I have a couple parts that are failing in simple shear and I need to determine the pressure that causes the failure.  They are parts of a rocket motor test apparatus and we were unable to record the failure pressure as it far exceeded our transducer's limit.  Basically, I need a source to find the ultimate shear strength of the materials - a paperclip (low carbon steel) and 1020 steel.  Below are models of the items in the assembly before failure.

The above photo shows a blowout plug (yellow) and the paperclip (grey).  The chamber pressure is applied to the front (yellow) face of the blowout plug while the paperclip is held fixed by another part.  The blowout port was ejected from this other part after shearing the paperclip.  Similar to a pin and clevis double shear failure.

This model shows a gauge mount (yellow) which is held in place by a steel retaining ring (brownish).  The gauge mount was ejected from the assembly, shearing the retaining ring exactly at the outside diameter of the gauge mount.  Again, this is simple shear, but not as easy as the pin and clevis example. 

Basically the problem I'm running into is (as mentioned above) not being able to find the ultimate shear strength.  All I can find is a shear modulus, but what would I use for shear strain to back out shear stress?  One thought I had is applying mohr's circle - is the ultimate yield strength the same as the ultimate shear strength since they both would lie on the same radius circle?  Or am I way off base?

Thanks in advance!

peace-

Matt Kennedy