The peak stress produced in the 3-pt flexure fixture is at the specimen mid-point (or point contact) with reduced stress elsewhere. Hence, this analysis (stress localization) is ideal to test for specific isolation of stress on the specimen.

On the other hand, the 4-pt flexure fixture produces peak stresses along an extended region of the specimen surface. Hence, exposing a larger area of the specimen is possible with more potential for defects and flaws to be highlighted.

For other information's http://www.nordson.com/en-us/divisions/dage/support/Literature/Documents/3_4point%20bend%20Flexural%20AppNote%20Nov11.pdf

http://www.instron.us/wa/acc_catalog/prod_list.aspx?cid=833&cname=3%20and%204%20Point%20Flexure%20Fixtures

Also, I am curious to know other reasons if any.

Thank for your information, Pathikumar Sellappan.

From your link, I also found some further differences between 3- and 4-point test. It is said that 3 point test is recommended for homogeneous materials like plastic, while 4 point bending test is more appropriate for non-homogeneous materials like composite, which is stiffer and more brittle, to avoid premature failure.

http://instroncommunity.blogspot.jp/2011/02/what-is-difference-between-single-point.html

Regards

It depends on whether it is important to measure the influence of shear in the bending test. Some materials like orthotropic materials, wood for example, the influence of shear in the displacements is around 15 % , whereas for isotropic materials,steel, this influence is not significant.

For me, the main difference is:

4-point bending produce pure bending state over significant area of specimen.

3-point bending introduce significant normal stress, so stress state in specimen is much more complicated.

Simply stress state of 4-point bending is in my opinion more suitable to check mechanical properties or verify numerical models.

It is also to be considered that in the case o 4-point bending, the volume under stress is bigger than the one under 3-point bending. If we consider the Weibull statistics, it is to be expected that the mechanical strength measured through 4-point bending will be lower than the one measured through 3-point bending. Weibull statistics says that the bigger the volume, the higher will be the probability to find a longer crack or flaw in general. Therefore, if you whant to have a more reliable mechanical strength from the point o view of tensile stress, I recommend that you use 4-point bending. That is important in the case o ceramic materials, which are very brittle and it is very difficult to prepare specimens for pure tensile tests. In the case of 3-point bending, the results of the modulus of rupture will be very sensitive to the finishing of the botton surface of the specimens.

Exactly. Having significant volume under well defined stress state is a great advantage. In this aspect, 4-point bending is similar to tension/compression test.

The main reason for using 4-point bending is, as already said, that the volume under stress is bigger than the one under 3-point bending and you get a more relevant answer from the statistical point of view.

The major difference between the three point and four point flexural tests is the location of the bending moment. The advantage of four point bending method is that it allows for uniform distribution between the two loading noses, while the stress in the three point bending method is located under the loading nose. The  four point bending test  is used for both reinforced or unreinforced materials including high modulus composites and for materials that do not fail within the limits of three point loading (especially brittle materials like ceramic).

OK I Agree. I have no futher commnts.

Thank you for all shared  worthy info.

For a thin piece of plywood (1.5mm, 3 veneers), what effect will the 3 point bend test have on the sample versus a 4 point bend test in determining the modulus of elasticity. Will a 3 point test give higher values of MOE that do not truly reflect the nature of the material?

Dear Son, as you see it depends on what are testing! The material, the sizes (nano versus macro) and the applications should influence which methods one takes. If you specify your case than, you will get dedicated answers!

So far, the discussions on this thread have highlighted the major differences between the 4- and 3-point bending tests, and the advantages the former over the latter in that it enables the exposure of large volume of the material body to the peak stress. As for me, I agree with these facts. These ideas are based on theoretical considerations, which have been shown to be correct in a numerous published works. However, I believe there are more questions to be answered from the practical point of view.  I would expect the failure locations to be random between the 2 loading noses, since the bending stress is assumed uniform. The random location of the failure should be the weakest point. From my personal humble experience over the years, I found failure occurred, more often than not, under the 2 loading noses.The question here is: how do we explain this theoretically? Thanks in advance for your contribution on this point.

