Today's KNOWLEDGE Share : Why do we always see an L/D of about 20 in our conventional GF filled polymers

Today's KNOWLEDGE Share

Why do we always see an L/D of about 20 in our conventional GF filled polymers

Although it may surprise some of you, the main mechanism (if we neglect fiber-fiber or fiber-steel attrition) of fiber damage in a shear flow is COMPRESSIVE BUCKLING.

The flow stress is not sufficient to pull enough on the glass fibers to break them in tension.

However, during the well known fiber rotation in a shear flow field, there will be moments where such a fiber will be loaded in COMPRESSION.

And very similar to what happens if you try to snap raw spaghetti by squeezing one between your thumb and index , the flow imposed compressive load will exceed the critical buckling load of a thin glass fiber, leading to bending and snapping.

Once the fibers become smaller in length, the critical buckling load (Euler formula) grows quadratically with 1/(L/D), quickly leading to the impossibility of further breaking the fibers with the available flow stress and the reduced fluid to glass stress transfer on a shorter fiber.

In the picture from a 1959 paper, the authors show the relationship between the fluid shear flow and the “critical” aspect ratio of the glass fiber.

What the figure tells us, for instance, is that if you develop a 1 MPa flow stress (10e6), all fibers longer than about L/D=20 will break by buckling.

And, in a nutshell, this is why all commercial GF filled polymers end up with typical average fiber length in the vicinity of L/D=20, i.e. about 200 micron length.

This also means that once you get to that level of breakage, the fibers will now survive multiple passes in Injection Molding with a good retain of fiber length. Something of interest for recycling of course !

source : Vito leo