The real reason I chose an oil-quenched AISI 4140 steel was because of the great improvement in ultimate tensile strength that the quenching provides. This is because the expression we use to determine our design endurance limit is a function of the ultimate tensile strength. We naturally expect the shock absorber to experience severe loading conditions, especially repetitive shock loading due to poor road conditions (in fact, here in Australia, road trains regularly travel over corrugated unsealed roads, the corrugation primarily a result of wind erosion). Hence the we must be heavily biased towards a design against fatigue failure, thus a high ultimate tensile strength (hence high design endurance limit) would be desirable. Furthermore, the outer tube, being placed near or beside the wheel, is likely to experience impact, abrasion and corrosion from anything and everything thrown up by the tyre. Oil quenching greatly improves the hardness of the metal used and will assist in the outer tube's physical and checmical resistance to such effects. I guess that such hardness would be useful, because significant scratches could act as stress concentrators. However I'm worried about the material's toughness. It experiences about 14% elongation before rupture (according to matweb.com). Should the shock absorber fail by yielding, the post-yield behaviour would allow some (albeit tender) structural integrity, rather than rupturing completely. Hence maximising this would be of great use. However, I decided that the ultimate tensile strength was the deciding parameter for which material to use.
Is my reasoning correct or is there a different way of approaching the problem? I emailled some companies and, since I still await any reply, I thought I might send a news post to see if anyone could help :)
thanks!
prasanna