Evaluation of CFRP-Reference Samples for Porosity made by Drilling and Comparison with Industrial Porosity Samples by Means of Quantitative X-ray Computed Tomography

The main objective of this work was to find a new way to create reference samples for the quantitative evaluation of porosity in carbon fibre reinforced polymers (CFRP) by means of X-ray computed tomography (XCT). Artificial porosity samples for an exact determination of porosity were made by drilling several hundreds of holes into CFRP laminates with a diameter between 200 to 300 µm. The diameters were evaluated by optical microscopy and XCT to obtain reference porosity values. Results show void contents between 0.96 and 4.81 vol.%. Using these artificial porosity samples five different threshold methods were compared. An adapted method from Airbus and ISO50 threshold led to best results for the artificial porosity samples. In addition to the reference samples two different types of industrial porosity samples were investigated. The void morphology differed between both types. Type 1 is characterized by a high amount of micro- and meso voids (> 90 %) and type 2 contains only macro voids. Varying voxel size from (2.75 µm)³ to (120 µm)³ we showed that for the type 1 sample it was more important to choose a proper threshold method than for a type 2 sample. For type 1, FHW method and for type 2 ISO50 methods led to the best results for a broad voxel size variation. Further it was shown, that at high resolution it is less critical to choose a proper threshold. In a next step, the evaluated porosity at high resolution can be used to calibrate or adapt a threshold method to get useful results for lower resolution (larger sample size). Our final conclusion is that for each individual sample and material type a proper threshold method in combination with optimised XCT measurement parameters has to be chosen to get reliable quantitative porosity values.