Comparative Study of State of the Art Nondestructive Testing Methods with the Local Acoustic Resonance Spectroscopy to Detect Damages in GFRP

Abstract

This paper evaluates and compares the application of current state of the art methods and the new local acoustic resonance spectroscopy (LARS) method for nondestructive evaluation of damages in glass fiber reinforced polymers. The innovation of the LARS is the combination of the analysis of the acoustic signals and the force excitation. Generic plates of a standardized material (Vetronit EGS 619) and segments of rotor blades of wind turbines were tested. The generic specimens, 2 and 6 mm in thickness, were damaged with various impact energies caused by a spherical impactor with a diameter of 16 mm, which generated impact damages ranging from barely visible to clearly visible on the generic specimen as well as on segments of real rotorblades of windturbines. The impacts have been measured to account for damage diameter, form and area, indentation depth and bulge height. In addition, blind holes of different depths have been drilled to assess the depth of penetration of the methods tested. As observed in scientific literature as well as in current research, impact damages exhibit a peanut-shaped damage area when impacted with minimum threshold energy. The current research tested several of the specimens using X-ray computed tomography as a reference measurement. These results were compared to the data obtained by ultrasonic methods, LARS and optical lock-in thermography. Finally, all methods have been applied to evaluate rotor blades of wind turbines. The results are shortly discussed in respect to practical applications and accuracy.