Sensors for monitoring early stage fatigue cracking

Three sensor systems were evaluated for their ability to detect early stage fatigue cracking in open holes. The sensor systems were (1) a Meandering Winding Magnetometer Array sensor system that induces eddy currents to monitor conductivity changes; (2) a through-transmission ultrasonic technique that monitors energy loss, and (3) the Electrochemical Fatigue Sensor which detects fatigue-induced changes in a metal surface through their effects on the electrochemical double layer. The sensors were mounted individually or in tandem in samples of 7075-T651 aluminum alloy which were then fatigued using a spectrum loading sequence. The samples were also examined at various stages during fatigue cycling using optical or scanning electron microscopy. Detection thresholds of approximately 100 μm were observed, and calibration curves for crack size in terms of the sensor outputs were obtained.