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 materia ls poses a challenge to instrumentation and inspection technology. A r eliable method has yet to be developed anywhere in the world which is sufficiently rugged to be used on components in an industrial environm ent. The application-oriented studies conducted to date have demonstra ted the complex overlapping of influences exerted on nondestructive me asurement variables by the damage to be detected and by other material states and properties. The nondestructive detection and characterizat ion of material damage will not offer any realistic chance of success until the material and component state is adequately characterized pri or to the occurrence of the damage and variables are used which provid e various types of information on the states and properties resulting from various interactions with the material. The process approaches an d measurement techniques developed by the Institute for Nondestructive Testing are presented and evaluated below. The characterization of th e initial state of a component with respect to its homogeneity and iso tropy of its properties is possible by way of ultrasonic and magnetic techniques. Both techniques are also successfully employed in characte rizing 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 in dividual cases. The same applies to the analysis of the anisotropy of material properties. Detecting areas of plastic deformation is possibl e via ultrasonic and magnetic techniques: quantification requires cali bration, 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 detect ion and characterization of creep damage with the required degree of d etection sensitivity is not possible to date. The potential of ultraso nic and magnetic techniques for the early detection of damage, i.e. fo r 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 an d monitoring technology. Experimental studies for detecting hydrogen-i nduced embrittlement and stress-corrosion cracking demonstrate the pos sibilities offered by electric and magnetic techniques. In summary, th e present level of knowledge and state of the art have to be evaluated so that the possibilities and limitations of the individual methods a re recognized and the detection potential improved by the combined uti lization of several techniques. (C) 1997 Elsevier Science S.A.