Report on composite material studies at TU Dortmund University (WPT) using an X-ray tomography tensile stage
Ronja Scholz is a scientific assistant and PhD student in the group of Prof. Dr. Frank Walther, Head of the Department of Materials Test Engineering (WPT) at TU Dortmund University. She specialises in the use of computed tomography (CT) to characterise composite materials contributing to the scientific and industrial research projects. These deal with investigations concerning metals, additively manufactured materials and composites using modern measurement and inspection processes, supported by optimised analysis and evaluation techniques.
WPT provides the database for construction and production as well as for virtual development of reliable high-performance products for different industries and is used for decision-making guidance for material selection, quality control, component monitoring and damage analysis. This is facilitated by determining the chemical composition, analysing microstructural properties with light, electron beams and X-rays, determining material properties and characteristic values by means of destructive (DT) and non-destructive testing (NDT) methods.
Scholz describes her work applying CT: “Our investigations provide microstructural information of a material as a non-destructive testing technique. The ability to look inside a specimen or component and getting 3D volume information, which can be characterised concerning different structural issues like defect volume, fibre orientation or wall thickness, expanded the test technology of our department in a very efficient way. Thus, to really understand the interdependencies between microstructure, mechanical loading and climatic implied processes in materials as accurate as possible, in situ investigations are of great importance.
“Therefore, the use of Deben’s CT5000TEC stage in combination with Nikon XT H 160 kV microfocus X-ray source enables the capture of the current state of a material in situ under mechanical tensile or compression loads up to 5kN and variable temperatures in the range of -20°C to 160°C. These measurements give essential information about defects such as pores or delaminations in the material’s initial condition as well as the progression and development of these defects under mechanical loading.”
This strategy allows the evaluation of different process routes for the manufacturing of a material, allowing the study of the resulting microstructure as well as the performance of the material or component under application-oriented conditions such as elevated temperatures.
When comparing CT scans of a damaged composite specimen, the in situ scan results reveal significantly more damage than the results of a conventionally performed scan. The image shown in Fig.2 visualises the damage development in a fibre-reinforced plastic. This material shows great potential as a substitute material for metals in automotive and aero applications providing economic efficiency thanks to their high specific strength.
To gather sufficient information about the load capacity, damage development and the use of composites under changing environmental conditions, detailed investigations are necessary. In Fig.2, the development of the defect volume due to increasing tensile load is shown for a cellulose-based composite, which shall be used as resource-efficient construction material. These results generated at room temperature can now be correlated with investigations at elevated or degraded temperatures, depending on the area of application.
Scholz adds: “Lastly, it is important to note that before using the integrable Deben CT5000TEC stage, we focused on conventional CT investigations, where specimens are scanned while resting (no load applied) to characterise their specific structure, e.g. fibre orientation, pore content, wall thicknesses, etc. Now our experiments produce data that is much realistic to the in-life use of these exciting, new composite materials.”
