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# Obtaining Average Stress & Strain From Load Vs Deflection

## Obtaining Average Stress & Strain From Load Vs Deflection

(OP)
Hello All

I was hoping someone could help me with my understanding of obtaining Stress & Stress From load deflection data.

I have a timber sample which underwent a compression test (dimensions of 30 x 30 x 30).

The test was done 3 times in order to obtain an average.

1) One the first test I had 100 rows of load vs deflection data, on the second test I had 112 rows of load vs deflection data and the 3rd test I had 96 rows. I was wondering if someone could tell me how i would obtain an average of the Load vs Deflection and an Average Stress and Strain???

2) The question relates to Strain which I know is obtained from Change in Length / Original Length - My question is if I had the data below and given the fact that the dimension of my sample was 30 x 30 x 30 then would the following values of strain be correct:-

2, 2100, 2/30 = 0.0667
2.5, 2300, 2.5/ 30 = 0.083

Any help or guidance would be greatly appreciated.

I have attached a screen shot of my data of all three tests.

Thank you.

https://files.engineering.com/getfile.aspx?folder=...
Replies continue below

### RE: Obtaining Average Stress & Strain From Load Vs Deflection

Your data, as shown, is rubbish

Typically with multiple runs of real test data I'd use Matlab's interpolation function to reduce all the different runs to a common x axis and interval.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Obtaining Average Stress & Strain From Load Vs Deflection

MatthewMansfield,

I am looking at the first column of your chart. At 1.67N force, your deflection varies from 0.006mm to 0.010mm. When you double the force to 3.33N, there is no (immediate?) change in deflection. Is there a time factor you are not showing us?

Are you really able to measure a piece of wood to micron accuracy? Most theory about stress and strain is highly functional on homogenous material, but wood isn't. Is it flat? Over a deflection of ten microns, all sorts of weird things could be happening.

Stress and strain are supposed to be linear, and your data isn't.

--
JHG

### RE: Obtaining Average Stress & Strain From Load Vs Deflection

(OP)
Hi Drawoh

The data set I supplied was only a snap shot.

I have attached the full data set below slightly different but its all there.

My key question is really about how to combine all three data sets to get an average because the number of rows for test 1 is 8190, test 2 has 8668 and test 3 has 7264 rows of data.

https://files.engineering.com/getfile.aspx?folder=...

### RE: Obtaining Average Stress & Strain From Load Vs Deflection

MatthewMansfield,

I am looking at your data. Try working out the spring rate of your wood block. This is force divided by deflection. With elastic deformation and accurate measurements, this should be constant. It isn't. I recommend that you fire up your spreadsheet and graph the results.

I don't know how wood behaves when you exceed its elastic limits. Is your block visibly damaged?

--
JHG

### RE: Obtaining Average Stress & Strain From Load Vs Deflection

"My key question is really about how to combine all three data sets to get an average because the number of rows for test 1 is 8190, test 2 has 8668 and test 3 has 7264 rows of data."

Ahem "Typically with multiple runs of real test data I'd use Matlab's interpolation function to reduce all the different runs to a common x axis and interval."

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Obtaining Average Stress & Strain From Load Vs Deflection

Suddenly we've learned 5 different things (1) you need to truncate the data, (2) the results aren't linear (3) the data doesn't look real or it has been heavily processed (4) the 3 tests are quite different so averaging them together is a bit suspicious (5) perhaps do a polynomial curve fit if you don't like interpolating them to a common x

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Obtaining Average Stress & Strain From Load Vs Deflection

GregLocock,

It looks like you have more time on your hands than I do.

--
JHG

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