RU2793559C1 - Method for calibrating measuring systems of a drop test rig for measuring the work of destruction of samples during tests with a falling weight of a drop test rig - Google Patents

Abstract

FIELD: testing equipment.

SUBSTANCE: invention relates to methods for calibrating systems for measuring the work of destruction of samples during tests with a falling weight. The method consists in comparing the values of energy absorbed during testing by a falling load of a drop test rig with a sample with a sharp notch, obtained by two independent methods: a drop test rig measuring system, which needs to be calibrated, and direct determination of the potential energy by the calculation method. On each sample, the same type of incision is made, a preliminary calculation of the required impact energy of the drop test rig required to destroy the sample is carried out, and in the process of testing, the minimum energy required to destroy the sample is determined, then the impact energy is reduced to obtain at least three samples, which during the test reached a deflection of at least 30 mm, but did not collapse into two halves, for each non-destroyed sample the relative measurement error is determined, while the maximum of certain relative errors is taken as the metrological criterion of accuracy, the calibration is recognized as successful with a maximum relative error of not more than 1,0%.

EFFECT: high calibration accuracy.

1 cl, 2 dwg, 2 tbl

Description

The proposed invention relates to measuring systems of the impact tester, in particular to methods for calibrating systems for measuring the work of destruction of samples during tests by a falling weight and extends to contact and non-contact systems for measuring the work of destruction of samples during instrumented testing by a falling load (IPG) on vertical impact testers according to the "DWTT" method » (Drop-Weight Tear Test, see for example: https://mash-xxl.info/info/166657/). This test method is described in the following regulatory documents: GOST 30456-97 “Metal products. Rolled sheet and steel pipes. Impact test methods" (since 04/01/2022 - GOST 30456-2021), STO-07516250-259, API RP 5L3-e4 -2014 "Tear drop tests for main pipes", ASTM E436-03-2014 "Standard drop load test method for ferritic steels”, DIN EN 10274:1999 “Metallic materials. Drop load test.

Instrumented tests on vertical impact testers consist of the impact of a falling weight on a specimen with a sharp notch, loaded according to a three-point bending scheme. Sample dimensions correspond to one of the listed standards, table 1.

The work of crack initiation from cuts of various types is different. In this case, it is not possible to always use one type of notch. It is believed that a mechanical (milled) notch (M) introduces uncertainty into the test result, since it can be performed in different cutting conditions, that is, to introduce different work hardening. However, for high-strength steels (with a yield strength of more than 690 MPa), it is difficult to make a workable knife for making a pressed cut, the knife will either crumple or break off. The API chevron notch is recommended in cases where it is desirable to reduce the work of destruction of the sample, since its net section under the notch is less than that of other samples.

The height of the net section of the samples can be in a fairly wide range (see Table 1), which also affects the work of destruction. Differences in the span between the supports are insignificant. The speed of the load is also in most cases allowed to vary within wide limits.

Thus, the work of destruction of samples manufactured and tested according to different standards may not be the same. From this it follows that in order to compare the test results obtained on different headframes, taking into account the different masses of the falling load, the following requirements must be met:

- the same nominal height of the net section and the thickness of the samples;

- the same type of incision;

- impact speed, close to the minimum required for the destruction of the sample, with a certain specified weight of the load.

The proposed calibration method is applicable to copra measuring systems, which allow recording not only the work of destruction of a sample completely destroyed during testing, but also the energy absorbed by the sample that was not destroyed during testing into separate halves.

The following information is included in the state of the art:

1. It is known from GB1349248(A) that “In order to analyze the performance of a repetitive impact tool, it is often necessary to determine the distribution of energy in a series of impacts. In particular, it is important to note how many beats are above one given level, how many are above the next level, and so on. To do this, the pulse counter can be powered by an amplitude recognition circuit connected to the output of the second capacitor. This recognition circuit is adjusted to a level corresponding to a predetermined impact energy level, below which impacts are not counted. As a consequence, the pulse counter measures the number of shocks in a series with an energy above this value. Thus, using a range of levels of the recognition circuit, it is possible to separate the impacts into different impact energy ranges. Different levels of the recognition circuitry can be calibrated by lowering the same weight from a series of lowering heights as described above. By successively adjusting the recognition circuit until the count drops to a very low value, the magnitude of the biggest beat can be measured.”

