In-situ damage detection and extrusion kinetics with light optical microscopy
The lifetime of materials is a critical parameter for the reliability of devices. Further, rising requirements on materials lead to the necessity of a more distinct understanding of microstructural influences on fatigue mechanisms. The progress of fatigue in alloys can be divided in several stages driven by cyclic irreversibilities, such as the formation of dislocation structures and PSBs, growth of extrusions at the surface and crack initiation and growth. The lifetime of stress scenarios beyond the high cycle fatigue regime is mainly dominated by early stage fatigue mechanisms until crack initiation. Currently used in-situ fatigue crack initiation characterization methods mostly operate either under vacuum or with interrupted experiments.
In [Straub et al.] a multiaxial resonance fatigue setup was designed for the detection of the early stages of fatigue occurrence in atmospheric testing conditions. Due to operating frequencies of 2 kHz the setup enables fast and easy measurements and therefore data acquisition up to the very high cycle fatigue regime. Complementing in-situ image acquisition systems in the multiaxial resonance fatigue setup allow damage localization on the microstructure scale. Combinations with two different imaging systems will be presented and compared.
One experimental setup uses stroboscopic bidirectional illumination with blue LEDs to acquire dark field-like images of the sample’s surface. This setup is suited to capture statistical in-situ data on damage initiation and crack formation for the whole micro sample.
The second image acquisition technique is founded on a novel microscopy technique, which allows high resolution in-situ observation of extrusion formation at a mechanistic scale. Namely, the ROCS-microscopy (Rotating Coherent Scattering Microscopy), up to now developed and used for the observation of biological cells, allows time and position dependent imaging (up to 190 nm image resolution during fatigue experiments). This method employs illumination of the sample’s surface with a collimated laser beam with a defined tilt and a variation of the azimuthal angle and integrates the back scattered light from different angles into an image.
Both experimental setups allow the occurrence and propagation of extrusions. The differences in the multimodal datasets will be presented and their potential use for validation of theoretical models on the microstructure or mechanisms level will be discussed.