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Is your Laminate Special?

### Is your laminate special?

I would like to think that all my laminates are special…and I’m sure that, well, all of your laminates are special too. Hmmm…this is starting to sound like an intervention! Well, not quite…what I’m actually referring to is a type of layup that excludes the use of angle plies, greatly simplifying an upfront analysis and allowing for the use of a closed-form solution (you mean no FEA!). You’re kidding right! No…in fact I’m not…I’m referring to laminate construct commonly referred to as Specially Orthotropic.

theory plate orthotropic deflection analysis

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Are you Balanced or Unbalanced?

So how do we know that we have an unbalanced laminate in the first place? Moreover, if you end up with an unbalanced layup, what are the implications? Good questions to have answers to before signing-off on that laminate design.

Firstly, a perfunctory definition is in order regarding a balanced laminate. A designer will need to ensure that for every -α ply there is a +α ply (with the same material and thickness) somewhere within stacking sequence irrespective of location. Examples of balanced laminates are: [0/30/-30/0] or [45/-45/0/0]. An unbalanced laminate is: [0/30/30/0]...notice that the negative 30-degree ply is no longer present. Now maybe you desire an unbalanced laminate or maybe it’s simply unavoidable but in a majority of design cases this layup scheme should be avoided at all costs. Why? Well, perhaps you have already surmised, this layup scheme has adverse implications, one of them significant…the often dreaded and undesirable in-plane extension-shear coupling.

unbalanced shear plate laminate extension coupling balanced

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Flatwise Tension in a Curved Plate

### Composites…It’s not just a 2D world…got Flatwise Tension?

Too often I hear both analysts and designers toss around the term quasi-isotropy…a mythical monster to some, an in-plane idealization to simplify a 3D problem for others. I for one am guilty…because I appreciate the fact that you can take a complex 3D composite problem and knock it down a dimension...in this case, the out-of-plane direction.

tension stress radius plate flatwise failure curved