RU2279657C1 - Method of determining mechanical characteristics and physical strength criterion for workpiece - Google Patents

Abstract

FIELD: mechanical engineering.

SUBSTANCE: method comprises introducing indenter in the material to be tested and determining the mechanical characteristics. The physical criterion of the strength of the workpiece material is the maximum permissible stress assuming the viscous material stress to be equal to the production of the permissible strengthening coefficient by the minimum value of the yield point.

EFFECT: reduced cost.

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Description

The invention relates to the field of engineering, construction and metallurgy, in particular to the determination of operational properties (elasticity, ductility, toughness, crack resistance, etc.) and the physical strength criterion for material of machine parts.

A known method for determining the mechanical characteristics (AS USSR 2079831), which consists in the fact that a conical indenter is introduced into the test material by the shock method, while the sensor registers the kinematic indentation characteristics and determines the mechanical properties by the dependences of the nth degree polynomial.

Known RF patent No. 2128330, which is implemented as follows: the indenter is introduced into the test material and registered by the sensor, while the sensor records the kinematic characteristics of impact indentation and is transmitted by an analog-to-digital converter to an electronic device. They are processed by the software systems embedded in the device, as a result of which the mechanical properties and their dispersion are determined, a small sample is formed for statistical control and an output form is prepared containing the maximum, minimum and average values of all mechanical characteristics with the indicated errors in their determination.

Closest to the proposed invention is RF patent No. 2234692, 2004, the essence of which is that the indenter is introduced into the test material, the kinematic characteristics are recorded by the sensor and the mechanical properties are subsequently determined, then the units of material with the lowest values of elongation and impact are selected viscosity and standard samples are made from them for uniaxial tensile testing, then, using a tensile testing machine, stresses are applied to the material of the first sample equal to the yield strength, and a greater stress is applied to the material of each subsequent one, higher than the yield strength by the accepted value of the stress increment, and so on to the stress at which the specimen breaks down, from the stretched specimens, excluding the broken, standard specimens for impact testing are made which are destroyed on the pendulum head, after which a factual analysis of the nature of the fracture is carried out to determine a sample with an 80% viscous component, and the hardening coefficient et th sample, and then calculates the physical strength criterion material.

The disadvantage of this method is the inability to move from the physical criterion of the strength of the sample to the physical criterion of the strength of the material of the part.

The essence of the invention lies in the fact that the indenter is introduced into the test material, the kinematic characteristics of the shock indentation are recorded by the sensor and the mechanical characteristics are determined, then the samples of the starting material are selected, which have the lowest relative elongation and impact strength values, standard samples are made from these units for uniaxial tensile tests and alternately subjected to tensile testing on a tensile testing machine, with stresses from the yield strength ty and higher to fracture with a given step, after stretching on all stretched specimens, excluding fractured, conduct standard tests for impact strength, after which a fractographic analysis of the nature of the fracture is carried out, the i-th specimen is determined, in the fracture of which the viscous component is at the level of 80% determined hardening coefficient K _i = σ _i / σ _Ti of this sample, which is the ratio of the applied thereto σ _i to the yield stress σ _Ti given sample, the K _i is considered valid from the condition of hardening coefficient Knitted one fracture, K _i = K _ext, the obtained mechanical properties of the starting material statistically processed using a three-parameter Weibull distribution law

(где X_i - i-я механическая характеристика исходного материала), и тогда для i-ой механической характеристики материала детали уравнение трехпараметрического закона Вейбулла примет видwhere β, μ, k are, respectively, the parameters of shear, scale, and shape, then one piece is made of the source material, in the same way as in the source material, the mechanical characteristics of the material of the part _{Ximd are determined} , the obtained mechanical properties are also processed by Weibull's law, after what determine the hardening coefficient for a given mechanical characteristic

(where X _i is the ith mechanical characteristic of the source material), and then for the ith mechanical characteristic of the material of the part, the equation of the three-parameter Weibull law takes the form

then the minimum value of the ith mechanical characteristic of the material of the part is determined

and calculates the minimum yield strength of the material of the part

Then the physical criterion for the strength of the material of the part is the maximum allowable stress from the condition of viscous stress of the material of the part σ _{before MD} is equal to the product of the allowable hardening coefficient K _add to the minimum value of the yield strength of the material of the component σ _{Tmd min}

The essence of the invention is justified by the following.

