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.
NDE of a 3-D Surface Crack Using Closely Coupled Probes for DCPD Technique
A procedure of applying the d-c potential drop technique using the closely coupled probes to NDE of a 3-D surface crack is newly developed. The calibration equation for three sensors which differ in the distance between the probes is derived. Experiments validated the use of the calibration equation for the NDE of cracks. The method to use the three sensors properly based on the measuring sensitivity is shown.
"Flying Triangulation" - a new optical 3D sensor enabling the acquisition of surfaces by freehand motion
In order to obtain complete 3D surface information of an object it is often necessary to acquire many partial 3D views. For this purpose, the sensor has to be moved (e.g. for the acquisition of sculptures or rooms). Further, in many cases a motion of the object relative to the sensor is unintended but inevitable (e.g. for medical applications such as intraoral measurements of teeth). Nearly all existing optical 3D sensors either lack an easy handling or require a standstill during the acquisition of a single partial 3D view, since usually multiple camera shots have to be taken for this purpose. This makes such measurements elaborate or even impossible.