Got Shear Lock?
Funny…I was at the barber the other day and he kept complaining about his shears locking up…pesky scissors keep jamming on me he said! Well…obviously this is nothing but banal witticism, because that’s surely not what this article is about. I draw your attention to an experience I had regarding a fabric drapeability study that I performed inorder to evaluate fabric compliance over non-developable tool geometry…huh…bear with me. First the basics, when draping a composite fabric (or tape) over a tool, particularly a tool with compound curvatures, it is incumbent upon the designer to check for wrinkling and/or bridging in the fabric; especially near a radius or doubly curved surfaces. Fabric distortions of this type are clearly undesirable, and often yield poor laminate performance in terms of stiffness and lower than expected margins.
shear-lock fabric draping
Quasi-Isotropy | Wait...what is that?
Quasi-Isotropy…wait…what is that?
What is it you ask…well, this is one of my favorite meals. A large quasi-isotropy with a side of fries and coke. LOL…but of course is isn’t…actually, this is a laminate that when constructed correctly emulates a metallic material that follows the isotropic relationship defined as: Ex= Ey = Eθ. Quantitatively this can be described using a general rule for a quasi-isotropic layup. Simply apply the following equation that defines the angle between the plies for a symmetric laminate having an identical number of plies at each orientation:
Equation: π/n → n≥3
stiffness quasi matrix laminate isotropy composite
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
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
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