Engineering & Analysis Methods > Material Strength Composites: Laminate Approach

Quasi-Isotropic laminate failure

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**narcisu**:

Hi there,

I have a model with a number of quasi and non quasi-isotropic laminates. Due to lack of available shear test data (allowable), I am currently using max strain principle to derive MoS in a ply-based analysis, using a combination of laminate OHC/OHT/FHC/FHC data and CLT to define lamina strength. This approach is conservative, therefore, to be able to use a pure max strain principle, I am assuming a quasi-isotropic laminate can only fail first in fibre direction and orthogonal, rather than shear. Can a QI laminate fail in shear? Can AML help in deriving new allowables? Not very familiar with this method though.

Kind regards

Narcis

**BStier**:

Hi,

Max. strain first ply failure is conservative as well (it is still a first ply failure theory). A QI laminate under macroscopic pure shear (stress state) has the 45's under tension/compression, but the 0's and 90's under pure in plane shear. Under pure macroscopic shear assuming linear elasticity, (+/-)S12 = -SII = SI holds until (first ply) failure. Thus, which ever stress component reaches its limit first, will dictate the failure. Since typically the (ply) longitudinal strength is so much higher than any other strength, we can rule out fiber failure. Also, since the transverse compressive strength is in most cases greater than the transverse tensile strength, we can rule that out as well. Since the transverse tensile fail strain is much lower than the shear fail strain (typical for many composite materials), it will basically always be a transverse tensile failure of the -45 ply that dominates the first ply failure (this depends on your definition of positive shear and positive angle - I mean the 45 ply under transverse tension for pure positive applied shear force).

However, theoretically everything is dependent on the ratios of the tensile, compressive and shear moduli of your composite and the respective strength ratios!

My recommendation is to generate a discrete QI laminate in HyperSizer with the ply material you have:

- Create a non FEA Project with only one Component (1 Stack Unstiffened Panel)

- Assign your Material (with the material allowables you know - assume a conservative shear strength) to the Component

- Generate a Discrete Laminate

- Right click and select 'view/edit Material'

- Review the Laminate (Check that it is QI, and change if necessary)

- When the Laminate is correct, click 'Analyze' (at the top of this form)

- On the left, apply a constant Nxy load.

Now you can toggle through all the different tabs and review the ply by ply stress and strain plot as as well as the MS plot for the various failure criteria. Check the strain state of the plies with low MS.

For most typical composites, the MS of plies under pure shear strain have relatively high MS (as expected).

However, for your special material system this might not be true!

But you can use the tool to easily change the shear strain allowable, and see if a conservative value would still be critical! If not, you could just use a conservative shear strain allowable while sizing, and use max strain criterion for your analysis.

Best Regards,

Bertram

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