Book excerpt: An analytical and experimental program was conducted to evaluate the crack growth gage concept for application as an individual aircraft tracking device. Three gage designs were evaluated. Volume I discusses the evaluation of two versions of a stepped thickness gage and a stepped width gage. Volume II discusses the evaluation of a side-grooved constant thickness gage. An analytical evaluation of the change in transfer function corresponding to gage design parameter changes was performed to determine the configuration of the gages to be tested. Initial spectrum tests were conducted on the first stepped gage design to determine the ranking ability of the gage. Then constant amplitude tests were conducted on the second stepped gage design. A modification of the gage was made to improve spectrum test response. The modified gage was subjected to both constant amplitude testing and spectrum testing. The analytically derived crack growth rates were compared to the test data. In general, the analysis predicted slightly higher growth rates than observed.