Elongation and reduction of the area of metal materials can specifically reflect some characteristics of materials.
Elongation is a broad concept, which includes: maximum force total elongation (uniform elongation), elongation after fracture, yield point elongation, maximum force plastic elongation, total fracture elongation, etc.
Each elongation reflects different material characteristics.
For example, the elongation after fracture can consider the uniform plastic deformation ability and local deformation (necking) ability of metal materials, while the maximum force total elongation (uniform elongation) can more accurately reflect the uniform plastic deformation ability of metal materials with strain hardening characteristics than the elongation after fracture.
Therefore, elongation is widely used as an important index to measure the mechanical properties of metal materials.
This paper introduces the main points of using common extensometers in the tensile property test, points out the differences and effects of different instruments in the test process, and analyzes the methods to improve the precision of the metal elongation test in combination with the precision requirements of the standard for the test results.
1. Test accuracy of elongation
Elongation is a conventional testing item of metal materials in a third-party testing agency.
The tensile test of metal materials is carried out using a microcomputer-controlled universal testing machine, and the elongation of materials is calculated by measuring the permanent deformation of material fracture.
However, there are many methods to measure the deformation.
To control the accuracy of the test process and obtain correct test results, the following aspects should be paid attention to.
1. Select the appropriate test method according to the type of materials tested.
The materials to be tested can generally be divided into metal and nonmetal, and the conditions produced by these two materials in the tensile process are also different.
Since some non-metallic materials have good toughness, and the vibration generated during fracture is relatively small (negligible), a clamping extensometer can be used to ensure the accuracy of the test results.
However, some metal materials will produce large vibration when they break, which may cause uncontrollable displacement of the clamped extensometer and affect the accuracy of the test.
Therefore, when measuring the elongation after fracture of metal materials, the fractured samples are usually spliced together, and the vernier caliper is used to measure the final deformation.
But at this time, we only get an approximate value close to the real deformation, because the process of metal fracture is an irregular deformation process, and there will be some gaps when splicing.
If the influence of irregular deformation and splicing gap is ignored, due to the limitation of human eye resolution, the surveyor cannot guarantee the accurate positioning of the caliper and the benchmark, and the fracture location will significantly affect the test accuracy, etc., so the test accuracy of elongation after fracture is usually far lower than the accuracy of the caliper.
The laboratory usually uses a caliper with an accuracy of 0.02mm.
Other parameters such as yield point elongation, maximum force total elongation, etc. can be tested with a clamping extensometer.
2. Select the appropriate extensometer.
In the test of mechanical properties of metal materials, extensometers are often used in the test of material deformation.
At present, the common extensometers are mechanical extensometers, optical extensometers and electromagnetic extensometers.
Mechanical extensometers are the most commonly used extensometers at present, which can be divided into large deformation extensometers, metal extensometers and full-automatic extensometers.
These extensometers need to be clamped on the surface of the sample to measure its deformation.
The metal extensometer needs a rubber band when it is installed and fixed, so the following three points should be noted when using the rubber band for fixing:
First, the elasticity is moderate, and the rubber band is generally stretched to about 80% of the maximum length or about 4 times of the original length for use;
The second is that the winding is straight, and the resultant force of winding should be perpendicular to the sample and in a straight line with the knife edge, otherwise there will be a component force pulling the extensometer arm to produce an unexpected rotation, which will directly affect the direction of the test curve;
The third is that the gauge distance is accurate.
During installation, ensure that the two arms of the extensometer are open to the positioning rod without pressure, and generally leave a gap of about 1mm.
Generally, the large deformation extensometer is specially designed for non-metallic materials (such as rubber) with large elasticity.
The deformation can reach 80cm or more, and the accuracy is 0.008mm.
The maximum deformation of the metal extensometer is generally 25 mm, and the test accuracy is 1.5 μ m.
These two types of extensometers are the most commonly used extensometers at this stage.
They are widely used because of their high-cost performance.
However, they also have certain limitations:
① The range and gauge length are poorly adjustable.
The metal extensometer can only be used in a small range or a specific gauge length, and the range is relatively fixed;
② The fracture of the sample needs to be removed, and the elongation after fracture cannot be directly measured;
③ If the extensometer is not firmly connected with the tested sample, it may slip;
④ It is difficult to adapt to special test environment, such as sample deformation in the measuring environment box;
⑤ The large deformation extensometer has a large measuring range, but its measuring accuracy is relatively low.
During the test, slight sliding will occur with the change of sample thickness.
3. The use problems of mechanical extensometers listed above can be avoided by using grating extensometers.
The grating extensometer is fast, accurate, with high resolution, and the accuracy can reach 1 μ m. It is the best tool for measuring elongation at present.
However, the price of such extensometers is relatively high, and general measuring equipment is not equipped with such extensometers.
2. Elongation test results
The extensometer is a basic device used to measure the axial and radial deformation between gauge lengths of specimens.
The parameters that need to be measured with the extensometer usually include elongation, Poisson’s ratio, tensile strain hardening index, plastic strain ratio, etc.
The use of appropriate tools and methods in the measurement process can reduce some measurement errors.
We can not only focus on the accuracy of the testing instruments, but also ignore the standardization of the testing methods in the actual testing process.
Fig. 1 Appearance of two types of common extensometers
At present, there are two kinds of extensometers commonly used in the laboratory: large deformation extensometer and metal extensometer, as shown in Fig. 1.
The gauge length (original gauge length) of three commonly used metal extensometers is 25, 50, 100mm, and the deformation is 10mm.
It is not recommended to use an extensometer when measuring the elongation after fracture of metal, because the metal fracture will produce a lot of vibration, which will affect the accuracy of the extensometer, while the extensometer is required to measure the elongation before fracture (such as yield point elongation).
The maximum deformation of the large deformation extensometer is 80cm, and the original gauge length can be adjusted at will.
It is suitable for measuring the elongation of plastics, rubber and tape.
However, very thin samples, such as plastic wrap, are not suitable because the dead weight of the extensometer will damage the sample when the extensometer is clamped.
The accuracy of manual measurement with caliper is 0.02mm, and the accuracy of extensometer measurement is 0.0015mm, with a difference of more than ten times.
However, in GB/T 228.1-2010, the requirements for rounding off of test results specify that the yield point elongation is rounded off to 0.1%, and other elongation and elongation after fracture are rounded off to 0.5%.
The final results obtained by these two methods are basically the same after rounding off.
It can be seen that when the equipment accuracy meets the requirements, the test accuracy will be affected by the extensometer accuracy.
(1) At present, there are more and more extensometers on the market, and the accuracy is getting higher and higher.
However, the use and limitations of each extensometer are also quite obvious.
Therefore, it is necessary to understand the measurement characteristics of the extensometer used in the test, make reasonable use of its own advantages, avoid its limitations, and select appropriate measuring instruments according to different test items.
(2) The extensometer has a great influence on the measurement results of metal elongation, but it is not the extensometer with high accuracy that can obtain accurate test results.
It is necessary to select appropriate measuring instruments according to different test materials to obtain accurate test results.