Computed tomography gives us a glimpse into inner worlds. It has become an essential tool in medical applications, used to perform routine investigations in the human body. However, it also lends itself well to X-raying composite materials such as reinforced concrete and provides a non-destructive method of examining them. So what happens inside a concrete component when it is exposed to mechanical strain? Where do cracks occur? How are they created? And how do they grow as they are exposed to more and more strain?
These are the questions that researchers at Fraunhofer ITWM are addressing. In the BMBF-funded project »Detection of Anomalies in Large Spatial Image Data« (DAnoBi), they are developing mathematical and statistical methods together with working groups at the universities in Kaiserslautern, Ulm, and Magdeburg in order to find, completely segment, and record crack structures in concrete in a robust and automatable way using computed tomographic data. »Even in noisy CT data of small concrete samples, we were able to not only detect tiny, micrometer-sized cracks, but also identify the voxels (data point in a three-dimensional grid) belonging to them. To do this, the cracks need not be wider than a voxel. This means that in a concrete cuboid with an edge length of 15 centimeters, we can find cracks that are 100 micrometers wide,« says Dr. Katja Schladitz, a scientist at Fraunhofer ITWM. To achieve this, Schladitz and her team combined machine learning methods, structural modeling and imaging, and statistical methods to detect the cracks. »Their thickness and shape allow conclusions to be drawn about how post-crack behavior and microstructure are related. In the institute's own CT device, however, we have so far only observed them before or after, not during, loading,« says the mathematician.