Page 276 - Aircraft Stuctures for Engineering Student

8.7 Fatigue 257

loads arise from aileron application, in undercarriages during landing, on engine
mountings and during crash landings. Analysis and discussion of these may be
found in Ref. 6.

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7
8.7 Fatig
Fatigue is defined as the progressive deterioration of the strength of a material or
structural component during service such that failure can occur at much lower
stress levels than the ultimate stress level. As we have seen, fatigue is a dynamic
phenomenon which initiates small (micro) cracks in the material or component and
causes them to grow into large (macro) cracks; these, if not detected, can result in
catastrophic failure.
Fatigue damage can be produced in a variety of ways. Cyclic fatigue is caused by
repeated fluctuating loads as described in Section 8.2. Corrosion fatigue is fatigue
accelerated by surface corrosion of the material penetrating inwards so that the
material strength deteriorates. Small-scale rubbing movements and abrasion of adja-
cent parts cause fretting fatigue, while thermal fatigue is produced by stress fluctuations
induced by thermal expansions and contractions; the latter does not include the effect
on material strength of heat. Finally, high frequency stress fluctuations, due to vibrd-
tions excited by jet or propeller noise, cause sonic or acoustic fatigue.
Clearly an aircraft's structure must be designed so that fatigue does not become a
problem. For aircraft in general, BCAR require that the strength of an aircraft
throughout its operational life shall be such as to ensure that the possibility of a
disastrous fatigue failure shall be extremely remote (that is, the probability of failure
is less than under the action of the repeated loads of variable magnitude
expected in service. BCAR also require that the principal parts of the primary
structure of the aircraft be subjected to a detailed analysis and to load tests which
demonstrate a sefe life, or that the parts of the primary structure have fail-mfi.
characteristics. These requirements do not apply to light aircraft provided that zinc
rich aluminium alloys are not used in their construction and that wing stress levels
are kept low, Le. provided that a 3.05m/s upgust causes no greater stress than
14 N/mm2.

8.7.1 Safe life and fail-safe structures

The danger of a catastrophic fatigue failure in the structure of an aircraft may be elimi-
nated completely or may become extremely remote if the structure is designed to have a
safe life or to be fail-safe. In the former approach, the structure is designed to have a
minimum life during which it is known that no catastrophic damage will occur. At the
end of this life the structure must be replaced even though there may be no detectable
signs of fatigue. If a structural component is not economically replaceable when its safe
life has been reached the complete structure must be written off. Alternatively, it is
possible for easily replaceable components such as undercarriage legs and mechanisms
to have a safe life less than that of the complete aircraft since it would probably be
more economical to use, say, two light-weight undercarriage systems during the life