要

As composite laminates find ever greater application in safety critical components, itbecomes increasingly important to be able to measure the accumulated damage within these newcomposite structures. There has been no small amount of research into possible methods of monitoringaccumulated damage, including piezoelectric patches, fibre optics, and even the intrinsic piezoresistiveproperties of carbon fibres themselves. Some of the problems encountered by the above methods includemanufacturing considerations, as well as the complexities associated with some of the monitoringequipment needed to interpret damage levels. As an alternative avenue to the delicate systems above, amore robust concept has recently seen development, with simplified sensor equipment, and the possibilityfor smart element incorporation at primary manufacturing stage. The smart composite structural healthmonitoring system under development at the University of Natal is based on the embedding of metastableferrous alloy wires or films within the laminate. Not only can these elements easily withstand post-cureprocessing temperatures, but correctly used may impart significant load-bearing capacity. The detectionsystem is simple to the point of damage assessment being carried out by a person with no special training.The metastable ferrous alloys under discussion may be classed as smart because they possess a metastableaustenitic crystal structure at room temperature, but upon application of strain, this transforms to athermodynamically stable martensite. The fact that the austenitic structure is paramagnetic, and themartensitic structure is ferromagnetic, implies that there are levels of magnetic susceptibilitycorresponding to the percentage of martensite present in the material. Because this martensite forms inproportion to the amount of strain incurred by the material, a correlation between peak strain andmagnetic signature is possible. It should be emphasized that it is peak strain that is recorded within thematerial, because the transformation is irreversible. So, even if the strain level drops, the amount ofmartensite remains the same. What this in effect means is that the maximum strain incurred by thelaminate will be recorded, and this corresponds to the level of damage experienced by the laminate. Thelevels of sensitivity attained by the sensor elements are directly affected by two parameters: thetransformation characteristics of the material used as the sensor-element, and the sensitivity of themagnetic sensing equipment. Although strain memory alloys have been successfully developed and usedin other structural health monitoring systems, there are a number of challenges unique to the applicationof producing a smart composite laminate, since the volumes of martensite produced within such smallelements will be minute.