The area under the stress-strain curve theroretically represents the energy which is absorbed by the specimen during the compression test. This is fully correct and acceptable, provided that it is specified that the test was quasi-static (i.e. carried out at low speed). What is not correct is assuming that this energy corresponds to what can be absorbed by an impact. Indeed, the stress-strain curve can be modified significantly at high deformation speed (thus corresponding to a true impact), therefore the absorbed energy can be very different.

Alain

Dear Alain,

Many thanks for comprehensive answer.

To be more precise my issue is not measuring impact properties but there is a direct correlation between toughness and amount of absorb energy.

As I mentioned in my question, I just considered Charpy and Drop weight impact test which are adequate for low velocity impact. As a result, I performed tensile test based on ASTM D3039 with 2 mm/min cross head speed. That implies low velocity test has been performed.

I wonder to know if measuring composite material's toughness through stress-strain curve is acceptable for journals?

As you said, theoretically, measuring toughness is acceptable according many mechanical and material academic books, particularly for woven an-isotropic composites which the numbers of wrap and weft are unequal.

I don't know how to convince journals while there are certain experimental tests for this purpose.

Alain, you have published so many papers and expert in this way. Could you advise me how to solve this problem?

Dear Amin,

The toughness really is the absorbed energy, which truly is the area under the stress-strain curve. Please take a look there :

http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Toughness.htm

So, for me, there is no problem that you calculate the toughness as you suggested. You'll just have to recall the definition in your paper, as possibly other authors won't have the same definition as you. What is important is explaining what you did in case of the defiition may not be universal.

Alain

Thanks Dear Alain,

It's a truly very informative paper.

You can use Indentation techniques either experimentally or utilizing finite element methods.

You can see:

http://link.springer.com/article/10.1007%2FBF02745672#page-1

Or

http://www.lbl.gov/ritchie/Library/PDF/Indentation%20in%20bone%20JOMBBT%208-09.pdf

Or

http://www.chemicke-listy.cz/docs/full/2011_s5_s842-s853.pdf

Dear Abdulaziz Alaboodi,

Thanks for reply and consideration but fracture toughness property is not my issue.

Bend test also gives an idea about the toughness if the sample dimensions permit.

Dear Syed Abbas,

If you are referring the three points bending test (Flexture Test), your idea would be true for isotropic materials while Flexture Modulus and Young's Modulus for an-isotropic materials is roughly different.

calculation of stress intensity factor, using CT samples is one of the best industrially recognized method of calculating the "toughness" of material.

Dear Amin,

How were you able to perform tensile tests when you did not have sufficiently large specimens for Charpy?

The area under the tensile diagram is indeed an acceptable method for measuring toughness. Also keep in mind that your material can behave differently under low-speed impact and tension. You have no guarantee that your tougness will be equal under such different loading conditions.

High. you can use nanoindentation to calculate fracture toughness by simply using information like a pdf in below link:

http://www.nanovea.com/Application%20Notes/nanofracturetoughness.pdf

then have in mind that:

Toughness is given by the critical strain energy release rate (Gc) while fracture toughness is given by the critical stress intensity factor (Kc). The strain energy originates from the elastic stresses around the crack. Toughness and strain energy are thus closely related. The formula relating the two is:

Kc = (E'Gc)^0.5

where E' depends on the stress state and is close to or equal to the Young’s modulus.

In your case "composite material" you should define what kind of composite you have . If it is Fibrous Composite or some other kind it is acceptable to use indentation test result as bulk specimen result. If your specimen is inhomogeneous you may repeat the test for different area and samples and use average value as your final result. I hope it help u.