The objective of this program was to determine the feasibility of the crack growth gage as a more direct method for monitoring potential crack growth damage in fatigue critical areas of F-4 aircraft structure. The major task consisted on mounting crack growth gages on a full-scale F-4C/D fatigue test article and collecting crack growth data from the gages at specified intervals. Two additional tasks were comprised of (1) conducting a gage qualification test program in accordance with MIL-STD-810C (Environmental Test Method) requirements, and (2) determining an appropriate method for collecting data from the gage. Inconsistencies in constant amplitude test results and bonding problems encountered on the F-4C/D full-scale fatigue test article prevented the MIL-STD-810C qualification tests from being started. Results of this investigation indicated that further research and development of the crack growth gage concept is required before the gage can be recommended as a fleetwide tracking device.

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.

Changes in the crack growth behavior of 7075-T651 aluminum specimens which has been exposed to temperatures between 150 degrees and 355 degrees F (66 deg and 179 deg C) were evaluated. Specimens were fatigue tested at room temperature under flight-by-flight loading conditions. Results from these tests were then compared with data from the baseline (as received) material. Exposure to 250 deg F (121 deg C) or more produced a definite increase in speciment life, apparently due to a decrease in crack growth rates.