Structural damage detection in a sandwich honeycomb composite rotor blade material using three-dimensional laser velocity measurements

A method for damage detection in sandwich honeycomb core composite materials based on point laser velocity measurements is presented in this paper. Current localized methods for detecting the presence of sub-surface damage (ultrasound scans, tap testing, and x-ray) require large time investments to implement on components such as helicopter rotor blades and fuselage sections. Thermography and ultrasonic inspections are not capable of detecting deep sub-surface damage in composites in or behind the material core. Experiments using a piezoelectric actuator to excite a fiberglass sandwich panel, which is representative of the materials used in rotor blades, are conducted to demonstrate a method for inspecting composites in aircraft. A three-dimensional scanning laser vibrometer is a non-contact measurement method that provides several benefits for inspection, including elimination of the need to remove components from the aircraft for inspection. The forced frequency response data measured using the scanning laser vibrometer at two distinct excitation amplitudes is processed to identify areas of the panel that exhibit significant nonlinear response characteristics. These local nonlinearities are shown to coincide with sub-surface damage in the panel. Because the nonlinearities distort the frequency response, it is possible to locate the damage without comparing the response to an undamaged, or baseline, measurement. In addition, rudimentary modeling has shown results confirming this reference-free method of detecting damage by direct comparison of frequency response functions excited by multiple amplitude signals, particularly at higher modal frequencies.