Paper 30: Low Endurance Fatigue of Very Thick-Walled Cylinders

Abstract

The repeated application of fluid pressure in a vessel poses a fatigue problem, which at high pressures can be in the low endurance range. Therefore for the design of high pressure vessels, it is important to know the dependence of endurance on applied pressure. This involves a knowledge of the steady cyclic stress-strain state in the vessel. In the present work various models of cyclic stress-strain distribution in a thick monobloc tube (diameter ratio 3–7) are examined assuming different degrees of stress shakedown during cycling. It is possible to determine upper and lower bound solutions on the stress-strain state and also an intermediate solution which agrees well with experimental data on En 1A steel. Once the steady stress-strain state is known, it is possible to predict the dependence of endurance on applied pressure by use of the Coffin–Manson failure criterion. Experimental fatigue tests were performed on monobloc vessels of En 1A and the results were in reasonable agreement with the theoretical predictions. Fractographic studies justified the use of the Coffin-Manson law. It is expected that this steady state approach to the fatigue problem in high pressure vessels can be extended to vessels made from stronger materials and provide a useful basis for design.