Analysis of the crack opening displacement of a surface crack using X-ray tomography

摘要

Fatigue cracks were initiated in a ferritic martensitc steel under fully reversed loading. The test continued until the dominant cracks had reached a size corresponding to several microstructural units, and, consequently, had turned into a continuously growing crack. Miniaturized specimens containing one dominant crack were cut the test specimens and analyzed in the SPring-8 facility in Japan using X-ray tomography. Crack extension could be studied using an in-beam loading stage. A pre-defined load cycles, the fatigue test was interrupted and CT- images were taken at different load levels. Hence the displacement of the crack faces can be determined by comparing the CT-results at different load levels. This comparison can be performed using digital image correlation (DIC). This method is nowadays widely using in analyzing deformation fields on surfaces, but can also be generalized to volumetric correlation, from which the three-dimensional displacement can be derived. This method is applied to the CT results yielding estimates of the crack face displacement. The three-dimensional crack shape, on the other hand, can be transferred into a CAD model which, in turn, is the basis for a Finite Element model. Matching the corresponding values of the crack face displacement can be used to qualify the results of the digital volumetric correlation.