Bolt tension on b7 rm stud

[Boater2235]

I am stretching a b7 rm stud. The stud is 14 in long and hollow for mic test. The shank is cut down to 2 Inches, the actual thread size is 2 1/4 inches. With 8 pitch thread. We are using a thread tensioning tool that will exhibit 305,500 axil force when maxed out. Not sure how high to tension before getting into proof load, but may have to exceed to get proper stretch. What woul the ultimate tensile strength be on this stud as well as proof load. I would appreciate any suggestions.

 

[CoryPad]

I have not heard of b7 rm - do you mean Grade B7M according to ASTM A 193?

What is the hole diameter? How deep does the hole extend? How long is the reduced diameter shank?

A solid stud with a 2.25-8 thread and B7M material would require a minimum of 285 000 pounds applied force for yield stress and 356 000 pounds applied force for ultimate stress. Your values will be lower based on the (unspecified) dimensions.

 

[Boater2235]

The stud is actually 15 inches long. The mic hole extends 14 1/2 inches and is a 1/2 inch in diameter. The thread engagement of the nut is 2 7/8 and 3 inches on the steam chest. The reduced shank is 8 inches long and the pitch is 8. Thanks for the help corypad. I would appreciate if u showed me the math also or software used so i can show it to my supervisors.

 

[rmw]

Do you think that hole might be for a bolt heater to heat the bolt so that it will lengthen instead of having to stretch it with torque?

rmw

 

[Boater2235]

The hole is for both a rod heater and taking mic readings for stretch

 

[desertfox]

hi Boater2235

I am not clear on how accurate you wish to be with the pre-tensioning of this stud,or indeed why you believe you might need to exceed the proof load to obtain a proper stretch.
The strength of a grade B7M material if thats what it is can be found at this link:-

http://www.fastenal.com/content/feds/pdf/Mechanical Properties of Inch Fasteners.pdf

You will need to scroll down several pages to find your grade and they are 100000psi tensile and 85000 for yield.

Now the calcuation for your stud is not straight forward because you need to get the stiffness of each part ie the threaded area and plain shank area, there are approximations for this but these are for full fasteners as opposed to hollow ones the theory can be found here:-

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html#Joint

Stiffness

A very rough calculation and to give you some idea if you just take the hollow shank area of your stud:-

3.142 * (2.25^2-0.5^2)/4 = 3.77973"sq

now multiply the above figure by 85000psi to give the tensile load needed to generate the yield stress in the shank of the stud.

85000*3.77973= 321276.9362lbf or 143.427tonsf

Upto this value the Modulus of elasticity is valid assuming 29000lbs/in^2

One can now work out the elongation of the stud from:-

E = Stress/Strain.

desertfox

desertfox

 

[Boater2235]

Thank u these are the numbers I was looking for

 

[desertfox]

Hi Boater2235

Your welcome!

desertfox

 

[CoryPad]

Boater2235,

ASTM B 193 specifies Grade B7M to have a minimum yield strength of 80 ksi and a minimum tensile strength of 100 ksi.

Your reduced diameter shank (2" OD and 0.5" ID) has a cross sectional area of 2.95 in².

The fastener yield force is 80 ksi x 2.95 in² = 236 kips or 230 000 pounds.

The fastener ultimate force is 100 ksi x 2.95 in² = 295 kips or 295 000 pounds.

Proof stress is defined in various books/standards/etc. (but not ASTM A 193) as 90 % of yield stress. So, proof force would be 0.9 x 230 000 pounds = 207 000 pounds.