Comparison of Signal Filtering Techniques for Ultrasonic Waves Used in Inspection of Composite Materials

The use of composite materials is growing worldwide as the number of its applications. Regarding the quality of components made from composites especially in fields that require high safety standards, such as aerospace, reliable inspection techniques must be used. Nondestructive inspection techniques using ultrasonic waves are largely employed in metals for fault detection, characterization, and stress measurement and they are also being applied in composites. However, composites are heterogeneous in nature and thus the signals acquired from ultrasonic transducers in these materials exhibit high noise, leading to inaccurate measurements. The objective of this work is to evaluate digital filtering techniques for signals from ultrasonic longitudinal bulk waves and longitudinal critically refracted (L_CR) waves propagating in a carbon fiber-epoxy prepreg. Samples of unidirectional composites were manufactured to study the signals of waves propagating at different angles in relation to the carbon fibers direction. For bulk waves, we acquired signals at 0, 15, 30, 45, 60, 75 and 90° from the fiber directions; for L_CR waves, the signals were measured at 0 and 90°. We compared the techniques based on digital filters IIR (Infinite Impulse Response), FIR (Finite Impulse Response) and Discrete Wavelet Transform (DWT). The results show that the filters FIR and IIR have the best signal-to-noise ratio (SNR) for most propagation directions, both for bulk and L_CR waves.