Evaluation of Ductile Fracture Using Micro-samples

Abstract

The FEM simulations in the field of design and safety assessment represent very powerful tools. Because of the growing demands on safety, reliability and longer lifetime period of structural facilities it is neccesary to use precise material data in a numerical simulation that considers ductile damage. Ductile fracture occurs after all the material plasticity is consumed. The plastic strain represents one of the most crucial parts in the process of ductile damage. If the damage additionally affects plastic behavior of the material then we can talk about the so called coupledcontinual damage model.The FEM simulations are strongly limited by material input data and available material models. Many of the current calculations are still performed on the basis of standard tensile tests, if not only on database data or data from literature. Such a material description is not sufficient for accurate design assessment and detailed material behaviour description has to be used for a reliable results.Standard tensile test is mainly based on uniaxial sample loading and small strains. Gained standard tensile test results are useful for elastic solutions or elastic-plastic solution for small plastic strains. If states near to fracture are to be considered, more complex material description taking into account multiaxial loading conditions should be applied. Thus samples of various geometries tested under various loading modes have to be used. On the basis of these tests a complex material behavior model covering elastic and plastic material behavior for various triaxiality states can be obtained. This kind of material behaviour description allows a wide range of applications from calculation of the component limit loading conditions to material properties conversion for samples of different sizes.The current paper is dealing with ductile damage parameters determination based on small samples for the structural ferriticsteel 15Ch2NMFA widelly used in the nuclear industry. The newly proposed specimen size is very small in comparison to standard size specimens. Different shapes of these test samples were tested and evaluatedin order to cover the whole range of the Stress Triaxiality.The obtained material plastic damage parameters are subsequently applied to FEM simulation and to the performed experimental tests. The comparison of the experimental and simulation results exhibits very good agreement and thus confirms applicability of the material models considered for reliable simulation of the material investigated.