The failure of a structural element corresponds to the case of exceeding the load strength of its material. Therefore, it is obvious that the absence of failure will be ensured when the load is less than the strength. However, the load and strength values are random values.

In accordance with the theory of probability, a random variable is a quantity that, as a result of experience, can take one or another value, and it is not known in advance which one. Since the values of load and strength have values that continuously fill some gaps, it is a continuous random variable.

Obviously, when calculating the strength, it is necessary to take into account not all possible loads, but only its maximum value. Therefore, it is necessary to introduce an event - load exceeding the maximum value N _max . To prevent such an event from happening, it is necessary that the probability of the event - exceeding the load of the maximum value have a low probability.

It is also obvious that when calculating the strength of an element, only the minimum strength values P _min must be taken into account. From here it is necessary to keep in mind the event - the strength is less than the minimum value. To make this event impossible, it is necessary that the probability of its occurrence is also small.

Obviously, a probabilistic calculation should consist in determining the equality for a given, low probability of the maximum load and minimum strength

This very simple formula is fundamental to probabilistic calculations. Indeed, it determines for machine parts and structural elements the simultaneous absence of their failures throughout the entire time and the exclusion of the safety margin, which inevitably causes “redundancy” in their mass and quality of materials. The transition to such a calculation takes future machines and metal structures to a fundamentally new qualitative level.

As is known, strength calculations of machine parts and structural elements should be carried out under the assumption of the least favorable combination of external loads and strength properties of the structural material: the loads are used in the calculation and the strength of the materials is minimal in order to avoid breakdowns and accidents in operation. However, in practice, both the load and the strength of the materials exhibit random variations under the influence of numerous factors that develop in different ways during the manufacture and operation of structures. These variations can practically be taken into account only by safety factors, which are established by strong-willed decisions in the design of structures.

The asymptotic distribution of the extreme members of the sample was obtained by R. Fisher and L. Tippet in 1928. They showed that under certain regularity conditions, one of three types of distribution can be used to approximate the extreme members of the sample:

where μ, β, and k are the parameters of shear, scale, and shape.

The first and second types of distributions characterize the distributions for the maximum member of the sample, and the third type characterizes the distributions for the minimum member of the sample.

The minimum strength P _min from equation (1) can be represented as the minimum value of the mechanical characteristics, the distributions of which correspond to the third type of distribution according to Fisher and Tippet. Later in the literature, this distribution became known as the three-parameter Weibull law.

Distribution density of this law

One of the most significant advantages of the Weibull law is the fact that the value of the shear parameter μ allows us to judge the minimum value of the mechanical property described by this law, that is, it can be concluded that in the sample under study there are no values of the mechanical property less than μ . Naturally, this parameter, depending on the sample size n, has some scattering. R. Dhabai found that estimates of the parameter μ are asymptotically distributed normally with dispersion

The determination of the minimum value of the mechanical characteristic in this case reduces to the well-known problem of finding the confidence interval for the mathematical expectation, which is the shift parameter μ. Confidence interval for μ

where β is the confidence probability opposite to the value of the previously assigned low probability of the occurrence of the event - the appearance of the minimum value of the mechanical characteristic. You can recommend β in the range of 0.95-0.99.

t _β is the number of mean square deviations of the normal law that must be put aside to the right and left of the scattering center so that the probability of falling into the interval I _β is equal to β. The value of t _β is determined depending on the accepted value of β.

Minimum value of mechanical characteristic:

If the mechanical characteristics of the rolled metal material and the part material are distributed according to the Weibull three-parameter law and have different parameters, then two special properties of the Weibull three-parameter law can be used to move from the distribution of the mechanical characteristics of the rolled metal material to the distribution of the material of the part. The first property is the invariance of the parameters of scale β and shape k with respect to the shift along the Ox axis. In this case, only the parameter μ changes by a certain amount. The second property: when the sample values are multiplied by some constant number λ ≠ 0, the scale and β parameters are multiplied by this number, and the shape parameter k remains unchanged. These two properties will be used later in constructing the transition coefficient from the parameter of one distribution law to the parameter of the other.

