Carbon/Glass and Rule of Mixtures


Maybe once a week I hear a suggestion that someone wants to slightly punch up a glass laminate with a bit of carbon. Or even a bit of Kevlar. I have even seen laminates on a roll with a small bit of carbon mixed in with the largely glass roll.  It is seen like a vitamin I guess.
Bad news,  it doesn’t work that way. The problem is the difference between each material’s stretch to failure percentage. E-glass is around 6%. Carbon is typically around1%. So, for example, if you had half of each in a laminate, when the carbon started to fail, the e-glass would be only contributing some 17% of its possible strength. Another way I have heard to visualize this is to imagine two boards several feet apart. A steel wire, and a rubber band connect them to each other. Imagine that each will fail at 100 lbs pull. The rubber band is doing nothing yet, as the steel fails and breaks.

E-glass can be laminated with carbon, if they are not both oriented in the zero degree axis for example.  The carbon could be on the 0 degree and e-glass could be on the off axis.  The off axis will contribute almost nothing to the strength, but will help keep the zeros in column and add some shock absorbing ability to the laminate.

3 thoughts on “Carbon/Glass and Rule of Mixtures”

  1. Is it also safe to assume that it’s best to use carbon fiber in conjunction with woods (& other materials) which have the same/similar moduli of stiffness? Unless, of course, the wood will be totally encapsulated with carbon, & only be functioning as a core, not as a structural componnent of the part in question?

    1. Yes plywood works very well with carbon. Both have similar stretch to failure. Is called a live core as it also helps with the loads. I have a paper on that I will publish soon. Based on lecture I gave to SCCC Boatbuilding School.

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