Page 256 - Aircraft Stuctures for Engineering Student

8.2 load factor determination 237

I I I I I I
IO io2 io3 lo4 io5 io6 lo7

No. of repetitions
Fig. 8.2 Typical form of S-N diagram.

Prior to the mid-1940s little attention had been paid to fatigue considerations in the
design of aircraft structures. It was felt that sufficient static strength would eliminate
the possibility of fatigue failure. However, evidence began to accumulate that several
aircraft crashes had been caused by fatigue failure. The seriousness of the situation
was highlighted in the early 1950s by catastrophic fatigue failures of two Comet
airliners. These were caused by the once-per-flight cabin pressurization cycle which
produced circumferential and longitudinal stresses in the fuselage skin. Although
these stresses were well below the allowable stresses for single cycle loading, stress
concentrations occurred at the corners of the windows and around rivets which
raised local stresses considerably above the general stress level. Repeated cycles of
pressurization produced fatigue cracks which propagated disastrously, causing an
explosion of the fuselage at high altitude.
Several factors contributed to the emergence of fatigue as a major factor in design.
For example, aircraft speeds and sizes increased, calling for higher wing and other
loadings. Consequently, the effect of turbulence was magnified and the magnitudes
of the fluctuating loads became larger. In civil aviation, airliners had a greater utiliza-
tion and a longer operational life. The new ‘zinc rich’ alloys, used for their high static
strength properties, did not show a proportional improvement in fatigue strength,
exhibited high crack propagation rates and were extremely notch sensitive.
Despite the fact that the causes of fatigue were reasonably clear at that time its elim-
ination as a threat to aircraft safety was a different matter. The fatigue problem has two
major facets: the prediction of the fatigue strength of a structure and a knowledge of the
loads causing fatigue. Information was lacking on both counts. The Royal Aircraft
Establishment (RAE) and the aircraft industry therefore embarked on an extensive
test programme to determine the behaviour of complete components, joints and other
detail parts under fluctuating loads. These included fatigue testing by the RAE of some
50 Meteor 4 tailplanes at a range of temperatures, plus research, also by the RAE, into
the fatigue behaviour of joints and connections. Further work was undertaken by some
universities and by the industry itself into the effects of stress concentrations.
In conjunction with their fatigue strength testing, the RAE initiated research to
develop a suitable instrument for counting and recording gust loads over long periods