13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- of coke drums. There are studies on the fatigue life estimation for the coke drums [2-4], these are based on base metal under uniaxial isothermal fatigue lives which do not consider cyclic temperature conditions. Xia et al. [5, 6] conducted a finite element study on heat transfer and stress analysis of coke drum for a complete operating cycle. It is found that significant stress and strain values are observed at the clad of the coke drum, which can exceed the yield limit of the material. Therefore, the fatigue life evaluation of coke drum based only on base material is not sufficient. The more accurate evaluation of the fatigue life behavior should be carried out under loading conditions similar to the operational condition, such as under thermal-mechanical cyclic loading. In this paper, a successfully-developed thermal-mechanical fatigue testing system is presented. A selected set of base and clad materials of coke drums are investigated under isothermal cyclic loadings. In addition, in addition, a comparative study between isothermal and thermal mechanical fatigue lives of clad materials is conducted. 2. Experimental Setup In order to experimentally investigate fatigue life of coke drum materials under thermal-mechanical cyclic loading, a thermal-mechanical fatigue (TMF) testing system has been successfully installed in our lab. The system mainly consists of a closed-loop servo-controlled hydraulic MTS testing machine which is used as a principal loading frame, a heating device, a control system, and gripping fixtures. 2.1. Heating Device The TMF testing involves a temperature cycling. Therefore, a relatively fast heating and cooling is essential for conducting a fatigue test. There are various techniques including induction, direct resistance, radiant, or forced air heating. By evaluating the scope of this research and the interested regime of fatigue life, an induction heating with power rated at 5 kW is selected as a effective heating source. The induction unit mainly consists of a power unit, a working coil, and a cooling system. By positioning the conductive material such as metal specimen inside the working coil, the specimen can be heated up at adjustable rate. Because the working coil provides an open environment, this approach also offer an opportunity to install active specimen cooling (for example, forced air) to achieve desired cooling rate. One of the difficulties using this system is to minimize the dynamic thermal gradient along the axial-direction of the specimen. The configuration of working coil plays an important role affecting the thermal gradient along the axial direction. Variables such as number of coil turns and patterns can have significant effect on the thermal gradients. There are several researchers [7-10] investigated the effect of working coil configuration on the thermal gradient of the specimen. In reference [7] it was found that by using ten-turns one direction helical configuration of the working coil, the thermal gradient along the gauge length of a solid cylindrical specimen could be within ±10°C at 800°C. In reference [9] an investigation on the effect of working coil configuration on
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