FEATURES OF THE STRESS-STRAIN STATE IN THE NECK OF A RING SPECIMEN UNDER TENSION ON SEMICIRCULAR CYLINDRICAL MANDRELS

In this paper, the stress-strain state of a ring specimen under tension on semicircular cylindrical mandrels is analyzed by three-dimensional finite-element modeling. The material of unirradiated fuel cladding, namely cold-worked EK164-ID austenitic steel, is used as an example. The hardening curve of the material is determined by the iterative inverse method. The finite element model is validated by the variation of the geometrical parameters of the specimen gauge areas. The modeling results show that, before failure, the neck of the ring specimen experiences a triaxial stress state characterized by a pronounced non-uniform distribution of plastic strains and stresses along the neck cross-section. This limits the possibility of interpreting the stress-strain state in terms of plane-stress or plane-strain approximation. The analytical expressions proposed in the literature for interpreting the stress-strain state in the neck of a flat specimen with a rectangular cross-section under tension have limited applicability to similar areas of a ring specimen, and they may cause significant errors in the experimental determination of the hardening curve and the stress triaxiality factor. The effectiveness of applying 3D finite element modeling combined with the iteration procedure of plotting the hardening curve in order to analyze the stress-strain state in a ring specimen under tension on cylindrical semicircular mandrels is demonstrated. The critical values of equivalent stress, equivalent plastic strain, and the stress triaxiality factor in the neck of a ring specimen before failure are determined for the cold-worked EK164-ID austenitic steel.