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

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
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?

Trevor Gore
Blackwood
Posts: 1437
Joined: Mon Jun 20, 2011 8:11 pm

Re: Static measurement of Young's modulus

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

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.