MetroLaser has demonstrated a system to perform nondestructive inspection (NDI) of a variety of critical parts ranging from small turbine blades to large sections of the wing or fuselage. The system consists of a unique and versatile multi-beam laser Doppler vibrometer (MB-LDV) that can project multiple beams onto a small area (such as a turbine blade) or onto a large area (several meters) from a standoff distance that can be many meters. The system should be applicable to composite, metallic, and other target compositions. The system measures vibrational patterns, which depend on the stiffness and mass of the structure and thus reveal internal flaws. By simultaneously measuring the vibrational pattern at multiple target locations, the sensor would respond to either impact or continuous excitation. In addition to expediting the diagnostics, this global measurement reveals many hard to find flaws.
Structural damage or conversely structural integrity may be assessed with the system in one of three ways: 1) measuring the modal pattern and comparing it to that of a healthy baseline structure, 2) measuring the localized vibrational pattern and comparing it to that of a healthy baseline structure, and 3) measuring the localized vibrational pattern as a function of position and searching for significant changes in the vibrational pattern. The first method uncovers substantial structural problems since it takes substantial structural problems to change the modal patterns. The second and third method would uncover the small, localized defects (such as disbonds, delamination, and cracks).
The system would be complementary to other NDI techniques. It would be particularly relevant to inspect composite materials since many of the other NDI techniques would not work on these materials. The system would also complement modal analysis efforts that are presently performed with a large number of accelerometers. The multi-beam laser system could measure the modes for cases where loading the structure with accelerometers may disturb the measurement and/or cases where rapid deployment is necessary. Figure 1 shows a 17×17-beam pattern created at MetroLaser using a custom diffractive optical element (DOE). Notice the presence of low intensity parasitic beams. They are typical of a DOE and do not interfere with the measurements. Each beam on the target is imaged by one and only one detector and there is no cross talk. Therefore the additional beams have no influence on the measurements.
During a Phase 1 SBIR program with NASA Dryden, MetroLaser used an LDV system with acoustic excitation to find defects engineered into a Plexiglas coupon and a carbon coupon. The results are summarized in Figure 2 and Figure 3.