The data of extensive experimental research (at the Rostselmash association in the 70s of the last century, steel grades st. 3, 20, 35, 45, 40X, at the end of the 80s of the last century st. 3, 15, 20, 30 , 35, 45, 40X, also supplied to Rostselmash, at the Amurstal plant, the control data for determining the mechanical characteristics of four steel grades were examined: st.3 _kp , st.3 _ps , st.3 _sp and 09G2S, at the Taganrog Metallurgical Plant at The data from the control tests examined samples for three steel grades: 20, 30KHGA and 12Kh1810G, at the Azovstal plant they examined Samples for four grades of steel: 09Г2С, RSD-40, RSD-32, 40Х, and recent studies of the mechanical characteristics of the following grades of steel: St.3, 15, 20, 25, 09G2S, reinforcing steels 3ps ⌀16, 35GS ⌀14, 35GS ⌀16 [Belenky DM, Kosenko EE Minimum values and dispersion of the mechanical characteristics of building steels. / Izv. High schools. Construction. 2003. No. 6; Belenky DM, Vernezi NL, Kosenko E .E. On the strength capabilities of reinforcing steels. / Concrete and reinforced concrete. 2004. No. 3]) show that in the vast majority of cases for the analytical description of the distribution of the mechanical characteristics of rolled steel, the three-parameter Weibull law is preferable, in the smaller - the three-parameter log-normal law, and only in a small number of cases the normal law.

For a final solution to this issue, the mechanical properties of the most common carbon steel grades were investigated - St.3, 15, 20, 30, 35, 45, 40X.

When compiling samples for tensile and toughness tests, an attempt was made to achieve maximum representativeness, i.e. maximum conditions for the implementation of the studied random variables. For this, the samples were prepared as follows: each sample was cut from a different rolled metal bar; no more than three samples of the same heat were present in the sample; samples were cut from the supply of various metallurgical plants producing this steel grade.

To obtain the values of yield strength and strength, elongation, standard tests were carried out on an IR-200 tensile testing machine, and the impact strength was determined on a pendulum head type PSWO-30. Before the destruction of the samples, the reverse surfaces of their heads were measured with Brinell hardness.

To obtain an unambiguous ω ² Mises criterion (correspondence of the empirical distribution to the expected theoretical distributions), the volume of each sample was taken to be the same value equal to 100. In addition, in accordance with GOST 11.006-74, the significance level for testing the null hypothesis was taken to be 0.1. As the proposed theoretical distributions, three were chosen: 1) a three-parameter Weibull; 2) three-parameter log-normal; 3) normal.

The parameters of these distributions were estimated using the maximum likelihood method.

The criterion ω ^{2 is} calculated by the formula

where x _i (i = 1, 2 ..., 100) are the results of observations;

F (x _i ) is the value of the theoretical distribution function with an argument value equal to x _i .

The inevitability of the appearance and growth of cracks during rolling and processing of the part raises the question of controlling the growth of cracks during operation of the structure. In principle, the “exact” probabilistic calculation can limit the maximum stresses arising during operation that would correspond to the minimum values of the yield strength of the material of structural elements. However, in most cases, emergency situations are inevitable when loads can cause significant stresses and significant plastic deformations corresponding to them for a short time. In addition, the yield strength is a macroscopic quantity at which shearing stresses in the slip system of the most “weak” crystals and the appearance of plastic deformations in them are possible. In some cases, when there are no strict restrictions on the magnitude of deformations, it is economically feasible to provide plastic deformations in structural elements. And, finally, the yield strength is only a characteristic of strength and does not take into account the plastic properties of the material, which are very important to know at elevated and low temperatures, shock loads, and alternating stresses. Therefore, it is economically feasible to have one physical strength characteristic that ensures the strength and ductility of the material, which allows for the design (and at the lowest cost) of the optimal material for the design and subsequent technological preparation of the production of machines and structures.

