Numerical Investigation on Cyclic Plastic Deformation and Ratcheting Behaviour of a C-Mn Steel Crack Geometry

An essential component of ensuring the structural safety of mechanical components is the assessment of damage caused by fatigue in its operating temperature range. Conventionally, fatigue crack growth is attributed due to crack driving force range derived from classical fracture mechanics approach. However, mechanistically it is caused due to cyclic plastic deformation ahead of the crack tip. The nuclear power plant piping steel, SA333 Gr.6 employed in this study, is prone to dynamic strain aging in its operating temperature range. The tensile and low cycle fatigue tests conducted at temperatures ranging from 75 °C to 375 °C in literature, indicate that the temperature range for the dynamic strain aging phenomenon is between 175 °C and 300 °C. The present study investigates the cyclic plastic zone ahead of the crack tip, both at room temperature and at a temperature of 300 °C. Two-dimensional finite element analyses are conducted on full compact tension specimens with the Chaboche nonlinear kinematic hardening material model. In order to demonstrate the effect of crack sizes, a total of 12 distinct crack sizes are simulated. The study aims to analyze the behaviour of ratcheting at the maximum triaxiality point and at the midpoint of the cyclic plastic zone for a given crack size. The impact of the load ratio is also examined on the ratcheting behaviour. It has been shown that negative load ratios result in early shakedown.