Newsletter October 2002
Structural Health Monitoring in Marine Structures
Paper presented at the June 2002 Offshore Mechanics & Arctic Engineering conference in Oslo, "Structural Health Monitoring in Marine Structures", Belinda Burton, Viktor Verijenko, School of Mechanical Engineering, University of Natal, Durban, SA.
Marine structures are both costly and often have the capacity to effect loss of human life if they fail. But, as well as being safety critical in nature, these structures must also endure extremely adverse conditions. Out of this same concern for structural integrity, was born the science of Structural Health Monitoring.
Currently under investigation are a group of smart materials, which can be used to monitor the actual stresses and strains in an inexpensive and reliable manner, without having to employ conventional NDT methods, which usually require components to be taken out of service for inspection, as well as requiring specially trained personnel. Structural Health Monitoring based on these smart. materials, by comparison, can be performed without any interruption to the operation of equipment, and may be carried out by anyone who is semi-literate in the operation of technical equipment. The reliability or safety of structures may be determined in a number of ways, but one of the most accurate must surely be by direct measurement of the stresses or strain incurred by the infrastructure. Structural Health Monitoring (SHM) as it has become known, is usually accomplished by means of sensor placement at strategic locations within the given structure. While some may argue that this becomes expensive, a new method of monitoring structural health may make use of cost effective smart materials, which possess not only monitoring capabilities, but also significant load-bearing capabilities.
Research is being conducted on various metastable alloys, which exhibit an austenitic phase at room temperature, but will transform to a martensitic phase structure upon application of strain. The martensitic phase has not only a greater strength than the austenitic phase, but also displays significant ferromagnetism, while the austenite is paramagnetic. Since the martensite forms in direct proportion to the strain experienced by the material, the magnetic signature increases in direct proportion to the strain. A measurement of the magnetism therefore yields a strain reading.
Because some of these metastable alloys can be produced in forms, which possess significant strength, various key structural members may then be replaced with the smart material generating self-monitoring components. Only the peak strain is recorded within the material though, since the transformation is irreversible. This peak strain is however usually the limiting variable in determining the remaining life of the structure. The varying degrees of strength, corrosion resistance and sensitivity to strain, which can be engineered into the material, mean that a structural element correctly designed using either a metastable alloy or TRIP steel, forms the perfect passive peak strain sensor. This becomes particularly useful in inaccessible situations where non-destructive assessment is required but difficult to carry out in-situ.
Contacts: burtonb@nu.ac.za, verijenk@ac.za
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