Laws of plastic strain and fatigue fracture of metals in torsion

Troshchenko, V. T.; Dragan, V. I.

doi:10.1007/bf00767589

Strength Mater

1982

DOI: 10.1007/bf00767589

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Laws of plastic strain and fatigue fracture of metals in torsion

V. T. Troshchenko

V. I. Dragan

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Cited by 9 publications

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“…Investigation into the development of the plastic deformation zones and fatigue crack initiation therein in torsion of steels 45 and 12KhN3A revealed [27] that there is a good correlation (Fig. 3) between the ratio of the product of the microcrack average length by the number of microcracks per unit surface area and that at stresses equal to the fatigue limit, q q τ −1 , and the ratio between inelastic strain per cycle and inelastic strain at stresses equal to the…”

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“…Logarithmic vibration decrement δ determined from the diagram of free damping vibrations [71,72] by the formula δ = + ln , a a i i 1 (27) where a i and a i + 1 are the amplitudes of two neighboring cycles of damping vibrations; the quality factor of the vibrating system Q, which is defined fairly accurately by the ratio of the specimen vibration amplitude at resonance a to the specimen strain a st from a statically applied force equal to the amplitude value of the disturbing force …”

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

Troshchenko

2005

Strength Mater

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“…The inelastic strain per cycle equal to the width of a hysteresis loop in the stress-strain coordinates was taken as a characteristic of inelastic deformation of the steels [22][23][24][25][26][27]. Figure 1 illustrates variation of the stress and strain signals in a specimen.…”

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Scatter in the fatigue characteristics of steels and its analysis with allowance for cyclic inelastic strains

2007

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The regularities of the scatter in the characteristics of fatigue and inelasticity of steels 45 and 1Kh13 have been studied under steady-state, step, and block loading. It is shown that the use of the inelastic strain per cycle as a measure of the fatigue damage accumulation rate makes it possible to take into account the scatter in the number of cycles to fracture and in the fatigue limits and to substantiate the cumulative fatigue damage rule.Keywords: fatigue, inelastic strain per cycle, scatter in mechanical properties, number of cycles to fracture, cumulative damage rule. Introduction.It is known that the number of cycles to fracture under high-cycle loading, and under step and block loading in particular, and the magnitudes of the fatigue limit of metals and alloys are subject to appreciable scattering [1-3 and others].It is important to investigate the regularities in this scatter and to justify the characteristics of the fatigue damage accumulation rate that would enable presenting the results of the investigation of the fatigue of metals and alloys without essential scatter.Cyclic inelastic strain is a promising characteristic for assessing the fatigue damage accumulation rate at the stage of nonlocalized fatigue damage, which is a stage where the main fatigue crack leading to final fracture of a specimen or a component is absent [4, 5, and others].Investigations of cyclic inelasticity (cyclic plasticity) are mainly performed in the low-cycle fatigue region at low loading frequencies. In this case, primary consideration is the investigation of interrelation between structural changes in the metal, which lead to the initiation of a fatigue crack, and the characteristics of cyclic inelasticity [6-8 and others], as well as justification of the strain and energy criteria of fatigue with allowance for the mean stress in a cycle, complex stress state, and loading regime [9-13 and others]. At the same time, analysis of many classical regularities in the high-cycle fatigue of metals and alloys such as, for instance, an essential scatter in the number of cycles to fracture and fatigue limits, inconsistency between the test results under step and block loading and the linear cumulative damage rule, etc., does not always involve the use of the results of studying inelasticity.Currently, much attention has been given to the consideration of the process of the fatigue of metals and alloys as a process of the fatigue crack development [14-17 and others]. No matter how attractive these approaches, one should bear in mind that, under high-cycle loading, the initiation stage for a fatigue crack of size 0.05 mm takes up to 90% and more of the total life of smooth specimens [18]. Therefore, the processes occurring at the stage of nonlocalized fatigue damage, for which cyclic inelasticity is an integral characteristic, can be determining for the regularities observed in high-cycle fatigue of metals and alloys.

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Fatigue strength, laws of inelastic deformation, and microcrack nucleation in steel 15Kh2MFA

et al. 1993

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Laws of plastic strain and fatigue fracture of metals in torsion

Cited by 9 publications

References 1 publication

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

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

Scatter in the fatigue characteristics of steels and its analysis with allowance for cyclic inelastic strains

Fatigue strength, laws of inelastic deformation, and microcrack nucleation in steel 15Kh2MFA

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