Local Acoustic Resonance Spectroscopy (LARS) for Glass Fiber-Reinforced Polymer Applications

Polymer composite materials combine high strength with low weight. This makes composites an interesting material for different industrial applications. In the aerospace industry, the use of composites is already common practice, while in the automotive industry carbon fiber-reinforced polymers have begun to replace metal in some parts. However, the nature of damage within composites is different from that within metal parts, so common techniques available for damage detection in metal may not work for composites thus new techniques for damage detection need to be developed. A technique that is often used but requires experienced technicians is the so-called coin tapping test where changes in sound waves generated by the impact of a hard object are detected. LARS is a (new) technique that avoids the errors due to variations in operator technique by using a instrumented impact device to generate controlled sound signals. If a hammer is used as an impact device it could be equipped with a dynamic force sensor to measure and record the excitation force of the sound signal. The force and the excited sound signal are related to the contact stiffness between the hammer and the test part. Flaws such as voids and delaminations affect the contact stiffness and can be detected under certain conditions. To the knowledge of the authors, no such technique has appeared in the literature. In regard to the frequencies, LARS is operated at much shorter wavelengths than in vibration analysis techniques (making it “local”) and at much larger wavelengths than in ultrasound. The material is excited to frequencies that are recorded by a microphone. To demonstrate the method, it is applied to the inspection of wind turbine rotor blades.