2. From patent application CN113029828(A), an intelligent test system is known that automatically derives the calibration factor B based on the calculated data performed according to the formula E=mgh, where E is the potential energy of the hammer (equal to the impact energy entered into the test system), J; m is the mass of the falling hammer, kg; g - acceleration of gravity, m/s2; h is the height of the fall of the falling hammer, m;

3. The calibration method known from CN patent application 112082726(A) implies that "Comparing the simulated shock waveform and the actual shock waveform to determine whether the protection recorder's strong impact test equipment has a deflection."

The technical solution according to the last of the mentioned patent documents CN 112082726(A) was chosen as a prototype.

In the case of complete destruction of the sample, it is not possible by simple physical measurements, without the use of instrumented strikers or non-contact measuring systems, to separate the energy spent on the destruction of the sample, and the residual energy that is absorbed by the shock absorbers of the impact tester.

This invention is directed to solving the problem of creating a technique in which the method of the sample that did not collapse during testing is used. The falling weight of the copra is completely stopped upon contact with the sample, while the sample undergoes plastic deformation, but does not completely collapse. In this case, the entire potential energy of the load will be spent on the plastic deformation of the undestroyed sample and the propagation of a crack in it, and this energy can be determined by the calculation method, neglecting the energy that turns into heat.

EFFECT: high calibration accuracy achieved with simple tools.

This problem is solved by the calibration method, which is applicable to the measuring systems of the impact tester, which makes it possible to register not only the work of destruction of a sample completely destroyed during testing, but also the energy absorbed by the sample that was not destroyed during testing into separate halves. In this case, such a level of potential energy of the copra should be used, which makes it possible to achieve a deflection of the sample of at least 30 mm.

It is preferable that the deflection of the specimens be sufficient to destroy the "scoring part" of the net section (the height of the undestroyed part of the section is not more than 19 mm), which allows a comparative assessment of the fracture work and the proportion of the fibrous component in the fracture (optional).

The method is based on comparing the values of energy absorbed during testing by a falling load by a sample not broken into two halves, obtained by two independent methods: 1) by a measuring system, the calibration of which must be carried out; 2) direct determination of potential energy by calculation method.

The minimum set of measuring instruments (SI) is presented in Table 2.

Before carrying out the calibration, it is necessary to make sure that all the necessary measuring instruments are available, that they are in good condition and that there are valid certificates of verification. Make sure that all elements and systems of the copra, as well as the necessary auxiliary equipment, are operational. Make sure that a series of identical samples in the amount of 15 pieces for testing by a falling weight, made in accordance with GOST 30456, are available at the workplace. Check that the samples have the necessary markings.

Samples must be taken from rolled sheets, the entire series is taken from one rolled product in the same position relative to its width, the samples themselves are cut across the rolled product.

It is recommended to test at least two of the specified series of samples, which, if possible, cover the working energy range of the copra and at the same time represent the typical product range of the enterprise: samples of the minimum thickness (at least 7.5 mm according to GOST 30456) from rolled products of the lowest strength category, and maximum thickness from rolled products the highest category of strength. The maximum thickness of the samples is chosen so that the energy of a particular calibrated tester is sufficient to meet the conditions for the correctness of the test. If it is necessary to reduce the thickness of the samples compared to the total thickness of the rolled product, such a reduction should be done in the same way for all samples: symmetrically on both sides (recommended), or on one side (from the same surface of the rolled product).

The type of notch on the specimens should be the same and should be in accordance with normal testing practice at the facility where the impact driver is located.

To implement the proposed method, it is necessary to carry out tests, realizing the drop of the copra load so that the impact energy is obviously less than required for the complete destruction of the sample.

The impact energy is selected experimentally, the value of the work of destruction obtained during testing of the first sample in the series is taken as the reference point.

Before starting the calibration, it is necessary to determine the weight of the falling load and the height of the drop of the load using reference SI.

