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WHAT IS METAL FATIGUE

We have all heard of metal fatigue.  It is a subject studied in great depth by metallurgists and materials scientists.  It is frequently mentioned in reports prepared by Burgoynes and is occasionally mentioned in the printed press or broadcast media.  We are often asked to explain what it is.  The following should help.

 

  • Metal fatigue is the relatively slow growth of cracks through metal structures or objects.  For it to occur the object must be subjected to a tensile, cyclic load.  In other words, there must be some force tending to pull the object apart and the force must vary over time.  These conditions occur with rotating or vibrating machinery and when sea-going vessels are worked in the seaways.
  • Fatigue cracks are very slow to develop initially but their rate of growth increases dramatically as the crack grows.  In essence, this acceleration results from a localised increase in the stress at the top of the expanding crack, which comes about quite naturally because the forces on the object are supported by an ever-diminishing cross-sectional area.
  • The initiation of fatigue cracks is promoted by the presence of defects in the original material and by sharp notches in the object.  Whilst it is good practice to avoid sharp corners (round port-holes rather than square ones) not all stress-raisers can be avoided.  Screw threads, for example will always incorporate some form of stress raisers.  Blemishes caused by poor drilling or machining frequently act as origins for fatigue cracks.
  • Careful examination of the fracture surface can reveal details about the development of the crack.  In some circumstances, it is possible to establish how quickly the crack grew.
  • Some materials have a fatigue limit.  For example, mild steel will not normally admit fatigue crack growth if the applied stresses are below about 10% of the strength of the material.  However, other materials, such as aluminium alloys, do not have any such limit.  If a cyclic load is applied, aluminium alloys will always fatigue.  As a consequence, aluminium alloys cannot be used for shafts where an infinite fatigue life is specified.
  • It is possible to calculate the likely rate of growth of a crack by fatigue in any particular situation.  This provides a basis for using non-destructive-testing (NDT) to certify a component for further use for a specified period of time.  The inspection interval should be chosen so that a crack which is large enough to be detected cannot grow to a size which would cause a catastrophic failure before the next scheduled test.  In practice, engineers normally add a margin of safety so that a detectable crack cannot grow to a critical size before the next but one scheduled inspection.