Nondestructive material 
condition monitoring and assessment is accomplished by placing, mounting, or scanning magnetic and 
electric field sensors and sensor arrays over material surfaces. The material condition can be inferred directly from material property estimates, such as the magnetic permeability, 
dielectric permittivity, electrical property, or thickness, or from a correlation with these properties. Hidden cracks in 
multiple layer structures in the presence of fasteners are detected by combining 
multiple frequency magnetic field measurements and comparing the result to characteristic signature responses. The threshold value for indicating a crack is adjusted based on a 
high frequency measurement that accounts for 
fastener type. The condition of engine disk slot is determined without removal of the disk from the engine by placing near the disk a fixture that contains a sensor for scanning through the slot and means for recording position within the slot. 
Inflatable support structures can be placed behind the sensor to improve and a guide can be used to align sensor with the slot and for rotating the disk. The condition of an interface between a conducting substrate and a 
coating is assessed by placing a 
magnetic field sensor on the opposite side of the substrate from the 
coating and monitoring at least one 
model parameter for the 
material system, with the 
model parameter correlated to the interfacial condition. The 
model parameter is typically a magnetic permeability that reflects the 
residual stress at the interface. Sensors embedded between material 
layers are protected from damage by placing shims on the 
faying surface. After determining the areas to be monitored and the areas likely to cause sensor damage, a shim thickness is determined and is then placed in at least one area not being monitored by a sensor. The condition of a test fluid is assessed through a 
dielectric sensor containing a contaminant-sensitive material layer. The properties of the layer are monitored with the 
dielectric sensor and correlated to contaminant level.