My view on the matter:

If you need to increase the thickness of your material in order to match the experimental results (presumably the stiffness?), then that means your material model (or some other aspect of your FEM) is in error somehow. Increasing the thickness may give you the correct numerical result in this case. However, presumably you are doing a FEM analysis because you want to extrapolate from your experiments to a more complex structure or loading condition.

In that case you need to ask the question: 'Can I be confident that increasing the thickness in the FEM model will still give me the correct answer in this case?'

Since you are using an artificial value for the thickness to correct for some unknown error, I'd say you can only answer that question with 'No'.

If you want to use your method of increasing the thickness  you need to be able to justify, based on physics, not just experimental correlation, why the thickness needs to be increased, and by how much. In other words, you need to be able to explain why inputting the real thickness in your model does not give you the correct answer.

Hi Mehdi

In FEM, a fundamental assumption is that you use first principles, i.e. describe the geometry to your best ability using elements and material properties using known material data or material data retrieved using recognized test methods. If results then do not match, it is either because you have a discrepancy between test/FE geometry, are using inaccurate material data and/or a material model that does not match your material properties.

If you are interested in matching simulation and test, feel free to take a look at my ramblings here http://qringtech.com/learnmore/why-simulate-measure-correlate-automate/

You may want to take a look at this discussion as well

https://www.researchgate.net/post/What_are_the_disadvantages_limitations_of_inverse_finite_element_method_for_soft_tissue_experiments

Sincerely

Claes

Dear Mehdi, It is better to use the actual thickness of the FRP material to better capture the actual behaviour. You can use two ways; the first is using the design thickness of composite matrix of about 1 mm as recommended by the FRP manufacturer and implement the design properties of the composite matrix, the second is using two different materials with relevant material properties (the CFRP sheet and the resin matrix) using the actual thickness of the CFRP sheet and about 1 to 2 mm thickness for the resin matrix. I believe that the thickness of the resin material can be checked against the used design standards.

My view on the matter:

If you need to increase the thickness of your material in order to match the experimental results (presumably the stiffness?), then that means your material model (or some other aspect of your FEM) is in error somehow. Increasing the thickness may give you the correct numerical result in this case. However, presumably you are doing a FEM analysis because you want to extrapolate from your experiments to a more complex structure or loading condition.

In that case you need to ask the question: 'Can I be confident that increasing the thickness in the FEM model will still give me the correct answer in this case?'

Since you are using an artificial value for the thickness to correct for some unknown error, I'd say you can only answer that question with 'No'.

If you want to use your method of increasing the thickness  you need to be able to justify, based on physics, not just experimental correlation, why the thickness needs to be increased, and by how much. In other words, you need to be able to explain why inputting the real thickness in your model does not give you the correct answer.

Dear Mehdi, If your verification between test and FEM results is not compatible in term of stiffness, you may be error in including Modulus of elasticity, if Incompatible results is in term of strength, your material model is in error, please present a picture from your test and FEM comparsion.

Thanks a lot for your contribution. I have used FEM for in several projects and the results are compatible with experiments. I am  also aware that how critical is   assumptions in FEM. By the way, I am looking for published papers or scientific discussion  that investigate  the assumption like changes in the thickness of FRP or any other similar issue. 

Regards,

Lima

Even if increasing the FRP sheet thickness gave your better results it won't be simulation to real behavior. The main point of FE simulations is to use the material properties and geometry characteristics as they are actually are. Otherwise your simulations are pointless. I am sure there are other errors in your model or constitutive law that caused the discrepancy

 Thanks a lot for your contribution. Is there any limitation about the ratio of E1/E2 (I mean maximum  or minimum ratio) for the FRP sheet if there is not testing on  FRP properties for E2 or E3.  

Regards, 

Lima

Hi Mehdi

Sometimes, you have materials with random properties, e.g. composites where the orientation should be, say at 30 degrees but due to production tolerances is at say, 28 degrees.

If you know, or suspect, this to be the situation, then you are of course entitled to try various physically sound models and assumptions to better macth your results.

I believe that my reaction and many of the above is that some researcher use a blck-box approach to modelling.

About E1/E2. I am not exactly sure as to what you are referring to as I am not unaccustomed to composites. That said, I guess Poisson's ratio must be sensible such that you avoid producing mass when deforming your structure and similar. Most FE software issues warnings or refuse to execute if one happens to input such nonsense data.

All the best.

Claes

I dont think Mahdi.The FEM model must be as the actual in the experiments.