[Flieger]

Originally Posted by Weissach911

...I agree that preload should be a minimum and I think I said in an early post that anything that increased the pull out force was undesirable.

I don't think fatigue in the studs us a major concern but studs will generally be designed to have enough preload to avoid fatigue loading.

I am not sure what you mean about the 'bad' orientation of grains in a Aluminium based material. Aluminium is face centred cubic which means it has 12 slip systems available to allow deformation so its its behaviour is generally quite isotropic.

Most metals that exhibit fatigue endurance limits have 'interstitial' alloying elements. These interstitial atoms block the movement of dislocations and inhibit the formation of the slip band intrusions and extrusions that usually initiate fatigue cracks. Interstitial atoms such as carbon are quite small.

Aluminium atoms are already quite small and the vast majority of alloying elements are subsitutional and these don't much change fatigue behaviour other than to increase strength.

The precipitation hardening alloys also don't really produce fatigue endurance limits as precipitate particles are again relatively large.

Steel at room temperature is Body Centred Cubic and there is a large octahedral interstital site which can be occupied by a Carbon atom and this is generally responsible for the endurance limit.

If a 'bulk' material is loaded in compression the orientation of individual grains will not lead to the production of fatigue cracks. As you say you will always need a traction vector to generate a crack. If you load a material in hydrostatic compression it will not fail even if it is polycrystalline as are the majority of day to day metals (Gas Turbine blades are single crystal to eliminate grain boundary sliding). I think you will only see resolved shear stresses at surfaces where buckling or other geometric instabilities occur.

I do think, however, that at around 200degC Aluminium/Silicon alloys can exhibit fatigue softening due to variations in tensile loads. The more highly loaded the stud the more likely this is to occur. This mechanism would almost certainly cause a loss of preload....

What I was trying to say was that this is not a case of uniform, triaxial normal stress, at least the far field stress. The compression is in one direction only and so Mohr's circle tells us that there will be orientations that will see shear stress. Combine that with the pressure in the cylinder which sets up a tensile hoop stress and there will be tension in the material. Aluminum's many slip planes combined with the polycrystalline nature should mean that there will always be a stress to cause fatigue. And since Aluminum alloys do not show an endurance limit there is nothing that can be done to the stud torque that will cause them not to fatigue, though their lives can be extended by using minimal pre-load.