A Combined ACFM-SMFM System for Real-Time Detection and Sizing of Surface Cracks in Metals

Abstract

<p>In some applications such as quality control of metal components or inspection of large metal structures, inspection systems with rapid scanning capability are required. Of particular interest are systems capable of real-time detection and sizing of surface breaking cracks in metals. Since such systems have to perform the data processing in the course of scanning, they should be based on NDE techniques with easy detection and inversion capabilities. One technique with these virtues is the alternating current field measurement (ACFM) technique [1]. In this technique, a high frequency uniform field is made incident on the work-piece and the resulting surface potential is sampled using a two-leg contacting probe. The crack signal is essentially a rectangular pulse, Fig. 1. For long cracks it can be shown that the crack depth can be determined from the crack signal using the simple expression (1) d=V2−V1V1⋅Δ2 where V1 and V2 are voltages picked up by the probe just before and just after the crack, Fig. 1, and Δ is the probe length. For a non-uniform crack or a non-uniform incident current, however, the above expression is not accurate. In such cases the measured depth dm resulting from (1) should be modified using a multiplier. Multipliers are based on mathematical models relating dm and d for different crack shapes and for different incident fields [1].</p>