Creep Testing Help - Secondary creep rate

In summary, the conversation involves testing creep in the laboratory and creating a graph of extension against time. The equation for determining the secondary creep rate is E= A stress^n e^-Qc/RT, where A and n are constants, Qc is the activation energy, R is the universal gas constant, and T is the temperature. To calculate the secondary creep rate, the experiment helper suggests plotting the stress vs. creep rate on a log-log scale and finding the slope of the line, which is an estimate of the exponent. However, the conversation also reveals confusion about how to use the equation, as it requires knowing the values for A and Qc. Taking the natural log of the equation simplifies it and allows for the estimation of
  • #1
DdotT
9
0
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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  • #2
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?
 
  • #3
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.
 
  • #4
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?
 
  • #5
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.
 
  • #6
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.
 
  • #7
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
 
  • #8
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?
 
  • #9
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?
 

1. What is creep testing and why is it important?

Creep testing is a method used to measure the deformation or strain of a material under a constant load over a period of time. It is important because it helps us understand how a material will behave when subjected to continuous stress, which is crucial for designing safe and reliable structures.

2. How is secondary creep rate different from primary and tertiary creep?

Primary creep is characterized by a decreasing strain rate over time, while tertiary creep is characterized by an increasing strain rate. Secondary creep, on the other hand, is a period of relatively constant strain rate. It is often referred to as the steady-state creep, where the material is deforming at a constant rate.

3. What factors can affect the secondary creep rate of a material?

The secondary creep rate of a material can be affected by several factors, including temperature, stress level, and microstructural properties. Generally, higher temperatures and stress levels lead to higher creep rates, while materials with finer microstructures tend to have lower creep rates.

4. How is secondary creep rate measured?

Secondary creep rate can be measured by conducting a creep test, where a material is subjected to a constant load at a specific temperature and the resulting strain is recorded over time. The slope of the strain-time curve during the secondary creep portion represents the secondary creep rate of the material.

5. What is the significance of understanding the secondary creep rate of a material?

Understanding the secondary creep rate of a material is crucial for predicting the long-term behavior of structures and ensuring their safety and reliability. It also helps in selecting the appropriate materials for specific applications, as different materials have different creep behaviors and can withstand different levels of stress and temperature before experiencing significant deformation.

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