This work presents the application of an ultrasonic method to measure stresses in unidirectional carbon fiber composites with epoxy matrix (HexTow Õ AS4/Hexply Õ 8552). This kind of composite is largely employed as a structural material in the aeronautical industry. The ultrasonic method is based on the acoustoelastic principle, a principle that holds that wave speed is affected by variations in strain in the material. We employ critically refracted longitudinal waves (Lcr waves) and relate their time-of-flight with applied strains and stresses. We first performed tests on a polygonal specimen evaluating the influencing factors on the results, that is, temperature and transducer positions, as well as their effects on each fiber direction: 0 , 45 , and 90 . Tensile tests were next performed on rectangular specimens, as we sought, for each fiber direction, the relation between stress and wave speed variation, otherwise known as the acoustoelastic coefficient. The results showed that the wave speed was sensitive to the stress variation along the fiber direction (0 ) and nearly insensitive to any other direction. Also, temperature (between 20 C and 27 C) was not a relevant factor for waves propagating along the fiber. These findings support the notion that the method holds promise as an alternative to measuring stresses in multidirectional 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 (LCR) 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 LCR 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 LCR waves.