Characteristics of fracture toughness of metals during cyclic loading. I

Troshchenko, V. T.; Prokopenko, A. V.; Покровский, В. В.

doi:10.1007/bf01524626

Cited by 9 publications

(3 citation statements)

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“…In the literature there is conflicting evidence on the proportion of static and fatigue fracture toughnesses of materials [1][2][3][4][5][6][7][8][9][10][11][12]. Cyclic fracture toughness is frequently shown to be higher than the static value [ 1-31.…”

Section: Introductionmentioning

confidence: 99%

“…In some investigations, where alloyed steels were tested at low temperatures, a decrease in fatigue fracture toughness as compared to the static fracture toughness was observed [6-121. In some of those investigations a specific feature of fatigue crack growth was observed during the final stages of growth when portions of stable growth alternate with those of crack jumps (cleavage zones) [ 7,9,11,12]. However the conditions of the transition from fatigue crack growth to brittle fracture, as well as the proportions of static and fatigue fracture have not been studied in depth.…”

Section: Introductionmentioning

confidence: 99%

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Unstable Fatigue Crack Propagation and Fatigue Fracture Toughness of Steels

Troshchenko

Pokrovsky

Ясній

1994

Fatigue Fract Eng Mat Struct

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This paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high-pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions.It has been shown that steels in an embrittled state, caused primarily by thermal treatment and lowtemperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature.An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour. NOMENCLATURE a,, of= initial, and final, crack length B = specimen thickness d = crack jump (cleavage zone) length da/dN = fatigue crack growth rate ep = preliminary plastic strain F = specimen cross-sectional area after plastic prestraining F, = specimen original cross-section K,,, K,,, = minimum, and maximum, stress intensity factor (SIF) KO =initial maximum SIF KO, = SIF corresponding to crack opening Kth = A&/( 1 -R) = threshold SIF K , = dynamic fracture toughness K,,, K , = static plane-strain, and plane-stress, fracture toughness Kit, KE = fatigue fracture toughness corresponding to the first, and final, crack jump Pmi,, P, , , = minimum, and maximum, load PM = maximum load under static loading R = stress ratio (R = Kmin/K,.,) T = temperature W = specimen width Aa = fatigue crack extension between two sequential cleavage zones AK = K,, -Kmin = SIF range AK,,, = range of the threshold SIF = 0.2% offset yield stress AK,,,,, = K,,, -KO, = effective range of the threshold SIF o, = ultimate strength (I/ = reduction of area 6 = relative elongation SIF = stress intensity factor

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Unstable Fatigue Crack Propagation and Fatigue Fracture Toughness of Steels

Troshchenko

Pokrovsky

Ясній

1994

Fatigue Fract Eng Mat Struct

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“…The size of such a crack generally decreases with increasing strength of the alloy and increases in torsion as compared with tension-compression [2,3].…”

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Nonlocalized Fatigue Damage of Metals and Alloys. Part 1. Inelasticity, Investigation Methods, and Results

Troshchenko

2005

Strength Mater

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Analysis of data presented in the literature and results of unique studies revealed that nonlocalized fatigue damage of metals and alloys, which manifests itself in the initiation and propagation of a large number of microcracks distributed arbitrarily in the bulk of the material, can be considered with the use of the characteristics of inelastic deformation such as inelastic strain and inelastic strain energy per cycle. The author studied the nature of the material inelasticity and methods of its investigation. Analysis has been made of the hysteresis loop shape, width, and area, as well as of the main regularities of inelastic deformation of metals and alloys.Keywords: nonlocalized fatigue damage, inelastic strain, inelastic strain energy.Introduction. The process of fatigue of metals and alloys can be divided into two stages: the stage of nonlocalized fatigue damage and that of localized fatigue damage where the process of fatigue is defined by the propagation of the dominant crack that leads to final fracture.As numerous investigations showed, the stage of the fatigue crack initiation is longer lasting than the stage of its propagation. Figure 1 shows the results of generalization of some of those investigations performed in [1], which are presented in the form of the relation between the ratio of the number of cycles to the initiation of a fatigue crack of length about 0.05 mm under axial loading of smooth specimens from various materials to the number of cycles to fracture and the number of cycles to fracture.It is seen that under high-cycle fatigue, which corresponds to the number of cycles to fracture equal to 10 5 and more, the stage of the fatigue crack initiation accounts for more than 90% of the total number of cycles to fracture.In the case of the fatigue fracture in the presence of stress concentration, fretting, etc., the stage of crack initiation is of shorter duration, yet it remains dominating.The size of a fatigue crack corresponding to the transition from the stage of nonlocalized to the stage of localized fatigue damage depends on the material structure, the level of acting stresses, loading regime, stress state type, and other factors. The size of such a crack generally decreases with increasing strength of the alloy and increases in torsion as compared with tension-compression [2,3].Thus, in [3] for steel 45 (σ u = 516 MPa) it is shown that, at stresses equal to the fatigue limit for the test duration 10 7 cycles, the size of the maximum crack in tension-compression and in torsion is 0.05 and 0.17 mm, respectively, and for 12KhN3A steel of higher strength (σ u = 950 MPa), it is 0.03 and 0.08, respectively. These crack dimensions are close to those used for plotting the relation in Fig. 1.

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