Small-scale testing methods based on nanoindentation and focus ion beam (FIB) milling have attracted increasing attention in the last decades due to its ability of evaluating mechanical properties with high spatial resolution. The development of techniques to evaluate fracture at the micrometre scale have been started 2005 by Di Maio and Roberts conducting micro-cantilever bending on thin coatings. Later, micro-bending was used to evaluate toughness for brittle materials by applying the linear elastic fracture mechanics (LEFM). In 2012 Wurster et al. pioneered in applying the elasto-plastic fracture mechanics (EPFM) on micron-sized samples which enables the applicability to other material classes but very brittle materials, as instance steel.

Ferritic-pearlitic steels in rail-wheel contacts are exposed to severe mechanical and thermal loadings. Under certain conditions, a formation of so-called white etching layers (WEL) and/or brown etching layers (BEL) - named after the appearance in light optical microscopy after etching with Nital - can occur. Acting as probable crack initiation sites, the knowledge about fracture behaviour and toughness of these near-surface microstructures is of utmost importance.

Within this work, V-notched micro-cantilever bending tests are executed in a stratified surface layer (combination of WEL and BEL) of a rail wheel. The fracture behaviour is investigated qualitatively by in-situ and post-testing SEM observations. Further, a quantitative evaluation is done by continuous stiffness measurement (CSM) enabling the calculation of the local fracture toughness K_IQ using EPFM.

Results show certain degree of plastic behaviour with fracture along the randomly orientated acicular martensitic grains in the WEL, calculated fracture toughness values K_IQ of around 16 MPam^1/2. Within the BEL region an increase in plastic behaviour and K_IQ can be observed. Correlation of the fracture behaviour of the micron-sized samples to the rail wheels from field can be outpointed.Hence, the presented results of micro-bending tests within this work provide important insights to understand the characteristic failure mechanism of stratified surface layers in rail wheel materials.