Fatigue damage in polycrystals – Part 2: Intrinsic scatter of fatigue life

Rodopoulos, C. A.; Chliveros, Georgios

doi:10.1016/j.tafmec.2007.10.001

Cited by 8 publications

(13 citation statements)

References 16 publications

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“…To simplify the above distinction in terms of variation of available crack paths (local flow resistance), while engaging parameters like crack shape, crack size, material and stress level, the author in 10,11 suggested the near threshold/short crack growth (Stage I) intensification factor, R C . This was done in order to maintain the engineering need to work with Δ K ().…”

Section: The Local Flow Resistancementioning

confidence: 99%

“…The conditions signifying the end short crack or near threshold crack growth is given by, 11 where the parameter R c can be selected from . The corresponding crack growth rate is, where S = G for short crack growth and near threshold long crack growth.…”

Section: Fatigue Damage Map: the Stress/crack Length/crack Growth Ratmentioning

confidence: 99%

“…(h) The transition from fatigue limit to threshold in every polycrystal follows a general behaviour described by the Kitagawa-Takahashi diagram. Considering that the above are truthful, the author in 10,11 concluded that there is a local flow resistance ahead of the crack, which is a function of the crack shape, the stress level and the material. This local flow resistance evolution is such, as to begin from a value equal to the fatigue limit for a crack limited within a single surface grain (single crystal), and reaching a plateau when the crack develops to some critical size.…”

Section: T H E L O C a L F L O W R E S I S T A N C Ementioning

confidence: 99%

“…In other words, the shape of naturally initiated cracks is determined by the shape of the strain field, while in pre-cracked specimens the shape of the crack or notch or slit determines the shape of the strain field. To simplify the above distinction in terms of variation of available crack paths (local flow resistance), while engaging parameters like crack shape, crack size, material and stress level, the author in 10,11 suggested the near threshold/short crack growth (Stage I) intensification factor, R C . This was done in order to maintain the engineering need to work with K (da/.dN = C R C K m ).…”

Section: T H E L O C a L F L O W R E S I S T A N C Ementioning

confidence: 99%

“…9. To assist with the Delaunay triangulation and contouring of the data, the crack growth rate has been expressed as, da/dN = −Log 10 (da/dN), (11) while σ has been transformed into σ max via division by (1 -R). Herein, it is clear that up to a certain K value, depending on the stress level, the R ratio, the shape of the crack and the maximum far-field stress the similitude exhibits various crack growth ranges or stages in the near threshold region.…”

Section: T H E M U L T I P L E S I M I L I T U D E S T a G E Smentioning

confidence: 99%

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Fatigue damage discrepancies, the intrinsic fatigue damage map locus and the multiple stages of the similitude concept

Rodopoulos

2010

Fatigue Fract Eng Mat Struct

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A B S T R A C TThe work demonstrates how the theory of the fatigue damage map (FDM) can be used in order to define a closed form locus where fatigue crack growth can be sought. The Intrinsic FDM Locus depending only on readily material properties represents a tool able to accurately predict crack growth of polycrystals. If the Locus is expressed in terms of a surface plot after triangulation of the data, it concludes into visualization of the potential for multiple similitude stages. The stages are defined as a function of the maximum far field stress and K. Multiple similitude stages are found to dominate the short and near threshold area (Stage I growth) and represent direct result of the effect of polycrystalline behaviour to flow resistance. The work concludes that interrogation of the points defining the Intrinsic FDM Locus and related to the three thresholds can provide potential characteristics in the quest for an ideal damage tolerance material.Keywords fatigue damage map; fatigue damage map locus; fatigue life reliability; fatigue property; fatigue threshold; grain size. N O M E N C L A T U R Eb = Burger's vector D = average grain size E = elastic modulus G = shear modulus K Ic = fracture toughness plane strain m i /m 1 = grain orientation factor R = stress ratio R C = stage I intensification factor S = stiffness modulus α = crack length K = stress intensity factor range σ c y = cyclic yield stress σ max FL(R=−1) = maximum value of the fatigue limit, R = −1

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Section: The Local Flow Resistancementioning

confidence: 99%

Section: Fatigue Damage Map: the Stress/crack Length/crack Growth Ratmentioning

confidence: 99%

Section: T H E L O C a L F L O W R E S I S T A N C Ementioning

confidence: 99%

Section: T H E L O C a L F L O W R E S I S T A N C Ementioning

confidence: 99%

Section: T H E M U L T I P L E S I M I L I T U D E S T a G E Smentioning

confidence: 99%

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Fatigue damage discrepancies, the intrinsic fatigue damage map locus and the multiple stages of the similitude concept

Rodopoulos

2010

Fatigue Fract Eng Mat Struct

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Combined effect of both surface finish and sub‐surface porosity on component strength under repeated load conditions

McMillan

Jones

2020

Engineering Reports

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High duty engineering component life is usually demonstrated through extensive testing and statistical analysis applied to empirical curve-fit equations. Because of this, the extent of the testing required is huge and costly: it must consider the load cycle range and test to high numbers of cycles. Additive Manufacturing (AM) for high duty components has brought to the fore the question of the effect of porosity and surface roughness on fatigue life, and how the true life of a critical component can be assessed conservatively. The authors propose the first step toward the development of a fatigue model based on well-established engineering physics principles, by creating computational specimens with modeled surface roughness and porosity, and subjected to cyclic loading using Finite Element Analysis. They show that the combination of roughness features and sub-surface pores leads to an equivalent plastic strain distribution pattern that suggests an emergent physical process that has not been reported before, and which indicates that the component strength and life reduction arising from surface roughness can be made significantly worse by the presence of porosity. The development of such phenomenological understanding should lead to improved life prediction techniques, more cost effective test procedures, and the development of better AM methods.

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Crack growth: Does microstructure play a role?

Jones

Singh

McMillan

2018

Engineering Fracture Mechanics

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The experimental data presented in this paper reveals that even if the growth of long cracks in two materials, with different microstructures, have different ⁄ versus ∆ curves the corresponding small crack curves can be similar. We also see that long cracks in a large range of steels with different microstructures, chemical compositions, and yield stresses can have similar crack growth rates. The materials science community is challenged to explain these observations. The experimental data also suggests that the threshold term ∆ thr in the Hartman-Schijve variant of the NASGRO crack growth equation appears to have the potential to quantify the way in which small cracks interact with the local microstructure. In this context it is also noted that the variability in the life of operational aircraft is controlled by the probability distribution associated with the size and nature of the material discontinuities in the airframe rather than the probability distribution associated with the scatter in the growth of small cracks with a fixed initial size.

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Fatigue damage in polycrystals – Part 2: Intrinsic scatter of fatigue life

Cited by 8 publications

References 16 publications

Fatigue damage discrepancies, the intrinsic fatigue damage map locus and the multiple stages of the similitude concept

Fatigue damage discrepancies, the intrinsic fatigue damage map locus and the multiple stages of the similitude concept

Combined effect of both surface finish and sub‐surface porosity on component strength under repeated load conditions

Crack growth: Does microstructure play a role?

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