Young's modulus of wood

[allthewayhome]

Hello,
I am currently carrying out an dynamic investigation of young's modulus for various linear materials such as copper, steel, and nylon but I am unsure how to find the modulus for wood.

I have several ideas including fixing one end of a sample length of wood to bench while placing a fixed load on the other and measure the extension, then work out the tensile force and strain from that.

Would this work?
Does anyone have any ideas or advice/criticism?

Regards,
Mark

 

[radou]

Yes, it should work. Note only that you'll get a greater modulus of elasticity if the applied load is in the direction of the fibers, since wood is anisotropic.

 

[allthewayhome]

This is the basic idea here with the load perpendicular to the fibres.

Wood (oak)
______________________ initial heigth under zero load
---------------- l
Bench l
l
l
l
(load X)

I was planning to measure the extension using a traveling microscope under a load x, increasing X by 0.1 kg.

The reason I am doing it this way is that the apparatus that the lab i use may not be able to accomadte the wood sample vertically with the load parallel to the fibres.

Can anyone point me in the direction(online) with details for this method or any other dynamic method for measuring young's modulus.

 

[FredGarvin]

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch04.pdf

I have a suspicion that the correlation you get between calculated stress and strain will not be in the ballpark. The moisture content in actual test conditions is controlled. Mark's Handbook has a pretty good chart of wood mechanical properties.

 

[siddharth]

This is the basic idea here with the load perpendicular to the fibres.

Wood (oak)
______________________ initial heigth under zero load
---------------- l
Bench l
l
l
l
(load X)

I was planning to measure the extension using a traveling microscope under a load x, increasing X by 0.1 kg.

The reason I am doing it this way is that the apparatus that the lab i use may not be able to accomadte the wood sample vertically with the load parallel to the fibres.

Can anyone point me in the direction(online) with details for this method or any other dynamic method for measuring young's modulus.

You could use a strain gauge to measure the deformation.

These links may help
http://midas.npl.co.uk/midas/content/mn016.html
http://www.materials.ox.ac.uk/teaching/practicals/1P2youngsmodulus.pdf [Broken]

 

[...harry...]

hey,
I am not sure whether this is directly related, but I am trying to understand why the Elastic modulus of wood is different under compression and tension.
I believe it has something to do with the fact they are an organic substance made up of cells rather than a pure element or alloy.
Is this the case?

 

[RedondoB]

Just want to add that in order to measure the flexural modulus accurately, the span-to-depth ratio that you use should be at least 6, preferably up to 16. Span to depth ratio is the length from the cantilever point to the load application point divided by the thickness of the beam. If the wood is laminated, the modulus so measured can differ from the tensile modulus by 15% to 40%. What if you have a short S-T-D ratio, the shear coupling effect can affect your results.

 

[RedondoB]

I would think that the tensile modulus is higher than the compressive modulus - one can assume that the fibers are naturally misaligned in wood. When loaded in tension, the fibers tend to align with the loading axis, thus making the material stiffer. Under compressive load, the fibers tend to deviate more from the loading axis, making it less stiff. When it comes to compressive testing for modulus, the tested specimen has to scaled to avoid buckling, i.e. the radius of gyration has to be carefully determined ...