The Two Thresholds of Fatigue Behaviour

Miller, K. J.

doi:10.1111/j.1460-2695.1993.tb00129.x

Cited by 219 publications

(110 citation statements)

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“…Investigations from Miller [32,33] support this argumentation. He proposed that cracks are always present with every kind of inhomogeneity and the only determining factor for the fatigue resistance is the question whether they reach a critical length that gives them the possibility of further growing.…”

Section: Discussionsupporting

confidence: 59%

Fatigue Behavior of Ultrafine-Grained Medium Carbon Steel with Different Carbide Morphologies Processed by High Pressure Torsion

Ruffing

Kobler

Courtois-Manara

et al. 2015

Metals

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Abstract:The increased attention ultrafine grained (UFG) materials have received over the last decade has been inspired by their high strength in combination with a remarkable ductility, which is a promising combination for good fatigue properties. In this paper, we focus on the effect of different carbide morphologies in the initial microstructure on the fatigue behavior after high pressure torsion (HPT) treatment of SAE 1045 steels. The two initial carbide morphologies are spheroidized as well as tempered states. The HPT processing increased the hardness of the spheroidized and tempered states from 169 HV and 388 HV to a maximum of 511 HV and 758 HV, respectively. The endurance limit increased linearly with hardness up to about 500 HV independent of the carbide morphology. The fracture surfaces revealed mostly flat fatigue fracture surfaces with crack initiation at the surface or, more often, at non-metallic inclusions. Morphology and crack OPEN ACCESSMetals 2015, 5 892 initiation mechanisms were changed by the severe plastic deformation. The residual fracture surface of specimens with spheroidal initial microstructures showed well-defined dimple structures also after HPT at high fatigue limits and high hardness values. In contrast, the specimens with a tempered initial microstructure showed rather brittle and rough residual fracture surfaces after HPT.

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Section: Discussionsupporting

confidence: 59%

Fatigue Behavior of Ultrafine-Grained Medium Carbon Steel with Different Carbide Morphologies Processed by High Pressure Torsion

Ruffing

Kobler

Courtois-Manara

et al. 2015

Metals

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“…A different two-threshold approach was discussed by Miller, who also acknowledged the importance of local conditions, in particular microstructure, in determining crack growth. [41]. He characterised fatigue behaviour in terms of a microstructural threshold which was important for short cracks, and a mechanical threshold which applied for long cracks.…”

mentioning

confidence: 99%

Short crack initiation and growth at 600°C in notched specimens of Inconel718

Connolley¹,

Reed²,

Starink³

2003

Materials Science and Engineering: A

115

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The natural initiation and growth of short cracks in Inconel ® 718 U-notch specimens has been studied at 600 o C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred microns in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life. IntroductionNickel-base superalloys like Inconel ® 718 (IN718) are used extensively for turbine discs and other components in industrial and aerospace gas turbines. Since turbine discs are safety-critical components, considerable effort is expended in determining their safe operating life. The lifing procedure adopted must enable the reliable prediction of the safe life of the disc, without being overly conservative. Once engines are in service, regular inspection of the engines and refinements of lifing models may enable life-extension programs to be considered. In the traditional predicted safe life philosophy, the life is declared based on laboratory specimen and component testing. In recent years a damage-tolerant approach to lifing has been adopted, in which it is accepted that components contain defects or inhomogeneities from which cracks can initiate. Assuming an initial flaw size c o , the fatigue life is determined as the number of cycles required to propagate the crack to some critical size c c . The initial size c o can be based on the maximum defect size present in the material, or on the minimum defect size detectable by nondestructive testing (NDT) [1]. Selection of c c is based on knowledge of the fracture toughness of the material, the limit load, the maximum allowable strain or maximum permitted compliance change for a particular component. Damage-tolerant lifing requires accurate information on fatigue crack initiation and growth rates. Fatigue lives are determi...

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“…The above pictures show that, according to Tomkins [18] and Miller [45,46], also in the presence of very severe stress concentration phenomena, the formation of micro/meso crack goes through an initial Stage I process which is shear dominated, followed by a Stage II propagation that is Mode I governed. With respect to the particular material considered in Figure 8, it is worth observing here that, by testing U-notched flat specimens characterised by stress concentration factors (calculated with respect to the gross sectional area) ranging in the interval 3.8 to 25 [44], the experimental average value of the angle between Stage I planes and notch bisector was seen to be equal to approximately 25°, that is, lower than the value of 45° which would be calculated by determining, according to continuum mechanics, the orientation of the plane experiencing the maximum shear stress/strain amplitude in a homogenous and isotropic material.…”

Section: Point Methods and Structural Volumementioning

confidence: 99%

Notch and mean stress effect in fatigue as phenomena of elasto-plastic inherent multiaxiality

Susmel

Atzori

Meneghetti

et al. 2011

Engineering Fracture Mechanics

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The present paper summarises an attempt of estimating fatigue lifetime of notched metallic materials by directly accounting for the degree of multiaxiality of the local elasto-plastic stress/strain fields acting on the fatigue process zone. In more detail, the proposed approach takes as its starting point the assumption that Stage I is the most important stage to be modelled to accurately estimate fatigue damage, and this holds true independently of the sharpness of the assessed geometrical feature. According to this initial idea, and by taking full advantage of the socalled Modified Manson-Coffin Curve Method (MMCCM), the hypothesis is then formed that the crack initiation plane is always coincident with that material plane experiencing the maximum shear strain amplitude. Subsequently, to devise an efficient design method capable of taking into account the detrimental effect of stress/strain gradients arising also from severe stress/strain concentration phenomena, the MMCCM is suggested here as being applied in terms of the Theory of Critical Distances (TCD), the latter being used in the form of the Point Method (PM). Further, in light of the well-known fact that the value of the mean stress/strain components in the vicinity of the stress/strain raisers' apices can be different from the corresponding nominal values due to the actual elasto-plastic behaviour of the material being assessed, it is shown, through the MMCCM itself, that also the mean stress effect can directly and accurately be treated as a problem of inherent multiaxiality. Finally, as a preliminary validation, the accuracy and reliability of the proposed approach is checked through several experimental results taken from the literature and generated by testing, under uniaxial fatigue loading, samples containing a variety of geometrical features, the effect of different nominal load ratios being investigated as well.

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The Two Thresholds of Fatigue Behaviour

Cited by 219 publications

References 1 publication

Fatigue Behavior of Ultrafine-Grained Medium Carbon Steel with Different Carbide Morphologies Processed by High Pressure Torsion

Fatigue Behavior of Ultrafine-Grained Medium Carbon Steel with Different Carbide Morphologies Processed by High Pressure Torsion

Short crack initiation and growth at 600°C in notched specimens of Inconel718

Notch and mean stress effect in fatigue as phenomena of elasto-plastic inherent multiaxiality

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