The importance of surface finish and flaw size on fatigue life. Flaw size counts!!

MARCH 2015

Uncontrolled surface damage and poor surface finish can play a significant role in a components service life. Make every effort to prevent inadvertent mechanical damage and poor machining finish.

In a cyclic stress environment failure of components by fatigue mechanisms is likely if loads/stresses are of sufficient magnitude or number of cycles is sufficient.

 

Total fatigue life is a function of both initiation life (sub millimetre) and propagation life (crack growing to critical length).  Depending on the environment and magnitude of the cyclic tensile stress, propagation life can vary between 10 and 80% of total fatigue life.


Fracture mechanics characterization of fatigue life (propagation) defines crack growth rate (and hence fatigue life) as a function of the cyclic stress intensity ΔK =ΔσY √σa by means of the Paris equation da/dN= CΔKm where C and m are material constants for the particular environment and loading condition.
Integration of the Paris equation shows propagation fatigue life is a function of the difference of the inverse of the initial and final flaw sizes, raised to the power of (m/2-1), the number of cyles to failure Nf ~ 1/ai(m/2-1) - 1/acrit(m/2-1).  This hyperbolic function is strongly dependent on the initial and final flaw sizes.  The critical flaw size can be increased but this is dependent on material toughness (difficult/expensive to control or improve).  By comparison, with relatively little effort, the initial flaw size can be decreased with marked extension of life.  For example, in a case where a component had initial and final flaw sizes of 2 and 200 mm respectively and the material's fatigue life exponent m was 4, doubling the critical flaw size (doubling the materials toughness) would increase the life by a factor of 1.005, however, reducing the initial flaw size from 2mm to 1mm increases the life by a factor of 2.010.  Further reductions of the initial flaw size to 0.5mm or even 0.1mm would increase the life by factors of 4.05 and 10.13 respectively!!!

This is illustrated by a recent case involving a titanium alloy, the as-fabricated surface roughness had an RA of the order of 12 microns, from which fatigue cracking could readily initiate and propagate, under typical stresses.  By polishing the surface to a fine mirror finish (RA less than 0.5 micron) there was a three to four fold increase in fatigue life.

It is well known by fatigue researchers that simply light scoring associated with running a razor blade through the starter notch in a fatigue specimen (which is already machined with a purposefully small radius) will reduce the tedium of waiting for the crack to initiate by hundreds of thousands of cycles. 

Learn from these examples - try to remove flaws and surface discontinuities and improve surface finish......


Published in Technical Tips by Origen Engineering Solutions on 1 March 2015