RU2437077C1 - Procedure for testing thin-wall specimens under stress and device "flying saucer" for its implementation - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: specimen of tested material is subjected to effect of load, medium and field. Required parametres are measured and there is determined degree of characteristic of specimens change by results of calculation of modulus of elasticity. A loading body is introduced into the central orifice of the case of a tested unit at the first stage of tests. One specimen is set on each side of the case. Clamping rings are laid on the specimens. The rings are trussed to each other deforming the specimens and creating pressure tight cavities. Initial parametres of the testing unit are measured. The testing unit is held under effect of working mediums and fields during specified time. Further, the testing unit is retrieved from working medium and field. At the second stage the testing unit is secured on a measuring device. The cavities of the case are connected with a pressure source. Pressure is supplied inside the testing unit without its disassembly; change of shape of cupolas of both specimens is being watched and general thickness of the testing unit is measured in a specified interval of pressure build-up. Further, obtained information on change of cupolas shape is processed depending on kind of deformation. The device consists of a source of working medium, of a main for supply of working medium for exertion of one-side pressure on the specimen, of a device for pressure measurement and of a measuring complex for measurement of geometric parametres of the specimen. The specimens are assembled into one testing unit consisting of the case in form of a disk with the central orifice wherein there is located a loading body. On both sides of the case there are arranged by one specimen with a sealing padding. The clamping rings with fastening devices are located outside the specimens. The case is furnished with channels for communication of the cavity of the testing unit with the pressure source.

EFFECT: facilitation of autonomy and expanded possibility to maintain loaded specimens under various conditions of effecting external mediums and fields, increased accuracy of mechanical characteristics of specimens, simplified methods and procedure of tests.

6 cl, 6 dwg

Description

The invention relates to the field of testing materials, namely the determination of time-varying mechanical characteristics of materials, in particular, wear of a material under the influence of various factors, for example, corrosion wear of thin-walled structural elements, including metal-based membranes.

Known methods of accelerated testing of materials, namely, that the test sample is placed in a three percent solution of sodium chloride heated to a certain temperature and subjected to a cyclically tensile load, the loading being carried out at a constant speed of 1-1.5 kgf / min and exposure at a maximum load of 50 -100 min, with a minimum of 40-80 min and with a difference between the maximum and minimum loads equal to 10-15 kgf / mm ² (analogue) [USSR Author's Certificate No. 597948, M.Kl. G01N 17/00, publ. 03/15/78, Bull. No. 10].

The disadvantage of such methods is to test the sample under tension only under uniaxial tension, which does not cover all the features of the behavior of the test sample in real operating conditions.

Known methods for testing samples of metal materials under tension, according to which a sample of the test material is subjected to tensile load and corrosive environment. In this case, a cross-shaped sample is used, at least one beam of which is made of the test material, exposure to the corrosive medium is carried out by filling the cavities between the rays with various liquids and / or gases, and a tensile load is applied to the beam from the test material in two axes (analog) [ USSR patent No. 1777648, M.C. G01N 17/00, publ. 11/23/92, bull. No. 43].

The disadvantage of these methods is the need to use laborious in the manufacture of a cruciform sample and the low accuracy of determining the corrosion characteristics of the samples.

There is also a known method of testing samples of metal membranes under tension, including the operation of a tensile load and a corrosive medium on a sample of the test material, while the test sample of the metal membrane is fixed on the flange of the load tank, create one-sided pressure of the working medium on the test sample, and act on the other surface of the sample corrosive medium, measure at certain intervals the necessary parameters, such as the height of the rise formed I dome and the thickness of the sample in the center of the sample, the elastic modulus for a given time interval for each sample and the degree of corrosion are calculated from the measurements, based on the calculated values, they make a conclusion about the degree of corrosion wear of the material of the test sample (prototype) [RF Patent No. 2296976, M. Kl. G01N 17/00, publ. 04/10/2007].

The known method has the following disadvantages:

a) it is practically impossible to ensure the constancy of the load of the samples directly when keeping them in a corrosive environment, it is necessary to constantly compensate for the loss of load pressure over a long test period;

b) the use of bulky stationary equipment is required;

c) there are technological difficulties in effecting samples on physical fields and when testing samples under various conditions.

