I've looked around on the web for the tensile modulus of G10/FR4,
can't seem to find anything that quotes it, although there are many
places that list it's other materials properties. I don't understand
why not, perhaps someone here could shed some light on this. Again,
it's the tensile modulus, not the flexural modulus I'm looking for.
TIA

... and what exactly is that flexural modulus? and why people list this
instead tensile modulus? Are these two related by some equation?
I always run into this problem trying to find material properties of plastic
for FEA. And as for Poisson's ratio, this I have to guess...
thanks
Paul

For well-behaved materials
[homogeneous, isotropic]
the three moduli are related in this way:
for sigma is Poisson's Ratio,
for E is Young's modulus
for n = E/(2(1 + sigma))...
Young's Modulus E = 9 n k / (3k + n)
Rigidity or Torsional modulus n = E /(2(1 + sigma))
Bulk modulus k = E / (3(1 - 2 sigma))
Express E, n k in N.m^-2
Brian Whatcott Altus OK

Tensile modulus = Young's modulus
I entered "Flexural Modulus" in Google. The FIRST response of many
was this:
"Flexural modulus is the ratio of stress to strain within the elastic
limit (when measured in the flexural mode) and is similar to the
tensile modulus."
Brian W

Paul thought carefully and wrote On 5/30/2004 10:21 AM:
I might be able to shed some dim light on the difference between tensile
and flexural modulus particularly for a fiber-reinforced material like
G10. I work in a university lab and we play around with tensile and bend
testing of metals and plastics for an undergraduate strength of
materials course.
The bottom line is that tensile testing of fiber-reinforced plastic is
actually a very difficult thing to do. The biggest headache is that the
grips holding of ends of the material usually crush the sample before
dependable results can be found. This is a common problem and it usually
takes a small development program to figure out the "right" way to
perform a tensile test on a sample.
So to estimate the Young's Modulus (inelastic stress/strain ratio) of
such a material we are forced to perform a bend test, measuring strains,
forces and deflections to back-calculate a value for the modulus.
The accumulation of errors, deviations from assumptions, linear stress
distribution, etc makes the bending modulus values "different" from
the values obtained from a straight tensile test.
For real-world applications, the modulus results from a bend test (or
the "flexural" modulus) is sufficient for most needs and there is no hue
and cry for a real tensile modulus. Though I imagine someone, somewhere,
has gone through the effort and gotten some values.
Lance
*****

I think confusion may arise because in ceramics (and maybe in some
other fields, too) the phrase "modulus of rupture" has traditionally
been used to refer to the fracture stress in bending. Obviously,
this isn't really a "modulus" in the way that the tensile or shear
moduli are, and I don't know how the term originated, but it has
been in use for a long time.
Anyway, these days, the term "flex strength," which is more correct,
is starting to see some use. However, some people get the two terms
mixed together and say "flex modulus" when they really mean "flex
strength."
Since the units for strength and stiffness are the same, when
someone says "flex modulus," you're best off asking them which they
mean (unless it's already obvious from the number).
Dave Palmer

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