Re: intentional fracturing

[Jaap Bouma <j.h.bouma@domain.elided>]

Hi Robert,

You raise some interesting points in your reply to my posting on
intentional fracturing of connecting rod caps. My understanding is that the
conrods are made in one piece, including machining of the inner surfaces of
the big ends and their side faces. The caps are then broken away from the
rest of the rods. The fracture surfaces then provide a better alignment key
than any bolt, dowel or other machined shape fit (form fit? my english
deserts me here) could. This is the primary reason why manufacturers use
this method. 
The fact that all machining takes place before the caps are broken off to
me indicates that the rods are not subjected to any further heat treatment,
as that might cause deformation of the machined surfaces and the rod as a
whole, which obviously is not desirable. However, I may be wrong here.
It also leads me to the conclusion that the ductile deformation of the
fracture area is sufficiently small that the elastic deformation of the rod
and cap as the cap bolts are tightened returns the shape and size of the
'eye' to within tolerances. This means that the material must be quite
brittle. How this brittleness is achieved without compromising the strength
of the rod, I do not know.
Finally, I would think that the crack initiation sites you mention would be
a big drawback if the fracture surfaces were to be loaded in tension.
However, the tensile forces are taken up by the cap bolts, leaving the
fracture areas in compression, where crack initiation sites may be less of
a problem. Indeed, I would think that the bolt clamping action is the major
force that the material around the mating surface of the rods is subjected
to. Compared to the rest of the conrod, the cap is a relatively lightly
loaded part.

All this is a bit speculative though, as metallurgy is not my specialism.
Although I am a mechanical engineer by training, my field of expertise is
two-phase flow, not material science. 

I hope this is of some interest.

Best, Jaap Bouma.

At 08:30 AM 5/11/99 -0700, you wrote:
>Jaap,
>
>Just read your post on fracturing of a connecting rod to separate the cap.
 I could see that it might be handy to forge both rod and cap in one die
but can think of a lot of metallurgical reasons why fracturing is not the
best way to separate the parts:
>
>*  shear lip at fracture would prevent perfect mating of surfaces
(yielding at fracturing area
>
>*  residual stresses may not all be baked out
>
>*  ductile fracture face would be full of fatigue crack initiation sites
>
>And yet manufacturers still do it this way, apparently.  Do you know why?
There should little to no shear loading of the fracture face.  So using the
fracture surfaces as alignment keys wouldn't be an advantage.
>
>thanks,
>Robert Hardwick
>'79 spider
>'91 164
> 

Best, Jaap Bouma (Netherlands)
'87 GTV6 2.5 Grand Prix

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