Showing papers on "Time-of-flight diffraction ultrasonics published in 1999"

Determining Crack Depth and Measurement Errors Using Time-of-Flight Diffraction Techniques

Abstract: The objectives of this study were to demonstrate the capability of the time-of-flight diffraction technique for measuring the depth of surface-opening cracks in reinforced concrete structures and to determine errors in measurement. In this technique, two receivers were placed on opposite sides of a surface crack to monitor surface disturbances caused by the arrivals of stress waves generated by impact. One receiver and the impact generator were located on the same side of the crack. This receiver was used to trace the start time of the impact. The second receiver, placed on the opposite side of the crack, was used to find the arrival time of the P-wave diffracted from the bottom edge of the crack. Thus, the travel time of the P-wave from the impact point through the crack tip to the second receiver could be obtained, which can help determine the crack depth. Two reinforced concrete beams were constructed as experimental specimens. Cracks were created by applying loads to these specimens. The depth of the cracks was measured by the time-of-flight diffraction technique first. Subsequently, verification of the measured results was performed by drilling cores. Experimental results showed excellent agreement between the measured depth and the true depth. Because this technique is based on using digital signal measurement and analysis, there exist errors inherent in the digital measurement, as well as in the method of signal analysis. Factors that affect the accuracy of crack depth measurement are discussed. It was shown that the maximum value of error in crack depth measurement depends on the errors in determination of wave speeds, the errors in positioning the impactor and the receiver, the sampling interval of data, and the depth of the crack itself.

Modelling the ultrasonic inspection of smooth and rough planar defects [and nuclear power plant application]

Abstract: A suite of computer codes has been developed for the theoretical modelling of ultrasonic inspection in ferritic steel. The models predict the echo amplitudes from postulated planar defects in direct pulse-echo, "corner effect", tandem or TOFD configurations. In general the defects are assumed to be smooth, but for direct pulse-echo a model is also available for rough defects. The models are mostly used to establish the capability of a proposed inspection procedure to detect hypothetical defects which could be of structural concern. They have an important role in providing evidence for technical justifications when qualifying inspections using the methodology described by ENIQ (the European Network for Inspection Qualification). This paper will describe the range of models currently available in the authors' companies, concentrating on recent developments. In particular the important issue of the validation of models against experiment will be discussed. Finally, some typical examples of use of the models will be briefly described.

Deconvolution Method for TOFD Technique

Abstract: Time of flight diffraction(TOFD) method is used in nondestructive tests of piping and pressure vessels because of its advantages over a pulse echo technique: its speed, objectivity, repeatability and its insensitivity to specimen surface conditions and discontinuity orientation. But it is the one of weak points in TOFD method that it has the dead zone in sub-surface resolution induced by lateral waves. We solved the dead-zone problem near the sub-surface by using the deconvolution method and the developed ultrasonic testing system showed high performance.