Abstract
This paper presents the results of fatigue tests performed on dented steel pipeline specimens that were tested under different environmental conditions and subjected to cyclic internal pressure. Thirty-three pipe specimens were divided into three groups and tested under three different conditions. A first set of nine dented specimens was tested in air without any restrictions. A second set of eight specimens was tested while buried in the soil. A third set of sixteen specimens was tested in air, after the dents had been repaired by composite material sleeves. Hot-spot cyclic strain amplitudes were measured using two experimental techniques: Digital Image Correlation (DIC) and Fiber Optic Bragg Strain Gauges (FBSG). At first, all thirty-three specimens were tested in air along five full cycles in order to carry out full-field measurements using DIC to identify and quantify strain concentration at sites that were potential locations for fatigue cracks to initiate. Close to these point-locations, measurements of strains using FBSG were also made, and the results were then compared with the DIC results. FBSG were also used during the cyclic pressure loading process while the specimens were being tested, in such a way as to monitor the influence of the environment in the dented areas. The test results demonstrated that a simple uniaxial Manson-Coffin fatigue equation that uses the universal exponents proposed by Manson, together with the circumferential strain amplitude measured at the hot spots can be used to predict the fatigue life of the dented specimens. Moreover, it was determined that the measured strains at the hot-spot locations were not influenced by the soil coverage, although showing a considerable and beneficial decrease in their amplitudes caused by the composite repair reinforcements.
Highlights
Pipelines are important structures for transporting and distributing liquid or gaseous products
The first set concerns preliminary results for the in-lab tests that were performed with the objective of recording dent deformed shapes and full-field strain distributions using Digital Image Correlation (DIC), as well as hot-spot strains, measured using the fiber optic strain gauges
This paper shows that two experimental techniques (Digital Image Correlation (DIC) and Fiber Optic Bragg Strain Gauges (FBSG)) can be successfully applied to accurately determine strain distributions and to locate hot spots in dented pipeline specimens subjected to fatigue damage caused by the application of pressurized cycles
Summary
Pipelines are important structures for transporting and distributing liquid or gaseous products. In the transportation of oils and gases, structural failure concerns the loss of containment, that is, the release of the product transported through the pipe. The reliable and safe operation is highly dependent on maintaining the structural integrity of the pipeline. Among the different types of pipeline damage, dents are dangerous yet frequent forms of mechanical damage that are commonly associated with a loss of integrity. A dent is defined as a gross distortion of the pipe cross-section and may be caused by the impact of external elements such as construction equipment during the pipeline construction and operation, resulting in severe plastic deformation of the pipe wall
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