Fatigue damage in two-step loading of 316L steel. II. Short crack growth Vasek, A., Polák, J. and Obrtlik, L. Fatigue Fract. Eng. Mater. Struct. (1996) 19 (2–3), 157–163

Fatigue crack propagation was modeled by using the cyclic plasticity material properties and fatigue constants for crack initiation. The cyclic elastic-plastic stress-strain field near the crack tip was analyzed using the finite element method with the implementation of a robust cyclic plasticity theory. An incremental multiaxial fatigue criterion was employed to determine the fatigue damage. A straightforward method was developed to determine the fatigue crack growth rate. Crack propagation behavior of a material was obtained without any additional assumptions or fitting. Benchmark Mode I fatigue crack growth experiments were conducted using 1070 steel at room temperature. The approach developed was able to quantitatively capture all the important fatigue crack propagation behaviors including the overload and the R-ratio effects on crack propagation and threshold. The models provide a new perspective for the R-ratio effects. The results support the notion that the fatigue crack initiation and propagation behaviors are governed by the same fatigue damage mechanisms. Crack growth can be treated as a process of continuous crack nucleation.

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Modeling the Fatigue Damage Evolution in Welded Joints

Volume 4: Materials Technology ◽

10.1115/omae2017-61201 ◽

2017 ◽

Author(s):

Zbigniew Mikulski ◽

Vidar Hellum ◽

Tom Lassen

Keyword(s):

Crack Initiation ◽

Crack Growth ◽

Fatigue Damage ◽

Damage Evolution ◽

Threshold Value ◽

Phase Model ◽

Two Phase ◽

Initiation Phase ◽

Weld Toe ◽

Small Cracks

The present paper presents a two-phase model for the fatigue damage evolution in welded steel joints. The argument for choosing a two-phase model is that crack initiation and subsequent crack propagation involve different damage mechanisms and should be treated separately. The crack initiation phase is defined as the number of cycles to reach a crack depth of 0.1 mm. This phase is modelled based on the Dang Van multiaxial stress approach. Both a multiaxial stress situation introduced by the acting loads and the presence of the multiaxial welding residual stresses are accounted for. The local notch effect at the weld toe becomes very important and the irregular weld toe geometry is characterized by extreme value statistics for the weld toe angle and radius. The subsequent crack growth is based in classical fracture based on the Paris law including the effect of the Stress Intensity Factor Range (SIFR) threshold value. The unique fatigue crack growth rate curve suggested by Huang, Moan and Cui is adopted. This approach keeps the growth rate parameters C and m constant whereas an effective SIFR is calculated for the actual stress range and loading ratio. The model is developed and verified based on fatigue crack growth data from fillet welded joints where cracks are emanating from the weld toe. For this test series measured crack depths below 0.1 mm are available. The two-phase model was in addition calibrated to fit the life prediction in the rule based S-N curve designated category 71 (or class F). A supplementary S-N curve is obtained by the Random Fatigue Limit Method (RFLM). The test results and the fitted model demonstrated that the crack initiation phase in welded joins is significant and cannot be ignored. The results obtained by the Dang Van approach for the initiation phase are promising but the modelling is not yet completed. The fracture mechanics model for the propagation phase gives good agreement with measured crack growth. However, it seems that the prediction of crack retardation based on a threshold value for the SIFR gives a fatigue limit that is overly optimistic for small cracks at the weld toe. The threshold value has been determined based on tests with rather large central cracks in plates. The validity for applying this threshold value for small cracks at the weld toe is questioned. As the present two-phase model is based on applied mechanics for both phases the parameters that have an influence on the fatigue damage evolution are directly entering into the model. Any change in these parameters can then be explicitly taken into account in logical and rational manner for fatigue life predictions. This not the case with the rule based S-N curves that are based on pure statistical treatment of the bulk fatigue life.

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Cyclic analysis of propagating short cracks at notches and short-crack growth behaviour

Engineering Fracture Mechanics ◽

10.1016/0013-7944(89)90220-8 ◽

1989 ◽

Vol 34 (4) ◽

pp. 831-840 ◽

Cited By ~ 4

Author(s):

J.C. Wang ◽

Y.Z. Lu

Keyword(s):

Crack Growth ◽

Short Crack ◽

Growth Behaviour ◽

Crack Growth Behaviour ◽

Short Cracks

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Different Fatigue Dynamics Under Statistically and Spectrally Similar Deterministic and Stochastic Excitations

Journal of Applied Mechanics ◽

10.1115/1.4025138 ◽

2013 ◽

Vol 81 (4) ◽

Cited By ~ 4

Author(s):

Son Hai Nguyen ◽

Mike Falco ◽

Ming Liu ◽

David Chelidze

Keyword(s):

Crack Growth ◽

Fatigue Damage ◽

Damage Accumulation ◽

Growth Dynamics ◽

Experimental System ◽

Counting Method ◽

Random Loading ◽

Stochastic Excitations ◽

Fatigue Damage Accumulation ◽

System Coupling

Estimating and tracking crack growth dynamics is essential for fatigue failure prediction. A new experimental system—coupling structural and crack growth dynamics—was used to show fatigue damage accumulation is different under chaotic (i.e., deterministic) and stochastic (i.e., random) loading, even when both excitations possess the same spectral and statistical signatures. Furthermore, the conventional rain-flow counting method considerably overestimates damage in case of chaotic forcing. Important nonlinear loading characteristics, which can explain the observed discrepancies, are identified and suggested to be included as loading parameters in new macroscopic fatigue models.

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