It is interesting what Saheed Adekunle talk about. The failure can occur anywhere, but most probably at the center at half position between the supports on the botton, because there is the location with the highest value of tensile stresses. But if we think in the Griffith law, for a certain value of stress uniformely distributed in a volume, the failure will occur where the flaw is the bigest. Them, now, in the case of 3 or 4 points bending tests, the stresses are not uniform. It is maximum in the middle vertical plane in the case of 3-point test. Even so, failure can occur out of that plane in a place that contain a big flaw. It must be large enough to compensate the less stress at that place. Them, in conclusion, the failure will occur where the combination of the local stress and the size of the local flaw results in the condition of rupture. Therefore, using 3 points bending test, the failure will happen on the middle vertical plane only if the flaw size is the same along all the body, otherwise, the previously presented conditions will be the most important.

Thanks all. 

On the difference between flexural moduli obtained by three-point and four-point bending tests by F. Mujika is interesting have a look at it guys.

I have an answer to Saheed Kolawole Adekunle: In 4-point bending tests, central region between the 2 loading noses suffers no shear stress (pure bending). The bending moment is constant almost every where up to the loading noses. Our analysis indicates that  applied bending moments required to propagate fracture are larger under pure bending than under bending in presence of shear. Outside the 2 loading noses, the specimen is under bending with shear; the shearing stress comes into play in the fracture propagation process beginning from the loading noses. Smaller bending moments are sufficient to promote crack motion. In this region, the largest bending moment is achieved about the loading noses; nucleation of cracks from the bottom surface of specimen suffers largest tensile stresses there. Hope this is clearly presented.

Personally and this seems uncommon, I do not believe in any analysis that proposes a failure stress in the bending tests of brittle homogeneous materials with no reference to the conditions for fracture propagation in the compressive part of the bent material.

Our contribution: A theory of the fracture of rectangular bars bent by terminal transverse load and couple.

We consider large (unnotched) brittle homogeneous specimen so that initiation of crack motion does not imply failure. When crack propagation is the controlling mechanism of failure, the only difference between three-point and four-point bending is that in the former, the location of failure is at one loading nose and in the latter, the location of failure is at the two loading noses.

Sorry, to disappoint all the mechanicians, mainly they are involed in the discussion exactly here. In the first place the existence of two bending schemes in ceramics testing is connected with certain technological problems in the various preparation methods of samples. Unfortunately, the flexure (bending) test is employed in ceramic materials engineering not to study a fracture process characteristics, but primarily only to compare the properties of different ceramic materials with each other. I would be able to continue my explanations, but I believe it is interesting only for technologists, but not for the mechanicians, who have different aims in reseach...

Dear Igor,

happy new Year first!

We are very much interested on your explanations especially the relations to the

ceramics! I think everybody involved here thinks on Weibull factors, cracks and other defects.

Please, go on!

Sorry, we are deviating from the Q&A.

Dear Igor

Don't you find that it may be easier (in the interpretation of results) for technologists to use either tension or compression or both (separately) rather than the bending test in order to compare the properties of different ceramic materials (as you have mentioned) ? We should also add the shear test.

Very briefly:

1) certainly, 4-point bending test has some advantages (in comparison with 3-point bending test);

2) however, for too small/short samples the realization of point (1) could be sometimes rather difficult;

3) however, unfortunately, for novel/advanced materials very often only the samples from point (2) are available for researchers;

4) certainly, in powder technologies we have special procedures for the manufacture of the samples, which have 1 from 3 dimensions longer than others (x = y, z >> x, z >> y), to prepare the samples for point (1);

5) however, the samples, prepared by the procedures (e.g. cold extrusion of plastifisized powders with subsequent sintering or others) and mentioned in point (4), even with zero porosity have different structure in comparison with the samples, prepared by procedures having no preferences for any dimensions (x = y = z) (e.g. cold pressing with subsequent sintering or hot pressing);

6) so, the same composition, prepared by different methods mentioned in points (4) and (5) and structured differently, naturally possess different physico-mechanical properties, including strength characteristics;

7) so, according to point (6), 4-point bending and 3-point bending tests have only - complementarity to each other, but NO replaceability to each other;

8) having the samples mentioned in point (4), it might be better to use tension test in stead of 4-point bending test, if the problems with the holding of samples are solved successfully at your measuring device.