Determining the weight of a drop weight:

Install a compression dynamometer at the base of the headframe at the place of impact of the striker. Slowly lower the falling weight with the striker onto the dynamometer. Raising and lowering a falling weight with a striker during a free landing along the columns, take three readings of the dynamometer three times. Calculate the average of the three dynamometer readings in N.

Determining the drop height:

Place the drop weight in the drop position. Observing the requirements of the labor protection instructions when working on a pile driver and the safety requirements of the operating manual for a specific pile driver, measure the height of the drop of a falling load with a reference SI. Measurements should be taken from the upper horizontal surface of the sample installed in the clamping device to the lower rounding point of the drop weight striker.

Before testing the first sample, it is recommended to evaluate the required impact energy of a copra according to the formula STO-07516250-259-2014:

E[J]=KV [J]× t[mm]×σ _0.2 [MPa]/100,

where KV is the average value of the impact work on Charpy samples (type 11 according to GOST 9454), at room temperature; t is the sample thickness; σ _0.2 - yield strength of the material at room temperature. For example, for a material 33 mm thick with a yield strength of 400 MPa and an impact work of 200 J, this formula gives E = 24600 J, which is expected to be sufficient to destroy the samples.

It is allowed to use the modified formula of Nippon Steel and Sumitomo

,

,

which gives smaller values close to the expected work of destruction of the sample. For example, for the same material we get E = 22340 J.

Having estimated the necessary potential energy of the copra, the first sample from the series is tested with an impact energy that allows the sample to be completely destroyed, with the recording of the work of destruction using the measuring system being calibrated. If the destruction of the first sample has not been achieved, the energy of the copra is increased by 1.5 times and the second sample is tested.

Measurement of the work of destruction with the help of a measuring system is carried out in accordance with the operating manual and operational documents of the manufacturer of a particular measuring system, and is not described in this method.

The next steps in the calibration procedure are aimed at obtaining at least three samples that, when tested, achieved a deflection of at least 30 mm, but did not break into two halves. At the same time, having received a certain deflection, the sample completely stops the falling load.

The value of the work of destruction of the sample, recorded by the measuring system on the first sample of the series, is used to select the impact energy of the copra for subsequent samples. For the following samples of the series, the energy of the copra is set to be slightly lower than the measured work of destruction of the first sample. If the samples continue to break down, it is necessary to further reduce the energy of the impact tester, if the samples cease to reach the required deflection, increase it. The magnitude of the change in the installed energy of the copra is selected empirically.

Registration using a measuring system of the work spent on the destruction of the sample (complete or partial) must be carried out when testing each sample in a series.

Before the accumulation of statistical information, it is sufficient to have a series of 15 identical samples.

The change in the energy of the impact tester is carried out in any of two ways: by reducing the mass of the falling load, or by reducing the height of the drop of the load, depending on the design and operational capabilities of a particular impact tester, if the speed of the striker at the time of the start of interaction with the sample remains within the limits required by GOST 30456. Therefore, one should not use a drop height of less than 1.3 m. This minimum height ensures the speed of the falling load at the moment of impact is not less than 5 m/s in full compliance with the requirements of the standards governing this type of test.

To test the second sample of the series, it is necessary to set the impact energy of the copra equal to the work of destruction measured on the first sample.

In the case of complete destruction of the second sample, it is necessary to consistently reduce the impact energy of the impact tester by 300 ÷ 500 J when testing subsequent samples, until the moment when it is possible to obtain a non-destructive sample.

If the second sample is not destroyed, and its deflection is sufficient (not less than 30 mm), the next sample of the series can be tested at the same impact energy of the copra, etc.

Since there is a natural spread of properties in the metal, samples of the same series may show different fracture work. Therefore, there are likely cases when, at the same impact energy of a copra, there will be both completely destroyed samples and not destroyed ones.

Therefore, it is allowed to both reduce the impact energy of the copra and increase it by the recommended 300÷500 J, at the discretion of the test operator. The ultimate goal is to obtain at least three non-failing samples with a deflection of at least 30 mm.

Further implementation of the method is illustrated in Fig. 1, which shows a scheme for determining the full path traveled by the striker 1 when testing an unbreakable sample 2.