Known devices for testing plate samples under tension by the method of constant deformation under bending, containing a body in the form of a bracket with supporting planes for the ends of the sample and a pressure screw located in the center of the bracket, and they are equipped with devices for pressing the ends of the sample to the supporting planes, which are rotated in opposite sides around the mounting axis of the bracket with respect to a plane perpendicular to the axis of the compression screw (analogue) [USSR Author's Certificate No. 355546, M.Kl. G01N 17/00, G01N 3/22, G01N 3/20; publ. 10.16.72 g., Bull. No. 31].

A disadvantage of the known devices is the need to place the device in the working medium together with a pressure screw, which wears out and affects the accuracy of the experiment.

Known devices for corrosion testing, containing an ampoule placed in a corrosive environment, a passive grip fixed on its bottom, an active grip, a loading device and a strain gauge, while they are equipped with intermediate grips installed in series along the axis of the passive and active grips, between which the test subjects are located samples, and the course stops of intermediate grips rigidly fixed on the ampoule, each of which is installed relative to the corresponding gripper at a distance and, determined by the specified degree of deformation of each of the test samples (analog) [USSR Author's Certificate No. 381972, M.Kl. G01N 17/00, G01N 3/08; publ. 05.22.73, Bull. No. 22].

The disadvantage of these devices is the complexity and high cost of the device and the need for careful preparation of devices for testing.

Known devices for testing hollow elastic samples for repeated deformation, containing chambers for an aggressive environment, movable and fixed grippers installed in the chamber for securing the ends of the sample, means for creating pressure in the cavity of the sample, means for acting on the sample with static and cyclic loads, and recording equipment. The means of influencing the sample with cyclic loads are made in the form of two punches symmetrically located relative to the axis of grips intended for simultaneous impact on the sample, and the working ends of the punches are made flat with rounding at the edges (analogue) [USSR Author's Certificate No. 1497512, M.Kl. G01N 17/00, publ. 07/30/89, bull. No. 28].

The disadvantage of these devices is the inability to test incomplete samples, the lack of accuracy of the results, as well as difficult to implement test technology.

It is also known a device for implementing the method of testing samples of metal membranes under tension, containing a container for an aggressive environment, means for influencing the sample with loads and recording equipment. At the same time, the load tank has a flange on which the test sample is installed to form a hermetically sealed cavity, and a line from the source of the working medium is supplied to the tank to exert one-way pressure on the sample, and a pressure measuring device is connected, in addition, a measuring complex for measuring geometric parameters is installed sample (prototype) [Patent RU No. 2296976 by M. Cl. - G01N 17/00, publ. 04/10/2007].

The known device has the following disadvantages:

a) does not allow testing of samples without being connected to a load source;

b) it is impossible to use the unit in a mobile mode;

c) difficulties arise when using the unit in natural conditions;

g) the device is relatively cumbersome to manufacture.

The aim (tasks) of the present invention is to ensure autonomy and to expand the ability to withstand loaded samples under various conditions of exposure to external environments and fields, to increase the accuracy of the mechanical characteristics of the samples, to simplify the test procedure and technology.

This goal is achieved by the fact that in the method of testing thin-walled samples under tension, including the operation of applying the load, medium and field to the sample of the test material, measuring the necessary parameters and drawing a conclusion about the degree of change in the characteristics of the samples according to the results of calculating the elastic modulus, at the first stage of testing an opening of the housing of the test unit is injected with a loading body of the appropriate size, one sample is placed on both sides of the housing, superimposed on the samples many rings, tighten the clamping rings to each other by means of fasteners with deformation of the samples and the formation of an airtight cavity, thus forming a test unit "Flying saucer", in which both samples are loaded simultaneously by applying a local load to the central portion of the sample from the load body when approaching clamping rings to each other. In this case, the dimensions of the load body are selected taking into account the level of predetermined stresses

where d is the diameter of the load body;

H _A is the total thickness of the package of the assembled test unit (case thickness together with the thickness of the samples and sealing gaskets);

h is the initial thickness of the test sample;

D is the diameter of the working part of the sample;

σ is the specified voltage;

E ₀ is the initial modulus of elasticity of the sample material.

The initial parameters of the test unit are measured, the test unit is kept under the influence of working media and fields for a predetermined time, the test unit is removed from the working medium and field, if necessary, the deposits are cleaned, the parameters (dimensions, shape and weight) of the test unit are re-measured.

