Edge Crack Detection: A Theoretical and Experimental Study

Detection of cracks close to an edge by conventional eddy current techniques is difficult, owing to the significant background signal from the edge. It is necessary to develop methods for minimizing the effect of the background signal, thereby increasing the probability of detection. The edge signal is known to be influenced by a number of factors and it is essential to characterize these in order to minimize its influence. This study aims at developing a good understanding of these factors so as to facilitate the development of such techniques. A boundary element method (BEM) approach was used to model the signal due to the edge and to compare with experimental measurements. Experiments were conducted on electro-discharge machined (EDM) slots in the vicinity of an edge using both absolute and differential probes. The influence of orientation of a differential probe on the signal from the crack and the edge was also studied. We report on the development of improved methods to reduce the influence of signal due to the edge by appropriate use of differential probes and with the aid of signal processing. An inexpensive physical technique which results in a improved detectability was also developed.