On the use of critical distance theories for the prediction of the high cycle fatigue limit stress in notched Ti?6Al?4V*1

High-cycle-fatigue (HCF) fracture mechanism of nickel-based superalloy IN 792 coated with Pt-modified aluminide outward-diffusion coating is studied with focus on the influence of coating cracks. It is found that cracking of the diffusion coating prior to HCF tests has little influence on the fatigue limit of specimens with thin coating (50 μm) but lowers the fatigue limit of specimens with thick coating (70 μm). By fractographic analysis, three types of fractural modes are established according to their crack initiations: internal, external and mixed. While external fractural mode is related to the propagation of existing cracks in the coating, internal facture initiates often at Ti-Ta-W-rich carbides and/or topological-close-packed (TCP) phases and grainboundaries in the superalloy. Increasing the thickness of diffusion coating or the amplitude stress promotes the fractural mode transition from internal/mixed to external. The influence of precracking of coatings on the HCF fracture mechanism can be qualitatively explained by its influence on the stress intensity factor.

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Fatigue Analysis of Metallic Components Using Critical Distance Methods

Volume 11: Mechanics of Solids, Structures and Fluids ◽

10.1115/imece2009-10901 ◽

2009 ◽

Author(s):

Somnath Chattopadhyay

Keyword(s):

Material Parameter ◽

High Cycle Fatigue ◽

Volume Effect ◽

Equivalent Strain ◽

Critical Distance ◽

Point Method ◽

Design Rule ◽

Material Surface ◽

Cycle Fatigue ◽

Number Of Cycles

The high cycle fatigue strength of crack-like discontinuities in metallic structures has been investigated using the critical distance approaches. Two methods have been employed, (a) the point method, and (b) the imaginary crack method. In the point method, the stress at a critical point within the material volume is chosen as the governing fatigue criterion. The effective parameter is the distance “d” from the material surface, which is a material property and the reference parameter is the fatigue limit. The imaginary crack method involves introduction of a sharp crack at the root of a notch and the length of the crack, “l0” assumed a material constant. The point method leads to a practical design rule that uses fatigue design curves expressed in terms of equivalent strain range versus number of cycles to failure. The equivalent strain is evaluated at a distance “d” from the crack tip. In the imaginary crack method, the effective crack length is taken as the sum of the actual crack and the material parameter “l0”. It is concluded that the high cycle fatigue has a volumetric character and the proposed methods introduce the volume effect in the determination of stress and strain fields as well as the fatigue life. Using the material parameter, the number of cycles to initiate a fatigue has been determined.

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Effects and mechanisms of low cycle fatigue and plastic deformation on subsequent high cycle fatigue limit in nickel-base superalloy Udimet 720

Materials Science and Engineering A ◽

10.1016/s0921-5093(01)01742-7 ◽

2002 ◽

Vol 332 (1-2) ◽

pp. 236-248 ◽

Cited By ~ 14

Author(s):

Weiju Ren ◽

Theodore Nicholas

Keyword(s):

Plastic Deformation ◽

Fatigue Limit ◽

High Cycle Fatigue ◽

Low Cycle Fatigue ◽

Nickel Base Superalloy ◽

Cycle Fatigue ◽

Nickel Base ◽

Base Superalloy ◽

Udimet 720

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Influence of Mean Stress on the Fatigue Behavior of 304L SS in Air and PWR Water

Volume 1: Codes and Standards ◽

10.1115/pvp2005-71064 ◽

2005 ◽

Cited By ~ 9

Author(s):

H. D. Solomon ◽

C. Amzallag ◽

A. J. Vallee ◽

R. E. De Lair

Keyword(s):

Fatigue Limit ◽

Fatigue Behavior ◽

Secondary Hardening ◽

304L Stainless Steel ◽

Mean Stress ◽

Cycle Fatigue ◽

Different Temperatures ◽

Limit Stress ◽

Conventional Models ◽

Stress Models

Load-controlled experiments on 304L stainless steel were run in Air and PWR water, at 150°C and 300°C, with and without a mean stress of 100MPa. These experiments were run to determine the influence of temperature, environment, and mean stress on the 107 Cycle Fatigue Limit stress amplitude. A 100MPa mean stress was found to have different effects at the different temperatures and environments. In contrast to all the conventional models used to describe the effects of mean stress, when the testing was done at 300°C (for both air and PWR water), a 100MPa mean stress was found to raise the 107 Cycle Fatigue Limit relative to that observed without a mean stress. This was ascribed to the effect of the hardening due to the initial straining and to secondary hardening, both of which are more pronounced at 300°C than at 150°C. The increased initial and secondary hardening resulted in the development of less non-elastic strain, thereby improving the fatigue behavior. In PWR water at 150°C, a 100MPa mean stress reduced the 107 Cycle Fatigue Limit by more than that predicted by conventional mean stress models, but in air at 150°C, the decrease in the endurance limit was more in keeping with the predictions of these models. This difference was ascribed to the effect of the PWR water, in the absence of significant initial straining and secondary hardening.

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Proposal of Fatigue Limit Design Curves for Additively Manufactured Ti-6Al-4V in a VHCF Regime Using Specimens with Artificial Defects

Metals ◽

10.3390/met11060964 ◽

2021 ◽

Vol 11 (6) ◽

pp. 964

Author(s):

Yoshihiko Uematsu ◽

Toshifumi Kakiuchi ◽

Yaodong Han ◽

Masaki Nakajima

Keyword(s):

Fatigue Limit ◽

High Cycle Fatigue ◽

Fatigue Tests ◽

Cycle Fatigue ◽

Design Curve ◽

Very High Cycle Fatigue ◽

Rotating Bending Fatigue ◽

Rotating Bending ◽

The Relationship ◽

Very High

Cantilever-type rotating bending fatigue tests were conducted under a very high cycle fatigue regime using conventionally manufactured Ti-6Al-4V specimens having drilled artificial defects with different sizes. The relationship between fatigue limit and defect size was defined as a fatigue limit design curve considering the transition from the fracture-mechanics dominating area to the fatigue-limit dominating area. A conventional Murakami’s equation was applicable as a design curve of additively manufactured Ti-6Al-4V with defects at 107 cycles. However, conventional equation gave un-conservative predictions for the fatigue limit at 108 cycles. Therefore, two kinds of modified Murakami’s equation were proposed as fatigue limit design curves for the very high cycle fatigue regime. Simple parallel shift of Murakami’s equation gave a conservative fatigue limit, whilst better result was obtained by changing the slope of Murakami’s equation. The proposed design curve was valid for the defect sizes ranging from 10 to 500 μm.

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