STRUCTURAL engineers have always been accustomed to think of the strength of a structure in terms of gradually applied, or “static”, loading. It was natural, therefore, that when strength tests were first made on aeroplane structures, the primary object was to find the load that, when gradually applied to the structure, just broke it. This “ultimate” load was easy to determine, because when it was reached the structure collapsed and refused to carry any more load. At a later stage attempts were made to define a “proof” load at which the structure, because of its deformation or other damage, just ceased to be regarded as airworthy. This proof load was a much vaguer load to determine experimentally; how long should it be left on the structure, should it be applied more than once, who should be regarded as competent to pronounce on the airworthiness of the deformed structure, and should the airworthiness be judged when the structure is actually underload or after the load is removed, were all difficult questions to answer. As a result, in spite of an increasing realization of the relatively greater importance of the proof load, or at least of some comparable concept, in practice reliance has continued to be placed on measurements of ultimate load.
Pugsley, A.G. (1945), "The Specification of Test Loads: Loading Conditions for Strength Tests on Aeroplane Structures", Aircraft Engineering and Aerospace Technology, Vol. 17 No. 12, pp. 352-353. https://doi.org/10.1108/eb031314
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