Quantitative ultrasonics


Ultrasonic non-destructive testing, as currently used in industry, is limited by its non-quantitative capabilities. In this sense, non-quantitative means that current technology is capable only of producing a signal that indicates the presence of a flaw, but it is unable to say anything about the characteristics of the flaw, e.g. its size, shape, orientation, and the m aterial of which it is composed (void or inclusion). This lim itation has been brought into sharp focus in recent years with the advent of fracture mechanics as a m ajor structural design and m aintenance philosophy. Since fracture mechanics is quantitative in nature, its effective utilization as an accept/reject criterion for flaws in materials and structures thus requires that quantitative inform ation be available from the non-destructive test procedures used to assure the design. W ith this lim itation in mind, the Defense Advanced Research Projects Agency (Darpa) and the Air Force M aterials Laboratory (A.F.M .L.) jointly initiated work at the Science Center, Rockwell International, to explore and to improve this situation. The work is structured to include both Science Center and university participants, and includes research and development in several areas necessary to achieve a quantitative capability. These areas include transducers, acoustic imaging, and defect characterization. In this paper principal emphasis is placed upon the defect characterization work that has been done. The approach that has been used is that of ultrasonic scattering, an approach which recognizes that several measurements must be taken at different angles and frequencies in order to acquire sufficient information to characterize a flaw. Items that are discussed include the design and preparation of a set of flawed samples in which the flaws are well characterized, theoretical developments that describe the ultrasonic scattering interaction with these flaws, experimental verification of these models, and inversion processes that have been developed to process the data and deduce flaw parameters from the ultrasonic measurements. A comparison of the deduced flaw param eters with the initially known values is given. Two other topics are discussed. One of these concerns recent developments in the analysis of long wavelength scattering which suggest that it should be possible to obtain stress intensity factors from ultrasonic measurements. The second item is concerned with recent developments in non-contact transducers (electromagnetic acoustic transducers (e.m.a.ts)) that are particularly related to weld inspection.