Four-point flexural test - Please help me understand

[Rich76]

According to wikipedia, this is how to calculate flexural stress where "the loading span is 1/3 of the support span (rectangular cross section)":

stress = force x length / (width x (depth x depth))

Stress ("outer fibers at midpoint"), (MPa)
Force ("load at a given point on the load deflection curve"), (N)
Length ("Support span"), (mm)
Width ("Width of test beam"), (mm)
Depth ("Depth or thickness of tested beam"), (mm)

Source: https://en.wikipedia.org/wiki/Four-point_flexural_test

Plugging in the numbers from this youtube video (four-point test of reinforced concrete):

Force = 42000 N (failure point)
Length = 3000 mm
Width = 152 mm
Depth = 269 mm

stress = 42000 x 3000 / (152 x (269 x 269))

stress = 11.45572 MPaI have two questions:

1. Is the math correct (I'm assuming "failure point" is "the load at a given point on the load deflection curve")?

2. What does a flexural stress of 11.45572 MPa mean? 11.45572 MPa "outer fibers at midpoint"?

 

[jrmichler]

The Wikipedia article is poorly written because it assumes that the reader fully understands free body diagrams (FBD) and area moment of inertia (search those terms). In order to calculate stress in four point bending, you need the distance between beam supports, the distance between the two loading points, and if the loading points are centered between the beam supports. Search free body diagram four point bending, then read the Wikipedia article very carefully. You could get a better answer if we had some idea of your background.