DescriptionShort fibre reinforced polymers are more often used for structural components especially in automotive industry. Superior mechanical properties, the possibility to produce complex shaped parts in short cycle times and the light weight are the main reasons why these materials are promoted by industry. The mechanical properties are defined by fibre content, aspect ratio, fibre length and orientation. X-ray computed tomography (XCT) is capable of determining all these parameters at once for the final product. For this study the characterization technique XCT was used to investigate damage development mechanisms. Therefor a tensile testing stage was mounted inside the XCT device. Between XCT scans the load was increased stepwise until break. The XCT analysis was done using the Sub-µm CT device Nanotom and an in- house software development. A reasonable resolution had to be chosen to reach an adequate data quality. For the glass fibres that were analysed in this study, which had a mean diameter of 12.5 µm, a resolution of 2 µm voxelsize was found optimal. The investigated composite consisted of Polypropylene filled with 30 wt. % short glass fibres. The test specimens were produced by injection moulding. The influence of fibre orientation was investigated by producing specimens with mean orientation 0°, 45° and 90° related to the direction of movement. Since the injection moulding process leads to a layered structure within the specimen, where each layer has different fibre orientation, it was not possible to align all fibres in exact one direction. The design optimisation of the tool and the regions where the specimens were cut out of the produced plate, the effect of layering could be minimised. The overall property of the investigated material is quite brittle. That means necking is weak, elongations are small and therefor the orientation of the fibres doesn’t change significantly. Significant change in the structure of the fibre network is seen not until a crack develops. During an XCT scan relaxation was detected by a reduction of strength since the scan time was almost 2 hours. A reduction of scan time was not possible because data quality required this time. The displacement was fixed during scanning which allowed avoiding motion blurring. The main damage mechanisms that were detected are fibre pullout, fibre breakage and fibre-matrix debonding. Depending on the mean orientation, pullout and breakage or debonding is the dominant mechanism. The XCT data analysis leads to quantitative values for each damage mechanism. For fibres aligned in direction of movement, pullout and breakage is dominant. The matrix can build bridges between the crack surfaces. The fibres that are aligned transversely are detached from the matrix longitudinal which leads to fast crack movement. In this case the matrix does not build bridges. XCT at the applied resolution does not allow characterising damage within the polymer matrix. The characterisation of matrix cracks, pores and crazes was done by SEM analyses on the broken specimens.
|Period||23 Jun 2014|
|Event title||ECCM 16 European Conference on Composite Materials: null|