Mixed Mode (I + Iii) Fatigue Thresholds in a Forging Steel

Abstract

The results of mixed mode (I + 111) fatigue tests on a low pressure steam turbine rotor steel are presented. Tests were performed on a uniaxial servohydraulic fatigue machine fitted with a torsion-bending loading frame. The onset of failure was marked by fatigue facets which grew perpendicular to the maximum principal stress direction. A criterion for mixed mode (I + 111) fatigue thresholds based on the orientation and the crack opening displacement of fatigue cracks is proposed. NOMENCLATURE u = crack length E = Young's modulus G = strain energy release rate A K , , A K , , , AK,,, = mode 1, mode 11 and mode 111 stress intensity factor ranges, respectively AK,,,, = mode 1 fatigue threshold stress intensity factor range N = cycles R = fatigue load ratio (minimum loadmaximum load) 6 = crack opening displacement 4 = twist angle of crack with respect to initial crack or slit (r, , ti = yield stress and shear yield stress, respectively u N , T ,~ = nett section tensile and shear stresses. respectively p = shear modulus Y = Poisson's ratio

Criterion for initiation of cracks under mixed-mode I + III loading

“…Observations in the literature, such as those of Yates and Miller (1989) also support a correlation between the twist angle and the ratio of stress intensity factors based on the idea of a locally opening mode crack. Cooke and Pollard (1996) also use the maximum energy release rate criterion, the requirement that mode III on incipient crack equal to zero (K III = 0), and the condition that mode I on the incipient crack K I is a maximum and show that all four criteria predict the same twist angle as a function of K ∞ III /K ∞ I as long as K ∞ III /K ∞ I < 1.4; beyond this level the maximum energy release rate criterion exhibits two solution branches.…”

Development of fracture facets from a crack loaded in mode I+III: Solution and application of a model 2D problem

“…The fact that similar phenomena are observed in such diverse materials as glass (Sommer, 1969), alumina (Suresh and Tschegg, 1987), steels (Hourlier and Pineau, 1979;Yates and Miller, 1989;Lazarus, 1997), rocks (Pollard et al, 1982;Pollard and Aydin, 1988;Cooke and Pollard, 1996), PMMA (Lazarus et al, 2008), gypsum and cheese (Goldstein and Osipenko, 2012) strongly suggests that the microstructure of these materials plays little or no role and that the standard tools of macroscopic Linear Elastic Fracture Mechanics (LEFM) should be able to explain the observations made.…”

“…The majority of these papers were of experimental nature; see the works of (Sommer, 1969;Knauss, 1970;Palaniswamy and Knauss, 1975;Hourlier and Pineau, 1979;Pollard et al, 1982;Suresh and Tschegg, 1987;Pollard and Aydin, 1988;Yates and Miller, 1989;Hull, 1993;Yates and Mohammed, 1994;Hubbard, 1995;Hull, 1995;Cooke and Pollard, 1996;Lazarus, 1997;Lazarus et al, 2001aLazarus et al, ,b, 2008Lin et al, 2010;Goldstein and Osipenko, 2012), among others. All these experimental investigations have shown that the crack propagates through formation of small fracture "facets" (or "lances" in Sommer (1969)'s terminology) which may either abruptly "tilt" or gradually "twist" about the direction of propagation; the wording "crack front segmentation" is often used to designate the phenomenon.…”

J‐integral in the description of fatigue crack growth rate induced by different ratios of torsion to bending loading in AlCu4Mg1

“…From the review of literature it appears that equations proposed by Sih [3] and Richard [1] give the best results in description of experiment results. Mixed modes I and III were considered by Yates and Miller [4]. In [4] they proposed a model for description of fatigue crack propagation for mixed modes (I+III), reduced to mode I.…”

“…Mixed modes I and III were considered by Yates and Miller [4]. In [4] they proposed a model for description of fatigue crack propagation for mixed modes (I+III), reduced to mode I. Specimens bended in three points were tested.…”