Experimental investigation of effects of residual stresses on fatigue crack growth in pipelines with allowance for the scale effects
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Abstract
In offshore industry, subsea pipelines are used for transporting oil and gas from wells to remote platforms and from platforms to nearby shores. A microscopic flaw in pipeline during installation and operation will grow normal to the principal stress if the threshold value of fatigue stress range is exceeded. In case, these microscopic flaws gradually grow into critical crack size, the pipe material will fail by fracture. Offshore oil/gas pipelines are typically constructed by joining steel pipes by welding. Residual stresses are generated in the vicinity of weld seam during welding process, which if tensile, can be detrimental to fatigue strength of the pipes. Four point bend full scale fatigue tests are carried out on notched pipes to investigate the effects of these residual stresses on fatigue crack growth. A four point bend test-rig is designed and constructed to carry out these tests. During analysis it is observed that the results for pipe specimens with residual stresses are comparable to pipe specimens without residual stresses. It might be due to phenomenon like plastic deformation taking place at the crack tip during the fatigue loading. Another gain of this thesis study is to investigate the scale effects on the fatigue crack growth. Scale effects include the effects of change of specimen geometry and size on fatigue crack growth. Codes (BS7910) allow the use of fatigue data of strip specimens with through thickness crack to relate to full scale pipe specimens with surface cracks. It is argued that residual stresses initially present in pipe are relieved when the strip specimen is cut from pipe. To investigate the difference, three point bend fatigue tests are carried out on notched strip specimens extracted from the pipes. On analysis of the results of full scale pipe tests and strip specimens, it is found that fatigue crack growth rates are comparable for both cases.