Layer Scheduling for Bulkhead Ring in Uniform Compression

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#1

Hello,

I am fabricating a 30" diameter carbon/epoxy bulkhead ring for handling a uniform compression load (ring has a revolved “L” cross section, each leg of the “L” about 2" long. Picture an overturned peanut butter jar lid with most of the center cut out). I will be using bidirectional carbon cloth, and shooting for 6-7 layers total.

What is the preferred fiber orientation for a ring in uniform compression? Keep in mind I will be using BID cloth, which will simplify things slightly.

My tentative layer schedule is [0, 45, 0, 0, 45, 0], without much basis behind it. Should I be more concerned with ±45 degree layers for a ring in uniform compression?

Any advice would greatly be appreciated. For that matter, if there is a good reference for deciding on fiber orientation of a layup (for the layman, not a composite design engineer), that would be amazing. Thank you!

#2

For maximum hoop strength (cross-sectional compression of a tube) you would want to use UD fiber that circles around the entire structure on both faces of the “L”. You could use woven on the inside and outside of the bulk head to hold the UD fibers together or use UD fibers that perpendicular to the primary hooped fibers.

The more fiber you have going around in a complete hoop the stronger the hoop strength will be. Fibers that are 45* to the loading axis can be 1/20 of the strength of fibers aligned with the loading axis. When working with wovens, having a fabric at 0/90 to the loading axis is stronger than having fibers at +/-45 to the loading axis…even though half of the 0/90 fibers are not utilized.

When you are talking about 0/90 and +/-45 what is the reference point for your fiber alignment in the hoop?

#3

Thanks for the helpful response.

The reference I had in mind was the axis of revolution of the compression ring, therefore 90 degrees are the “useful” hoop fibers according to your explanation, 0 degrees are the non-utilized fibers from my BID, and ±45 are relative to the axis of revolution once again.

I plan on using six separate pieces of carbon per layer (1/6 segment of an annular template), in order to facilitate laying the pieces in. This piece will be vacuum infused as well. I plan on 1" overlaps between neighboring pieces of each layer.

Are there any shear force concerns (suggesting a need for ±45 layers), or should I literally be using 6 layers of 0/90 BID (I only have BID on hand, hence no mention of UNI) with no ±45 layers?

#4

For ultimate strength the 0/90 will function better. The +/-45 may improve the strain which could improve the toughness but this would happen at the cost of lowering the yield compression strength (assuming equal weight). I wouldn’t consider myself of a laminate engineer though. I’ve just done a lot of comparative testing between samples of varying orientations, ordering, and combinations.

#5

Are you aware of any resources (I’m thinking along the lines of a handbook for a technician), that may illustrate suggested layups for given loadings?

#6

Technicians just follow the schedule determined by the engineer. I don’t know of such a handbook.