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.
An AC potential drop system for monitoring crack length
The design of an AC potential drop (ACPD) system for monitoring crack length in fracture specimens is outlined. The amplified voltage across a cracked specimen is compared with that across a reference specimen by employing a dual-slope analogue to digital convertor in the radiometric mode. The resultant digital output provides an accurate measure of the crack length. Because of the strong skin effect in steel even at relatively low frequencies, the output varies linearly with crack length. Although the output is insensitive to variations in specimen current and temperature, it is affected by the position of the sensing and current leads and by the presence of ferromagnetic materials in the vicinity of the specimen. In the case of steel, whilst the ACPD output is increased by crack growth it is decreased by tensile stress and plasticity in the specimen. The competition between the two opposing effects enables the ACPD to detect initiation of crack extension even when crack growth is accompanied by large scale plasticity.
Data interpretation in ACPD crack inspection
Abstract The voltages measured in the ACPD crack-sizing technique are sharply peaked at the crack edge for a short crack in a material of small skin depth, or for any crack in a material of large skin depth. The conventional sizing method uses these voltages as a first estimate of crack depth. A number of situations arise where these voltages may not be measured accurately, however, including rapid automated scanning and hand-held readings at weld toes. In such cases, more accurate sizing may be obtained using methods based on the saddle voltage with the probe straddling the crack. Multipliers have been calculated which allow crack depths to be estimated from the saddle voltage for circular arc or semi-elliptical cracks in the extremes of either small or large skin depth. These multipliers are presented in graphical form together with guidelines for their use.