A quantitative comparison of the capabilities of in situ computed tomography and conventional computed tomography for damage analysis of composites

The so called in situ computed tomography is a powerful non-destructive testing method to study the damage phenomenology while a material is loaded. However, the difference in the detected amount of damage compared to conventional CT scans after unloading of the specimen has not been reported so far and is therefore the focus of this study. Three different textile-reinforced composite materials have been experimentally investigated for that purpose. Carbon fibre reinforced epoxy composites with multi-layered flat bed weft-knitted fabrics and multi-axial non-crimp fabrics were uniaxially loaded in tension. In situ CT scans were taken at different load levels and conventional CT scans after unloading. The experiments have shown that the observed crack lengths and delaminated areas are significantly larger while the load is applied because of crack closure effects during unloading. Any damage diagnosis should therefore be performed under load. A ceramic C/C composite with woven reinforcement was taken as a third example. This composite was loaded by compression in thickness direction. It was found that the decrease of total porosity with increasing load can only be predicted by in situ CT measurements.