After testing the hypothesis that each empirical distribution corresponds to the three proposed theoretical distributions according to the value of the criterion ω ² and the accepted significance level of 0.1, it turned out that many samples are consistent with two and three different theoretical distributions. It should be noted that if the criterion ω ² for a given volume of the studied samples is unambiguous and precisely calculated by formula (9), then the question of choosing the value of the significance level is to some extent an uncertain question that cannot be solved from mathematical considerations. Therefore, testing the hypothesis that each used sample belongs to a specific theoretical distribution in terms of ω ² and a conditionally accepted level of significance cannot be unambiguous. For this reason, the question of the accepted theoretical distribution was solved by comparing the obtained values of ω ² for the considered theoretical distributions.

Next, we found the values of the Mises criterion for three significance levels of 0.08; 0.1; 0.12 (approximately determining the error in calculating the value of ω ² , depending on the number of experiments n, respectively, by 8%, 10%, 12%), respectively, amounted to ω ² = 2.12; ω ² = 1.94; ω ² = 1.8.

An analysis of the data shows that out of the 35 values of the Mises criterion (ω ² ) of each of the three approximating laws, the three-parameter Weibull law will correspond to a significance level of 0.08-29 times; log-normal - 21 times; normal - 22 times; with a significance level of 0.1, respectively 27, 20, 18; at a significance level of 0.12 - 26, 20, 15. Which tells us about the preference in terms of ω ² and the accepted significance levels for the investigated steel grades of the three-parameter Weibull law.

For each brand and each possible variant of its processing, the law of distribution of the mechanical characteristics of the material of the sample should be experimentally determined. Then, for each steel grade, it is necessary to choose a bar from which it is necessary to make a sample and a future part. After that, it is necessary to process the sample in the same way as the part. Then, using the "Strength" system (AS 2128330), it is possible to measure the mechanical characteristics of the material of the processed sample and the material of the part. As a result, the ratio of this mechanical characteristic of the material of the part to the same mechanical characteristic of the material of the sample is determined, which we shall conventionally call the hardening coefficient K _n .

Then, according to the already known law of distribution of the mechanical characteristics of the material of the sample, it is possible to determine the law of distribution of the mechanical characteristics of the material of the part due to the special property of the three-parameter Weibull distribution law: the invariance of the parameters of scale β and shape k with respect to the shift along the abscissa axis. Therefore, when all sample values are multiplied by a constant number (in our case by K _n ), the parameters of scale β and shift μ are multiplied by this number, and the shape parameter k remains unchanged.

We experimentally determined the law of distribution of the mechanical characteristics of the sample material - the three-parameter Weibull law:

where β, μ, k are the parameters of shear, scale, and shape, respectively. Then, only according to the results of measurements by the Strength system (AS 2128330) of the mechanical characteristics of the manufactured one part and the establishment of the K _n coefficients, we fairly accurately predict the distribution law of the mechanical characteristics of the part

The confidence shift parameter K _ni · μ in accordance with the Dabaye formula will be

With the accepted confidence probability β, the minimum value of the mechanical characteristic of the material of the part

At the same time, finding the physical strength index reduces to justifying the limiting value of plastic deformation, which can be allowed in the material from the condition of ensuring its strength.

According to patent No. 2234692, the dependence of the percentage of viscous fracture F% on the hardening coefficient K _y = σ / σ _T allows you to set the allowable hardening coefficient K _add for samples of the studied steel grades at room temperature. However, the allowable stress from the condition of viscous fracture must be taken into account with the dispersion of the yield strength σ _T. Obviously, the physical characteristic of the strength of the rolled material and the part in the form of permissible stress from the condition of viscous failure:

where σ _Tmin is the minimum yield strength of specific types of rolled products and parts.

The strength characteristic can be determined at any low temperature, since with it it is possible both an increase in σ _Tmin and a decrease in K _{in add} .

In the process of processing the material to the finished part, residual stresses unknown in magnitude remain in it, which in some cases can reach (0.7-0.8) σ _T. Since these unknown stresses reduce the strength of the material, they must be removed. For this purpose, the yield strength is measured on each manufactured part and the load is "organized" to relieve these stresses.

As follows from dependence (14), the physical strength index is a product of two values: the ultimate value of the hardening coefficient _K.sub.pred and the minimum value of the yield strength σ _T.min . The first value is dimensionless, therefore, if the initial sample (with minimum values of impact strength and relative elongation) is subjected to different processing options (pressure, thermal, mechanical), then we can talk about the similarity of the hardening value of the material of the initial sample and the material of the real part in the case of the same processing variant .

