WITHIN the limits set by practical considerations, stresses in engineering materials entail almost exactly proportionate strains. But in bodies of elongated shape, such as rods or thin sheets, displacements are not necessarily proportionate to the strains of which they are the integrated result, and hence, for such bodies, Hooke's law may hold in respect of some types of displacement but not of others. Applying end thrust to a long straight rod (or strut), and measuring the consequent approach of the ends, we find that this increases faster than in proportion to the thrust; under a load which would have been quite safe applied as tension, the strut bows largely and ceases to have value as a compression member.
Southwell, R.V. (1936), "A Classic Textbook: Professor Timoshenko's “Theory of Elastic Stability” Reviewed", Aircraft Engineering and Aerospace Technology, Vol. 8 No. 10, pp. 288-288. https://doi.org/10.1108/eb030104
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