Evaluation of a Testpiece for Porosity in Carbon Fibre Reinforced Polymers

Because of their outstanding high density specific strength carbon fiberreinforced polymers (CFRP) are the favourite material for many lightweight applications, especially in the aeronautic and automotive industry. However, defects like pores in CFRP cannot be completely avoided during manufacturing. Therefore, reliable and reproducible methods for porosity and defect detection are necessary. Common methods are ultrasonic testing and destructive methods like acid digestion and serial sectioning. All these methods lead to an uncertainty of ±1 vol.% in porosity or even worse. X-ray computed tomography (CT) is another promising method for porosity detection, because it is non-destructive, it is a 3D-method giving also the morphology of all pores and it has a very good reproducibility of around 0.04 vol%. However, up to now there are no CFRP test parts available with a well-defined porosity which can be used as reference parts for CT. Therefore, we propose a new CFRP-porosity test part for X-ray CT. CFRP-reference plates were made by drilling 300 - 1000 holes with a diameter between 200 to 300 μm in plates with a thickness of 1 mm. The diameters were evaluated by optical microscopy and CT to get reference porosity values and the corresponding uncertainty taking into account also machining induced diameter variations especially near the surface of the drilling hole. These hole-plates were mounted and pressed with porosity free 1 mm CFRP plates at the top and at the bottom. 3 samples with porosity values of 0.96 ± 0.02 vol. %, 2.58 ± 0.03 vol. %, and 4.81 ± 0.12 vol. % were prepared. These test parts were scanned by X-ray computed tomography and different data processing methods were applied to get accurate porosity values. These methods are evaluated and discussed within this contribution.