Damage and usage conditions in the vicinity of fasteners in joined
structures are nondestructively evaluated using the fasteners themselves.
Sensors or sensor conductors are embedded in the fasteners or integrated
within the fastener construct, either in the clearance gap between the
fastener and the structure material or as an insert inside the shaft or
pin of the fastener. The response of the material to an interrogating
magnetic or electric field is then measured with drive and sense
electrodes both incorporated into the fastener or with either drive or
sense electrodes external to the fastener on the material surface. In
another configuration, an electric current is applied to one or more
fasteners and the electric potential is measured at locations typically
between the driven electrodes applying the current. The potential is
measured circumferentially around the fastener at locations on the
material surface or across pairs of fasteners throughout or along the
joint. The voltage or potential measurement electrodes may be collinear
with the drive electrodes. State sensitive material layers can be added
either to the fastener or the test material layers in order to enhance
observability of the test material condition, such as the presence of a
crack, mechanical stress, delamination, or disbond.