FRACTURE INITIATION AND MATERIAL CHARACTERISATION OF INDIAN STEEL TO PREDICT THE PERFORMANCE OF STEEL SEISMIC FORCE RESISTING SYSTEMS

Abstract

One of the most important parameters affecting the performance of structures is material behavior. A material must be tested against various types of loading so that we get a range of possible material behavior, which in turn will help us in the properly predicting the behavior of structures. The material in focus for this research is Indian steel and its three grades – E250, E350 and E450 - and the phenomenon considered here is earthquake induced Ultra Low Cycle Fatigue (ULCF), hence the hysteretic behavior of Indian steel structures meant to be a part of seismic force resisting system of the buildings are in question. The primary focus of this research is to create a database of values of parameters of two well-established models – (1) Chaboche’s hardening model, which will help us in characterizing the plastic behaviour and, (2) parameters of Stress-weighted ductile fracture model (SWDFM) and thereby creating a fracture locus which will enable us in predicting the ductile crack initiation due to ULCF – both of them needed to define the complete hysteretic behaviour of varying grades of Indian steel. So to achieve this, this work comprises of a series of monotonic tension experiments on steel coupons of varying geometries and grades of Indian steel, performed in conjunction with FEM simulation. The computational simulations employ parameters of Combined Isotropic-Kinematic Hardening parameters which are calibrated iteratively via assessing against experimental data so that the simulations are accurately matching experimental behaviour at the end. The calibrated model of each specimen then has been used to extract critical values of triaxiality (T), equivalent plastic strain (PEEQ) and Lode parameter (X) at critical section of notches where ductile fracture is known to initiate. The range of triaxiality and Lode parameter values so obtained at material level have been used to predict the type of failures, at member level, which can be expected from above mentioned Indian steel grades. Furthermore, the equivalent plastic strain (PEEQ) values obtained for all our specimens, together with triaxiality (T) and Lode parameter (X) values has been used for two purposes – (1) To obtained critical values of calibrated parameters of SWDFM for each specimen and (2) To create a 3D plot representing fracture locus – both of them enabling us in predicting the ductile crack initiation for three grades of Indian steel.