At the second stage of the method, the test assembly is fixed to the measuring device, the cavity of the test assembly is connected to the pressure source, pressure is supplied inside the test assembly without disassembling, the shape of the domes of both samples is monitored and the total thickness of the test assembly is measured after a predetermined pressure rise interval. Next, the received information about the change in the shape of the domes is processed. For elastic materials, the mechanical characteristics of the samples are calculated from the values of the elastic modulus according to the formula

where E _i is the current modulus of elasticity of the sample material;

N is a matching coefficient;

p _i - uniformly distributed pressure inside the test unit;

D is the diameter of the working part of the sample;

ν is the Poisson's ratio of the material;

H _i - the thickness of the test unit at a pressure p _i ;

H _A is the total package thickness of the assembled test unit;

H ₀ is the initial thickness of the test unit before testing;

d is the transverse size (diameter) of the load body.

For materials with plastic deformations, mechanical

characteristics of the samples are determined by the conditional modulus E _{sr of} elasticity according to the formula

where E _sr - conditional elastic modulus of the sample material;

A is a parameter characteristic of a particular material, taking into account a possible change in the structure of the material during loading of the sample;

k is the coefficient characteristic of the material of this sample (0≤k≤1);

e _i is the strain intensity.

In the device for implementing the method of testing thin-walled samples under voltage, containing a source of the working medium, a line for supplying a working medium for exerting one-way pressure on the sample, a pressure measuring device, a measuring complex for measuring the geometric parameters of the sample, the samples are arranged in a single test unit consisting of a housing in the form of a disk with a central hole in which the load body is placed. On both sides of the housing are placed on one sample with sealing gaskets. Outside the samples are clamping rings. In the housing of the test unit there are channels for connecting the cavity to a pressure source. The load body usually has a spherical shape of the appropriate size.

Figure 1 presents a General view of the test site in the implementation stage of the first stage of the method, indicated: D is the diameter of the working part of the sample, H ₀ is the initial thickness of the test site; figure 2 - General view of the device for implementing the second stage of the method, denoted by H _i - the thickness of the test unit at a pressure p _i ; figure 3 is a transverse section of the test site, indicated: d is the transverse size (diameter) of the load body, N _A is the total thickness of the package of the assembled test site; figure 4 is a section aa in figure 3; figure 5 shows a graph of the change in the coefficient N from the Poisson's ratio of the material ν; figure 6 shows a graph of the thickness of the test node H _i versus pressure p _i for the considered example (the solid line corresponds to the deformation of the sample exposed to the aggressive environment, and the dashed line to the deformation of the sample without the influence of the aggressive environment).

The method of testing thin-walled samples under voltage is carried out in two stages.

At the first stage of the test, a loading body of the appropriate size is introduced into the central hole of the housing of the test unit. If the loading body has a spherical shape, then its diameter is taken as the corresponding size. On both sides of the housing place one sample with sealing gaskets. Apply clamping rings to the samples and tighten the clamping rings to each other using fasteners. To ensure uniform loading of the samples, it is recommended to tighten the clamping rings along diagonal sections, for example, by means of a screw and nut. In this way, the Flying Saucer test unit is formed in which both samples are loaded simultaneously by applying a local load (to the central portion of the sample) from the load body when the clamping rings come closer together.

In this case, the dimensions of the load body are selected taking into account the level of predetermined stresses

where d is the diameter of the load body;

H _A is the total thickness of the package of the assembled test unit (case thickness together with the thickness of the samples and sealing gaskets);

h is the initial thickness of the test sample;

D is the diameter of the working part of the sample;

σ is the specified voltage;

E ₀ is the initial modulus of elasticity of the sample material.

The initial parameters of the test unit are measured, including its geometric dimensions, which characterize the shape of the dome formed upon deformation. Withstand the test unit under the influence of working environments and fields for a given time.

Three options for the effects of media and fields at the first stage of testing can be considered: 1) only in a given environment; 2) only under the influence of a given field; 3) combined (joint) impact of both the medium and the field.

As working media can be used liquid, vapor-gas, vapor-gas, corrosive, acid, alkaline, soil, biological and other environments. As the fields can be applied temperature, electric and magnetic fields, electromagnetic radiation, including optical, x-ray and other radiation.

After the specified test time has elapsed in the given environment and fields, the test unit is removed from the working medium and field, and if necessary, the resulting deposits (residues) are cleaned. When cleaning, it is necessary to exclude the application of mechanical and other damages (defects) to the test samples. Re-measure the parameters (dimensions, shape and weight) of the test unit. In this case, the disassembly of the test unit is not performed.

