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in-situ X-Ray Material Characterization

In-Situ X-Ray Testing

 

The current understanding of how materials fail due to mechanical loading are based on observations made on the materials‘ surface. Alternatively, failure mechanisms are studied by dissecting the material after failure once the mechanical load has been removed, for instance by classical failure surface analysis. While these approaches define the current state-of-the-art, the situation can certainly be improved by developing techniques for observing damage and failure evolution on-line, i.e., while the material is still in the loaded state.

To this end, we are studying materials under applied loads close to their failure strengths using X-Ray Computer Tomography. CT data sets are recorded in an initial unloaded reference state, and at several loading states. From these data, full 3d displacements and strain maps are computed using Digital Volume Correlation and Image Registration algorithms. These strain maps allow the automated identification and evaluation of damage features such as microcracks within the full volumetric test specimen space, i.e., without any restriction to an arbitrarily chosen failure surface.

Figure above: studying emerging microcracks using in-situ X-Ray CT. A sample (“Materialprobe”) is subjected to tensile loading and a CT volumetric dataset is recorded. This dataset is then compared against a prior CT dataset recorded at the unloaded state. The differences between both datasets yield a deformation map. This deformation contains large rigid body translations and rotations (blue frame), which are irrelevant for computing strains. Once these are identified, the physically relevant strain field (green frame) can be computed.

Using this approach even very small emerging features may be identified, see the image below: Here, a basalt-fibre/epoxy specimen is subjected to tensile loading with the force direction normal to the image plane. While the 0°- and ±45° plies carry most of the load, the comparatively weak 90° ply exhibits a localized delamination feature, as indicated by the red color which is proportional to out-of-plane strain.