Failure Theory

[numbercraze]

Hello All,
In calculating the failure index using one of the failure theories is it an industry standard to use the strain or stress from the mid plane (average) of the ply rather than the strain or stress from the top surface of the ply. Also can you guys share your thought on the failure theories commonly used these days in the industries. Thanks in advance.

 

[SWComposites]

The typical practice is to use strains at the ply midplane, unless the laminate is very thin, the ply thicknesses are large, or there are other compelling reasons linked to test data. Most large aerospace companies use the max strain criteria with strength values from multi-directional laminate (not lamina) tests. See my previous posts in this forum for more details.

Steve

 

[numbercraze]

Thanks SWC for your reply. But I am assuming that the allowable ply strain which is obtained from testing would be from the top surface. Correct me if I am wrong.

 

[SWComposites]

If the test is an axial load of a symmetric laminate, then the strain is essentialy constant thru the thickness. What is the issue that you are trying to deal with?

 

[numbercraze]

There are two different non-symmetric but balanced four ply laminates: one is a fabric laminate and the other is the Tape(UD)laminate. If i apply a load which is in the first quadrant the ply top and the bottom surface stress for the fabric is almost same and hence the average stress is similar. But for a tape laminate for the same kind of loading the critical ply top and bottom surface stress is offset by a big value and hence the average stress is not close to either the top surface stress or the bottom surface stress.
Now if i use a failure theory to predict the failure index it makes a big difference when i use the average stress of the ply instead of the top surface ply stress which is always greater than the mid plane ply stress. I know most of the companies use mid plane ply stress to calculate the failure index. Shouldnt they be using the top surface ply stress? The same holds good for strains too.Thanks

 

[SWComposites]

I can't think of a reason to use a 4 ply unsymmetric laminate that isn't stabilized by core or some other means. Like I said before, if you have a case with very large stress/strain gradients thru the ply thickness then you may have to use ply surface values; but in general this is not an issue with practical structures.

Are you trying to correlate to test data? or is this an acedemic problem? or what? What specific layups are you analyzing?

 

[numbercraze]

The stacking sequence is [45,0,0,-45. This exercise is just to figure out which failure theories are conservative and which are not. Through the course of this exercise I found out that some use the ply surface stress/strain and some others use the mid plane values.
I was wondering if you have any material on modeified Tsai-Wu criterion or any good paper which discusses the failure criteria's.Thanks.

 

[SWComposites]

For the results of an extensive evaluation of composite failure theories, see this paper:

M. J. Hinton, et. al., “A Comparison of the Predictive Capabilities of Current Failure Theories for Composite Laminates, Judged Against Experimental Evidence”, Composites Science and Technology, Vol. 62, 2002, pp 1725-1797.

along with many of the references listed therein.

The bottom line: lamina level composite failure theories, including Tsai-Wu, don't work very well (to be charitable).

How do you intend to determine if a failure theory is conservative? Are you comparing to test data?

 

[numbercraze]

Thanks SWC. Is that research paper available in the public domain.
The idea is to conduct test and generate a failure envolope to compare it with the failure theories. The three criteria's which i am looking at is Hashin,Yamada-Sunn and Tsai-Wu. I know that Hashin and Yamada does not take into account the transverse component and hence the theory is not so accurate. This is just a preliminary study.

Thanks.

 

[SWComposites]

You can purchase it here:

http://www.sciencedirect.com/scienc...serid=10&md5=9de0b7c5d94c31e99b5d1c2bd315e3f5

Or try your company or university library.