Sub project B04: Deformation and Fracture of Surface Structured Specimens

Surface machining processes affect the morphology of the near surface microstructure due to thermal and mechanical loadings. Hardening processes as well as notch effects are connected to these machining processes and may significantly change the mechanical behaviour of the material. The project focuses on the analysis of crack initiation behavior depending on machining process, notch-geometry, surface morphology (surface conditions, grain-size and grain-orientation-distribution) and mechanical loading. Another aim is the development of a reliable, non-destructive method to predict critical failure locations prior to fatigue testing.
The investigations are conducted on two materials: cp(commercially pure)-titanium and a case hardening steel 16MnCr5. Surfaces of the specimens are machined by different structuring methods including micro milling, micro grinding, particle structuring and additive manufacturing. Within a full metallographic characterization, all determining factors concerning the surface morphology of a specimen are analysed. Subsequently, the mechanical properties are tested under quasistatic and cyclic mechanical loading, including optical strain measurement technique and acoustic emission spectroscopy to validate a model to predict failure location.
In the first funding period metallographic investigations and nanoindentation tests were conducted in order to characterize the near surface microstructure as a function of distance to the surface, (Fig. 1). It could be shown that the fatigue behaviour of cp-titanium depends on the relation between notch size and grain size. If the notch-depth is smaller than the grain size, the endurance limit remains unchanged. If the notch-depth exceeds the grain size, the endurance limit is reduced and the crack initiation takes place at the notch root (Fig. 2). Beside the notch effect, different values influence crack initiation behaviour, such as grain size, grain orientation and surface conditions, as shown in Fig. 3.

Literature:

[1] C. Godard, A. Klingler, T. Junker, E. Kerscher: The Applicability of Nanoindentation for the Examination of Microstructured Areas in CP Titanium Samples, Pract. Metallogr. 52 (2015) 6, pp. 314-322

[2] C. Godard, E. Kerscher: Characterization of the Fracture Morphology of Commercially Pure (cp)-Titanium Micro Specimens Tested by Tension-Compression Fatigue Tests, Proc. Mat. Sc. 3 (2014), pp. 440-446

Contact: M. Sc. Luisa Böhme