Yield Strength
This discussion has been good in a number of ways, our budding rocket scientist, learned a lot, hate to use the cliché, but ‘when it comes to learning, ‘Failure IS an option’, provided that there is the proper safety provisions in place where failure doesn’t hurt anyone. Or as they say, live and learn.
Yield vs Tensile
When it comes to failure of a metal tube, one piece of information that I think needs to be clarified is yield vs. tensile strength. Lots of mis information has been put out there. Bottom line, for rocket motors, yield strength = failure NOT tensile strength.
Changing the Yield Strength
There are many ways of increasing the yield strength of a part to get it up close to the tensile strength. These involve temperature and mechanical work hardening of materials. For example, way back at the day job we would form a motor tube and then using water or oil pressurize the tube past yield but below ultimate. Without going into the mechanics of it (there are some really good books on this) we raised the yield strength of the tube. Really important, if we tried that with gas not water the tube would have burst once we hit yield, this is the difference between compressible and incompressible fluids.
Heat Treatment / Temper
Other ways of increasing yield strength are with heat treatments. Often at work we remove all the thermal stresses (called annealing ) to make the part easy to form and machine, and then using heating and rapid cooling induce thermal stresses increasing the yield strength (again some really good books on this stuff). Here is the problem, if you heat it up to high you lose the temper. This can be done by machining the tube without sufficient coolant or by getting the inside of the motor too hot.
How the tube is made can change the yield strength
How the tube is made can change the yield for steel tubing one way to make it is start with a flat plate, roll it, weld the seam, and then cold form it into the final diameter by forcing it over a mandrel. This is called DOM tubing. All of this ‘cold work’ increases the yield strength. Other processes cold roll the tubing and then weld it without the drawing step, hot rolled is easier to make, but has lower yield. Aluminum can be extruded to shape or extruded and then drawn, again resulting in different yield strengths. Even the difference between cutting threads and rolling threads can change the local strength
So, for rocket motors,
Understand what allow and temper you have when you start. Different manufacturing methods will give different properties even with the same alloy.
Make sure you don’t change the yield without knowing it, change it for better if you know what you are doing.
Use yield strength as your upper limit WITH A FACTOR OF SAFETY
Testing a motor tube can be done BUT NEVER WITH GAS only with incompressible fluids.
If you have a case with evidence of heat damage, don’t use it until you ask the manufacturer.
Mike K
P.S. Just for fun
6061 aluminum with a T-6 temper (heat treatment) has a yield of 40ksi, and an ultimate strength of 45ksi. If you a remove the heat treatment the yield decreases to 18ksi
