Flexure/Shear test data processing

[phoenix221]

Hello everyone,

I was wondering if anyone has a formula for deriving the flexural and shear strength of the composite from data obtained from a plain flexure/shear stress test using a 3 point flexure/shear test fixture (e.g.

http://www.wyomingtestfixtures.com/Products/page37.htm)

Thanks,
M

 

[tstanley]

I've been trying to do the opposite, that is why I mentioned about material properties in your thread on computer aided engineering. I did download "the laminator" as you suggested, but as far as I can see it just uses classical lamination theory. Using the micromechanics principles and Nastran 4W the results of a four point test could be predicted only to about ten percent with a sandwich panel of carbon, kevlar layup. Some were better, but how do you know which one ahead of time?

Tom S.

 

[phoenix221]

tstanley,

I am trying to correlate computed values myself. I don't use Nastran so I'm out of luck from that standpoint, therefore I am relying on "The Laminator". Perhaps there is a better program (cheap) out there, but for now I am focusing on getting the process all together... I'll optimize it later...

I understand that engineers routinely predict flexural and shear strength from multipoint (3 minimum) flexural test results. I suspect there is some formula behind all this... I would like to get that information and I hope someone would help me out

I can live with a 10% variance as long as the computed values are below the actual ones!

 

[sgdon]

Hi, assuming elastic regime,

Stress (flexure) = (0.75*L*P)/(b*d*d)

Stress (shear) = (0.75*P)/(b*d)

where P is the load of the 3-point test.

Regards
sgdon

 

[phoenix221]

>Hi, assuming elastic regime,
>
>Stress (flexure) = (0.75*L*P)/(b*d*d)
>
>Stress (shear) = (0.75*P)/(b*d)
>
>where P is the load of the 3-point test.

To make sure I understand the values are for the test specimen and are:
L - length
b - width
d - thickness ... right?

Now what would the formula look like for a 4 point test?

Also, I have recently come accross a similar formula for the (flexure stress):

F = 3*P*L/2*b*d*d ... Any comments on this one?

 

[sgdon]

Hi phoenix221

Apologise for the mistake here. Yes, you were right, the flexure stress factor should be 1.5.

For 4-pt test, you can similarly derive from 1st priciple, i.e. obtain moment from the triangular stress block and equate to the moment obtained from the free body diagram.

Thanks

 

[phoenix221]

sgdon,

>Apologise for the mistake here. Yes, you were right, the
>flexure stress factor should be 1.5.

No problem... Just beginner's luck on my end

>For 4-pt test, you can similarly derive from 1st priciple,
>i.e. obtain moment from the triangular stress block and
>equate to the moment obtained from the free body diagram.

This is really testing the limits of my current understanding :-(...So the formula for a 4 point test would be written how?
F = ?

 

[sgdon]

Ha..you are also trying to test my limits huh?!

For 4 pt and still assuming elastic, one can work out to be

Stress=Moment/Elastic modulus (For rectangular section, elastic modulus=bd^2/6)
Moment = (P/2)*l/3 (assume total load on the beam is P)
i.e. Stress=Pl/(b*d*d)

Having, i did come across people quoting 0.75Pl/(b*d*d). Now, this time round, you tell me why is that so?! I hope someone else may also be able to answer that..

sgdon