No. It's not a linear thing and there is no way I can answer the question, "What about damaged threads?"
It depends on the amount and type of damage and I can safely say, don't trust any damaged thread if it is used in a pressurized area on a firearm.
Trying to "put it in a laymans language :hmm:
Pressure measurements need to define the pressure. Duh. Pressure is good but it doesn't really mean anything unless it has something to push against.
In fact, if it doesn't have anything to push against it will quickly become no pressure at all.
OK. So we know that pressure will try to push stuff that's holding it in but how big is the stuff? (If the size of the stuff is large the pressure will be able to push against it with a lot of force. If it is small, the pressure will be able to push against it rather weakly. After all, if it is small, there isn't much area to push against.)
To define a usable value for size, in America we decided to use a 1 inch X 1 inch square. That would be a good area size to base the pressure value on and that is the reason we measure pressure in PSI or, Pounds (of pressure) per Square Inch (the area).
Using this value, if we know the pressure and the size of the thing the pressure is pushing against it is fairly easy to figure just how hard any object is going to be pushed by any pressure that wants to do the pushing.
You mentioned the 21000 PSI value?
OK, if the area it is pushing has a area of 1 square inch, that pressure will create a force of 21000 pounds trying to get the obstruction out of its way.
But, what if the object is less than one square inch? What if it is only 1/2 square inch?
If it is 1/2 square inch, there is only half as much area to push against so the result is it can only push with half of its power or 11500 pounds.
If the area is only 1/10 of a square inch in size, the 21000 psi pressure will only have 1/10 of the area to push against so the maximum force it can create on the object will be 2100 pounds.
Speaking of 1/10 square inch, a diameter that is .357 inches in size has that amount of area.
What if the object is only the size of a #10 screw? A number 10 screw is usually taken to have a diameter of .190 inches. A .190 diameter circles area is 0.0284 square inches.
That 21000 pounds of pressure applied to a .190 diameter object will be able to push the object with a force of .0284 square inches times 21000 psi for a net force of 595.4 pounds.
I've been talking about the effect of pressure on a round object like a pin or thread but it also works just the same on plane old surfaces too.
If we want to talk about the inside of a guns barrel for instance we need to set a limit for the length of the bore so we can calculate the surface area.
Lets say we have a breech pressure of 7000 psi inside a .50 caliber bore.
To calculate the surface area of a 1 inch length of that bore we need to find the circumfrunce of the .50 diameter circle. That would be equal to Pi (3.1416...) times 0.50 which is equal to 1.5708 inches. A 1 inch length of that size results in a total surface area of 1.5708 square inches.
Figuring out how much actual pressure is trying to enlarge the bore with a 7000 pound breech pressure we will multiply the area times the pressure to come up with an answer of 10995.57 pounds. :shocked2:
This is not how the barrel wall thickness is calculated but it does come into that. It also gives a person a hint of why the breech area of a barrel needs to be fairly thick.
Before saying good night I need to mention the strength of materials.
The strength of a material is measured several ways. One of these ways, and the one that is usually quoted (because it is the biggest number) is the tensile strength. That is the strength of the material when it fails.
One of the leaded steels people around here talk about has a tensile strength of around 68000 psi.
Does that mean it can withstand a internal pressure of 68000 psi?
No.
If there is less sectional area in the barrel than 1 square inch of material it will fail at pressures far lower than 68000 psi of pressure.
This makes sense if you think about it.
A paper thin barrel made out of ultra strong material with a tensile strength of 240,000 psi might not be able to hold in a pressure of just a few hundred psi without bursting.
Hope my rambling helps.