|FAILURE UNDER CYCLIC LOADING - Fatigue, Fracture and Crack Growth Rates|
1 Fatigue - Involves crack initiation followed by crack growth
Requires cyclic - repeated stressing - normally cracks only develop under tensile stresses.
Fatigue has been recognised and researched for 120 years - so - why is it still a problem?
Welding now used extensively - and is a potent source of defects
Higher mechanical efficiency is being required, leading to more highly stressed components.
Comparatively recent techniques enable calculations to be made predicting the life or remaining life of a structure containing defects.
There are two stages in fatigue: crack initiation and crack growth. For some materials, ferrous metals being an important group, low cyclic stresses, below the 'threshold limit', do not lead to crack initiation.
Fatigue damage normally starts where cyclic stresses are most severe, this will often be at somewhere on a component surface where there is some imperfection or notch. For smooth specimens with a gradually changing section impurities or inhomogeneities in the grain structure provide crack initiation points. For these reasons it is important to take care when designing components which contain changes in section and features such as - key ways, screw threads, 'O' ring grooves, etc. to ensure that their effects are properly assessed. For components subjected to very high cyclic stresses, high purity steels may be used to minimise potential crack initiation locations, an example of this is the steel used by some manufacturers of ball and roller bearings.
2 Design philosophies:
2.1 Safe life:
There were however two significant problems with this approach:
2.2 Fail Safe:
In addition to the multiple load paths, crack stoppers are often used. These may consist
of materials with a high fracture toughness used to supplement the residual strength of the
surrounding structure and to prevent cracks propagating to failure.
2.3 Defect (or Damage) Tolerant Approach:
For aircraft the objective is to detect cracks in 'Principal Structural Elements' (PSE)
before they propagate to failure. By establishing inspection intervals for the PSEs based
on the time it takes for a crack to grow from an initial detectable size to the critical
crack length, safe operation can be maintained. This computation is quite complex and will
involve working from the detailed usage programme of the plane.
This methodolgy means that undamaged components are not retired and factors of safety
can be reduced as fracture mechanics provides a more precise characterisation of crack
behaviour, the large scatter factors associated with fatigue results and methods are not
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