Thin Ply Composites show superior strength properties and delayed onset of damage (onset at 90% or higher of ultimate strength compared to 40-60% with standard ply thicknesses) (eg. studies by Amacher, Sihn etc.).Therefore, decreasing ply thickness seems to suppress damage/delamination and thus delay ultimate failure. However, various studies show reduced fracture toughness with decreasing ply thickness (e.g. studies by Frossard et al.). How does this fit together? What does that imply? Does it mean that it is more difficult to develop a crack/delamination with reduced ply thickness, but once the crack has developed, is it easier to grow?
I think you basically answer your question yourself. Thin-ply composites are stronger but also more brittle than standard ply composites. So yes, once damage has initiated the toughness is lower.
The reduction of fracture toughness in thin ply stems from the fact that the extrinsic toughening (presence of fiber bridging) is minimum in thin ply. The reason is just a statistic: in thin ply, you have lower number of fibers that help the bridging mechanism in comparison to ordinary ply. The implication: how can we improve both damage initiation onset and fracture toughness?
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