13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- feature of conventional and high-strain X80 linepipe material, some differences can be detected that the grain size of first one is much finer and the content of polygonal ferrite and M/A of second one is much higher. The tensile properties of both X80 pipe are listed in table 1. It can be found that the tensile strength, ductility and strain hardening property of X80 high-strain pipe are much better than conventional pipe. Since cyclic load that leads to fatigue failure always derives from axial stress, such as bending, compression and tension stress, round bar specimen with uniform section is cut from the pipe body along the longitudinal direction where is adjacent to the weld seam with 90°and machined with a gauge length of 12.5mm as shown in figure 2. Low circle fatigue test has been conducted on electro hydraulic servo fatigue testing machine Instron 1341 in terms of GB/T 15248-2008, in which tension-compression symmetric cycle and total axial strain control has been used. Triangle wave loading is adopted in tests to ensure a stable strain rate during the process of cyclic tension and compression and an obvious inflection point of sluggish loop. Strain amplitude of tests are 0.4%, 0.6%, 0.8%, 1.0%, 1.2% and 1.4%, strain rate used in tests is 4×10-3mm/s, and strain ratio is -1. Fatigue fracture has been investigated on SEM JSE-6700F. Specimen for TEM investigation was cut from the specimen in the range of gauge length along section direction after fatigue tests with EDM and machined by mechanical thinning and electrode polishing. Deformation microstructure observation has been carried out on the TEM JEM-200CX using an accelerating voltage as 200Kv. Figure 1. Micro-structure of X80 linepipe: a) conventional, b) high-strain Table 1. longitudinal tensle property of X80 linepipe (round bar, gaugh length 50mm, diameter 12.7mm) No. σs/MPa σb/MPa δ/% σs/σb UEL/％ n Conventional 580 653 24.1 0.89 7.25 0.0688 High-strain 569 729 24.3 0.78 7.87 0.1126 Figure 2. Specimen of low circle fatigue test (a) (b)
RkJQdWJsaXNoZXIy MjM0NDE=