At the second stage of the method, the test assembly is mounted on the measuring device and the body cavities are connected to a pressure source, for example, to a receiver or compressor. Serve through appropriate channels, for example, air under pressure inside the test unit (without disassembling it). Observe the change in the shape of the domes of both samples and measure the total thickness H _{i of the} test assembly through a predetermined pressure rise interval. Next, the received information on the change in the shape of the domes is processed depending on the type of deformation. Moreover, for elastic materials, the mechanical characteristics of the samples are estimated by the values of the elastic modulus according to the formula

where E _i is the current modulus of elasticity of the sample material;

N is a matching coefficient;

p _i - uniformly distributed pressure inside the test unit;

D is the diameter of the working part of the sample;

ν is the Poisson's ratio of the material;

H _i - the thickness of the test unit at a pressure p _i ;

H _A is the total package thickness of the assembled test unit;

H ₀ is the initial thickness of the test unit before testing;

d is the transverse size (diameter) of the load body.

The matching coefficient N is obtained from the analytical dependence

N = 0.35963-0.22425ν + 0.12623ν ² .

The graph of changes in the coefficient N from the Poisson's ratio of the material ν is shown in Fig.5.

For materials with plastic deformations, the mechanical characteristics of the samples are estimated using the conditional modulus E _{sr of} elasticity according to the formula

where E _sr - conditional elastic modulus of the sample material;

A is a parameter characteristic of a particular material, taking into account a possible change in the structure of the material during loading of the sample;

k is the coefficient characteristic of the material of this sample (0≤k≤1);

e _i is the strain rate.

When constructing the curve "stress intensity σ _i - strain intensity e _i " adhere to the following algorithm.

1. Consistently set different values of the coefficient k, characteristic of the material of the samples in the range from 0 to 1 with a certain step (for example, with a step of 0.1).

2. Enter the dimensionless parameter

3. The parameter c, which characterizes the equilibrium conditions of the loaded sample, is determined from the solution of the algebraic equation

3.67s ⁴ + (12-G) s ³ + G (G-23.5) s ² + G ² (26-0.67 G) s-10 G ³ - (1-k) [3.167 s ⁴ - (3.5 G-8) s ³ - (17.5-6 G) Gc ² -G ² (2.667 G-22) s-8 G ³ ] = 0.

4. Set the radial displacement u and the deflection w of the middle surface of the sample in the form

- безразмерная радиальная координата;Where

- dimensionless radial coordinate;

r is the radial coordinate.

5. Calculate the radial ε ₁ and circumferential ε ₂ strain according to the formulas

Where

6. Calculate the strain rate e _i from the expression

7. The auxiliary parameter J, which depends on the deformations in the material, the thickness of the test assembly H _i at pressure p _i and the coefficient k, is calculated

8. Calculate the parameter A (e _i ), which depends on the deformation and characterizes the strength of the material, according to the formula

9. Calculate the stress intensity σ _i according to the formula

.

.

10. Calculate the curvature of the dome of the sample in the radial K ₁ and district K ₂ directions, respectively. The curvatures of K ₁ and K ₂ are obtained from the formulas

11. Calculate the theoretical pressure p _m according to the formula

12. To clarify the coefficient k, the described procedure is repeated many times, based on the condition of maximum agreement between the theoretical pressure p _m and the experimental value of the loading pressure p.

13. Determine the value of the conditional module E _sr according to the formula

If necessary, build the appropriate graphs of the dependence of the parameters. Next, they make a conclusion about the strength properties of the sample.

A device for implementing a method for testing thin-walled samples under voltage contains a frame 1, a source 2 of the working medium (load tank), to which a line 3 is connected to supply the working medium to the test unit through valve 4. There is also a bleed pipe 5 with valve 6. K a pressure measuring device is connected to the line, which in the simplest version can be a pressure gauge 7. On the frame there is a measuring complex 8 for measuring the geometric parameters of the test unit .

The test unit is a single structure, which consists of a housing 8, made in the form of a doubly connected disk with a Central hole a. A load body 9 is placed in the central hole. The load body is usually spherical in shape of an appropriate size (diameter).

On both sides of the housing 8 there are two samples 10 and 11 with sealing gaskets 12 and 13. Outside the samples, there are clamping rings 14 and 15, which have fixing connections. Thus, when assembled, a sealed cavity b is formed in the test assembly. The casing 8 has channels c and d for connecting the cavity b of the test unit to the line 3. The channel c at the outlet has a threaded part into which the plug 16 is screwed in at the first stage of the test, and line 3 is connected at the second stage of the test. Measurements of the current geometric parameters of the test nodes are produced using a measuring complex 17.

The device operates as follows.

