19-04-2014, 11:17 AM
FATIGUE
FATIGUE.ppt (Size: 1.74 MB / Downloads: 18)
DE HAVILLAND COMET AIRLINER
The failure was a result of metal fatigue caused by the repeated pressurisation and de-pressurisation of the aircraft cabin.
All most all structures and machine parts are frequently subjected to varying loads (stresses) and it is therefore important to know the strength of materials under such conditions of dynamic loading.
In static loading the load is applied gradually, attains its full value and then it does not change. However, in dynamic loading the load goes on changing with time.
A component or a body is said to fail when the stress in the body due to a static load reaches the value of yield stress (or ultimate stress) of the material.
when a body is subjected to dynamic loading it fails under a load whose magnitude is much lower than the yield value.
This behavior of decreased resistance to failure under dynamic loading is known as fatigue.
Thus fatigue is defined as the progressive, localized, and permanent structural damage that occurs when a material is subjected to cyclic or fluctuating strains at nominal stresses that have maximum values less than (often much less than) the static yield strength of the material.
TYPES OF STRESS
FULLY REVERSED LOADING
In this type of loading the stress varies from a maximum value in tension to an equal value in compression. Hence the mean stress is zero and the body is subjected to complete stress reversal
Characteristics of fatigue failure
A fatigue crack starts at a microscopic discontinuity, called the initiation site, which then increases with each subsequent movement.
Failure is essentially probabilistic. The number of cycles required for failure varies between similar homogeneous specimens.
The greater the applied stress, the shorter the life.
Damage is cumulative. Materials do not recover when rested.
Fatigue life is influenced by a variety of factors, such as temperature and surface finish, in complicated ways.
Some materials (e.g., some steel and titanium alloys) exhibit an endurance limit or fatigue limit, a limit below which repeated stress does not induce failure, theoretically, for an infinite number of cycles of load. Most other non-ferrous metals (e.g., aluminium and copper alloys) exhibit no such limit and even small stresses will eventually cause failure.
As a means to gauge fatigue characteristics of non-ferrous and other alloys that do not exhibit an endurance limit, a fatigue strength is frequently determined, and this is typically the stress level at which a component will survive 107 loading cycles.