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Bob,
The fatigue behaviour of Titanium is interesting and for a number of years Titanium was always considered not to have an 'Endurance Limit' which is the stress at below which it will never fail.
Virtually all steels have the limit and can usually considered to have an infinite life when used within this limit.
In recent years 6AL4V which is a commonly used fastener alloy has much good quality published data providing Endurance Limits and I would be comfortable using this data as it can be found in many NASA and other 'Unclassified' Military Publications.
Certainly airframe fasteners used on wing structures can be problematic due to the effects of galling, particulalry when initially specified with slight interference and this would limit re-use but these type of issues will not be a problem for a head stud as this type of damage won't occur.
I am not sure what you mean by working a piece of steel but if you mean bending and reverse bending with significant plastic deformation until failure then the mechanisms are very, very different when we consider the elastic behaviour of a metal.
Severe plastic deformation leads to the formation of voids within the material and these voids grow and coalesce with increasing deformation and this reduces the bulk modulus of the material which is effectively softening.
Elastic processes don't form voids but they can form surface cracks which grow longer with time but in practice the components stiffness doesn't change until a crack initiates and then failure normally occurs very quickly.
There is another process that can occur and this is known as 'cyclic work softening' but this is also associated with plastic deformation and is more commonly classified as Low Cycle Fatigue.
I did some work in this area many years ago using a combined tension/torsion test machine and some 7000 Series Aluminium Alloys but I am not sure if I still have any of the data..... but this is way off topic.
The sad truth is that Ti has some great properties that would make very good head studs but it has other issues such as galling that make it difficult to consider.
I have asked ARP some questions via their Technical Helpline but I am not confident they will reply.
I am trying to find solutions for the problems which don't impact on the benefits. The levels of stress in the head studs even at the highest temperatures that are likely aren't high enough to worry about fatigue failure and I am confident that this isn't an issue.
If I can be confident that we can eliminate galling so that they can be tightened reliably and that they will not result in too much Bi-Metallic Corrosion then I would be happy.
The Bi-Metallic corrosion is unlikely to be much more of an issue than the Stainless Steel and/or Inconel Studs that are being used so the main issue is reliable torque to preload correlation and galling.
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