Advances in Structural Integrity pp 1-11 | Cite as

# High-*R* Fatigue Crack Growth Threshold Stress Intensity Factors at High Temperatures

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## Abstract

While knowledge relating to the determination and practical application of fatigue crack growth threshold stress intensity factors for defect assessment is relatively well established for many circumstances, this is not the case for materials and conditions which are sensitive to time-dependent mechanisms. There are two well-established international standard procedures for the determination of this fracture mechanics parameter, although their respective crack growth rate criteria differ by an order of magnitude. Unfortunately neither specifically addresses determination of the property for very high-*R* (*K* _{min}/*K* _{max}) ratios, under conditions when the environment can be influential, and Δ *K* _{th} can be even more sensitive to the d*a*/d*N*(Δ*K*) criterion adopted for its determination. In addition to a general state of knowledge review, particular attention is paid to circumstances concerning high-*R* Δ *K* _{th} in power plant steels at high temperatures for which oxide-induced crack closure and creep cracking can be influential. Evidence for low-alloy 1%Cr, martensitic 9%Cr and austenitic 17%Cr steels is examined.

## Keywords

High-*R*High temperature Δ

*K*

_{th}Oxide-induced crack closure Creep cracking

## Nomenclature

- a
Crack depth

*A*Constant in Paris mid-

*K*regime power law*B*Specimen thickness

- CT
Compact tension (specimen)

- CTOD
Crack opening displacement

- d
*a*/d*N* Fatigue crack growth rate

- DCPD
Direct current potential drop (electrical crack monitoring instrumentation)

*f*Frequency

- FIB
Focussed ion beam

- HCFCG
High-cycle fatigue crack growth (typically for 80 <

*f*< 100 Hz)- TDFAD
Time-dependent failure assessment diagram

*k*_{p}Oxidation parabolic growth constant

*k′*Inelastic strain constant in

*ε*(*σ*) relationship*K*, Δ*K*Stress intensity factor, range of stress intensity factor

*K*_{c}Critical stress intensity factor responsible for unstable fracture

- \(K_{\text{mat}}^{\text{C}}\)
Material creep toughness (for a given temperature and time)

*K*_{max}Maximum stress intensity factor (in cycle)

*K*_{min}Minimum stress intensity factor (in cycle)

*K*_{r}*K*Ratio representing proximity to fracture- Δ
*K*_{th} Fatigue crack growth threshold stress intensity factor

- \({\text{d}}\Delta K_{\text{th}}^{\text{ox}}\)
Enhancement to Δ

*K*_{th}due to oxide-induced crack closure*L*_{r}Stress ratio representing proximity to plastic collapse or creep rupture

*m*Exponent in Paris mid-

*K*regime power law*N*Number of cycles

*R*Load ratio (

*K*_{min}/*K*_{max})*R*_{p0.2}0.2% proof strength

*R*_{m}Ultimate tensile strength

*R*_{R}Creep-rupture strength

- \(R_{ 0. 2}^{\text{C}}\)
0.2% creep strength (stress responsible for 0.2% inelastic strain for a given temperature and time)

- RT
Room temperature

- SEM
Scanning electron microscope

*t*Time

*W*Specimen width

*x*Oxide thickness

*β*Inelastic strain exponent in

*ε*(*σ*) relationship*ε*,*ε*_{ref}Strain, Reference strain

*σ*,*σ*_{ref}Stress, Reference stress

- \(\sigma_{\text{ref}}^{ \hbox{max} }\)
Maximum reference stress (in cycle)

*υ*Poisson’s ratio

## References

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