A Comparison of Flow Criteria Applied to Elevated Temperature Creep of Rotating Disks with Consideration of the Transient Condition

Abstract

A study is made of stress distributions in rotating disks with holes, under steady-state creep at elevated temperature, based on various flow criteria (Tresca, Teesca-Mises or Mises), and assuming a creep ratestress -time function of the form ɛ = K σⁿ f (t). On this basis, comparisons are given between maximum stresses and creep deformations obtained using the various flow rules for disks having different ratios of outside to inside diameters and composed of material with different n values. Results of these show generally lower peak stresses and creep strains if the Mises criterion is used. Spin tests on steel disks at 1000 F and on lead disks at 85 F also indicate that the commonly used Mises criterion gives low creep deformations compared to average test values; better results are usually obtained using the Tresca or Tresca-Mises criteria. Further experimental data, however, are required before definite conclusions can be drawn.

Using the various flow rules and both strain-hardening and timehardening assumptions, methods are given for taking into account the additional creep resulting during the initial transient period after starting, during which the stress distributon in the disk changes from an initial elastic-plastic one to the steady state. These indicate that, for the spin tests reported, a negligible error is made by assuming the steady-state stress distribution during the transient period, first stage creep being taken into account by the time function in the expression for strain rate. In certain cases in practice it may, however, be necessary to consider this transient condition.