# Homework Help: Creep Testing Help - Secondary creep rate

1. Oct 17, 2011

### DdotT

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.

1. The problem statement, all variables and given/known data

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)

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.

2. Oct 17, 2011

### Mapes

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

3. Oct 18, 2011

### DdotT

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. Oct 18, 2011

### Mapes

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. Oct 18, 2011

### DdotT

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 dont understand how to use this.

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

6. Oct 18, 2011

### Mapes

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

7. Oct 19, 2011

### DdotT

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

8. Oct 19, 2011

### Mapes

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

9. Oct 19, 2011

### DdotT

lnε = ln A + nlnσ - Qc / RT

but we still need to know Qc and constant A? i just dont understand it x( sorry

10. Oct 19, 2011

### Mapes

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. Oct 20, 2011

### DdotT

yes, it makes more sense know. thank you.

12. Jan 13, 2012

### novice111

Hi
I am new to it. Still dont 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?