Experimental investigation of the strength of pipes loaded by variable internal pressure

Conclusions

• 1. The nature of the failure and the value of the limit stress amplitude for tubular specimens made from straight pipes differ from those for specimens made of bent pipes. The propagation of the longitudinal fatigue crack in tubular specimens made from bent pipes concludes with its unstable growth, and the fatigue limit of these specimens in a pulsating cycle is 27% lower than of specimens made from straight pipes. Consequently, the technology adopted for producing the bent pipes impairs the fatigue strength and the resistance to brittle failure.

• 2. The influence coefficient of cycle asymmetry for pipes loaded by internal pressure differs from a similar coefficient in uniaxial tests of standard specimens. On the section from the symmetric to the pulsating cycle, the influence coefficient of asymmetry in the loading of pipes is approximately 0.375. When the coefficient of asymmetry and the mean stress rise further, the influence coefficient of asymmetry decreases, according to the results of the executed experiment it is equal to 0.164.

• 3. On the basis of the generalization of the known expressions and the obtained experimental data a dependence was suggested for the approximate engineering evaluation of the limit amplitude of the cycle for straight pipes made of low alloy medium-strength steels.

• 4. The value of the influence coefficient of cycle asymmetry found and the dependence for calculating the limit amplitude of the cycle obtained with its aid make it possible to increase by 13–15% the accuracy of stress analysis of pipes (cylinders) for obtaining the practical coefficients of asymmetry and the mean stresses induced by variable internal pressure. With increasing mean stresses the accuracy of the calculations increases.