Such changes are limited in the case of composition and can be significant in the other two. The performance and life of such components will be seriously affected by these changes. To name a few examples, creep damage in boiler tubes operating at elevated temperatures is caused by microstructural changes and fatigue damage in rotating components are caused by residual stresses. Though the importance of changes in the material characteristics on the performance and functional properties are well-recognised and even understood, the means to monitor them in service are limited. In many cases, destructive techniques are the only solution.
Considerable research is in progress to develop such techniques.
A number of different NDE techniques based on ultrasonic, magnetic and X-ray or neutron diffraction principles have demonstrated their suitability for assessment of residual stresses. However, each technique has its own advantages and limitations. In spite of a higher speed and greater flexibility of measurements, use of both ultrasonic and magnetic methods is limited due to the strong impact of the microstructural effects on the stress measurements. When suitable means can be found to separate and quantify the individual contributions of microstructure and stresses, both techniques can offer much more attractive and wider industrial applications. When an integrated system is developed which incorporates the most reliable aspects of each of the techniques, a powerful inspection and condition-monitoring tool will emerge.