Static measurement of Young's modulus

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colin north
Sassafras
Posts: 45
Joined: Tue Sep 20, 2011 4:24 am

Static measurement of Young's modulus

Post by colin north » Wed May 17, 2017 12:43 am

Soundboards - I set up a test rig using normal type supports /centre loading, used results to calculate Young's modulus, got 19000 for a density of 0.36, looks rubbish

The formula I tried to use was on pg1-25

E = (P/A)/(dl/l)

where, and I can see this must be wrong because the thickness of the plate is not included in "my" interpretation of the formula
P=(loading mass in Kg x 9.81),
A = (length (between supports) x width of plate, in metres)
dl = deflection in metres
l - (length (between supports) (original dimension l is stated on the page above formula)

OK , I'm not a genius, but what am I missing?

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Trevor Gore
Blackwood
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Re: Static measurement of Young's modulus

Post by Trevor Gore » Wed May 17, 2017 9:06 am

That formula is the basic definition of Young's Modulus = Stress/Strain = (Force/Area (=stress), divided by extension per unit length (=strain)) and I'm using the formula there just to define Young's Modulus (and the other terms I use extensively from thereon). In a typical engineering example Young's Modulus is measured using an extension (stretch) method rather than a bending method and that formula is more directly applicable in those circumstances. In that formula "A" is area (says so at the top of p 1-25).

On p. 1-24, Equ. 1.3-2 is the formula to use for a bending test, but you need to invert it so E is on the front. I've done this on p 4-40, Equ. 4.4-5 and Equ. 4.4-6 in Section 4.4.4, where I talk in more detail about measuring Young's modulus.

colin north
Sassafras
Posts: 45
Joined: Tue Sep 20, 2011 4:24 am

Re: Static measurement of Young's modulus

Post by colin north » Wed May 17, 2017 10:01 am

Trevor, you did point this out recently on the OLF to me in reply to my query, my apology, got fixated on your first reference.
Thanks for your patience.
Average of 16 readings is working out at 11.1 Gpa, looks ballpark for this density Lutz, (which I thought was quite stiff originally) and was baked for an hour at 220deg F last year (according to Brian Burns, it does seem to increase MoE, (although my original reason for doing it was for stability with recently cut wood)

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