Fatigue tests

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Aeronautical phenomenon of fatigue is studied for obligation of law for the past several decades. The first aircraft to present problems of fatigue was in fact the De Havilland Comet, aircraft designed in 1946 and extremely innovative in several aspects, one of which consisted in being able to reach a cabin pressure of 8.25 psi, about double of any other aircraft. This airplane, however, was the victim of several incidents between the ’52 and ’54, in two cases this incedents led to the disintegration of the fuselage in 1954, both during flight on Italy sky. The investigations carried out on other fuselages demonstrated that the failure occurred at the edges of the windows, in which there was a great concentration of stress, due to the presence of sharp edges and to the technique used to riveted windows to the fuselage. The crack started from a rivet and propagated then to the fuselage, the stress concentration was caused by the continuing cycles of pressurization. Today, all the windows has round shape.
The study of fatigue behavior of mechanical structures has today become necessary in all fields because of the increasing need for lightweight, high-performance, yet secure in their use: sports cars and motorcycles and bicycles are just examples of fields in it is important to study the fatigue behavior.
A component or structure may be designed to deal with difficulty with different criteria:
  • Infinite life: that the structure (typically valid for steel) will never reach the stress limits determining a fatigue failure
  • Safe life: the level of stress achieved by the structure is such as to have a limited lifetime, but in its operational life is subjected to a number of cycles of fatigue significantly lower than those tolerable
  • Fail safe: it is a structure designed and manufactured in such a way that the propagation of any crack will occur within a limited area or in any case it stops before the collapse of the structure
  • Damage tolarance: the structure in this case is designed to support defects or breakages in a safe, until such breakage are eliminated by repairing or replacing the part.

The veracity of these design criteria be validated by experimental activity.

Perform good fatigue testing is not trivial and requires experience to be able to plan correctly cycles of load to be applied to the structures and how they are applied. The structure during test must be in fact subjected to the same loads to which it is subjected in its working life, trying to minimize the number of actuators to be able to reduce the costs of testing. It is therefore important to check that simplified loads can achieve the same distribution of stress that structure have in life. Strain gauge sensors, which measure the deformation of the structures, placed suitably are used to verify the state of stress during the fatigue test, in order to check on the one hand that the state of initial effort is the desired one from the other hand to verify the integrity of structure during the test. There are also sensors for crack propagation, which let you track the length of crack propagation. Vicoter for its expertise in the field of fatigue testing is able to provide a complete service from the design of the test rig to constant surveillance of tests to verify the occurrence of cracks and their propagation is by visual inspection both survey methods do not destructive as dye penetrant, X-rays and ultrasound.