Dear sirs,
I'd like to know about flexture strength. What is the flexture strength? How to test it? How to use it? Is it related to modulus of something? Thanks
Flexure "strength" as you put it, is, in a nutshell, how much stress a beam or similarly-acting structural member can take before it breaks. ASTM D790 is a good place to start.
There are many ASTM standards associated with flexural strength, so it depends upon the material you are testing. The one noted by DaveViking (D790) is related to the flexural strength of plastics and insulating materials. Since you asked about a specific modulus, I will answer your question with the assumption that you are testing concrete, since the flexural strength of concrete and its modulus of rupture are commonly interchanged terms.
The test for flexural strength most commonly used for concrete is ASTM C78, the "third-point" loading test. From this test, you can determine the modulus of rupture of the concrete in flexure. This is a commonly specified parameter for rigid pavement design. Typically, the modulus of rupture should exceed about 500 psi for standard duty rigid pavements, and should be greater than 650 psi for airfield or industrial pavements.
There are other similar standards in ASTM for concrete, particularly ASTM C293 for center point loading, and ASTM C1018 for flexural toughness and first crack index of fiber reinforced concrete.