Sensing Subsurface Structural Damage using Noncontact Tomography

Kenneth J. Loh, Sumit Gupta, Gaochen Fan, and Sashank Shivakumar
Active, Responsive, Multi-functional and Ordered-materials Research Lab (ARMOR)

Aerospace, civil, marine, and even biological structures and components are susceptible to damage that often initiate beneath the surface, which make them difficult to detect by conventional visual inspection. If damage is left undetected over time, they can propagate and cause component or even system failure. The ARMOR Lab at UC San Diego is actively developing new strategies that enable one to detect, localize, and assess subsurface damage, without having to install tethered sensors or even physically contact the structure. The method relies on a unique measurement scheme and algorithm called electrical capacitance tomography (ECT). The ECT hardware uses a set of electrodes arranged to form a circular ring. An electric field is propagated through the sensing area defined by the array of electrodes (Fig. 1). 
Depending on how the field interacts with the structural component in the sensing area (i.e., how they penetrate through various aspects of the component), this would affect the measured capacitance response at the other electrodes. The set of capacitance measurements are then used for solving the inverse problem of reconstructing the electrical permittivity distribution of the sensing area. Since damage can cause changes in a material’s electrical permittivity, the magnitudes and localizations of permittivity changes determined by ECT allows one to directly detect, localize, and assess damage. 
Three example application areas are presented in this article. First, epoxies and polymer-based materials are seeing greater adoption in various industries, such as for fiber-reinforced polymer composites in the aerospace sector or for building additively manufactured (i.e., 3D- and 4D-printed) parts. Improper builds or errors during manufacturing can create voids and defects in components. To demonstrate noncontact detection of voids in polymer-based materials, a cured epoxy specimen was subjected to ECT testing after drilling different sized holes.