Creep Testing Help - Secondary creep rate

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SUMMARY

The discussion centers on understanding secondary creep rates in materials, specifically lead, using the equation E = A stressn e-Qc/RT. The user, DdotT, seeks clarity on how to derive the secondary creep rate (E) without knowing the constants A and Qc. The conversation emphasizes plotting extension against time to identify the secondary creep region and suggests using a log-log scale to analyze stress versus creep rate, where the slope provides the stress exponent (n). The universal gas constant (R) is noted as 8.31 J/mol K, and the experiment is conducted at room temperature.

PREREQUISITES
  • Understanding of creep testing and material science principles.
  • Familiarity with the equation E = A stressn e-Qc/RT.
  • Knowledge of logarithmic functions and their application in data analysis.
  • Experience with graphing techniques, particularly log-log plots.
NEXT STEPS
  • Research the concept of activation energy (Qc) in creep testing.
  • Learn how to calculate constants A and Qc for different materials.
  • Explore the use of log-log plots in material behavior analysis.
  • Investigate the relationship between stress and creep rate in various metals.
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Material scientists, mechanical engineers, and students involved in materials testing and analysis, particularly those focused on creep behavior in metals like lead.

DdotT
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I tested creep in the labs at univeristy, one thing i am confused about is, we made a graph of the extension against time. Taking the gradient gives us E or secondary creep rate.

Homework Statement




The equation for this (without using gradient ) is, E= A stress^n e^-Qc/RT

A and n are constants ( i know n is the stress exponent which for the material (lead) was 10)

Qc is the activation energy, can't find anything about this.

R is universal gas constant 8.31 J/mol K

Note: the test was done at room temperature.

T is absolute temp.



Really confused by this, the experiment helper didn't explain any of this.
 
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DdotT said:
I tested creep in the labs at univeristy, one thing i am confused about is, we made a graph of the extension against time. Taking the gradient gives us E or secondary creep rate.

Homework Statement




The equation for this (without using gradient ) is, E= A stress^n e^-Qc/RT

A and n are constants ( i know n is the stress exponent which for the material (lead) was 10)

Qc is the activation energy, can't find anything about this.

R is universal gas constant 8.31 J/mol K

Note: the test was done at room temperature.

T is absolute temp.



Really confused by this, the experiment helper didn't explain any of this.

Hi DdotT, welcome to PF! What are you trying to do? Is the problem to estimate one of the parameters?
 
Mapes said:
Hi DdotT, welcome to PF! What are you trying to do? Is the problem to estimate one of the parameters?

No the 'thing' i need to do is draw up curves for the 3 specimens (extension against time), work out the stress , work out the secondary creep rate, then take natural logs of these, then plot a final graph with the natural logs, take the gradient of the line of best fit, that gives me the stress exponent (10 in lead's case).
However, this is what the experiment helper told us to do, in our booklets are the formula i stated, i do not understand how i would use this formula / get the secondary creep rate ε without taking the gradient.
 
OK, got it. You should first plot extension vs. time. Secondary creep occurs after an initial transient and before rapid failure; it is a region of relatively constant creep rate. Find this creep rate. This will be E in your equation above. Note that it depends exponentially on stress. So plot the stress vs. creep rate on a log-log scale; the slope of the line is an estimate of the exponent. Does this make sense?
 
Mapes said:
OK, got it. You should first plot extension vs. time. Secondary creep occurs after an initial transient and before rapid failure; it is a region of relatively constant creep rate. Find this creep rate. This will be E in your equation above. Note that it depends exponentially on stress. So plot the stress vs. creep rate on a log-log scale; the slope of the line is an estimate of the exponent. Does this make sense?

yh i understand this, and its relation to the exponent.
its the equation of: ε = A σ^n e^ Qc / RT

E is the secondary creep rate , stress is σ, n is the stress exponent of lead which is 10, Qc os the 'activation energy' for creep in the metal, R universal gas constant and T is temp.

How do i use this equation ?

i have stress, n , R, T (room temp i think).
I do not know Qc or A. It says A and n are constants.
So i really don't understand how to use this.

Sorry if i didnt make it clear what i needed help with.
 
DdotT said:
yh i understand this, and its relation to the exponent.
its the equation of: ε = A σ^n e^ Qc / RT

E is the secondary creep rate , stress is σ, n is the stress exponent of lead which is 10, Qc os the 'activation energy' for creep in the metal, R universal gas constant and T is temp.

How do i use this equation ?

i have stress, n , R, T (room temp i think).
I do not know Qc or A. It says A and n are constants.
So i really don't understand how to use this.

Sorry if i didnt make it clear what i needed help with.

You know that ε = E here, right? Just checking.
 
Mapes said:
You know that ε = E here, right? Just checking.

yes, i just don't know what to subsitute for things like A and Qc
 
DdotT said:
yes, i just don't know what to subsitute for things like A and Qc

Does it matter? What happens when you take the log of ε = A σ^n e^ Qc / RT?
 
Mapes said:
Does it matter? What happens when you take the log of ε = A σ^n e^ Qc / RT?

lnε = ln A + nlnσ - Qc / RT

but we still need to know Qc and constant A? i just don't understand it x( sorry
 
  • #10
DdotT said:
lnε = ln A + nlnσ - Qc / RT

but we still need to know Qc and constant A? i just don't understand it x( sorry

When one considers dlnε/dlnσ = n (i.e., the slope of ε vs. σ on a log-log chart), all other parameters go away. Know what I mean?
 
  • #11
Mapes said:
When one considers dlnε/dlnσ = n (i.e., the slope of ε vs. σ on a log-log chart), all other parameters go away. Know what I mean?

yes, it makes more sense know. thank you.
 
  • #12
Hi
I am new to it. Still don't understand. Can you help with this:
I have known: 1 stess and 1 creep rate; 2 stress and 2 creep rate; 3 stress and need to calculate 3 creep rate based on the above.
The temperature is constant. the same all the time.
s1; e1
s2; e2
s3; ?
What is the easiest way to do it?
 

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