NONDESTRUCTIVE MEASUREMENT OF THE RESPOSE OF REAR-WALL FLAWS IN THICK-WALLED STRUCTURES USING DC FOUR-POINT-PROBES TECHNIQUE
Repeatable and accurate evaluation of flaws from the inaccessible sides of thick-walled structures is still remained as one of the hurdles in the field of NDE of flaws. The present paper describes the development of an effective DC potential-drop measuring system for evaluating the response of rear wall flaws, especially in thick-walled structures, in which an ideal averaging scheme is introduced with the help of an adjustable fourpoint-probes system. The capability of the present measuring system is extended here to the evaluation of rearwall cracks as well as the reduction of wall thickness for the case of thick stainless steel structures. The reliability as well as accuracy of the measured potential drop responses is verified by comparing them with the corresponding responses obtained by theoretical analysis. The potential-drop method of evaluation is based on the principle that the electrical resistance of a body/location changes due to the presence of a geometrical defect or the change in the local electrical property of the material ( i.e., conductivity); the corresponding electrical potential across the defect/flaw is thus measured by sending a pulsed current to the test piece and compared with that at the position of no defect. The simplest version of the potential-drop measuring system uses two pairs of probes – one for current input and output, and the other for measuring the potentialdrop. Both the DC [1-2] and AC [3-4] versions of the potential-drop method have been investigated. The Direct Current Potential Drop (DCPD) technique has gained wide acceptance in the field of nondestructive evaluation (NDE) as one of the most accurate and efficient methods of evaluation for the material flaws. The application of the method is, now a days, not only limited to the evaluation of material flaws/defects, rather it also includes metal sorting and alloy identification, monitoring the heat treatment of metallic alloys, evaluation of thickness reduction of pressure vessels, measuring the surface topography to determine local material conductivity, and so on.
THE MEASUREMENT OF SURFACE BREAKING CRACKS BY THE ELECTRICAL SYSTEMS ACPD/ACFM
— The successful implementation of a fracture mechanics approach to the in-service structural integrity assessment of welded and machined components requires an accurate and reliable NDT method for detecting and sizing surface breaking cracks. This paper discusses and compares the crack depth sizing abilities of two theoretically-based NDT techniques: alternating current potential difference (ACPD) and alternating current field measurement (ACFM). Surface fatigue cracks of high and low aspect ratio are used in the investigation. It is demonstrated that ACPD is capable of accurately sizing long shallow cracks and that ACFM provides accurate crack depth predictions for small deep cracks.
Developmental aphasia: The perception of brief vowels and extended stop consonants
Abstract Developmental dysphasics and matched controls were examined for their ability to discriminate (a) two synthesized vowel-vowel syllables and (b) two synthesized consonant- vowel syllables. For both vowels and consonants, dysphasics were impaired when the discriminable components of the two stimuli were brief (43 msec) but unimpaired when these components were 95 msec or longer. It is concluded that developmental dysphasics have no difficulty in discriminating transitional auditory information as such and that their impaired discrimination of synthesized stop consonants is attributable solely to the brief duration of the discriminable components.