Page 255 - Aircraft Stuctures for Engineering Student

236 Airworthiness and airframe loads

8.2.2 Uncertainties in design and structural deterioration
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Neither of these presents serious problems in modern aircraft construction and
therefore do not require large factors of safety to minimize their effects. Modem
methods of aircraft structural analysis are refined and, in any case, tests to determine
actual failure loads are carried out on representative full scale components to verify
design estimates. The problem of structural deterioration due to corrosion and
wear may be largely eliminated by close inspection during service and the application
of suitable protective treatments.

8.2.3 Variation in structural strength

To minimize the effect of the variation in structural strength between two apparently
identical components, strict controls are employed in the manufacture of materials
and in the fabrication of the structure. Material control involves the observance
of strict limits in chemical composition and close supervision of manufacturing
methods such as machining, heat treatment, rolling etc. In addition, the inspection
of samples by visual, radiographic and other means, and the carrying out of
strength tests on specimens, enable below limit batches to be isolated and rejected.
Thus, if a sample of a batch of material falls below a specified minimum strength
then the batch is rejected. This means of course that an actual structure always
comprises materials with properties equal to or better than those assumed for
design purposes, an added but unallowed for ‘bonus’ in considering factors of
safety.
Similar precautions are applied to assembled structures with regard to dimension
tolerances, quality of assembly, welding etc. Again, visual and other inspection
methods are employed and, in certain cases, strength tests are carried out on
sample structures.

8.2.4 Fatigue

Although adequate precautions are taken to ensure that an aircraft’s structure
possesses sufficient strength to withstand the most severe expected gust or manoeuvre
load, there still remains the problem of fatigue. Practically all components of the
aircraft’s structure are subjected to fluctuating loads which occur a great many
times during the life of the aircraft. It has been known for many years that materials
fail under fluctuating loads at much lower values of stress than their normal static
failure stress. A graph of failure stress against number of repetitions of this stress
has the typical form shown in Fig. 8.2. For some materials, such as mild steel, the
curve (usually known as an S-N curve or diagram) is asymptotic to a certain
minimum value, which means that the material has an actual infinite life stress.
Curves for other materials, for example aluminium and its alloys, do not always
appear to have asymptotic values so that these materials may not possess an inhite
life stress. We shall discuss the implications of this a little later.