Jukka Kemppainen’s scientific contributions

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Publications (1)

Schematic difference between microstructurally small and short cracks at their respective fatigue limits. The microstructurally small crack faces consecutive microstructural barrier fronts, manifested as erratic crack growth rates. These barriers can also be seen in the cyclic R-curve as extra obstacles to be penetrated, which requires increased crack driving force. Similarly, the magnitude of the sudden drops in crack growth rate should contain information on the strength of these microstructural barriers. Microstructurally short crack has a relatively long crack front that more likely finds a weak spot in the microstructure so that the effect of microstructural barriers is diminished and the resulting fatigue limit is also smaller.
Predicted Kitagawa-Takahashi diagram with 90% confidence region. The x-axis corresponds to the initial defect size. The binary non-propagating crack findings in the experimental data are offset by 5% in the x-axis for better visibility.
Parameter identification scheme and predictive demonstration for the long crack threshold tests.
Measured non-propagating crack lengths a and corresponding estimated $\Delta K$ in the model parameter identification scheme. The envelope of non-propagating cracks is fit between the two curves drawn as solid black lines. The upper curve defines the minimum strength of microstructural barriers along the crack front, corresponding to 95% quantile in Eq. (19). A crack growing along this line has a 5% probability of arresting every time it faces a microstructural barrier front. The lower curve corresponds to the baseline crack closure $\Delta \hat{K}_\mathrm{th}$, defined in Eq. (2). If a crack grows below this line, probability of arrest is 100% once it faces a microstructural barrier front. The horizontal distribution is the initial resistance given a single microstructural barrier.
Specimen-wise fatigue test results and defect configuration of the experiments by [27]. The linkage of Kitagawa-Takahashi and Frost diagrams is demonstrated for defect size of 450 µm by drawing an additional vertical axis with increasing $K_t$. Keeping the defect size constant but making the defect sharper, as shown in the defect configuration, cracks initiate easier and fatigue limit drops until becoming constant when the cracks become non-propagating. El-Haddad [2] and Siebel-Stieler [31] models were used to describe the fatigue limit from crack non-propagation and initiation, respectively. A scaled prediction by Murakami-Endo model [3] is plotted for comparison.

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Probabilistic description of the cyclic R-curve based on microstructural barriers
  • Article
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April 2025

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International Journal of Fatigue

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A model for the probabilistic cyclic R-curve has been derived. The model is based on the commonly used hypothesis of consecutive microstructural barrier fronts defining the erratic behavior of microstructurally short cracks and the transition to physically short cracks with declining importance of the microstructural features. The model can describe the linkage between the traditional cyclic R-curve analyses and the El-Haddad type Kitagawa-Takahashi diagrams with the asymptotic fatigue limit at small defect sizes. The model fit against the experimental non-propagating crack lengths perfectly matches the observed and predicted fatigue limit for several defect types and sizes. The presented framework can be used to analyze any geometry, loading history, or defect configuration, including defect interaction problems.

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