An Electrochemical Probe for Characterizing Fatigue Damaged Metal Components

A new hand-held inspection tool was developed to assess the presence, type, and degree of fatigue and crack damage in metal components used in military equipment. An inspection tool is needed that can be used to differentiate the type of crack once found whether it is induced from a one-time static load, or induced from fatigue, or even induced from stress-corrosion cracking. A portable sensor that can help make these determinations without excising the material and analyzing it under laboratory conditions would help designers and maintenance personnel decide appropriate corrective actions to the damaged area. Toward this need, a new electrochemical sensor probe was developed and used to assess fatigue and crack damage on three representative aerospace metals including aluminum 2024, high strength titanium grade 5, and high strength 4130 steel alloy. Figure 1 shows a schematic of this sensor probe that packages the elements used in classic wet-electrochemistry methods behind a membrane transducer. This probe can be touched against the metal coupons to non-destructively interrogate the metal via electrochemical methods to characterize the presence and type of damage. The characterization mechanism we have developed is called Reactive Electrochemical Impedance Spectroscopy (REIS) that uses the presence of cracks and fatigue induced damage, if present, to react in situ when interrogated electrochemically. This electrochemical reaction response is simultaneously analyzed by electrochemical impedance spectroscopy giving a distinctive response pattern due to the nature, type, and extent of cracks and fatigue within the structure. The process interrogates the structure locally to react in situ absorbed moisture in its local environment while obtaining an impedance scan of the local structure. A comparison of impedance scans conducted with (energized) and without the induced reaction process (non-energized) is made where different signature responses indicate the presence and degree of cracks and fatigued damage. Equivalent circuit models coupled with reaction processes allow us to assess the location and severity of the cracks helping to characterize it in support of in-service field repairs. To demonstrate this electrochemical interrogation method, we assessed the degree of crack depth in a 2024-T3 Al component using the sensor probe. The process first measured an impedance scan on the Al coupon in the non-energized state. Subsequently, an applied potential less