Flexural vs tensile strength for plastics analysis? 1

[homutow]

I am doing FE analysis of the plastic part (similar to Bakellite). I have two material properties: tensile strength and flexural strength. I am little confused which value to use as a criteria for the design (I use first principal stress)? Till now I have used flexural strength because stress concentrations I obtained had big stress gradient but I am not sure if this is good justification (I used to work with metals). Does anyone have some experience with such task, problem??
Regards

 

[jerzy]

Hello Homutow,

I have found references that recommend using tensile strength or 2/3 of flexural strength. These references explain why because of test methods these two values are usually close in value.

A good text to consult is "Structural Analysis of Thermoplastic Components", by G. G. Trantina and R. P. Nimmer, McGraw-Hill, New York, 1994.

If you have access to the Society Of Plastic Engineers
database, a search there will turn up other references.

Jerzy

 

[homutow]

Hello Jerzy,
Thanks for answer. The material I use seems not to follow the rules of 2/3. Tensile Ultimate is min 80 MPa and flexural ultimate min 180 MPa. I am wondering why the flexural strength is so important if tensile strength is usually used? As I wrote previously I am new in plastics

 

[jerzy]

Hello Homutow,

The 2/3 rule does not hold in every case because of differences in testing. In fact just the other day I was working with a resin where the datasheet value of tensile strength using ISO 527-2 was higher than the flexural strength using ASTM D790: 5080 psi versus 4000 psi.
Because of its theoretical derivation, the rule probably better describes yield strength comparisons rather than ultimate strength comparisons.

I think that flexural strengths were at one time thought to work better with the classic strength of materials beam and shell formulas. Those formulas used the same assumptions that are used in deriving the flexural strength from the flexure test. But these assumptions aren't used in Finite
Element Analysis, so tensile strength better describes the material's strength when comparing with the FEA stress results to determine failure.

The CAMPUS database of resin properties does not even list flexural strength anymore.

I have been late in replying to your last post because I have been searching for references to your question that are freely available on the Web. There are many discussions of properties, but I found only one reference relevant to your question.

It can be found on Dupont's Website in their Delrin Design Guide and I quote a couple of sentences from it:

"The ISO test for flexural modulus and flexural
strength is similar to the ASTM procedure. However,
there is growing preference in ISO and within
the design community to report tensile modulus
rather than flexural modulus. For example, the
CAMPUS database of ISO physical properties
lists tensile modulus but omits flexural modulus."

So you will probably be seeing fewer datasheets that list flexural strengths with tensile strengths.

Good luck with your analysis.

Jerzy

 

[homutow]

Jerzy,
Thank you very much for the answer, I really appreciate
Homutow