Damage Characterization of Discontinuous Fibre Reinforced Polymers by X-ray Computed Tomography

Discontinuous fibre reinforced polymers have become important in recent years due to their wide range of industrial applications. Short fibre reinforced injection moulded thermoplastics and especially ones with longer fibres have outstanding mechanical properties, such as high strength, high stiffness, and good impact behaviour. Different degrees of anisotropic material behaviour can be achieved depending on the degree of fibre orientation. Detailed knowledge of the microstructure, as well as on the defect formation and failure behaviour is a key factor for the improvement of such materials and thus appropriate safety margins in the lightweight design. In this thesis the damage behaviour of short and long glass fibre reinforced polypropylene with 10 vol% fibre content has been investigated by X-ray computed tomography.

The main goal of this thesis was the application and further development of different methods for the quantification of the microstructure, including fibre characterization and defect characterization, as well as the determination of local strain development during the application of mechanical load. For this purpose, in situ experiments were performed in combination with X-ray computed tomography. Constant radius and double-edge notched test specimens were tensile tested in an interrupted manner. Such interrupted in situ investigations can be very challenging due to relaxation processes during scanning. Therefore, detailed investigations of the stress relaxation behaviour of glass fibre reinforced polypropylene were conducted and enabled the development of a methodology to achieve sufficient image quality for the characterization of defects and fibres. The influences of fibre orientation and local strains on defect formation for short glass fibre reinforced polypropylene on a qualitative and quantitative level were investigated. Long glass fibre reinforced materials pose further challenges for in situ testing and characterization. A workflow was suggested for the successful definition of the scan area prior to in situ testing. Additionally, different fibre characterization approaches for the analysis of long glass fibre reinforced materials were compared. Differences in the failure behaviour of the investigated materials were revealed by fibre and defect characterization. The three dimensional quantitative strain development at micro- and macrostructural level was analysed for different fibre lengths, main fibre orientations and test geometries. The induced plane strain and plane stress states were quantified by strain analysis and were in good accordance with literature.

The results presented, contribute to a comprehensive understanding of defect formation and failure behaviour in discontinuous fibre reinforced polymers. Furthermore, the results, obtained with the presented methods, appear promising for further developments of material and failure models for micromechanical simulations.