Talk:Impulse excitation technique

ASTM

Looks like much of this page is derived from information from the ASTM. Furthermore, no references are provided. I believe the equation $T=1+6,858\left({\frac {t}{L}}\right)$ is incorrect, the ASTM article I have here states $T=1+6,858\left({\frac {t}{L}}\right)^{2}$ under the condition that L/t >= 20.

Also the use of commas in $E=1,6067\left({\frac {L^{3}}{d^{4}}}\right)mf_{f}^{2}T'$ and $T'=1+4,939\left({\frac {d}{L}}\right)^{2}$ is misleading. Again here it is under the condition that L/t >= 20.

Seems $G={\frac {4Lmf-t^{2}}{bt}}\left({\frac {B}{1+A}}\right)$ is incorrect, it should read $G={\frac {4Lmf_{t}^{2}}{bt}}\left({\frac {B}{1+A}}\right)$

I'm going to go ahead and make these changes. I have ASTM E1875 in front of me which discusses the use of audio excitation and I believe these mathematical errors are fingerslips on the keyboard. Please revert the changes if they are found to be incorrect/unsubstantiated.203.91.84.7 (talk) 13:40, 29 July 2008 (UTC)[reply]

Disclaimer : I do not know anything about that particular topic. Still, there is a serious issue with the formulas here. There are neither derived (ok), nor explained (possibly too technical), and include magic numbers. Having these three together without any external link solving one of these issues makes them basically worthless - they cannot be used as such. For instance $E=0.9465\left({\frac {mf_{f}^{2}}{b}}\right)\left({\frac {L^{3}}{t^{3}}}\right)T$ : where does the 0.9465 come from ? It might be an expansion of some more complicated expression, or an empirical law to fit the data, or anything, but it has to be mentioned.

Also, the "L/t ≥ 20" condition is probably not a threshold where the formula go from completely wrong to exact, but probably more a condition like "t<<L" meaning the formula is valid in the low t/L region and becomes increasingly inaccurate as t/L gets larger. The magic number "20" is probably coming from a convention, but this should be mentioned ; right now the formulation ('The above formula can be used should L/t ≥ 20') means there is a threshold of validity.Tigraan (talk) 16:25, 27 June 2013 (UTC)[reply]

The magic numbers are probably from a reference, like the ASTM standard. With a more complete citation this might be a valid reference. Other books should have the numbers too, at least the more basic ones, possibly even showing the more complete mathematical form. The number 0.9465 should be some kind of numerical result - definitely not an empirical value. An other possible reference could be "Springer Handbook of Materials Measurement Methods". The formula is also only an approximation with respect to the third dimension, assuming b << L.--Ulrich67 (talk) 12:36, 31 August 2014 (UTC)[reply]

Dynamic nature

The existence of the variable $f_{f}$ suggests these values of E and G are frequency dependant - that they will become stiffer as they are excited at a higher frequency! I think this is noteworthy and will add it. 203.91.84.7 (talk) 13:57, 29 July 2008 (UTC)[reply]

It is wrong to state that by changing for example the length of a sample, also E or G would change. Yes, the natural frequency of the sample would change by doing this, but since we use both the natural frequency and the length of the sample, the calculated E and G will always have the same value. Therefore i think the portion you added was wrong and I deleted it. —Preceding unsigned comment added by Joris1bracke (talk • contribs) 11:56, 6 August 2009 (UTC)[reply]

There is a usually small (usually less than 1%) dependence of the (dynamic) moduli on frequency - giving a higher modulus at higher frequencies. Defects in the material can move under the influence of stress and thus relax stress after some time, thus lowering the modulus by a small fraction. This effect is directly related to the internal damping of vibrations in the material. One slightly different example is the thermoelastic damping - coming from the transition from isothermal to adiabatic moduli as the frequency goes up. So we may have to to note the this method measures the dynamic modulus.--Ulrich67 (talk) 12:54, 31 August 2014 (UTC)[reply]