Basswood Plywood Mechanical Properties

[atestani]

I have been searching for the subject information and have been unsuccessful.

I'm designing a 3" dia, fiberglass rocket and using laser cut basswood plywood for centering rings, bulkheads, etc.. An ejection baffle assy., built into a coupler, is epoxied into the main body tube and the shock cord connects to the top section of the baffle assy. with an 1/4-20 eyebolt, washer and nut. The other end of the shock cord connects to the bulkhead at the aft end of the coupler between sections. What I am trying to determine is how thick the basswood plywood needs to be to where the eyebolts are attached.

Right now I have 3/8" thick basswood plywood for both of the eyebolt mounting plates but this is just a guess. Can anyone point me to a source for the properties of basswood plywood so I can take the guess work out if this?

Thanks

 

[atestani]

Answering my own question for others, if interested. After many searches, I got the following in one try with ChatGPT.

Summarized from ChatGPT:

Basswood plywood:

Tensile Strength: Basswood plywood tends to have moderate tensile strength. The specific value can vary depending on the grade and thickness of the plywood. On average, the tensile strength can range from 10 to 20 megapascals (MPa). However, keep in mind that this value can be influenced by factors such as glue type, veneer quality, and fabrication techniques.

Compressive Strength: Basswood plywood has relatively low compressive strength compared to some other hardwood plywood types. It typically ranges from 20 to 30 MPa. Compressive strength refers to the resistance of the material to being squeezed or compacted.

Shear Strength: Shear strength represents the material's resistance to forces applied parallel to the plane of the plywood. Basswood plywood tends to have moderate shear strength, which can vary between 3 and 8 MPa.

Flexural Strength: Basswood plywood exhibits decent flexural strength, which is the ability to withstand bending or flexing forces. It can vary between 40 and 60 MPa.

Baltic Birch plywood:

Tensile Strength: Baltic birch plywood has excellent tensile strength compared to many other types of plywood. It typically ranges from 30 to 50 megapascals (MPa). The high-quality bonding and cross-grain construction contribute to its enhanced strength.

Compressive Strength: Baltic birch plywood is also known for its good compressive strength. The specific value can vary depending on the thickness and manufacturing process. On average, the compressive strength can range from 30 to 50 MPa.

Shear Strength: Baltic birch plywood exhibits high shear strength due to its cross-grain construction. The shear strength can range from 10 to 20 MPa.

Flexural Strength: Baltic birch plywood is well-regarded for its superior flexural strength. It can withstand high bending and flexing forces. The flexural strength typically ranges from 50 to 70 MPa.

 

[MidOH]

My Loc Onyx and Loc Micromag use 1/8'' ply centering rings. I backed those with a small scrap of 1/4" ply for the eyelet to screw through. Epoxied the ply, and a dab on both sides of the metal eyelet.

It's pretty darn secure. 3/8'' should be plenty.

 

[atestani]

In doing some calculations using flexural strength for a bulkhead and shear strength for a plate with a eyebolt, I *think* even 1/8" may be enough. However, the numbers are pretty large and I am not convinced. I think I am going to test it to be sure.

 

[JimByrne]

I believe (from my experience) that anything over 1/4" is overkill.

 

[MidOH]

1/8" is plenty with a through mounted eyelet and fender washers. But a wood screw eyelet, or small washers, I want more.

 

[bjphoenix]

In doing some calculations using flexural strength for a bulkhead and shear strength for a plate with a eyebolt, I *think* even 1/8" may be enough. However, the numbers are pretty large and I am not convinced. I think I am going to test it to be sure.

What background do you have in doing flexure and shear calculations? Flexure of a bulkhead is very complicated. You might find some formulas for flexure stress from someplace like Roark, and you might just do a simple 2-way flexure calculation for load from an eyebolt but then you would need to know not only the wood strength of the plywood but the section properties both directions. And shear calculations for plywood get even more complicated because you have to consider rollover of the plies. However pullout of an eyebolt is even a different kind of shear. Doing a test if you have a way to measure force would be the best approach.