A Critical Examination of Procedures Used in Britain and the United States to Determine Creep Stresses for the Design of Power Plant for Long Life at High Temperatures

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Abstract
The operating life of land power plant being far longer than allowable testing times for the constructional metals, the selection of a material, or the determination of its appropriate working stress, is dependent in an important degree upon the procedure followed in utilizing the creep test results obtained. Different procedures practised both in Britain and in the United States can result in different views regarding the potentialities of a material, and also about the working stress allowable. The paper examines the more commonly used procedures and focuses attention upon the factors present which would operate to introduce uncertainty and error as between the probable behaviour in the long time of actual service, and as yielded by a test procedure. The principal disturbing factor in altering the resistance to creep of the material is thermal action. In some procedures, especially where the creep tests are made at working temperatures, the influence of this factor may be small or negligible in the procedure, compared with its magnitude in service. The need is emphasized for thermal action to be taken adequately into account by the test procedure. Next in importance as a possible source of error is the method of extrapolation beyond the test times to the time of the operating life. The factor of thermal action and of its representation in the several procedures examined is considered. The circumstances of extrapolation are similarly investigated, especially in regard to whether the result would over-estimate or underestimate the safe working stress. Satisfying comparisons of working stresses using different procedures cannot at present be made. The position is therefore disappointing, and one which it is very desirable should be rectified.
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