Tensile strength increasing as material is removed? 1

[chadmc]

Hello, I'm having a hard time understanding why the tensile of the materials I'm testing are changing depending on what diameter i cut the sample to. Any help understanding this would be greatly appreciated.

 

[bobjustbob]

I think we need more information to determine what is going on. What material are you testing. What sizes of test bars are you testing and what were your results?

Bob

 

[EdStainless]

There are lots of reasons why this happens.
Tell us your material, any heat treatment, original size, final sizes, and actual property changes, and them maybe we can help you.

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[chadmc]

Hi Bob,

the material is 1/2" 360 brass H02. the original diameter is raw bar stock, UTS= 68 ksi. I cut down the material on a lathe to 3/8" and it increased to 71 ksi, then for 1/4" it went to 73 ksi. These are average values over several tests of the same heat lot. I've also noticed this pattern for shear tests. the products the company i work for makes are very dependent on the ultimate strengths of materials as they are designed to fail at specific loads.

-Chad

 

[chadmc]

my apologies i should have said strength is increasing as material is removed in the title.

 

[IRstuff]

Aren't you possibly changing the material properties during machining, as compared to the as-fabricated billets?

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[chadmc]

IRstuff,

I don't think so, the material is easily machinable and is cool to the touch right off the machine;I'm using coolant. One of the few things i can find on the internet is "size factor". But there isn't enough information to help me understand.

 

[BrianE22]

If it were steel I'd suspect work hardening. I don't know if your material tends to work harden.

 

[SWComposites]

Perhaps the grain structure is not uniform thru the bar stock thickness.

 

[MintJulep]

I'd guess diameter measurement error.

As you go smaller the same linear error from your caliper has an increasingly larger effect on the calculated area.

 

[SWComposites]

And what exactly is the test specimen geometry? Is there a grip stress concentration that is changing?

If you are designing essentially a fuse (fail at specific load) are you testing every lot of material?

 

[chadmc]

It’s not diameter measurement error. Even if my micrometer was off by 5 thousandths it wouldnt account for the difference. My gut feeling is that there are several things going on, dislocations and impurities increase with diameter and that mechanical properties aren’t uniform in the radial direction do to mill processes. We test each 12 foot bar on the ends and middle. I’ve been trying to find some theory on shear strengths but not much is published other stress=F/a. so I’m searching for an answer through tensile strengths. In reality I’ve found it’s much more complicated lol

 

Segregation carried over from the original ingot could be a factor.
A difference of 3% UTS might not be far outside the scatter range for tensile testing.
Take a full cross section of the original material and do a hardness survey to determine the pattern.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

The grain structure varies with location. If you took a piece and split lenghtwise and then made two mid-radius bars from it they would match each other. Different grain size and different thermo-mechanical history at different radius.
You don't say it but you are also seeing shifts in yield and elong correct?
You are making shear pins?
You must qualify each lot and adjust the root diameter of your notch for them.
There is no master calibration unless you use material that has been very specially processed (and therefor too expensive).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[Harvard26]

Hi Chadmc,

As you are measuring the ultimate tensile stress which depends on area and thus the diameter, so I think, if you are testing then cutting the same sample to test again probability is that it is due to strain Hardening, but also what I also think is, that the load sustaining capacity of the material does not change as the material does not change as all the samples are separate,then due to the reduction in the area, the stress is increasing.

To the point that it is not increasing proportionately, I really don't have a good point although I think it is due to the removal of material.

This is just my opinion to the problem, Kindly enlighten me if I have gone wrong anywhere as I am still learning.

 

[moon161]

I'd section or face cut a bar and see how uniform the hardness is. You mentioned a heat lot. How is it heat treated?

 

[mfgenggear]

on product I have worked with shear necks, are designed to fail at this area, the shear neck diameter is based on the hardness values,
higher on the Rockwell scale (tensile) the diameter is machined smaller to have the specific tensile shear.
is material is lower on the Rockwell scale, the diameter is larger to allow for tensile shear.

according data this material is cold worked H02 (1/2 hard ) clearly the tensile property changes thru the center and the outside.
just compensate for it, but it appears there is an issue with the tensile testing ( is tensile tester calibrated?)