Stress analysis in carbon/epoxy composites using Lcr waves

This work presents the application of an ultrasonic method to measure stresses in unidirectional carbon fiber composites with epoxy matrix (HexTow® AS4/Hexply® 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℃ and 27℃) 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.