Creep of Tubes under Internal Pressure

30 tubes made of an E110 zirconium alloy of standard and optimized compositions are subjected to creep tests under internal pressure at a temperature of 350 and 380 degrees C and a circumferential stress sigma = 70, 100, and 130 MPa. The circumferential strain is maximal during tests at a stress of 130 MPa for 4000 h: it is 1.5% at 350 degrees C and about 6% at 380 degrees C. The creep kinetics is found to obey the Li equation with exponential decay of the process at the first stage and transition to creep at a constant rate. The constants of the nonlinear kinetic equation and their reproducibility errors are found using the principle of maximum likelihood for each alloy composition, temperature, and stress. The characteristic transient process time is shown to be approximate to 1000 h and its contribution to the total strain does not exceed 1%. The steady-state creep rate depends on stress as epsilon' similar to sigma(m) with an exponent m = 3.82 +/- 0.18. The creep strain under external pressure and the same internal pressure are approximately the same.