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Episode 39 – Wood, what it is and how it fails.

Wood is a universal construction material, and needs to be understood if it is used to construct almost anything.  In this episode we discuss how wood grows and how it really is a composite material made up of fibers held together with a type of glue.

As the wood grows it’s growth rate is not uniform and it grows in rings around the inner center as is shown in the first picture.  This picture also shows what happens as the board is cut at the saw mill and why we see the familiar grain pattern in wood.Also discussed is the cutting of the wood using a wood chisel.  The wood could be cut along the face facing the camera.  This is called end grain and is tough cutting.  it could be cut down the top face and cutting from the camera toward the other end.  This is cutting “with the grain” and is relatively easy because it is cutting the glue holding the fibers and not actually cutting the fibers.   The final type of cut would be across the top of the board in the direction of my finger tips.   That would be “cross grain” and is much easier than cutting the end grain but since some fibers must be cut it would be harder than “with the grain”.

Most of the audio discusses splitting of wood as shown in the next pictures.  The first picture shows a split section formed when I did not support a piece of wood as I cut it and the weight of the piece cut off split a hunk of the wood.

The next picture is a close up of that same piece of wood.  It is easy to see how easily the pine separated along the “glue” lines between the fibers.

The final picture is a close up of a Popsicle stick that I supplied excess bending force until it failed.   The bending force was done exactly as shown in episode 38 but I bent it until failure.  The fibers at the top of the stick, the ones being stretched the most, reached their limit and broke. After that happened, the stick continued to fail by splitting the fibers from each other and splitting the wood.

Obviously from this talk,it is easy to see the failure mode we have to worry about with wood is splitting of the wood.




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