Talk:Flexural modulus

High means stiffer?

So, does a higher or lower flexural modulus mean stiffer? 216.70.247.242 (talk) 21:32, 29 January 2009 (UTC)

Stiffer would mean the material deflects less under a given load. The flexural modulus is inversely related to deflection - a lower deflection would result in a higher modulus. So a higher flexural modulus material is 'stiffer' than a lower flexural modulus material. 65.247.121.61 (talk) 21:03, 6 September 2013 (UTC)

Versus Young's modulus

What's the difference between this quy and the Youngs modulus? --157.24.10.32 (talk) 08:17, 3 September 2009 (UTC)

The Youngs modulus relates stress and strain in tension or compression only. A material in flex experiences both tension and compression simultaneously. I would expect that a material with a relatively high Youngs modulus would also have a relatively high flexural modulus. 65.247.121.61 (talk) 21:03, 6 September 2013 (UTC)

I came here to ask the same question. This page really needs to address that question since otherwise it's just confusing, since this page basically says it's the same as Young's modulus except when it's not. In response to the above response, beam bending is only a function of Young's modulus; for linear-elastic isotropic materials, tension and compression are basically the same thing (up to sign). For non-slender beams (which isn't what this page appears to be talking about), you have a shear component to deflection and so have to worry about Poisson ratio as well as Young's modulus (Timoshenko beam theory). But even then, the beam deflection is fully determined by the shape, loading, E, and $\nu$.

Note that in general 3D linear elasticity, the strain experienced by a small element of material is described by a 3×3 symmetric tensor and the stress-strain relationship is given by the stiffness tensor which, in general, has 21 degrees of freedom, but for isotropic materials boils down to being entirely a function of E and $\nu$.

All this leaves me still wondering: how is flexural modulus different from Young's modulus? Is it really only different for nonlinear materials (including linear materials after they yield)? —Ben FrantzDale (talk) 13:01, 9 October 2014 (UTC)

Googling some more, this source claims that flexural modulus is a kludge created by the plastics industry to incorporate nonlinearities. Then Instron's website says flexural modulus is the "[r]atio of maximum fiber stress to maximum strain, within elastic limit of stress-strain diagram obtained in flexure test." This page says similar things: "The flexural modulus is the value of the elastic modulus as determined by a spring bend limit testing device. It is calculated using bending formulas from the force-deflection relationship. It is typically equal to the elastic modulus in tension". So yea, it's typically the same as Young's modulus, but it can be different and that difference is due to material nonlinearity. —Ben FrantzDale (talk) 13:10, 9 October 2014 (UTC)

Flexural modulus is used to calculate bending effects in sheeting or fibres (the value of the flexural modulus will be different for the two cases - most general references for plastics give the value for sheets, typically as defined in ASTM D790). In this respect the article misrepresents its meaning - it should be rewritten including at least some of the background below (preferably both better written and more comprehensive than my efforts).

For sheets (w >> h) and fibres (w~h) the effective modulus under bending is (almost always) higher than the Young's modulus. With uniform isotropic materials the flexural modulus for a structure can readily be calculated fron the Youngs modulus and Poisson's ratio [it is approximately E/(1-ν²) for a sheet]. However, manufacturing methods mean that many types of sheeting are both anisotropic and non-uniform. This is particularly significant for plastics, as these typically have large Poisson ratio (ν > 0.4 in some cases); in addition, the properties of plastics are sensitive to molecular alignments, and these can be significantly affected by forming methods. This is a "natural" reason that the plastics industry would have taken the lead in defining a direct material constant for calculating bending effects in sheets and fibres.

Note on sources referenced by Ben FrantzDale: the first reference "this source" appears to be misinformed in all respects. Every standard I have seen references measurement in the linear range. The second reference (Instron) refers to the elastic limit rather than the linear range: assuming that the elastic limit exceeds the linear range, this aspect of the definition cannot be correct, as the flexural modulus is a single number and this definition would give rise to a non-constant value for the flexural modulus; Instron also oversimplifies the mechanics of formed plastics by assuming material uniformity; note also that this particular Instron reference is for fibres. The final (PDF) reference mis-defines flexural modulus almost in passing; even so, I can't see why a manufacturer of sheet springs (among other things) would have made this elemetary over-simplification (particularly as the difference for BeCu sheet can be in the order of 10%) PhysicistQuery (talk) 00:17, 29 May 2017 (UTC)