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Scale factor - the ratio of the boundary of endurance, which is
determined by testing geometrically similar specimens of large
size, the border of endurance reference sample
 
   1 або    1 . (6.10)
 
 
 1e  1e
Surface condition. Fatigue cracks usually begin from the surface.
Rough surface treatment of structural elements leads to surface
defects (cuts, scratches, streaks, etc.), which reduces the boundary
endurance. Influence of surface condition on the fatigue strength
estimated coefficient of surface quality
 
   1 або    1 , (6.11)
 
 
 1п  1п
where  ,  – border endurance sample with certain surface
 1  1
treatments;  ,  – border endurance sample with polished
 1п  1п
surface.
To eliminate the harmful effects of the above factors partially as
well as some others (temperature, aggressive environments etc.),
we should assume technological, designing and operational
measures to improve the resistance to fatigue. The technological
measures include the hard machining (grinding or polishing
surfaces), nitriding, hardening, surface hardening, blowing shot,
run-rollers. To the designing measures belong providing products
form, which would reduce the concentration of stress (design
halteley, fillets, grooves handling). To the operational – to grease
the surfaces that rub, protection from damage and corrosion,
periodic preventive maintenance and non-destructive testing
presence of fatigue cracks with replacement of damaged parts.

6.5 Diagram of boundary stresses. Fatigue safety factor

Usually, estimates of the strength of components operating at
variable stresses perform as tested. In this calculation we can
determine the actual safety factors n for one or more potentially
dangerous sections detail. Strength conditions has the form

n    n . (6.12)

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