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As part of the COST 501 Life Time Prediction project, the present work covers the determination of the multiaxial stress rupture criterion, MSRC, which is required in the design procedure for high temperature plant components operating under multiaxial stress. A widely used heat exchanger material, Alloy 800H, was prepared for the study in bar, plate and tube form and the latter was selected as the test piece for establishing the MSRC. In order to simulate the plant component closely, precision machined tubular test pieces were subjected to multiaxial stress situations including uniaxial tension, internal pressure and combined loading. Each range of multiaxial stress ratios was investigated at equivalent stress levels of 55 MPa and 70 MPa in the tube wall. The tests were all taken to failure and the rupture times compared with those predicted using a continuum damage based lifetime prediction model, applied to uniaxial creep data obtained from the bar form of Alloy 800H. It is shown that, at an equivalent stress level of 70 MPa, the Maximum Principal Stress criterion is the operative MSRC, whereas a transition to von Mises controlled rupture is found at the lower stress of 55 MPA. Metallographic investigation of the failed tubes supports the conclusions of a transition in the controlling criterion taking place between the two stress levels studied. A change in the failure mode of the tubes from circumferential to axial cracking is shown to take place at the same stress ratio for both stresses and is thus independent of the operative stress criterion.

Additional information

Authors: HURST R C, JRC Petten (NL);HUNTER C P, UNIREC, Firminy (FR);TAPLIN D M R, Trinity College, Dublin (IE)
Bibliographic Reference: Paper presented: High Temperature Materials for Power Engineering, Liège (BE), September 24-27, 1990
Availability: Available from (1) as Paper EN 35523 ORA
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