• Rich76
In summary, to calculate flexural stress according to Wikipedia, you need to use the formula stress = force x length / (width x (depth x depth)). The stress is measured in MPa, and the force is the load at a given point on the load deflection curve. The length is the support span in mm, the width is the width of the test beam in mm, and the depth is the depth or thickness of the tested beam in mm. By plugging in the numbers from a four-point test of reinforced concrete (force = 42000 N, length = 3000 mm, width = 152 mm, depth = 269 mm), we get a flexural stress of 11.45572 MPa at
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"?

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.

Rich76 and berkeman

1. What is a four-point flexural test?

A four-point flexural test is a mechanical test used to determine the strength and stiffness of a material by applying a bending load to a test specimen. It involves placing the specimen on two supports and applying a load at two points in between the supports.

2. What is the purpose of a four-point flexural test?

The purpose of a four-point flexural test is to measure the flexural strength and stiffness of a material. This information is important for understanding how a material will behave under bending or flexing loads, which is crucial in many engineering and construction applications.

3. How is a four-point flexural test performed?

To perform a four-point flexural test, a test specimen is placed on two supports and a load is applied at two points in between the supports. The specimen is then subjected to a constant rate of displacement until it breaks. The load and displacement are recorded to determine the flexural strength and stiffness of the material.

4. What types of materials can be tested using a four-point flexural test?

A four-point flexural test can be used to test a wide range of materials, including metals, plastics, ceramics, and composites. It is commonly used in the construction, aerospace, and automotive industries to test the strength and stiffness of materials used in these applications.

5. What are the advantages of a four-point flexural test compared to other mechanical tests?

One advantage of a four-point flexural test is that it can provide more accurate results for materials that are not perfectly homogeneous or isotropic. It also allows for the testing of larger and thicker specimens compared to other mechanical tests. Additionally, the four-point loading configuration creates a more uniform stress distribution along the length of the specimen, resulting in more reliable and consistent results.

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