It has been demonstrated that exoelectron emission phenomena can be used to observe and measure alloying, crack growth, fatigue, annealing, and grinding processes. For observation of alloying, crack growth, annealing, and grinding a fixed plate or wire collector biased at +90 V is used to collect the electrons. The current is measured by a picoammeter and recorded directly via a strip chart recorder. To find the fatigued area in a specimen a heat source is scanned along the coupon and the exoelectron emission is picked up by a moving collector wire connected to a picoammeter.
These experiments are usually done in ambient air without any special control of temperature and humidity. The initiation of crack growth is observed as a sudden change in exoelectron emission level. In a fatigued but uncracked specimen a significant increase in exoelectron emission is observed when the heat source and collector move over the area where the fatigue damage is most intense.
Most of the data to date has been obtained with aluminum but we feel that this technique can be extended to other aircraft structural materials.