The Effect of Mean Stress on Mixed Mode (I + Iii) Fatigue Thresholds

Abstract

Mode I, mode 111 and mixed mode (I + 111) fatigue crack growth threshold tests have been performed on a 3.5YoNiCrMoV steel at a range of mean stresses corresponding to load ratios R = 0.06 to 0.5. Angled slit three point bend specimens were used for mixed mode (I + 111) tests, circumferentially slit round bars for mode I11 tests and conventional three point bend specimens for mode I tests.Test results were divided into those specimens which had failed and those in which no extension of the initial slit had occurred. The results were compared with the response predicted by two existing mixed-mode threshold models. It was found that a model based on the magnitude and direction of mode I crack opening gave results which were in better agreement with the experimental results than a model based on branch crack formation. NOMENCLATURE E = Young's modulus AK,, AK,, , AK,,, = mode I, mode I1 and mode 111 stress intensity factor ranges, respectively AK,,,, = mode I fatigue threshold stress intensity factor range AKA = equivalent mode I stress intensity factor range for an angled slit specimen AK* = mode I stress intensity factor range for a crack in the direction of the maximum principal stress R = load ratio (rninimum/maximum load in a fatigue cycle), ! ? = orientation of initial slit to specimen axis 6 = crack opening displacement 4 =twist angle of a crack with respect to the initial slit p = shear modulus v = Poisson's ratio uy, T~ = yield stress and shear yield stress, respectively

Crack Propagation Under Mixed Mode (I + Iii) Loading

“…The threshold condition has not been considered in this paper, but may be equated to the pure mode I condition through the work of Pook [2] or Yates and Mohammed [4]. Interestingly, the straightforward macroscopic mode I component of the stress intensity factor, Eqn.…”

“…It does not seem to be necessary to calculate a local branch crack mode I stress intensity factor. The threshold condition is not directly controlled by the mode I component of the stress intensity but is a function of either the local branch crack stress intensity [2] or the local crack tip displacements [4].…”

“…The factors controlling the growth of fatigue cracks under mixed mode loading conditions have yet to be fully resolved and will require experimental, analytical and numerical work on all modes of crack extension. Already, several models exist to predict the onset of crack growth under mixed mode loading [1][2][3][4]7,8]. In this paper the results of fatigue tests on angled slit beams subject to three point bend loads are presented and a model describing the crack propagation rate is offered.…”