However, the situation with minimum values of yield strength is more complicated. For him, there is no similarity between the yield strength of the sample material and the material of the part, since the yield strength has a dimension. Veste, however, can use the method of transition from the three-parameter Weibull law of distribution of the yield strength to the law of distribution of the yield strength of the material of the part.

At the accepted confidence probability β, the minimum value of the yield strength of the material of the part

Thus, in the presence of reference data on the magnitude of the limiting value of hardening _K.sub.pred for a given steel grade and a given variant of its processing and manufacturing with technological preparation of production of only one part, it seems possible to determine the physical indicator of the strength of its material.

Then the physical criterion for the strength of the material of the part is the maximum allowable stress from the condition of viscous stress of the part σ _{pre md} equal to the product of the allowable hardening coefficient K _add by the minimum value of the yield strength of the material σ _{tmd min} ,

In the traditional method of calculating trunk pipelines tend to have higher values of the standard values of yield strength and strength. Therefore, only low alloy steels are used as the material. The use of the physical strength characteristics allows one to study the possibility of switching to cheaper high-quality carbon steel. The conditions for the manufacture and operation of pipelines require not only the strength of the material, but also its considerable ductility and toughness, which are more mild steel. This raises the problem of choosing the optimal grade of steel, which will provide minimal costs for pipeline transport due to the lower cost of the material, smaller wall thickness, no gaps and an increase in the standard service life. However, such a choice requires knowledge of the minimum and maximum values of all the mechanical characteristics of pipe steel. Note that these characteristics differ markedly from the characteristics of the original steel, because in the manufacture of the pipe its material, undergoing one or another plastic deformation, significantly changes its mechanical properties. So, for example, straight-seam pipes (they seem to us more preferable than spiral-seam pipes) are obtained by cold pressing, which increases the yield strength and strength and decreases the elongation and impact strength. However, the identification of statistics of these characteristics of pipe steel requires representative samples of samples cut from pipes, which must be made from the original sheets of different melts, which is a very expensive affair.

An example implementation of the method.

We experimentally determine the physical criteria of strength and mechanical characteristics of the material of pipes made of steel 09G2S, 20, 25.

First, the initial sheet of a particular steel grade is examined. For this, 100 pieces of 09G2S steel were tested at a temperature of + 20 ° C. The mechanical characteristics of the sheet _Xil were measured on all segments using the Strength system: tensile strength σ _ow , yield strength σ _T , elongation δ _l , hardness HB _l and impact strength KCU _l (5 measurements per sample). Next, 18 segments were selected with the lowest values of elongation and impact strength, from which samples of size 220 × 20 × 10 were made. Then 3 samples were destroyed on a tensile testing machine IR-200, to average the values of tensile strength, yield strength and elongation and confirm the average values of the mechanical characteristics of the sheet obtained by the "Strength" system. The results obtained are processed using the three-parameter Weibull law.

In general terms, the expression of the three-parameter Weibull distribution law in integral form for some mechanical characteristic of the sheet

where μ _l , β _l and k are the parameters of shear, scale and shape, respectively.

The value of the shear parameter μ _l allows us to judge the minimum value of the i-th mechanical property of the sheet described by this law. Naturally, this parameter, depending on the sample size n, has some scattering. R. Dhabai found that estimates of the parameter μ _{l are} distributed asymptotically normally with dispersion

The determination of the minimum value of the mechanical characteristic in this case reduces to the well-known problem of finding the confidence interval for the mathematical expectation, which is the shift parameter μ _l . Confidence interval for μ _L

where β is the confidence probability. We take β = 0.99.

t _β is the number of mean square deviations of the normal law that must be put aside to the right and left of the scattering center so that the probability of falling into the interval I _β is equal to β. Its value is 2.3263.

