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Abstract The crack propagation and initiation of small crack and the determining of the size effects of small defects in lower cyclic fatigue lives of steel specimens were investigated. The specimens chosen was Q345R steel， normally for fabricating pressure vessels and pipes in china， with laser hole of different sizes on specimens surface， say from [?]40-200 μm in diameter. The size of small defect affects the fatigue life apparently， but the effect weakens with increasing stress range level. It shows that one types of crack initiation character at the surface edge of small hole are possible. One is caused by slip bands in which the shear stress dominates. In certain stress range， there is a critical value of the size of small crack， and beyond which， the fatigue life will drop highly. The percent between crack initiation life and total life [（Nt/Nf）] has nothing to do with the small hole size under the same stress range and be about 10%-25%.
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Received: 23 June 2014
Published: 18 March 2015
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2015, Vol. 50 
