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Re: FEA analysis of 12" rotary spark gap disk

Original poster: "Scott Hanson" <huil888@xxxxxxxxxxxx>

Kevin -

Unfortunately, your FEA may not be completely valid, unless you used a special software
module optimized for analysis of fiber-reinforced composites. The basic FEA
program assumes completely homogenous material properties, whereas G10 is a
composite consisting of a matrix of relatively brittle, low-strength epoxy
resin reinforced with high-tensile strength glass fibers. As soon as any of
the fiber reinforcement is cut, the load-bearing characteristics of the
nearby material are radically affected.

I'm also curious about the "lay" of the glass reinforcement of the material
in the Matlab database, which shows significantly different tensile strength
values for "lengthwise" vs "crosswise" orientation. This data may represent
a material with unidirectional fiber orientation. G10 is available with a
number of different types of glass reinforcement, including
randomly-oriented "mat", parallel strands, and several types of multi-layer
woven glass cloth.

The type of G10 that would be best for RSG use would be the latter. Most of
the examples I've seen are a pale green color, and have many layers of very
fine-weave glass cloth like the linen reinforcement in LE grade phenolic.

I think that if the FEA program was capable of analyzing the effects of the
large number of glass strands that are cut by the .50 dia. electrode holes,
you'd see lower press-fit stresses, and significantly higher stresses in the
disk under centrifugal loading, especially just outboard of the electrode holes.

In a "real" composite structure (aircraft wing or fuselage, etc), load-bearing reinforcement is never cut, punched or drilled; the structure is "wound" or the reinforcing fiber is oriented prior to resin impregnation so that any holes, inserts, etc are surrounded by intact reinforcement.

Scott Hanson

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Tuesday, May 23, 2006 8:41 PM
Subject: FEA analysis of 12" rotary spark gap diskproperties, where

Original poster: "MakingLightning" <MakingLightning@xxxxxxxxxxx>

I am designing a 12" rotary spark gap.
I really wanted some hard numbers so I knew where I stood on the safety
margins. I also wondered how good a press fit electrode was held in. I had
it 3D modeled in SDRC IDEAS and then analyzed it with their FEA package
called Visualizer. It was calculated and the numbers matched what the FEA
came up with.
It was really nice to see how the press fit stress reacted with the
spinning disk.