fractured rods

["Hardwick, Robert C" <Robert.Hardwick@domain.elided>]

Dear non-geeks:  pardon this rather oblique thread...

Eric Storhok wrote that Ford rods are made from powder alloy and that the sintering mold could include features which ensured the fracture path.  He also noted that ANY (his caps) ductility would ruin things.

I don't know what properties can be obtained from powder alloys that make them fracture more reliably.  Certainly, a forging die can include notches.  If the powder is formed so that grain structure isn't produced then the part's missing out on a lot of potential strength.

As for ductility issue, on my desk is an intentionally fractured test part (we tested fracture toughness, K1c) made of bearing steel, H.T. Rc58 (~300ksi).  The tensile test coupon stress-strain "curve" was essentially a straight line - really brittle stuff!  And yet, there is a distinct shear lip on the fracture face, i.e. ductile overload.  So the matchup across the fracture face will not be perfect.  And then there are all the other metallurgical issues.

I'll bet with Greg Hermann that this a cost saving, not performance enhancing process.  Machined dowels hold alignment as tightly as you want to spend money to hold it!  Is the shear load from motion on the bearing high enough to be a structural concern or would it spin the bearing first?  What else would provide a non-radial load to the cap?

BTW, don't conrods see both tension and compression loads?  If so, wouldn't maraging steels be a bad idea?  Tney have great static properties but fail amazingly fast under reversed loading.

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