Brinell method: features and essence

To determine the hardness of the material, the invention of the Swedish engineer Brinell is most often used - a method that measures surface properties and gives additional characteristics of polymeric metals.

Evaluation of material

It is thanks to this discovery that the ways of the most effective application of plastics are now being evaluated. Not too high hardness of plastic is tested for elasticity and softness to be used as a sealing, sealing and cushioning material. Brinell development is a method that allows to determine the strength and hardness of a material that will serve in important structures - in gears and crowns, bearings under heavy load, details of threaded joints, etc.

It is this method that gives the most accurate estimate of strength. The value of the parameter, which is denoted by P1B, can not be overestimated. Most often for this purpose, Brinell's development is used - a method in which a five-millimeter steel ball is pressed into the material. According to the depth of the ball indentation and determined by GOST.

History

In 1900, the engineer from Sweden Johan August Brinell, the method he proposed to world materials science, made famous. It was not only named after the inventor, but also became the most widely used, standardized.

What is hardness? This is a special property of a material that does not experience plastic deformation from a local contact, which most often boils down to introducing an indexer (a more rigid body) into the material.

Restored and unrestored hardness

The Brinell method helps to measure the reduced hardness, which is determined by the ratio of the load to the volume of the print, the projection area, or the surface area. Thus, hardness is volumetric, projection and surface. The latter is determined by the ratio: the load to the area of the print. The volume hardness is measured by the ratio of the load to its volume, and the projection hardness is measured by the load to the projection area that the imprint left.

Unrestored Brinell hardness is determined by the same parameters, only the main measured value becomes the resistance force, the ratio of which to the surface area, volume or projection shows the index introduced into the material. In the same way, the volume, projection and surface hardness are calculated: the ratio of the resistance force to either the surface area of the embedded part of the index, or the area of its projection, or to the volume.

Determination of hardness

The ability to resist plastic and elastic deformation when exposed to a material of a more rigid index is a definition of hardness, that is, in fact it is a test of the material for indentation. The method for measuring Brinell hardness is a measurement of how deep the index has penetrated the material. To know the exact value of the hardness of this material, you need to measure the depth of penetration. For this, there is the Brinell and Rockwell method, and the Vickers method is used less often.

If the Rockwell method determines directly the depth of penetration of the ball into the material, Vickers and Brinell measure the imprint over the area of its surface. It turns out that the deeper the index in the material, the more the footprint is obtained. On the hardness you can test absolutely any materials: minerals, metals, plastics and the like, but the hardness of each of them is determined by its own method.

How to find a way

The Brinell hardness test method is very good for heterogeneous materials, for alloys that are not too hard. Not only does the type of material determine the measurement method, but also the parameters that you need to determine. The hardness of the alloys is measured, as it were, averaged, since materials with different characteristics coexist in them. For example, cast iron. It has a very heterogeneous structure, there are cementite, graphite, perlite, ferrite, and therefore the measured hardness of cast iron is averaged, composed of the hardness of all components.

Measurement of the hardness of metals by the Brinell method is carried out using a large index to make the impression on a larger area of the sample. Thus, in cast iron, it is possible to obtain in these conditions a value that is average over many and different phases. This method is very good when measuring the hardness of alloys - cast iron, non-ferrous metals, copper, aluminum and the like. Precisely this method shows the value of the hardness of plastics.

The Rockwell method in comparison

It is good for solid and superhard metals, and the obtained value of hardness is also averaged. The same steel ball or cone serves as an indicator, but in addition to them a diamond pyramid is also used. The imprint on the material in the Rockwell method is also large, and the number of hardness for the different phases is averaged.

The methods of Brinell and Rockwell differ in principle: in the first, the result is presented in the form of a partial after dividing the force of indentation onto the surface of the area of the print, while Rockwell calculates the ratio of the penetration depth to the scale unit of the depth measuring device. That is why the Rockwell hardness is practically dimensionless, and according to Brinnell it is clearly measured in kilograms per square millimeter.

