In a 3-point bend test, the convex side of the specimen is placed in tension, and the outer fibers are subjected to maximum stress and strain. Failure will occur when the strain or elongation exceeds the material's limits. Fracture toughness can be determined using a three-point flexural test.
The four-point bending test is therefore particularly suitable for brittle materials that cannot withstand shear stresses very well. It is one of the most widely used apparatus to characterize fatigue and flexural stiffness of asphalt mixtures.
The 4-point loading type has a constant flexural moment ihe middle third, so the probability to find a weaker cross section is bigger, than with 3-pint loadig, where the the position of failure is predicted. The 4-point loading therefore is more severe (you get smaller flexural strength), than with the other one. The road concretes are always tested with 4-point loading. (Attila ERDÉLYI, Ph.D. Civ.Eng. Budapest, Hungary, University os Technology BME
in addition to what Attila Erdélyi has rightly pointed out for 4-point loading, it also has the portion between the two loading points to be in pure flexure -- constant flexure with zero shear.
Our answer ( Rahul Leslie and me) is now fully complete Attila ERDÉLYI
Three point test: It produces its peak stress at the material mid-point and reduced stress elsewhere. Four point test: It produces peak stresses along an extended region of the material hence exposing a larger length of the material. The elastic modulus in bending (E) can be extracted from a flexural test
I thank you all of expert responses. Knowing the test methods philosophy behind is essential to design optimum structures based on expected loading conditions.
I think to have a middle and constant zone effect of flexure in order to see cracks clearly.
In concrete flexure testing, both three-point and four-point load tests are commonly used to evaluate the flexural strength and behavior of concrete specimens. Each test configuration offers distinct advantages and serves specific purposes:
By conducting both three-point and four-point load tests, we can obtain comprehensive information about the flexural behavior of concrete, including its strength, stiffness, and ability to resist cracking and deformation under load. This helps in designing structures with appropriate reinforcement and optimizing material usage for enhanced performance and durability.
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