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 Crack by Using Closely Coupled Probes for DCPD Technique
In order to enhance the sensitivity of the d-c potential drop technique, the principle of a technique using the closely coupled probes is proposed. A calibration equation for this technique is derived, where the path-independent integral j e for electric crack problems is utilized. Experiments on laboratory specimens are discussed which validate the use of the calibration equation for the nondestructive evaluation of cracks. It is shown that the sensitivity of the present method using closely coupled probes is higher than that of the usual method using a uniform current flow in the region far from the crack.
A model for predicting heat transfer through gypsum‐board/wood‐stud walls exposed to fire
To facilitate the development of cost-effective and flexible design options there is a need to develop models to predict the fire resistance of wood-frame building assemblies. Such assemblies often derive much of their fire resistance from a protective membrane composed of gypsum board. A simple two-dimensional computer model is presented to predict heat transfer through gypsum-board/wood-stud walls exposed to fire. Input data for the thermophysical properties of gypsum board were measured exploying standard bench-scale tests. Input data for wood were selected from the literature. Small-and full-scale fire resistance tests were conducted on gypsum-board/wood-stud wall assemblies to provide data for the validation of the model. The model is shown to predict heat transfer through these walls rather well.