material condition monitoring may be performed by electromagnetic sensors and sensor arrays mounted to the material surface . the sensors typically have a periodic winding or electrode structure that creates a periodic sensing field when driven by an electrical signal . the sensors can be thin and flexible so that they conform to the surface of the test material . they can also be mounted such that they do not significantly modify the environmental exposure conditions for the test material , such as by creating stand - off gaps between the sensor and material surface or by perforating the sensor substrate .