Analysis of a Thermal Fatigue Test of a Stepped Pipe

Results from thermal-structural finite element analysis (FEA) were used to predict cycles to crack initiation in thermal fatigue tests of stainless steel pipes. The pipes were fatigued by alternately pumping hot and cold low oxygen water every four minutes through 304 stainless steel pipes. The rapid change in water temperature imparted a thermal shock to the inner wall of the pipe. The pipes were stepped to four different thicknesses to give four different values of thermal shock stress depending on thickness. The pipes were pressurized to 17.2 MPa (2500 psi) and the temperature cycled between 38°C (100°F) and 343°C (650°F) in three seconds. This was followed by holding at 343°C for 237 seconds and then quenching to 38°C in three seconds followed by another 237 second hold period. Thermal cycling continued until significant cracking was detected on the inside surface of the pipes. Measurements of fatigue striation spacing on the fracture surfaces allowed determination of cycles to the initiation of defects 0.254 mm (0.01 inch) deep. Alternating stresses and strains were calculated using both elastic and elastic-plastic finite element analyses (FEA). The analysis results were used with a best-fit fatigue curve to predict cycles-to-crack initiation for comparison to the experimental data. Using elastic analysis corrected for stresses beyond yield in accordance with the ASME B&VP Code and the best-fit fatigue curve adjusted for low oxygen water environments resulted in under-estimates of the observed cycles to crack initiation from the tests. Improved predictions of cycles to crack initiation are possible by using an elastic-plastic FEA method with a kinematic hardening model along with the best-fit fatigue curve.