Crack growth in IN718 at high temperature

Crack growth tests have been performed on specimens with rectangular cross-section of Inconel 718 at 550 and 687°C in order to examine the low-cycle fatigue behaviour. Frequencies of 0.5 and 10 Hz were employed. Symmetric strain-controlled tests with a strain range of Δε=1.5% and load controlled zero-tensile tests with a maximum load of 700 MPa were performed. A 10-min hold time at maximum strain was also added to the strain controlled 0.5 Hz tests. Additional creep crack growth tests were performed at 687°C. Fracture surfaces were examined to determine the fracture mode. At 687°C the fracture surface was found to be intergranular at 0.5 Hz and to be a mixture between inter- and transgranular at 10 Hz. At 550°C the fatigued specimens showed a mixed fracture mode at 0.5 Hz and a transgranular fracture mode at 10 Hz. For the fatigued specimens showing an intergranular fracture mode, crack propagation rates as a function of stress intensity factor range proved to be time-dependent and to be slightly higher than the pure creep crack growth rates. Experimentally observed crack growth rates were used as input to a finite element simulation to determine the fraction of crack closure. The material was described in terms of the visco-plastic constitutive equations developed by Bodner and Partom. The material parameters were found by fitting simulations to the experimental data. Crack opening was found to take place almost immediately upon load reversal from a state of maximum compression.