Correlation of Fatigue Crack Growth Data Obtained at Different Stress Ratios

Fatigue crack growth tests have been carried out on a medium carbon structural steel over a wide range of stress ratios, i.e. from 0 to 0.7. All tests were conducted under constant amplitude loading conditions corresponding to growth rates in the Paris regime. Crack closure behaviour was observed experimentally by a surface strain gauge technique, and numerically by a finite element analysis under plane stress condition. While the crack closure could not be detected by experimental measurements at stress ratios equal to or greater than 0.5, the numerical results showed that closure occurred even at high stress ratios up to 0.7. The differences between experimentally and numerically determined crack opening levels were found for each stress ratio. A cause for these differences is discussed. In addition, two new types of crack tip parameters which have been proposed recently are evaluated by finite element analysis and their relevance to fatigue crack growth are discussed. It is concluded that fatigue crack growth rates are substantially determined by the effective stress intensity factor range which is based on the crack closure concept. NOMENCLATURE a = crack length d, = length of closed crack during loading da/dN = crack growth rate F = geometry factor AK = stress intensity factor range AK,= effective stress intensity factor range AKRP = stress intensity factor range which corresponds to the applied stress range (0--uRpG) AKtt = stress intensity factor range which corresponds to the applied stress range (urn -utt) L, = crack length that is close at umin P-, Pmax =minimum and maximum applied load R = stress ratio (umin/umu) ~y = strain in y-direction u,,, = crack opening stress uII =applied stress at which stress at crack tip becomes tensile oy =stress in y-direction ays = yield stress u-, urn= =minimum and maximum applied stress uRpG = applied stress just when plastic deformation occurs in the vicinity of crack tip

Section: Crack Closure Behavioursupporting

confidence: 90%

Section: Consideration On New Crack Tip Parametersmentioning

confidence: 94%

Fatigue Crack Growth and Closure at High Stress Ratios

Tsukuda

Ogiyama

Shiraishi

1995

“…Theoretical loading histories 1 –3 of rolling contact fatigue exhibit mixed‐mode I and II cycles with some degree of overlap, but the ratio of the stress intensity factors changes during cycles (nonproportional loading). In addition, crack closure and locking effects 10 –13 are fundamental parameters to characterise the crack tip condition, because the normal travelling load from the wheel will compress the crack faces together, thereby inducing sliding friction. In order to have a better understanding of the growth mechanism of the squat‐like cracks, and their growth rates, we need to have a wider range of studies relating laboratory tests to rail track conditions in the field.…”

Section: Introductionmentioning

confidence: 99%

Fatigue crack growth rates under sequential mixed‐mode I and II loading cycles

Wong

Bold

Brown

et al. 2000

One common mode of failure that occurs in rolling bodies such as gears, bearings and rails is due to the fatigue process. Several research workers suggest that rolling contact fatigue cracks are subjected to mixed mode I and II loading cycles. It is believed that the correct modelling of loading cycles can help us to study the mechanics of crack growth because fatigue comprises a major safety consideration in the design process. Experiments have been performed under nonproportional mixed‐mode I and II loading cycles with fixed degrees of overlap, so that coplanar cracks were produced. Three empirical crack propagation laws have been established which are related to both mode I and mode II effective stress intensity factor ranges.

“…have observed that the d a /d N versus Δ K eff data are still considerably separate for each positive R ‐ratio, the shift of a given plot in the positive direction of the Δ K eff axis increasing with the R ‐value. Results like those presented in Refs [6–8] are likely to imply that the CC concept cannot sufficiently account for the R ‐ratio influence on crack growth. However, Toyosada and coworkers (Toyosada et al unpublished observations) did obtain a good correlation of the d a /d N versus Δ K eff data from different positive R ‐ratio tests on specimens of the same material and geometry as those used by Skorupa et al 8…”

Section: Introductionmentioning

confidence: 73%

An algorithm for evaluating crack closure from local compliance measurements

Skorupa

Beretta

Carboni

et al. 2002

A numerical procedure for evaluating fatigue crack closure from load‐local deformation data is presented. The local compliance is assumed to vary during the entire loading cycle, including the portion when the crack is fully open. The closure mechanism‐related characteristic load levels are identified based on comparisons between the open crack compliance variations upon loading and unloading. This type of compliance analysis is conditioned by an appropriate smoothness of the measured data. Hence, the choice and optimization of the procedure for smoothing of the measured data. Hence, the choice and optimization of the procedure for smoothing the recorded signals is thoroughly addressed in this paper. The performance of the algorithm is evaluated from comparisons between the computed closure parameters and their reference values which have been previously shown to correlate the observed crack growth rate behaviour.