Abstract
Abstract— The effects of stress variables on the fatigue design of steel structural components (SAE 1010 steel) in the threshold region are investigated. The threshold and the closure threshold stress intensity ranges both decreased linearly as the stress ratio is increased. The threshold and opening threshold stress intensities also decreased linearly as the magnitude of compressive peak stress is increased. Crack opening stress measurements using a mechanical extensometer showed that the crack is not fully closed throughout the stress cycle at the threshold level. The crack opening stress is found to be independent of the crack length up to a certain crack length depending on the loading conditions. It is also found that the threshold stress intensity consists of two components: opening or closure stress intensity required to overcome crack closure, and intrinsic stress intensity range required to grow the crack. Linear relationships are obtained for the intrinsic stress intensity range as a function of stress ratio or compressive peak stress.
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