The minimum value of the i-th mechanical characteristic of the sheet:

After that, the averaged section from σ _T to σ _{in the} sheet material was conditionally divided into 3-4 levels, and the next 15 samples were stretched (five pieces at each stress level). As a result, samples with different hardening coefficients (K _yi ) by uniaxial tension were obtained. K _yi expressed the ratio of the applied stress to the stress sample to the average value of the yield strength. Further, standard samples with dimensions of 10 × 10 × 54 and a U-shaped notch were made from stretched samples, then they were destroyed on a pendulum head. After that, a factual analysis was carried out, and hardening coefficients (K _add ) with 80% viscous component were determined.

Further, 100 samples were cut from 09G2S pipe steel material with a diameter of 1420 mm. The sizes of the samples were respectively 180 × 15 × 8. On all the samples, the “Strength” system was used to measure the mechanical characteristics of X _itr : tensile strength σ _vtr , yield strength σ _Ttr , elongation δ _tr , hardness HB _tr and impact strength KCU _tr (5 measurements per sample).

In general terms, the expression of the three-parameter Weibull distribution law in integral form has for some mechanical characteristic of the pipe

After that, the ratio of the mechanical characteristics of the material of the pipe X _ipr and the sheet X _{il is determined} , which we conventionally call the _hardening coefficient for this mechanical characteristic

For the yield strength and strength, this coefficient will exceed unity, and for relative elongation and impact strength it will be less than unity. Then, according to the well-known distribution law of the ith mechanical property of the sheet, it is possible to determine the distribution law of this pipe property, thanks to the special property of the three-parameter Weibull distribution law: when we multiply all sample values by a constant number (in our case, by К _нi ), the scale and β parameters are multiplied by this number, and the shape parameter k remains unchanged, since it is invariant with respect to the shift along the abscissa.

Then, using the hardening coefficient for the i-th mechanical characteristic, this equation for the pipe material will take the form

The confidence interval of the shear parameter K _ni μ _l in accordance with the Dabaye formula will be:

With the accepted confidence probability β = 0.99, the minimum value of the i-th mechanical characteristic of the pipe

In accordance with formula (7), the maximum value of the mechanical characteristics of the pipe material and, accordingly, the amount of its dispersion is determined.

Having thus determined the minimum and maximum values of the mechanical characteristics, we obtain their dispersion X _imax / X _imin .

To assess the dispersion of the mechanical characteristics of the material of pipes made of 09G2S steel, Table 1 presents the minimum and maximum values of its mechanical characteristics.

Table 1
Table of minimum and maximum values of the mechanical characteristics of pipe steel 09G2S. Mechanical characteristics Pipe steel grade 09G2S Yield strength σ _T , MPa σ _t min 351.7 σ _T max 512.2

1.46 Tensile strength σ _in , MPa σ _in min 554.7 σ _in max 720.8

1.3 Elongation δ% δ _min 23.6 δ _max 30.7

1.30 Impact strength KCU, J / m ² KCU _min 3.05 KCU _max 5.07

1.66 Hardness HB HB _min 135.6 HB _max 177.2

1.31

The obtained experimental minimum values of the mechanical characteristics of a sheet of steel 09G2S and material of pipe steel 09G2S are summarized in table 2.

The hardening coefficient for each mechanical characteristic for steel 09G2S is presented in table 3.

We use the found values of the hardening coefficient of 09G2S steel to predict the minimum and maximum values of the mechanical characteristics of the material of pipes made of steels 20 and 25.

table 2
The minimum values of the mechanical characteristics of steel 09G2S and pipe steel 09G2S steel grade Mechanical characteristics σ _T σ _in δ% KCU HB 09G2S 295.23 397.67 73.65 4.3 220.88 Pipe steel 09G2S 351.7 554.7 23.6 3.05 135.6 Table 3
Hardening coefficient values for each mechanical characteristic of 09G2S steel Mechanical characteristics _Hardening coefficient K _ni Yield strength σ _T , MPa 1.19 Tensile strength σ _in , MPa 1.39 Elongation δ% 0.32 Impact strength KCU, J / m ² 0.71 Hardness HB 0.63

Then, for the yield strength of the pipe material, the Weibull equation takes the form

в соответствии с формулой Дабая будетConfidence Interval Shift Parameter

according to the formula Dabay

.The determination of the minimum value of the mechanical characteristic in this case reduces to the well-known problem of finding the confidence interval for the mathematical expectation, which is the shift parameter

.

