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National Advisory Committee for Aeronautics, Technical Notes - Experimental and Theoretical Determination of Thermal Stresses in a Flat Plate

Thermal stresses induced in a flat, rectangular, YES—T6 aluminum-
alloy plate by nonuniform heating are determined both experimentally and
theoretically. The characteristics of commercially available bonded
resistance wire strain gages are first investigated to determine their
suitability for measuring stresses under simple conditions of stress
and temperature. The gages are then used to measure thermal stresses
in the flat plate in order to study their suitability under more com-
plicated conditions. The experimental results are found to be in
satisfactory agreement (within I5 percent of the maximum calculated
stress) with an approximate theoretical solution of the problem.

Aerodynamic heating of aircraft flying at supersonic speeds has
become important in structural design because material properties are
changed by exposure to elevated temperatures, and thermal stresses which
result from nonuniform heating may have a significant effect on the
strength of the structure. Methods for determining thermal stresses
are therefore required if an efficient design is to be attained.

References l and.2 considered the thermal-stress problem in air-
craft structures theoretically and presented some procedures for thermal—
stress analysis. The methods presented varied in their accuracy and
involved various assumptions and approximations. These theoretical
approaches alone, therefore, are not sufficient, and experimental methods
are needed to check the accuracy of approximate thermal-stress—analysis
procedures and to help analyze those structures which are too complex
for theoretical analysis.

An investigation has therefore been made of the characteristics
of commerically available bonded resistance wire strain gages to deter-
mine their suitability for measuring stresses under simple conditions of
stress and temperature, and the results are presented in this paper.

The strain gages are used to measure the thermal stresses induced in a
flat plate subject to a known, nonuniform temperature distribution to
study their suitability under more complicated conditions. The stresses
so measured are then compared.with an approximate theoretical solution,
the details of which are included in the appendix.