This paper presents a novel Fiber-Bragg Grating interrogation system and its validation for detection of Lambwaves
and acoustic emission events on both aluminum and composite substrates. The system utilizes a robust laser
demodulation technique for FBG interrogation, based upon a simple laser wavelength tracking scheme. This technique
enables detection of much higher frequency strains than previous FBG interrogation techniques, enabling the use of FBG
sensors in acousto-ultrasonic structural health monitoring schemes such as Lamb-wave pitch-catch and acoustic emission
detection in the presence of a quasistatic strain background. The principles of the FBG interrogation system are
presented, including validation of the system for detection of ultrasonic Lamb waves, and results from a 4-point bending
test of a braided composite tube wherein the FBG system was used to detect crack-growth induced AE events on the
braided tube. The AE data agreed well with damage index values measured by a commercial acousto-ultrasonic system.
Fatigue cracks initiating at fastener hole locations in metallic structure are among the most common form of
airframe damage. Current methods for inspecting airframes for these cracks are manual, whereby inspectors rely on nondestructive
inspection equipment or hand-held probes to scan over areas to be monitored. Use of this equipment often
demands disassembly of the airframe to search appropriate hole locations for cracks, which elevates the complexity and
cost of maintenance inspections.
In this study an Additive, Interleaved, Multi-layer Electromagnetic (AIME) sensor was developed and
integrated with the shank of a fastener to form a Structural Health Monitoring Fastener, a new technology targeted at insitu
detection of fastener hole cracks. The major advantages of the Structural Health Monitoring (SHM) Fastener over
other SHM technologies are its installation, which does not require joint layer disassembly, its capability to detect inner
layer cracks in a multi-layer joint, and its capability to operate in a continuous monitoring mode.
The AIME sensor design, SHM Fastener, and complete SHM system are presented along with experimental
results from a series of single-layer and bolted double lap-joint aluminum specimens to validate the capability of these
sensors to monitor metallic joints for fastener hole cracks and loads. Fatigue cracks were successfully tracked to over
0.7 inches from the fastener hole in these tests. Sensor output obtained from single-layer fatigue specimens was
compared with analytical predictions for fatigue crack growth versus cycle number showing a good correlation in trend
between sensor output and predicted crack size.