After receiving at least three non-destructive samples with a deflection of at least 30 mm, it is possible to determine the energy absorbed by the sample using the formula

E= Р⋅h, [J]

P - cargo weight, N ;

h - the full path traveled by the cargo until it stops completely, m (Fig. 1).

h \u003d h ₁ + h ₂ , [m]

where h ₁ is the height of the load drop, the distance between the lower point of the rounding of the striker 1 before the drop and the upper horizontal surface of the sample 2 installed on the anvil;

h ₂ - the distance traveled by the striker 1 from the moment it touches the upper surface of the sample 2, until it stops completely.

In FIG. 2 shows the simplest and most reliable way to determine the deflection of the sample. Sample 2 is placed on the anvil supports 3 together with a thin "dummy sample" 4 having a height equal to the height of the test samples.

When measuring the deflection, it is necessary to use template 5, which exactly repeats the profile of the striker 1 (usually the radius of the striker is 25 mm). In this case, the true value of the deflection (h ₂ ) will be equal to the vertical distance from the upper horizontal surface of the mockup 4 to the lower rounding point of the template 5 of the striker 1, attached to touch the deformed sample.

If the design features of the copra do not allow using the method proposed above, it is necessary to use a sketch of the anvil, made on a scale of 1: 1.

Next, the measurement results are processed.

The relative measurement error that the copra measuring system has is determined for each non-collapsed sample, rounded to one decimal place according to the formula

[%],δ =

[%],

where E is the energy absorbed by the non-destroyed sample, determined by the calculation method, J;

E1 is the energy absorbed by the non-destroyed sample, determined by the measuring system, J.

As a metrological criterion of accuracy, the maximum of certain relative errors is taken.

It is allowed to adjust the system for measuring the work of destruction of the sample during calibration by introducing one additional numerical multiplier coefficient into the algorithm for calculating the absorbed energy. This coefficient takes into account the loss of the stored energy of the hammer for friction during the fall and during the destruction of the sample. Coefficients can be adjusted for different ranges, one per range. For example, for a range of 0-10 kJ, the factor is 1.01, for a range of 10-20 kJ, the factor is 1.02, and so on for the entire calibration range.

Test experience [See, for example: Larionov A.V. Ilyin A.V. Leonov V.P. Malyshevsky V.A. Determination of the energy intensity of destruction during testing by a falling load of pipe steels / / Proceedings of the international scientific and technical conference - PIPES 2010, 2010, report No. 41] shows that on identical samples the measured work of destruction has a spread of about 10%, therefore, in order to recognize the calibration of the measuring system of the copra as successful It can be considered sufficient to obtain a maximum relative error of no more than 1.0%.

For example, for the specified material with a thickness of 33 mm with a yield strength of 400 MPa and a Charpy impact work of 200 J (paragraph 1 on page 6), the drop load failure energy is 17400 J.

The application of this method is possible to the measuring systems of the copra, which are not MI of the approved type. For systems that are MI of an approved type and included in the State Register of MI, it is recommended to apply the methods established for this type of MI by the Federal Agency for Technical Regulation and Metrology (Rosstandart).

Claims (5)

A method for calibrating copra measuring systems for measuring the work of destruction of samples during tests with a copra drop weight, which consists in comparing the values of energy absorbed when testing with a copra drop weight by a sample with a sharp notch, obtained by two independent methods: a copra measuring system, which needs to be calibrated, and direct determination potential energy by the calculation method, characterized in that each sample is made of the same type of cut, a preliminary calculation of the required impact energy of the copra required to destroy the sample is carried out, and in the process of testing, the minimum energy necessary to destroy the sample is determined, then the impact energy is reduced to obtain, at least three specimens that, during testing, reached a deflection of at least 30 mm, but did not collapse into two halves, for each specimen that did not collapse, determine the relative measurement error that the copra measuring system has, according to the formula δ=

[%], where E is the energy absorbed by the non-destroyed sample, determined by the calculation method, J, E ₁ - energy absorbed by a non-destroyed sample, determined by a calibrated measuring system copra, J, in this case, the maximum of certain relative errors is taken as the metrological criterion of accuracy, and the calibration of the measuring system of the copra is recognized as successful with a maximum relative error of not more than 1.0%.

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