Hello everyone,
I have the attached Excel file containing the results and equations required to calculate the modulus of flexural for injection molded plastics (in this case PHAs) as per ASTM D790-17. Yet, the flexural modulus value I am getting doesn't make sense. I was wondering if I am doing something wrong.
** My doubt is about the deflection in the flexural strain equation . I believe this value is equal to the column "extension" in this Excel file, no?
Any response is highly appreciated.
I'm not sure if I've understood your question, so pardon me if this is off the mark. Your spreadsheet shows
flexural strain = 6*deflection*thickness/(span^2)
I haven't gone to the ASTM D790 method to check the formula, but lets assume you have it right for a three point bend. You can see right there in cells C4, D4 that flexural strain is NOT equal to deflection.
Also that three point bend formula gives maximum stress at "outer fibers" in the flexural situation, and makes some assumptions about compressive and tensile stress-strain curves being mirrors of each other. But that might be minor details versus what you are asking.
If it helps, tensile modulus is often very close to flexural modulus, at low strains.
Your data gives a flexural modulus of about 0.33GPa, which is 1/10 of the known value. Check the load cell and LVDT readings.
Sun Kyoung Kim thank you for taking the time to go through this. Would you please explain how you reached to this value? I keep doing the math and ending up with a very high value.
Nothing wrong in the red marked flexural modulus formula in your sheet but please use Force/Deflection instead of the slope. The slope should be from Force-Deflection curve not from anything related to stress or strain. Then, if you use the mm and N unit, it would result in N/mm^2 or MPa.
Thank you so much Sun Kyoung Kim I believe that’s the mistake I am doing. I was finding the slope of the linear region from the flexural stress-flexural strain curve. Regards.
I can not see your Excel file, but as I worked with ASTM E 790, I recommend using a local measure deflection and do not use measuring displacement of the movable head of the testing machine.
Actually I use an LVDT located between the sample and a closer fixed point on the sample holder. The displacement difference of the total chain involved in the automatic measurement of the machine may introduce some error.
Thank you Edgardo Santarelli . Do you mean like using an extensometer for the deflection?
That's right, here I attach a diagram of how we measure deflection
Thank you Edgardo Santarelli
Would you please provide a picture that shows where to place the extensometer during the 3 points testing?
it’s very obvious where to place the extensometer during tensile testing, but I don’t have enough experience in case of 3 points bending.
Sorry I don't have an image on hand at this time. Anyway, we use a magnetic mount to attach an LVDT to the base of the device. The measurement point should preferably be on the neutral axis of the specimen. This is not always possible, therefore it can be measured at the bottom or at the point of application of the load.
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