Influence factors on stress measurements using LCR waves for unidirectional carbon-epoxy composites

Acoustoelasticity has been widely studied for stress measurements in metallic materials, such as steel and aluminum, mainly for structural health evaluation and monitoring. Applying it requires careful consideration of the influence factors on wave propagation, particularly temperature, texture and non-uniformities. However, for fiber composite materials, it is crucial to consider additional factors such as delamination and waviness of the fiber. Due to the thickness of structural composites parts, longitudinal critically refracted (L_CR) waves are one of the most suitable NDE methods to be used to measure stresses, especially because of the high sensitivity of its speed to strain. This paper focuses on evaluating the effects of the main influence factors, aiming to verify if it is possible to use L_CR wave to measure stresses in carbon-epoxy composites even in the presence of them. The factors are temperature, non-uniformities, delamination and waviness. Because these waves travel a fixed distance, the effects are measured on the wave time-of-flight. The results show that the magnitude of the influence is similar to manufacturing non-uniformities, as well as delamination and waviness. A standard deviation of 15 MPa (< 1% S_ut) is found for samples with and without defects when the stress-free reference time is taken in the same sample where the measurement is performed and about 62 MPa (< 4% S_ut) using a global reference time for this composite material.