How do you explain the difference between flex, tensile and shear strength

[The Mekon]

I was asked a question by a customer the other day which I know that I
should know the answer for:

Why is the flex strength of a polymer greater than the tensile strength, and
why is the shear strength lower than the tensile strength. (In essence all
these values are taken as the 'yield' point on the stress strain curves)

I think it must be to do with the direction of testing and the way in which
stress is calculated from the technique, but I'd like to know more.

Does anyone have any links / references which may help?

Thanks

Craig

[J.Harding]

"The Mekon" <the_mekonukP...@yahoo.co.uk> wrote in message news:<ahqvej$v8m8l$1...@ID-115312.news.dfncis.de>...

> Why is the flex strength of a polymer greater than the tensile strength, and
> why is the shear strength lower than the tensile strength. (In essence all
> these values are taken as the 'yield' point on the stress strain curves)
>
> I think it must be to do with the direction of testing and the way in which
> stress is calculated from the technique, but I'd like to know more.
>
> Does anyone have any links / references which may help?
>
> Thanks
>
> Craig

From Roger Brown's "Handbook of Polymer Testing, Physical Methods",
Marcel Dekker (New York), 1999 :

"When a beam is bent, a continuous gradation of stress occurs from a
max. tensile stress on one surface through a neutral axis to a maximum
compressive stress on the other surface. It is the maximum tensile
stress and strain that are calculated. Because of the geometry
differences and the fact that in bending tests the surface stress
rather than a homogenous stress is considered, values for strength and
modulus cannot be simply equated with those from normal tensile test,
although in theory, they are equal".

We tested a glass reinforced polyamide in both flexural and tensile
stress-strain modes. The maximum detected lb (load cell) for the
flexural test was 112. The maximum detected lb (load cell) for the
tensile machine was recorded at 1,600. Starting with the easier
calculation, the tensile strength is 1,600 lb / Area (square inches)
. Our initial cross sectional area was about 0.0606 in^2 , so the
calculated engineering strength is 26,400 psi.

Flexural strength calculation uses: 3PL/2bd^2 where P is the
maximum load, L is the support span, b is the beam width, d is the
beam depth. Our flexural bar was about 0.495 inches width, the depth
0.1225 inches. With a span of 2 inches, our maximum of 112 lbs
calculates to about 45,230 psi (representing the strength of the outer
surface).

Here's an interesting note: the calculated flexural strength is
higher, even though it required less force to bend and break the bar
than was required to pull and break the tensile bar. Its all about
how the 'state of stress' is defined.

I will add that in most cases, one can plot flex versus tensile
strength results and usually see a nice correlation.

Other references:
ASTM D790 or ISO 178 (test methods)
ASTM D638 or ISO 527 (test methods)

[the mekon]

Thanks for the answer Jeff, it is the sort of thing I expected, I'll
take a look at the references you suggest
Regards
Craig

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