Prediction of crack-opening stress levels for 1045 as-received steel under service loading spectra

Abstract A crack growth analysis based on a fracture mechanics approach was used to model the fatigue behavior of as-received 1045 steel specimens for three load spectra scaled to various maximum stress range levels. The crack growth analysis was based on an effective strain-based intensity factor, a crack growth rate curve obtained during closure-free loading cycles, and a local notch strain calculation based on Neuber’s rule. The crack-opening stresses were modeled assuming that the crack-opening stress when it is not at the constant amplitude steady-state level for a given stress cycle builds up as an exponential function of the difference between the current crack-opening stress and the steady-state crack-opening stress of the given cycle unless this cycle is below the intrinsic stress range for crack growth or the maximum stress in the cycle is below zero in which cases the crack-opening stress does not change. The crack-opening stress model was implemented in a fatigue notch model and the fatigue lives of the notched annealed 1045 steel specimens under the three different load spectra scaled to several maximum stress levels were estimated. The average measured crack-opening stresses for the various histories and levels were within between 8 and 13% of the average calculated crack-opening stresses. The fatigue life predictions based on the modeled crack-opening stresses were in good agreement with the experimentally obtained fatigue data. The averages of the measured crack-opening stresses and those calculated using the model were nearly the same for all the histories examined. When these average crack-opening stresses were used in the life prediction model they gave predictions as good as those obtained by modeling crack-opening stress on a cycle by cycle basis. The use of a crack-opening stress level corresponding to the cycle causing a reduction to a crack-opening stress reached for 1/200 of the cycles in the history gave a conservative estimate of average crack-opening stress for all the histories.