To Dr. P.N.B. Anongba: Thank you very much for your comments! I can agree with you and be happy to collaborate with you and your team. However, please, don't forget about points (2), (3) and (8, in bold)

Happy New Year to everyone,

Igor 

Dear Son Thanh

Test methods are always compromises between the real behaviour of a material used in a technical application and the need to have measurable properties independent from application. This is also true for the bending test which should deliver a measure of the flexible behaviour of plates or beams.

Metalls, ceramics and polymers have different structures with different bonding forces between the atoms. This causes different failure behaviour based on the difference between metallic and covalent bonds. In case of polymers the strength of the covalent bonds are superposed by more ore less occurring structural deformations within the macromolecule. These structural deformations are reduced by intermolecular interactions causing crystallin and amorphous areas as well as by crosslinking reactions in between the polymer chains.

The bending strength test, independent from the material, produces tensile stress in the convex side of the specimen and compression stress in the concave side. This creates an area of shear stress along the midline.

To ensure the primary failure comes from tensile or compression stress the shear stress must be minimized. This is done by controlling the span to depth ratio; the length of the outer span divided by the height (depth) of the specimen. For most materials S/d=16 is acceptable. Some materials require S/d=32 to 64 to keep the shear stress low enough (see also attached link).

The major difference between the three point and four point flexural tests is the location of the bending moment. The four point bending method allows for uniform distribution between the two loading noses, whilst the three point bending method’s stress is located under the loading nose.

Since the flexural properties of many materials can vary depending on temperature, rate of strain and specimen thickness, it may be appropriate to test materials at varied parameters.

With best regards

Michael

http://www.instron.us/our-company/library/test-types/flexure-test

There is no shear stress along the midline. Why? In the tensile region (1) of the bent sample, the tensile stress tends gradually to zero from arbritrary point P1 inside the region (1) to a point P on the midline. In like manner, in the compression region (2) of the bent sample, the compressive stress tends to zero on the midline. Any slab of material with very small thickness, encircling the midline, is free of tensile and compressive stresses. As a result, no shear stress exists along the midline.

In bending tests, there are shear stresses in the tested specimen; in 3-point bending experiments, the shear stress is present every where in the bent specimen except on the vertical plane containing the central loading nose. In 4-point bending, the shear stress is zero in the region between the two loading noses and has a non-zero value outside this central region (out of the two loading noses). The shear stress direction is almost parallel to the applied loading direction throughout.

Now that we are speaking of shearing stresses! A question is: how shearing stresses manifest themselves during the fracture of brittle solids under bending?

In three-point bending tests:

In a number of cases, the broken surface is smooth and flat and identical to the central vertical plane ( Π ) containing the central loading nose. In this situation, fracture proceeds without shear.

However, in most cases, the fractured surface is non-planar; it fluctuates about the central vertical Π plane and consists of planar facets of micron size. In this latter case, the fracture develops with the help of shearing stresses. The applied loading required to promote crack motion in the latter situation is smaller than in the former case where the broken surface follows a single plane (i.e. the Π plane). Experimental evidences are numerous (see Anongba et al. (1995) " a study of the brittle fracture characteristics of CoSi2 using laser beam reflections" and Anongba (1996) "A non-planar crack analysis using continuously distributed sinusoidal edge dislocations and linear elasticity" and references therein).

A recent "Q&A" with additional comments on this subject is " Why in brittle homogeneous materials is the fracture toughness lower when the fractured surface is rough (wavy crack-front) than when it is flat?".

Zero normal stresses means no tension and compression. This does not occur in bending tests! I am afraid not to understand better your question.

plz can any one tell me the scope of six pointing bending test in present research.

Greetings for everybody,

I think the reasons are depend on material type (e.g. in Asphalt mixture the 4 point test is used compared to 3 point test for Concrete) it is related to the true behavior of the material in the true life.

Also the test is depend on the Standard test method (e.g. ASTM, AASHTO, or else).. Thank you for question and all helpful answers.

I want to consider the subject from another perspective. The ductility concept is an important parameter in the investigation of the usability of the elements exposed to bending loads. Since the area under the load-displacement curve will be larger in the 4-point bending test, the calculation of the ductility factor can be made more realistic. In addition, since the crack distribution will spread more homogeneously between the loading points, the failure mechanism is easier to understand.