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Pressure Vessels: The Determination of the Relation between the Fatigue Strength and the Hydrotest Pressure


Quality Improvement Proof Testing of Pressure Equipment (QIPTEST) Summary Proof testing, with a test pressure higher than the maximum allowable pressure, is one of the final tests after manufacture of pressure equipment. The procedure shall prove leak-tightness and it shall serve to detect undesirable, potentially catastrophic defects.
At the same time it can create beneficial residual stresses, can improve material properties and, thus, improve the reliability. To achieve some of these goals, at leas partially, the test pressure has to be high enough. Obviously too high a test pressure can impart undesirable deformations or other kinds of damage.
It is the aim of this research project to determine the relationship between the proof pressure factor on the subsequent cyclic pressure loading, in dependence of the various influencing parameters, in support of CEN activities (to justify also that pressure equipment tested accordingly will have a fatigue life of more than 500 cycles).
For pipelines high pressure testing (stress test) belongs to the "state of the art" in Germany, with many successful applications for cracked pipelines as well; and it is used now successfully also for shell boilers (more than 3000). A statistical investigation (multivariate analysis) of a series of experiments, performed in preparation of this proposal on rather thin-walled vessels with defects in single-pass longitudinal welds, has shown a statistically significant positive of test pressure and fatigue life for lack of penetration and proof-pressurization factors up to 2.0 but showed no improvement for lack of penetration plus pores, but still a full pressure cycle fatigue life over 9000 cycles.
Having in mind that, according to a German statistic more than 80% of pressure vessel have thicknesses not exceeding 6 mm, and that thicknesses up to 6 mm will show additional influences, not encountered in thicker ones, and that thicknesses up to 6 mm will show additional influences, the study will focus on thicknesses of 6 mm and below It is intended to run two series of experiments: One with appr. 25 pressure vessels of 3.5 mm thickness, material St 1.1 with strengths rather at the high end and rupture elongation (reduction in area) at the low end, single-pass welds with weld defects, mainly lack of penetration. The other - appr. 30 tensile specimen and 5 cylinders - with thicknesses of 4 and 6 mm, similar material, similar welding, but natural and artificial cracks. The results of the following statistical analysis will then be compared with those of a parallel theoretical analysis using proven models for various effects. The results will then be checked by means of special complementary vessels: pressure vessels with 6 mm thickness and low strength, high rupture elongation St 1.1 material, 5 pressure vessels of approximately 3.5 mm thickness of St 9.1 material, pressure vessels of St 1.1 material with lack of penetration in weld joint between a flat end and the shell, and pressure vessels of St 1.1 material with lack of penetration in weld joint of weld-on nozzles.
Possible crack initiation during the proof test will be monitored by acoustic emission analysis (and strain gauge measurements), crack propagation during fatigue testing by strain gauge measurements, calibrated by Finite Element Analysis' results. Based on past experience (pre-proposal experiments), the tests will concentrate on limit defects. As shape of vessels in the first series will be monitored in detail before and after proof testing-, a by-product will be the influence of proof test on geometrical defects - offset and peaking.

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Technische Universität Wien
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1040 Wien

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Participants (12)