Fatigue Crack Characterization for Rotorcraft Structures under Varying Operational Conditions

The Rotorcraft industry has ongoing technology initiatives to develop highly usable and reliable Structural Health Monitoring (SHM) capabilities. This paper will discuss the testing of an acousto-ultrasonic sensor based SHM system for fatigue crack characterization of rotorcraft structures. Specifically, the paper will focus on the efforts to improve signal reliability in varying environmental and operational conditions thus minimizing erroneous decisions. The SHM system that is tested utilizes ultrasonic guided waves that have known sensitivity to these variations. Operational conditions such as temperature and load can affect both wave speeds and propagation distances in this type of system. In this paper, both effects are considered and their impact on detection, localization and quantification of fatigue crack using guided waves is explored. Especially, a noble nondestructive damage quantification method called as damage simulator is proposed. The advantage of the damage simulator is that it can generate the needed calibration curves from different locations and orientations of the target structure in a nondestructive manner. Also, the obtained results can be used to calibrate the SHM system regularly during long term operations in the field. Experimental results are shown from fatigue tests performed on aluminum test structures that incorporate various structural design elements from typical rotorcraft airframe structures. The work presented in this paper has widespread applicability to any rotorcraft platform and is not limited to a single configuration.