Fatigue Design in Long Life Region under Variable Amplitude Loading

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In order to investigate the load sequence effect on fatigue crack growth behavior in the long life region under variable amplitude loading, fatigue tests were carried out on non-load carrying fillet welded specimens. A comparative study of fatigue crack growth analysis was also carried out. And then, fatigue crack growth rates were measured under constant amplitude, single overload, periodic overload, block loading and variable amplitude loading on center-cracked tension specimens. At last, a fatigue crack growth prediction model based on crack closure concept was established. A summary of the research results is given as follows.(l)There exists a variable amplitude fatigue limit under variable amplitude loading. It is located below the constant amplitude fatigue limit by a factor equal to the ratio of the highest to the equivalent stress ranges of the spectrum.(2)Miner's rule or equivalent stress range concept predict well the fatigue life under VA loading in the short life region, for example, less than two million cycles, whatever the type of stress range spectrum is. In the long life region, for example, over five million cycles, the computed fatigue crack propagation life is much dependent on the VA loading spectrum patterns.(3)In the fatigue crack growth measurements, load sequence effect was found for single overload, periodic overload, block loading spectra. However, for variable amplitude loading under the current loading conditions, though the stress ratio effect was proved to be significant, no load sequence effect was found during the loading procedure.(4)By using the established fatigue crack growth model, good agreements between experiments and predicted results are achieved for all of the loading spectra used in the fatigue crack growth measurements.