tensile test specimen

[cs39]

If a tensile test specimen is held a machine by its top and bottom with a pressure applied at both of these points. The uniform shear stress is Txy= (coefficient of friction)x p.
sigma y is uniform at all horizontal sections and rises linearly from zero at the top. I have the length of the top part which is held in the machine and the dimensions of the gauge length .including a safety factor, I need to find the width of the section held in the machine, so that the yielding will happen first in the gauge section.
Has anyone got any idea how to work this out?

 

[CESSNA1]

CS39: If you are doing only a tensile test then your shear equation is incorrect. The shear is obtained from the tensile test by Mohr's Circle. There are criteria for the design and manufacture of test specimens, I would start with the American Society of Testing Materials (ASTM). The number and type of threads on the end is also a concern.

Regards
Dave

 

[JStephen]

I'm not sure I understand the post. Is this the normal dog-bone (sort of) shaped specimen? If so, I don't see the problem. And even if you did get some yielding in the clamped part, would that make much difference, since you don't gauge that part?

 

[NickE]

The width of the clamped part of a tesile specimen only needs to be greater than that in the gauge section. That is why the gauged section has it particular dimensions.

IE: ASTM A370- 2"x.5"xt -- for sheet materials.


I used to cut lots and lots of tensile bars on a tensilkut machine, the starting width was .749" the jig allowed me to achieve .5"+/- .001" in the gauge section.

JStephen- yielding in the clamped section is kinda ok only if you have properly mounted extensometers on the guage section. Even then its not desireable. I think that the ASTM spec says that any noticable slippage in the grips, or fracture out side of the gauge length is grounds for re-running the test.

 

[yates]

Yielding in the clamped portion is not strictly allowed because it may introduce undesirable bending stresses into the test-piece, and there are rules in tensile testing on machine alignment (ASTM E1012 or MIL-STD-1312) to avoid bending stress. Rupture outside the gage length is very tricky if you're trying to measure elongation, and of course, yield or slippage outside the gage length is going to upset your yield strength reading. Apart from this, if you just need to know UTS, provided your test-piece ruptures in the gage section (known cross sectional area), it doesn't matter if end portions slip or yield. However, if this does occur, there is little chance of the test piece rupturing in the gage length anyway, so it's best to stick with standardized test pieces.

 

[Ron]

This sounds like a homework problem. Are you a student?