Crack Detection and Characterization

Surface-breaking crack detection and characterization is another application of interest in Microwave NDE. While microwaves cannot penetrate conductive materials well (recall the skin effect), they can interact with the surface of conductive materials. As such, when a microwave signal from a probe illuminates a conductive surface it induces a surface current density. This means that any surface anomaly, such as a crack or pitting, will perturb this surface current density and scatter or reflect some of the signal from the probe. By combining knowledge of the electromagnetic properties of the structure under test and analysis of this scattered/reflected signal, information about the cause of the perturbation can be determined. This can include information, such as the crack size and orientation. Using a linearly-polarized probe, such as open-ended waveguide, is the most common approach for detecting cracks and pitting. However, other probes such as open-ended coaxial lines have also be used for this purpose. Dual-polarized probes, having either two linearly-polarized signals that have orthogonal polarizations with respect to each other, or two orthogonal circularly-polarized signal, can also be used to obtain accurate crack orientation. In addition to using near-field probes, synthetic aperture radar imaging using these varieties of probes can also be implemented for crack detection and evaluation.

There are a few advantages to this method. One of them is that since microwaves can see through dielectric materials, cracks and other surface defects on a conductive surface under paint can also be detected. Other advantages of this method include that it is a one-sided non-contact measurement method and that it is highly sensitive to the presence of relatively small flaws. 

When making a measurement to try to detect and characterize a surface-breaking defect, the probe will be scanned over the sample under test. 

Open ended rectangular waveguide

Schematic of an open-ended rectangular waveguide scanning a conductor with

a surface-breaking crack (not-to-scale).

Simulations of waveguide

Simulations of a waveguide being scanned over a narrow surface-breaking crack in metal.