Vickers Method

If the sample is too small or you need to measure an object smaller than the index of the indictor that measures Rockwell or Brinell hardness, microhardness methods should be used, among which the most popular is the Vickers method. The index is a diamond pyramid, and the imprint is studied and measured by an optical system similar to a microscope. The averaged value will also be known, but the hardness is calculated over a much smaller area.

If the scale of the measured object is very small, then a microhardness meter is used that can make an imprint in a separate grain, phase, layer, and the indentation load can be selected independently. Metal science allows using these methods to determine both the hardness and microhardness of metals, and materials science in the same way determines the microhardness and hardness of nonmetallic materials.

Range

There are three ranges for measuring hardness. In the macro range, the load is regulated from 2 N to 30 kN. The micro-range limits not only the load on the index, but also the depth of implementation. The first value does not exceed 2 N, and the second value exceeds 0.2 microns. In the nanoscale only the depth of introduction of the index is regulated - less than 0.2 microns. The result is nanotvurdost material.

The measurement parameters depend, first of all, on the load that is applied to the index. This dependence even received a special name - the size effect, in English - indentation size effect. The nature of the size effect can be determined by the form of the index. Spherical - the hardness increases with increasing load, so this dimensional effect is reversed. The Vickers or Berkovich pyramid reduces hardness with increasing load (here the usual or direct size effect). The cone sphere, which is used for the Rockwell method, shows that increasing the load first leads to an increase in hardness, and then, when the spherical part is introduced, it decreases.

Materials and methods of measurement

The hardest to date of existing materials are two modifications of carbon: lonsdaleite, which is half the hardness of the diamond, and fullerite, which exceeds the diamond by half. Practical application of these materials is just beginning, but while the most widespread of diamonds is a diamond. It is with its help that the hardness of all metals is established.

The methods of definition (the most popular ones) were listed above, but in order to understand their features and understand the essence, it is necessary to consider others that can be conditionally divided into dynamic, that is, shock, and static, which have already been considered. The method of measurement is otherwise called a scale. It should be recalled that the most popular is still the Brinell scale, where the hardness is measured by the diameter of the print that leaves a steel ball pressed into the surface of the material.

Determination of the number of hardnesses

Brinell's method (GOST 9012-59) allows you to write the number of hardness without units of measure, denoting it HB, where H - hardness, and B - actually Brinell. The area of the print is measured as part of the sphere, not the area of the circle, as is done by the Meyer scale for example. Rockwell's method is distinguished by the fact that the determination of the depth of the ball or cone from the diamond that has entered the material is hardness. It is designated HRA, HRC, HRB or HR. The formula for the calculated hardness is as follows: HR = 100 (130) - kd. Here d is the depth of the indentation, and k is the coefficient.

By Vickers method, the hardness can be determined from the print left by the tetrahedral pyramid pressed into the surface of the material, in relation to the load that was applied to the pyramid. The area of the print is not a rhombus, but a part of the area of the pyramid. The unit size of Vickers should be considered as kgf per mm ² , denoted as unit HV. There is also a method for measuring Shore (indentation), is more often used for polymers and has twelve measuring scales. The corresponding Shore scales of the Asker scale (Japanese modification for materials of soft and elastic) are in many respects similar to the previous method, only the parameters of the measuring instrument are different and other indices are used. Another Shore method - with a rebound - for high-modulus, that is, very solid materials. Hence, we can conclude that all methods that measure the hardness of the material are divided into two categories - dynamic and static.

Tools and devices

Instruments for determining the hardness are called hardness meters, these are instrumental measurements. Testing affects the object differently, so the methods can be destructive and non-destructive. There is no direct relationship between all these scales, since none of the methods reflects the fundamental properties of the material as a whole.

Nevertheless, approximate tables are constructed in sufficient measure, where scales and different methods are connected for the categories of materials and their individual groups. The creation of these tables became possible after a number of experiments and testing. However, theories that would allow one of the calculation methods to move from one method to another, does not yet exist. The concrete method by which hardness is determined is usually chosen, based on the available apparatus, the measurement tasks, the conditions for carrying it out, and, of course, the properties of the material itself.