Investigation into the plastic material behaviour up to fracture of thick HSS using multi-DIC and FEMU.

Steel is one of the most important materials employed in heavy duty machinery and constructions. Evidently, the improvement of the steel properties is a key element to enhance those steel structures regarding the strength and weight. The development of High Strength Steel (HSS) grades enabled to reduce the weight of steel structures while maintaining or even improving the strength. Unfortunately, the increase in strength mostly does not correspond with an increase in ductility. Therefore, brittle fractures are a safety risk when adopting HSS grades. Consequently, a profound understanding of the plastic material behaviour up to fracture of HSS is mandatory.

To characterise the plastic material behaviour up to fracture, no straightforward method exists. Consequently, a general approach is compulsory to address the latter and the issues mentioned hereafter. Therefore, an general inverse approach, such as Finite Element Model Updating (FEMU), is optimal. FEMU is a powerful, robust, intuitive and frequently used approach for the inverse characterisation of material properties. It allows the characterisation on the most complex geometries which have the advantage of containing more diverse deformation information than a simple dog bone specimen. Hence, more parameters can be characterised with a single test, which drastically reduces the amount of experimental work. To measure the complex deformation, multiple synchronised stereo-DIC (Digital Image Correlation) systems have been employed.

The first optimised complex specimen was a Double Perforated specimen which enables the characterisation of the strain hardening behaviour and a 3D anisotropic yield criterion of thick materials and is employed on thick HSS in this work. At present, however, no 3D anisotropic yield criterion exists which can describe the through-thickness inhomogeneity. Consequently, another complex geometry has been designed, namely a Descending Disk specimen. This geometry enables the characterisation of the through-thickness inhomogeneous material behaviour of thick materials and has successfully identified it on thick HSS in this work. Thereby, several independent material layers are identified simultaneously through the thickness of a thick material. Further, a pipeline was subjected to material identification. The material properties need to be obtained along the circumferential direction of which the specimens are curved. Due to the curvature, the conventional methods are not applicable any more. Moreover, the specimens cannot directly be clamped in a regular tensile bench. As a result, the curved specimens have been adapted with shims to compensate the curvature followed by an inverse material identification. The results of the general procedures employed in this dissertation have all been compared with the according conventional methods. As a result, more accurate and reliable results have been obtained with a substantial decrease in experimental work.

On route of the research, a peculiar phenomenon was observed, namely delamination. Since the origin is not completely understood at present, it has been scrutinised together with the influence of the strain rate and the according temperature on an S690QL grade. Thereby, the steelmaking process could be optimised to exclude delamination cracks in the S690QL an possible other thick HSS in the future.