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Prediction Of Creep Effects In Aircraft Structures

Report Number: WADD TR 60-411 Part 1
Author: Alesch, C. W.
Author: Riparbelli, C.
Author: Steurer, W. H.
Author: Robe, W.
Corporate Author: Convair, A Division of General Dynamics Corporation Engineering Department
Laboratory: Flight Dynamics Laboratory
Date of Publication: 1960-04-15
Pages: 246
Contract: AF 33(616)-6567
Project: 1367
Task: 13584
AD Number: AD0261976

A step-by-step numerical process for introducing thermal and creep effects into a redundant structural analysis was developed. This process involves seven consecutive steps: (1) elastic thermal stress determination; (2) unit displacement determination in terms of stiffness patterns; (3) relaxation increment determination; (4) real thermal stress determination resulting from summation of thermal and relaxation stresses; (5) incremental stress determination; (6) determination of stress redistribution attendant to creep; and (7) summation of elastic, thermal and creep stresses. Calibrations with model materials and structural elements, seeking verification of structural criteria introduced into numerical processes are demonstrated. An example problem for a delta wing illustrates method of calculation.

A critical re-examination of interpretative approaches for reducing creep data derived from materials tests into simplified form suitable for numerical analysis of redundant structures is introduced.

The creep test independent variables, time, temperature and stress, were examined with reference to their influence upon the dependent variable, strain. These examinations explored three fields of creep activity operative under conditions of (1) constant stress and constant temperature, (2) varying stress and constant temperature, and (3) constant stress and varying temperature. Comparisons of effects arising from constant stress and constant temperature exposures also are presented.

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