| In this study, a series of monotonic tensile tests and fatigue crack growth testsare conducted at room temperature on base metal of API X80pipeline steel which iswidely used in West-East Gas Pipeline Project to investigate the mechanicalbehaviors under various loading conditions. The fractography of specimens areobserved by scanning electron microscope. The structures of specimens are studiedby finite element analysis. At last, a new fatigue crack growth model was proposed topredict the X80steel with high toughness. The experimental results showed that (1)X80steel exhibited slightly anisotropic mechanical properties. The tensile curve wasa characteristic of yielding continuously with the dimple pattern fractography.(2) Theincreasing stress ratio R and loading amplitude both could decrease the fatigue lifeand crack growth length. In the same stress-strain field, the increase of stress ratio Rcould decrease fatigue crack growth rate. However, the loading amplitude had littleeffect on the fatigue crack growth rate.(3) The fatigue crack growth rates were almostthe same in different specimen orientations, while the fatigue crack length was longerin the T-L direction due to the low fracture toughness.(4) The fractography of X80steel showed that slip was the main mechanism of fatigue crack growth. Staticfracture phenomenon appeared in rapid growth area under high loading. At the sametime, a lot of blunting happened in the crack tip.X80pipeline steel is easy to perform a full yielding. Therefore, the classicalmodel is useless to predict fatigue crack growth rate of X80steel. So, the new lawwhich based on Walker's model was proposed. The effective J-integral range wasmodified by ?Jp in order to correlate plastic zone's effect to predict the fatigue crackgrowth rate. This new model takes crack closure and blunting of crack tip intoconsideration. The model was proved to fit well for fatigue crack growth rate of APIX80at various stress ratios of R>0and loading amplitudes in both L-T and T-Lorientations. |
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