RU2207537C2 - Method for determining the tensile strength of metals - Google Patents

Abstract

FIELD: mechanical engineering, applicable in calculations of tensile strength of machine elements. SUBSTANCE: the method consists in manufacture of specimens, testing them for tensile strength with registering of the value of load in the process of tests and determination of their tensile strength as a result of tests. After loading to the maximum load the specimen under test is unloaded, the residual dimensions of the section are measured and its area is determined, strength σ_м is calculated by division of the maximum load into the section residual area after unloading. For prediction of the increase of strength Δσ_м a linear equation of the dependence of Δσ_м from ultimate strength σ_u, is determined, to this end, a series of specimens with a different level of strength in the preset range of its values is tested. EFFECT: provided a tensile strength with due account made for the actual area of section of the working part of the specimen at the instant corresponding to its maximum load. 1 dwg, 2 tbl

Description

 The invention relates to the field of mechanical engineering and can be used in testing metals for strength, in calculating the strength of machine parts, assessing the strength characteristics of metals in their research and development of new alloys.

 Known methods for testing metals with unloading under loading, for example, when determining the elastic limit, conditional yield strength according to GOST 1497-84 "Methods of tensile tests".

 Known methods for determining the strength of metals taking into account the deformation of the fracture cross section, for example, determining the reduced stress taking into account the relative narrowing of the destroyed sample (MITOM magazine, 1984, 12, p.21-22).

 GOST 1497-61 provided for the determination of the true stress, taking into account the cross section of the neck of the destroyed sample, however, this strength indicator did not find practical application.

 Almost all the methods used to determine the strength characteristics, except for determining the reduced voltage, are based on the ratio of the load to the cross-sectional area of the sample before testing (loading). In this case, the reduction in the cross-sectional area in the loading zone and, accordingly, the change in the stress state are not taken into account. Of practical interest are the stresses at the time preceding the loss of plastic stability of the loaded metal (the beginning of the decline of the stress-strain diagram).

где F_о - площадь сечения рабочей части образца до испытания (нагружения).The closest analogue is the method for determining the temporary resistance (σ _in ) according to GOST 1497-84. In this case, the sample is subjected to stretching under the influence of a gradually increasing load. The greatest load preceding the destruction of the sample is taken as the load P _max corresponding to the temporary resistance:

where F _about - the cross-sectional area of the working part of the sample before testing (loading).

The disadvantages of the closest analogue: does not take into account the increase in voltage by reducing the cross section of the working part of the sample upon reaching P _max .

The objective of the invention: to provide a strength test taking into account the actual cross-sectional area of the sample at the moment corresponding to P _max .

The problem is solved by manufacturing a sample, measuring the cross-section of the alleged fracture with the calculation of the cross-sectional area, loading to the maximum tensile load with its fixation, loading to fracture, calculating the strength by dividing the maximum load by the cross-sectional area of the sample. In this case, after loading to the maximum load, the sample is unloaded, the residual cross-sectional dimensions are measured and its area is determined, strength with unloading is calculated by dividing P _max by the residual cross-sectional area after unloading. The analysis of publicly available sources of information on the level of technology did not allow to identify a technical solution identical to the declared one, on the basis of which it is concluded that the latter is unknown, that is, the method presented in this application meets the criterion of "novelty."

 A comparative analysis of the claimed solution with known technical solutions revealed that the presented set of distinctive features is unknown to a person skilled in the art and does not follow explicitly from the prior art, on the basis of which it is concluded that the invention presented in this application meets the criterion of "inventive step".

 To implement the method perform operations.

1. Make samples for tensile testing, measure the dimensions of the cross section of the working part, calculate the area F ₀ .

2. Load each sample to P _max .

 3. Unload the sample.

 4. Measure the size of the cross section at its calculated length.

5. The strength with unloading σ _{m is} calculated by dividing P _max by the residual cross-sectional area after unloading.

 6. If necessary, determine the plastic properties of the sample again loaded to failure.

The method has been tested practically during a comparative test of carbon and alloy steels with different strength levels with or without hardening heat treatment. Samples of type 2 6 ("long" in accordance with GOST 1497-84) were made from rods. The tests were performed on a universal machine "INSTRON" with a record of the tensile diagram according to GOST 1497-84. The averaged results for the five tested samples of each steel are shown in Table 1. It can be seen that the test with unloading gives improved results of strength σ _m compared to σ _in in accordance with the residual reduced cross-section. The practical result is the value of this increase in strength Δσ _m for each steel with its strength level - table 1.

The drawing shows a characteristic linear dependence of the increase in strength with unloading Δσ _m from the temporary resistance, although steel with different strength characteristics is taken:
Δσ _m = 44,418-0,2502 • σ _in
where strength values are taken in kgf / mm ² . The pair correlation coefficient is 0.974, i.e. a close correlation is observed.

 On the samples of steel 20, plastic characteristics were additionally determined during testing with unloading and additional (re) loading and without unloading, i.e. according to GOST 1497-84, - table 2. Re-loading does not significantly reduce the plastic characteristics with constant strength.

The technical result of the proposed method is the ability to determine the actual stresses withstood by the metal immediately before the loss of plastic stability. When automating the measurement of the cross-sectional dimensions of a stretched sample, it is possible to automate the determination of σ _m without unloading. The increase in strength σ _m in comparison with σ _in does not depend on the grade of steel, but depends on the level of strength.

Claims (1)

The method for determining the tensile strength of metals, including the manufacture of samples, loading for tension with fixing the magnitude of the load during the test, calculating the cross-sectional area of the working part of the sample, calculating the strength, characterized in that after loading to maximum load the sample is unloaded, the residual cross-sectional dimensions are measured and determined its size, strength σ _m is calculated by dividing the maximum load on the residual cross-sectional area after unloading, and to predict the increment of strength build graph Δσ _m or to find a linear equation Δσ _m depending on the time of resistance _in σ, for which a series of test samples with different levels of strength in a predetermined range of its values.

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