Characterization of creep-fatigue crack growth behavior in 2.25 Cr-1 Mo steel using (C t)avg

Fatigue Fract Eng Mat Struct

Narasimhachary

2019

Self Cite

Creep‐fatigue crack growth (C‐FCG) rates in a P91 steel at 625°C were correlated as the average time rate of crack growth during hold time, (da/dt)avg, with (Ct)avg. At 60‐second hold time, the rates were lower than for 600‐second hold time. At 600‐second hold time, the crack growth rates converged on to the creep crack growth rate (CCGR) trend. Thus, the CCGR trend represents the upper bound for time‐dependent crack growth rates in P91 materials. The analytical expressions based on considering just the elastic and secondary creep deformation rates overestimated the magnitudes of (Ct)avg by as much as a factor of 10 for the 600‐second hold time tests. After accounting for the effects of cyclic plasticity during unloading, and accounting for only partial reversal of creep strains accumulated during hold time, the estimates of (Ct)avg compared well with the measured values. CR represents the extent of crack tip creep strain reversal, and tpl is the time required for the crack tip creep zone during the hold time to become equivalent in size to the cyclic plastic zone in terms of stress carried by that region. Together, these parameters accurately account for the effects of crack tip cyclic plasticity on the magnitude of (Ct)avg. Both tpl and CR depend on material properties, and the latter also depends on the hold time. A parameter ϕ is introduced that is dependent only on material properties and from which CR can be estimated for a given hold time. tpl and ϕ can be reported as part of the test results from C‐FCG testing.

Section: Inclusion Of Crack Tip Cyclic Plasticity In the Estimation Omentioning

confidence: 99%

Accounting for crack tip cyclic plasticity and creep reversal in estimating (C_t)_avg during creep‐fatigue crack growth

Fatigue Fract Eng Mat Struct

Narasimhachary

2019

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“…6. For longer times, the validity of the small-scale-creep conditions cannot be assured so the (Grover and Saxena 1995) validity of Eq. 1 also comes into question.…”

Section: Figmentioning

confidence: 99%

“…When hold time is added to the cyclic loading waveforms of creep-ductile materials, the fatigue crack growth rates tend to show a hook in the behavior when correlated to K , Fig. 7 (Grover and Saxena 1995). This behavior is due to ratcheting of the minimum loadline deflection with each cycle because the specimen accumulates creep strains near the crack tip that are not fully reversed during unloading.…”

Section: Figmentioning

confidence: 99%

Creep and creep–fatigue crack growth

2015

Int J Fract

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Creep and creep-fatigue considerations are important in predicting the remaining life and safe inspection intervals as part of maintenance programs for components operating in harsh, high temperature environments. Time-dependent deformation associated with creep alters the crack tip stress fields established as part of initial loading which must be addressed in any viable theory to account for creep in the vicinity of crack tips. This paper presents a critical assessment of the current state-of-the-art of time-dependent fracture mechanics (TDFM) concepts, test techniques, and applications and describes these important developments that have occurred over the past three decades. It is concluded that while big advances have been made in TDFM, the capabilities to address some significant problems still remain unresolved. These include (a) elevated temperature crack growth in creep-brittle materials used in gas turbines but now also finding increasing use in advanced power-plant components (b) in predicting crack growth in weldments that inherently have cracks or crack-like defects in regions with microstructural gradients (c) in development of a better fundamental understanding of creep-fatigue-environment interactions, and (d) in prognostics of high temperature A. Saxena (B) College of Engineering, University of Arkansas, 130 A John A. White Hall, 790 W. Dickson Street, Fayetteville, AR 72701, USA e-mail: asaxena@uark.edu component reliability. It is also argued that while these problems were considered intractable a few years ago, the advances in technology do make it possible to systematically address them now and advance TDFM to its next level in addressing the more difficult but real engineering problems.

“…Fracture mechanics approaches have been previously used 4 –6 to study CFCG in materials. These aim at correlating the crack growth rates in a component to a crack tip parameter, which permits data generated on laboratory‐size specimens to be used for predicting the life of actual components.…”

Section: Introductionmentioning

confidence: 99%

“…and Yoon et al 6 . proposed analytical schemes for calculating ( C t ) avg , Grover and Saxena 5 used the experimental approach, developed in Ref. [1], to obtain ( C t ) avg .…”

Section: Introductionmentioning

confidence: 99%

Modelling the effect of creep–fatigue interaction on crack growth

Grover

Fatigue Fract Eng Mat Struct

1999

Time‐dependent creep–fatigue crack growth (CFCG) is a major consideration in estimating the remaining life of elevated temperature components. Fracture mechanics approaches have proven useful in providing a framework for characterizing crack growth under service conditions, and in defining safe operating conditions and selecting inspection criteria and intervals. Experimental and analytical approaches have been developed to characterize crack growth under combined creep and fatigue loading conditions using (Ct )avg as the crack tip parameter. The analytical approaches that have been proposed to characterize CFCG are limited in their application because they do not completely account for the effect of creep–fatigue interactions in modelling crack tip deformation, and thus, accurately estimating the (Ct )avg value. A new creep‐reversal parameter, CR , is defined in this study to quantify the extent of creep–fatigue interaction at the crack tip, and is used in an analytical scheme, suitable for components, for calculating (Ct )avg . This approach does not rely on any simplifying assumptions regarding the extent of reinstatement of Ct , which is dependent on the amount of creep reversal due to cyclic plasticity, during the unloading part of a trapezoidal loading waveform cycle. The (Ct )avg values calculated by this approach compare well with the experimentally obtained values for compact type (CT) specimens, thus providing an experimental verification of the approach.