Investigation on the plastic zone during crack growth experiments in high-alloy CrMnNi TRIP/TWIP steels by means of the complementary in situ characterization techniques: digital image correlation, infrared thermography and acoustic emission measurements
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This thesis describes the analysis of the material behaviour of two high-alloy CrMnNi steels with TRIPand TWIP effect respectively. The focus of the investigations was the systematic characterisation ofthe mechanical behaviour under uniaxial tensile load as well as under cyclic load in the context offatigue crack investigations using advanced in situ techniques. These complementary in situ techniquesare (1) digital image correlation (DIC), (2) infrared thermography (IR-TG) and (3) acoustic emission (AE)measurements. The simultaneous application of these methods is accompanied by a multitude ofchallenges that had to be overcome. These challenges consisted mainly of the experimental setup ofthe entire equipment, the synchronization of all methods and equipment and the selection of thecorrect experimental parameters.The high-alloy CrMnNi steels have excellent mechanical properties in terms of high strength combinedwith high ductility. In order to better understand the damage behaviour of these austenitic steels,crack growth investigations under cyclic loads were carried out in addition to the tensile tests. Inparticular, the conditions as well as the development of the plastic zone in front of the growing crackwere central points of the investigation.Interesting results could be obtained from the tensile tests and the crack growth tests with the help ofthe in situ methods used and compared or connected with each other. These include, for example:Tensile test: Kinetics and damage mechanisms (AE), Dissipated energy and power (IR-TG), Displacements and strain fields (DIC)Crack growth test: Crack growth data (DIC and IR-TG), Process zones and total strain fields (DIC), Temperature development (IR-TG)Some of the data comparisons between the different techniques showed a very good agreement, suchas the start of necking in the tensile test or the crack growth data in the fatigue tests. The basic aim ofthe investigation, to bring all in situ methods into agreement, could be fulfilled for the tensile test. Forthe crack growth investigations, first important findings could be collected with the methods of DICand IR-TG.