Eddy-current measurements of conductivity spectra are needed to gauge the  
near-surface residual stress in surface-treated turbine engine components for  
life extension, where conductivity and stress are related via the  
piezoresistive effect.  Broadband (0.1-100MHz) planar spiral-coil eddy-current  
probes are currently being developed using photolithography for the purpose of  
efficiently measuring the conductivity spectra for subsequent inversion to  
residual stress versus depth profiles.  

The key problem is that the probes do not perform well at high frequencies  
(\$\>30\$MHz) due to stray capacitive effects, which begin to dominate as the  
resonant frequency of the probe is approached.  This parasitic effect is  
mainly due to capacitive coupling between adjacent traces of the coil, which  
makes it especially difficult to predict in terms of sensitivity.  At high  
frequencies, the lift-off sensitivity of current spiral coils becomes unstable  
above about 25MHz.  The objective of this effort is to develop models to  
account for these problematic parasitic effects, and allow for probe  
optimization in the design stage, rather than through the tedious and  
inefficient trial-and-error approach.  The key parameters of interest to the  
probe designer are coil diameter, trace width and trace spacing.  The probes  
could be single or multi-layer, circular or rectangular, shielded or  
unshielded.  

In this project we will apply the same volume-integral algorithm of VIC-3D to  
develop a computer code to design spiral-coil  
eddy-current probes for optimal broadband performance.  This code will allow  
the designer to predict probe performance as a function of frequency.  We will give a number of simulated examples of the performance of the spiral-coil probe.


Model Development for Broadband  Spiral-Coil Eddy-Current Probes

No proceedings papers in 2020. Video only.

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