UCS Test and Young's Modulus 2

[kb38765]

Hi all,

Hopefully a basic question. I am looking through some UCS data (image attached) and am trying to understand where the Young's Modulus has been derived from. The strain at failure is given (and a strain range) but none of these produce the Young's Modulus shown when plugged into E = Δσ / Δεa. I have read that E is calculated at a stress-strain level of about 50% of the maximum load, but this doesn't seem to yield the "correct" Young's Modulus provided either. The stress/strain are already area corrected so felt that I had ruled out this as a potential issue too.

Am I missing something, or is the data I need to calculate E myself hidden within the lab processing?

Cheers.

 

[EireChch]

Its not stress strain at a single point, its a tangent or secant modulus.

There are many ways to determine E mod form USC, tangent modulus of strain %, secant mod at 50% of ult strength or tangent mod at 50% strength.

Yours is likely E determined using tangent modulus measured at 50% ultimate strength.

I guestimated the values so 215MPa is correct.

Also, the lab report should clearly state how it calculated it.

 

[TigerGuy]

The report indicates the Young's Modulus was calculated for strain range of 0.66-1.24%

 

[kb38765]

Thanks both, must have had a brain fart yesterday as I plugged everything in as per EireChch methodology but using their provided strain ranges and resultant stress range and it works fine.

Not sure how their provided strain range matches any of the potential ways to calculate E you list EireChch - looking through the small batch of UCS test results I received, the provided strain ranges do not align around the 50% strength mark, rather they correspond to a range between 16-25% and 100% of strength. No further commentary within the lab report.

 

[TigerGuy]

I get spoiled having direct access to all lab sheets and data, with in house testing. When I read other lab's reports, I can't make sense of most of the results. Makes me wonder if my own results make sense to others.

 

[BigH]

If y'all let me regress a bit - but TigerGuy's comment about "makes me wonder if my own results make sense to others." brought to me something that I ran into in Indonesia back in '83. I was about 8 years out of school and assigned to a port masterplan study for a number of ports in Sumatra. I would read the various geotechnical reports and noted that the c-phi' value of a triaxial test with pore water measurements gave phi values not much different than undrained phi (5 deg or so). This seemed very strange but these tests were being done by "good" labs and even professors at some of the universities. Made me wonder if I knew what I was talking about . . . so finally I had a chance to go to a lab and questioned the lab engineer who, by the way, was a graduate of the Asian Institute of Technology in Bangkok. I asked him: "when you do a triaxial test with pwp measurements, how long does it take to do the test?" He replied about 20 minutes. Hmmmm - my Toronto technician would take 8 hours or so per stage. The chap was convinced that he was right so he went to get his book. I looked at the book and found the sentence that when taking pwp measurements the time of each stage was dependent on the coefficient of permeability but that it would be a "long time" - like 8 hours?? This is a problem that I see overseas - they sometimes miss that crucial sentence. Thanks for letting me "reminisce" . . .

 

[TigerGuy]

Big H,

Sadly, it's not just overseas. I could not convince one of my bosses that we were shearing the triax samples way too fast, due to the low permeability of the material we tested.

His response was effectively, "That's how we've always done them, there's no reason to change."

Let's just ignore the ongoing research that provides improved test results for better analysis, right?

 

[Mccoy]

I myself do not waste time at all with anomalous values. If a parameter goes outside the known range, from literature or professional practice, I simply discard it as garbage, unless extensively justified with valid technical reasonings.

I'm utterly horrified that the university guys cited by bigH were all right with a 5° phi-drained value. That would be an absolute outlier. outliers must be technically explained, otherwise, they must be regarded as mistakes.

Speaking of UCS tests, I found that many of those executed in my place do not yield reasonable values of Su, let alone Young's modulus. They go straight in the trash can (or the OS trash bin).