:Confidence Interval for Value

:

where β is the confidence probability, we take 0.99;

t _β - Student's coefficient, taken according to the table depending on the confidence probability and the number of degrees of freedom n-1, its value is 2,3263.

At the accepted confidence probability β, the minimum value of the yield strength of the pipe material

From formula (11) we determine the maximum yield strength.

Similarly, the minimum and maximum predicted values of the mechanical characteristics of pipes made of steel 20 and 25, as well as their hardening coefficients, are determined.

The physical criterion for the strength of the material of pipes made of 09G2S, 20, and 25 steels was determined by the formula

where K _add - permissible hardening coefficient (determined in the same way as in RF patent No. 2234692, 2004);

σ _Tminр - the minimum value of the yield strength of the pipe material.

The results of the experimental determination of the acceptable values of the hardening coefficients and physical criteria of the strength of the pipe material for three grades of steel are presented in table 4.

Table 4 steel grade The minimum yield stress σ _Tmin , MPa The limiting value of the hardening coefficient K _add Physical characteristic of strength σ _prev. MPa

Dispersion coefficient

09G2S 351.7 1,03 362 1.46 twenty 321.6 1.42 455 1.32 25 327.8 1.40 460 1.34

From the table it follows that for the manufacture of pipes you can use the cheapest grade of steel - steel 20. It has significant strength in the pipe (455 MPa) and sufficient impact strength (4.9-13 J / m ² ).

The use of this method for determining the mechanical characteristics and physical strength criterion of a material of a part allows in cases where it is not possible to produce samples from the material of any part and there is no information about its strength capabilities, and it is also not possible to conduct large-scale tests with a large number of parts themselves, having studied the source material from which the part itself is made, obtained its mechanical properties and the limiting value of the hardening coefficient (K _add ), and then, having made one the part and measuring its mechanical characteristics using the "Strength" system, then obtaining the hardening coefficient, obtain the distribution law of the mechanical characteristics of the material of the part itself and, accordingly, information about the minimum values of the mechanical characteristics of the material of the part, and then, calculating the physical criterion of the material of the part, information about the strength material reserve parts.

Claims (1)

A method for determining the mechanical characteristics and physical criterion of the strength of the material of a part, including the introduction of an indenter in the test material, registration of the kinematic characteristics of the shock indentation by the sensor and determination of the mechanical characteristics, then samples of the starting material are revealed, which have the lowest values of relative elongation and impact strength, are made from these units standard samples for uniaxial tensile testing and alternately subjected to tensile testing with stresses from the yield strength and higher to fracture with a given step, after stretching on all stretched samples, excluding the broken one, conduct standard tests for impact strength, after which a fractographic analysis of the nature of the fracture is carried out, the ith specimen is determined, in the fracture of which the viscous component is at a level of 80%, determined hardening coefficient K _i = σ _i / σ _Ti of this sample, which is the ratio of the applied thereto σ _i to the yield stress σ _Ti given sample, the K _i is considered acceptable to ffitsientom hardening conditions of ductile fracture, K _i = K _ext, characterized in that the mechanical properties of the obtained raw material is statistically processed using a three-parameter Weibull distribution law

where β, μ, k are the parameters of shear, scale, and shape, respectively, then one part is made from the starting material, in the same way as in the starting material, the mechanical characteristics of the material of the part are _determined , and the obtained mechanical properties are also processed _according to the Weibull law, after which the hardening coefficient for this mechanical characteristic is determined

where X _i - i-th mechanical characteristic of the source material, and then for the ith mechanical characteristic of the material of the part, the equation of the three-parameter Weibull law takes the form

then the minimum value of the ith mechanical characteristic of the material of the part is determined

and calculates the minimum yield strength of the material of the part

where t _β is the student coefficient; n is the sample size; To _nσT μ is the shear parameter; Д (К _нσT μ) and Д _i (К _нi μ) - confidence interval of the shift parameter; then the physical criterion for the strength of the material of the part is the maximum allowable stress from the condition of viscous stress of the material of the part σ _{pre md} equal to the product of the allowable hardening coefficient K _add to the minimum yield strength of the material of the part σ _{Tmd min}