Porosity measurements by X-ray computed tomography: Accuracy evaluation using a calibrated object

Accurate identification and measurement of internal voids and porosity is an important step towards improvement of production processes to obtain high quality materials and products. Recently, the importance of knowing the exact size, shape, volume and location of defects has become even higher as tighter requirements and new standards have been introduced in industry. There are several well-established methods for defects evaluation based on various principles (both destructive and non-destructive). However, all conventional methods have various deficiencies and the information about internal voids/porosity that can be extracted is limited. Most of these drawbacks can be overcome by using X-ray computed tomography (CT). Unlike other methods, CT provides full three-dimensional information about shape, size and distribution of internal voids and porosity; however, the accuracy of measurements is still under investigation. Hence, further evaluations on CT porosity measurements must be performed in order to consider X-ray computed tomography a reliable instrument for the assessment and detection of internal defects. A reference object with artificial defects was used in this research work in order to evaluate the accuracy of porosity measurements by CT. The reference object was manufactured by ultra-precision micro milling. The object contains dismountable components with embedded internal hemispherical features that simulate internal porosity. The artificial porosity was micro-milled on top surfaces of dismountable cylindrical inserts. The hemispherical calottes were thereafter calibrated by traceable coordinate measuring systems and calibrated values were compared to actual values measured by a CT system. The accuracy of CT porosity measurements was then evaluated based on results obtained on various measurands, using different software tools and measuring procedures, comparing real scans to numerical simulations and investigating the influence of CT system parameters settings on measurement results.