Hi Roman,
Thanks for posing the question about interlock. The question that remains is whether spherical or crushed particles are better and I think that that's been an age old argument.
I'm going to make an argument from the strict perspective of the theory below for the sake of argument even though I suspect there may be other mechanisms at work. I'm curious what others think of the quality of the argument.
From the perspective of the two models I work with*, it's much more difficult to get a high packing density with crushed particles. A given size of crushed particles tends to get Beta Packing Density Coefficients in the high 50's to low 60's. Round particles tend to be between the mid 60's and the theoretical max around 71%.
Looking at the graph representation of the Hashin-Shtrikman model I posted back in post 3119, it's fairly clear that a small change in packing density makes a large change in the modulus.
http://www.cnczone.com/forums/444339-post3119.html
Because spheres have a better packing density, the models would suggest spheres as the lower effort approach to obtain a given modulus. The reference aggregate design in Hexion's mineral casting epoxy flyer also bears this out. See attached reference originally posted on page 215 attached in post 2902: http://www.cnczone.com/forums/418868-post2902.html
I'd also posit that aspect ratio 1:1 particles uniformly covered in a thin layer of epoxy exactly satisfy the conditions for validity of the Hashin-Shtrikman equation which equates the elastic strain energy between a shell of epoxy and a contained particle. When every particle is covered in a thin layer of epoxy there is no direct particle-particle contact and no direct interlock can exist unlike in the dry case. Thus, from the model perspective, interlock isn't relevant to the obtained modulus.
Having given the model perspective, I'd have to say that I think that there may be some positive effects from interlock when a high enough packing density is achieved since in practice the epoxy layer might not be quite so uniform. I'll also say that I remember seeing an article in an academic journal pointing to the effects of interlock in arresting crack propagation.
On the other hand, fracture mechanics tells us crushed aggregates are more likely to create flaws in the epoxy matrix with a very small crack tip radius which provide stress concentrations and lower the critical stress for crack propagation. Along this line, One of the articles in Kinloch's book found an increase in strength at a given packing density when smaller particles were used.
*Note: I use the Compressible Packing Model for predicting packing density and the Hashin-Shtrikman equation for predicting modulus from packing density.
I've probably gone on long enough on this topic today.
Regards all,
Cameron