Co-authors​ :

 William LUCAS (FRANCE) 

Carbon Fiber Reinforced Polymers (CFRPs) are widely used in the aerospace industry for their outstanding mechanical properties and the mass savings resulting from their low volumetric mass. During the manufacturing process, defects such as porosities due to entrapped air can be introduced in the material, affecting its mechanical properties. Uniformly distributed and small-sized porosities, also called residual porosities are tolerated up to a threshold limit of 2% of the volume fraction for aircraft applications. However, the distribution is frequently non-uniform, leading to the formation of local clusters of porosity that leads to a significantly higher risk to the mechanical integrity of the structure compared to the residual porosity. Therefore, accurately characterization of porosities is crucial to avoid unnecessary rejection of expensive components and ensure their integrity. Ultrasonic methods, which rely on velocity and attenuation measurements, are commonly employed for this purpose but are limited to samples with a uniform distribution of porosity. As of now, addressing clusters of porosity continues to be a scientific challenge.
The objective of this study is to investigate the localization and characterization of clustered porosity content using ultrasonic methods. On the experimental side, samples of thermosetting composite materials with unidirectional fiber orientation, featuring diverse distributions and volume fractions of porosity, were prepared in an autoclave. These samples underwent testing through an immersion ultrasonic scanning method, employing two planar transducers in the far field with central frequencies ranging from 2 to 10 MHz to measure the transmission through the sample. This experimental setup includes goniometers and microcontrollers, ensuring optimal alignment of the transducers.
Simultaneously, numerical simulations were conducted to explore ultrasonic propagation in such materials. Various multiple scattering models were used in order to estimate effective mechanical parameters that describes both the composite and the porosity content in terms of the radius and volume concentration of the pores. Subsequently, an inverse problem was undertaken, aiming to fit the experimental data to an analytical propagation problem based on a three-layers model: semi-infinite fluid, solid, semi-infinite fluid. Different effective parameters for the solid, obtained by testing various models of multiple scattering of ultrasonic waves were assessed.