Nondestructive characterization of materials. A growing demand for describing damage and service-life-relevant aging processes in plant components
The nondestructive detection and characterization of damage to materials poses a challenge to instrumentation and inspection technology. A reliable method has yet to be developed anywhere in the world which is sufficiently rugged to be used on components in an industrial environment. The application-oriented studies conducted to date have demonstrated the complex overlapping of influences exerted on nondestructive measurement variables by the damage to be detected and by other material states and properties. The nondestructive detection and characterization of material damage will not offer any realistic chance of success until the material and component state is adequately characterized prior to the occurrence of the damage and variables are used which provide various types of information on the states and properties resulting from various interactions with the material. The process approaches and measurement techniques developed by the Institute for Nondestructive Testing are presented and evalualed below. The characterization of the initial state of a component with respect to its homogeneity and isotropy of its properties is possible by way of ultrasonic and magnetic techniques. Both techniques are also successfully employed in characterizing surface and bulk stress states in components. Adaptation of the sensor systems and material-specific preliminary testing are possibly required prior to actual application under practical conditions in individual cases. The same applies to the analysis of the anisotropy of material properties. Detecting areas of plastic deformation is possible via ultrasonic and magnetic techniques: quantification requires calibration, e.g. via a tensile test. By contrast, positron annihilation--a technique which is still in the laboratory stage--offers the advantage of being independent of residual stress for the most part. The detection and characterization of creep damage with the required degree of detection sensitivity is not possible to date. The potential of ultrasonic and magnetic techniques for the early detection of damage, i.e. for reliably detecting advanced porosity, is limited. It seems necessary not only to improve methods for detecting pore formation, but also to utilize other creep-induced structural changes via instrumentation and monitoring technology. Experimental studies for detecting hydrogen-induced embrittlement and stress-corrosion cracking demonstrate the possibilities offered by electric and magnetic techniques. In summary, the present level of knowledge and state of the art have to be evaluated so that the possibilities and limitations of the individual methods are recognized and the detection potential improved by the combined utilization of several techniques.