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Tensile or Flexural Strength/Stiffness – is there really a difference?

I have often found myself in a situation where a datasheet states material strength and modulus as flexural or tensile; sometimes both but at other times only one of them.  

 

I find this confusing and decided to research the subject. Here are the results:

The difference between the two testing methods can easily be understood by watching these two short videos. If all is clear to you, just skip the clips and move on.

Video: Tensile  

Video: Flexural

 

This is pretty understandable. But how does it relate to actual values? How close are they to each other? I made a little comparison. In Prospector database you can compare different material properties as a diagram. The options that I chose were injection-mouldable grades (from Albis Plastic Gmbh’s offering), commodity/high-performance, amorphous/semi-crystalline and reinforced/unreinforced. (Albis, incidentally, is a random choice).  

How is it with Modulus?

This is how it looks when flexural and tensile moduli are set on the x- and y-axes:

I’ve added a red line to illustrate where the 1:1 ratio would go. As we can see, the values follow the red line relatively closely, confirming that for most polymers, flexural modulus and tensile modulus do not significantly differ from each other. This is in line with my experiences. I would claim that it is possible to compare these two as apples to apples and not go too much astray. It is much more likely that a designer falls into the pothole of considering material properties in room temperature only, as they are usually listed in the datasheets, without taking into account changes due to elevated temperature or moisture absorption.

How about Strength then?

Let’s do a corresponding comparison:

There is more deviation now and the values no longer follow the red 1:1 line. I have drawn a blue line to roughly illustrate the average tendency. It appears that the flexural strength values are roughly 1.5 times higher than the tensile strength values. There are some exceptions and it would be unwise to simplify the case by making it a rule of thumb, but the tendency is probably worth making a note of.

So, which value to use?

In case of clear bending or pulling loads the corresponding values are without a doubt usable. In practice, however, the load is usually some sort of a combination of them, and compression or torsion may be added to the mix. Apart from the load, the geometry of your design is likely to be quite far from a test-lab arrangement. The standardized specimens do not take into account weld-lines or fiber-orientation, either, both of which exist in real products. Plastic product designer should therefore never rely on datasheet values too closely.

The one most important but often overlooked thing, however, is that the design should meet its long-term use requirements. If elevated service temperature and/or continuous/cyclic loads are expected the strength of your design should be many times higher than what the static strength calculations suggest. For this reason, the decimals do not matter. In calculating the strength in demanding applications I would increase the safety factor by using the lower value, usually the tensile value.

Comments

Stephen Wootton, Design Engineer, GE-TDI
Thank you so much for this information. Very nicely laid out and the data does show a beautiful trend. Keep up the great work.
Markus Paloheimo
Thank you Stephen, Markus
Manfred Staat
What you have "found" is a simple geometrical fact. If you bend a specimen with rectangular cross section the plasctic section modulus is 1.5 the elastic section modulus. The elastic section modulus determines the first yield in the section and compares with tension test. The plasctic section modulus determines the fully plastic cross section which leads to a socalled plastic hinge. So the strength is 1.5 larger in fully plastic bending compared the elastic limit (first yield). The factor is different for different cross sections. It is a geometrical effect and not a material property.
Markus
Manfred, thank you for bringing that up. But as the ratio is different for different cross sections, is giving the flexural strength value on the datasheet really helpful for designers or is it only confusing? How do you see it?
Ram
Well explained
Sathish
Superb
Adwait
Hi Markus, Thanks for posting this analysis, it was very helpful. I have always wondered about the same, especially because the mechanical datasheets are often missing the Tensile Modulus value.
YP
Thanks for the technical analysis & it is really helpful & informative.
Essi Lepistö
Thank you for explaining the difference or alikeness of these two figures. The problem is that either these both are in datasheet or just one of them.
Jon Boomgaarden
Manfred is mostly correct. As long as the material is nicely behaved, the strength of materials bending failure calculation based on the extreme fiber reaching the tensile strength will underestimate the strength measured in the test, because failure in the test requires yielding across the section. One thing which will cause a deviation from the 1:1.5 ratio is if the compressive strength is different from the tensile strength. (See BMC 350 for an example of this) I suppose materials with odd shear strength or elongation properties could cause deviations also. I would tend to use flexural properties whenever the failure would look like the flexural test - a thin part in bending, especially for failure analysis. For design, I wouldn't use the flexural properties if the maximum distance from the neutral fiber was large, since bending of thick sections could result in exceeding the elongation of the extreme fiber before the elastic limit near the center of the part. But always - test your design. (we test stuff because we don't know everything)
Markus
Jon, thank your for your comment. Great addition. And yes, everything should be tested...if possible before the mold is done. : )
Steve Nieves
That was very interesting and made clear what was confusing to me. Thanks.
Mohsen Aliyari
It was so useful. Thank you.
Vikas Tyagi
Thanks a lot for all use full information with good representation.
Seongmin Pae
It is very useful to me. Thanks
M Ezzat
Many thanks .it is very useful to me
Garman Sin
Nice demonstration. It's very clear to show how the behavior of plastic under tensile and flexural test. The graph is well described. And explain the mechanical properties of different plastic in the datasheet for comparison. Thanks
Chaitanya Gandhi
Thanks for such a proper explanation and clarifying the basics.
Mayank Dave
Very nice presentation Markus !
ywen
Great comparison and clear explanation!

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