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Composite Materials/Wooden stacking chair design


I am a student studying final year product design at Brunel Universiy, London.

I am designing a wooden stackable chair, with leg sizes of 22mm by 28mm and about 420mm high.
The front legs both bend 90 degrees to suppot the seat, the back legs continue up to support the backrest; both these components are joined.
I want to ask the following:

1)would it be more suitable to use long grained laminate wood or cross grained ply-wood?(canadian birch 0.8mm)
2)would the resulting material be isotropic or orthptropic?
3)would mortis and tenon joints weaken the structural strength significantly?

My main concerns are whether the chair will break under 100kg to 200kg weight, and whether the 90 degree bends are still achievable.

Many thanks for any advice in advance.

Kind Regards,   

Charles Kah-Ho Li

Hi Charles - this is a little outside my expertise - I ussually offer assitance with composite (fibreglass/carbon and resin) queries.

From what I do understand, I'm guessing you're going to bend the front legs to support the chair, therefore the greatest strength will be with multiple layers of timber, all glued together.  I'm not sure of the material properties of the various plywoods, but would think that long-grained laminate ply (with longest fibres in length direction) would be greater strength than cross grain - although the cross grain will be easier to bend.

The resulting laminated legs and back would be quasi-isotropic - by that I mean as close to consistent mechanical properties as is possible with a non-metallic (cast or forged etc) item.

A simple length of cross-grain is (very) orthotropic, which helps with bending without the use of steam, heat, or cutting very fine flitches and then stacking and glueing them together.

Mortice and tenons are ok - use good timber adhesive or preferably a good modern epoxy with proper surface preparation and sealing methods on the timber first.

I hope this helps - perhaps a professional woodworker or furniture maker can assist further?


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andy mackay


Composite fabrication and testing, plug and mould design & build, surfacing materials, gelcoats, resins (epoxy, polyester, BMI, urethane) GRP, FRP, carbon, aircraft and marine composites, infusion techniques, Light RTM methodology, troubleshooting materials and process engineering.


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