That is the key problem with sandwich structures. The flexural strength is not given information of the core. I usually gives you information of the material present in the surface, that use to be stronger than the core in many cases.

I do not understand your problem, if you follow the standard, step by step, it is quite easy

my question is: for sandwich structure, does it mean the flexural propeties represented by its core shear strength and face bending strength? it is quite different with simple beam where we can get its flexural properties by flexural strength and flexural modulus

That is the key problem with sandwich structures. The flexural strength is not given information of the core. I usually gives you information of the material present in the surface, that use to be stronger than the core in many cases.

Try this Hexcel resource --> http://www.hexcel.com/Resources/DataSheets/Brochure-Data-Sheets/Honeycomb_Attributes_and_Properties.pdf

very good technical information - beginner research

It is necessary to specify your task. You need properties of the whole cellular structure (its flexural rigidity) or its components. If that is necessary properties of all structure 4th the dot bending(ASTM C393)  is suitable for this purpose. But there are instrumental subtleties. First. You need to measure a deflection between the center of a beam and the top punches, the testing equipment  with the probe (or to carry out test on points) is for this purpose necessary, secondly punches can press through a surface of honeycombs.

on my research, i did flexural test on sandwich structure with polymer foam core (Rohacell foam).

Anyway, thank you so much for the responses

Danny, check out this page below. Hope this helps! 

http://www.admet.com/testing-applications/testing-standards/astm-c393-sandwich-construction-beam-flexure-shear-testing/