A split-ring resonator probe for assessing subsurface wood defects.

The Microwave Nondestructive Testing (MNDT) method has been increasingly used for inspecting wood characteristics. In this work, a MNDT method is established for effective and accurate detection and analysis of wood subsurface structural defects. A microwave near-field probe loaded with a split-ring resonator is proposed to bring in strong electric field, which is directly attached to the trailing end of the coupled microstrip line. High-resolution, light weight, low-cost, and compactness are some of the advantages of the sensor. The designed sensor is used for detecting subsurface structural defects under the thin paint layer of wood. Here, S21 values are used to analyze the characteristics of wood. Different width and depth defects of wood samples with different moisture content levels were measured. The experimental results show that the maximum relative error between the actual widths and measured widths is less than 10%. The spatial resolution is approximately 0.42λ at a resonant frequency of 9 GHz. The results demonstrate that the designed probe can accurately trace defects in wood with different moisture content levels without striping the upper coating of the paint.