Composite structures

Porosity analysis in a carbon fibre pressure vessel

Filament winding process used in manufacturing fibre-reinforced composite pressure vessels is known to lead to some microstructural variability. In particular, presence of voids has been reported. The knowledge of composite material microstructure is of great importance for accurate modelling of damage development and failure. In the present study, a thick 40-litre carbon fibre pressure vessel was scanned to observe and characterize porosity. In order to preserve the structure for future pressurization testing, it was necessary to perform the scans on a complete structure. This presented a technical challenge due to its significant dimensions and thickness.

Role of Computed Tomography 
The tomography images allowed characterizing the composite stacking sequence and revealed the presence of intra- and interlaminar porosity. To observe the structure in more detail, a higher resolution scan was also carried out locally. The images showed a dependence of porosity on composite ply orientation as well as through-thickness position. Damage was observed in the inner hoop layer in the form of circumferential transverse cracks. The CT results were used to feed a multiscale numerical model for pressure vessel strength, accounting for the variation in mechanical properties due to the presence of porosity. Fibre break and matrix cracking phenomena were further taken into account. This gave a deeper insight into damage development mechanisms in composite pressure vessels.

The research leading to these results has been done within the framework of the FiBreMoD project and has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 722626. The pressure vessel for the study was kindly supplied by Luxfer Gas Cylinders.