At the first stage, the test assembly is assembled with samples 10, 11 installed on the housing 8 using clamping rings 14 and 15. The load body 9 located inside the housing 8 causes stresses to appear on the test samples due to deformation (due to size difference). The tightness of the formed cavity is ensured by the installation of gaskets 12 and 13. The channel is drowned out by the plug 16 so that the acting medium, foreign objects or chemical and biological contaminants do not get into the cavity of the test unit. After assembly, the test unit is placed in the coverage area of the media and fields for a given time. In this case, the samples are exposed to media and fields, which can cause a change in the physicomechanical characteristics of the materials of the studied samples.

At the second stage, the test assembly is removed from the coverage of the media and fields. If necessary, the external surfaces of samples 10, 11 are cleaned from corrosion products. Remove the plug 16 of the test unit and the channel in is connected to the line 3 of the device. In this case, the test unit is fixed on the frame 1. When the valve 4 is opened from the source 3, a working agent (for example, compressed air) enters the cavity of the test unit through the channels c and d. In this case, the bleed valve 6 is closed. Then, a surface load is created on the test samples 10, 11 due to the supply of pressure of the working medium from the source of the working medium 2 inside the test unit. Thus, one-sided pressure is exerted on the inner surfaces of the samples 10, 11 (the samples additionally bend). The pressure change is observed using a pressure measuring device 7. Simultaneously with the increase in pressure, the current geometric parameters of the test unit are measured using the measuring complex 17.

According to the obtained measurement results for each fixed pressure, theoretical processing is performed and a conclusion is drawn on the degree of change in the material characteristics of the samples under study.

Example 1. It was carried out according to the described method, the test of two thin-walled metal samples of steel 3 with an initial thickness h = 0.5 mm The transverse size of the load body was d = 20 mm. The initial thickness of the test unit prior to testing was H ₀ = 21 mm. The total thickness of the package of the assembled test unit was H _A = 15.8 mm. The working diameter of the samples was D = 110 mm.

At the first stage, the test unit was aged for 168 hours in a corrosive environment (50% HCl hydrochloric acid solution). The thickness of the test unit after testing was 20.02 mm (i.e., the thickness of the sample after testing 0.49 mm).

At the same time, a control test unit with the same parameters was prepared that did not fit in an aggressive environment.

At the second stage, the test units were investigated by supplying inward air units under pressure with measuring the thickness of the test units. The measurement results are shown in table 1.

As you can see, the samples begin to additionally bend at an internal pressure of 0.16 MPa.

For the initial values of the loading pressure, the conditional elastic moduli of the samples were calculated. At a pressure of 0.18 MPa, for a sample aged in an aggressive environment, the conditional elastic modulus was E _conv = 54,200 MPa, and for a sample not exposed to an aggressive environment, the modulus was E _conv = 53560 MPa.

Table 1 Dependence of test unit thickness on pressure Pressure p _i inside the test unit, MPa The thickness of the test site H _i at a pressure p _i , mm aged in an aggressive environment control 0.16 20.06 21.00 0.18 20.26 21.02 0.20 20.60 21.04 0.24 21.36 21.22 0.30 22.58 21.86 0.38 24.42 22.84 0.46 26.64 24.08

The proposed method and device for its implementation make it possible to determine with a sufficient degree of accuracy the moduli of elasticity of loaded samples exposed and not subject to fields and media and to establish the degree of wear. In addition, the test procedure, test technology and device for implementing the method are greatly simplified. The autonomy of the study is ensured, and the possibilities of withstanding loaded samples under various conditions of the acting external media and fields are expanded. Test nodes are portable nodes (the device has small dimensions), allowing you to set them in the required coordinates under investigation (under water, underground, in the air, in a fluid stream, etc.). The method allows without disassembling the device to test samples by the experimental-theoretical method and to constantly monitor the deformation process.

Claims (6)

1. A method of testing thin-walled samples under tension, including the operation of applying a load, medium and field to a sample of the test material, measuring the necessary parameters and drawing a conclusion about the degree of change in the characteristics of the samples according to the results of the calculation of the elastic modulus, characterized in that at the first stage of the test into the central hole the body of the test unit is injected with a loading body of the appropriate size, one sample is placed on both sides of the body, clamping rings are applied to the samples and with they are laminated to each other with deformation of the samples and the formation of a sealed cavity, the initial parameters of the test assembly are measured, the test assembly is held under the action of working media and fields for a predetermined time, the test assembly is removed from the working medium and field, and in the second stage, the test assembly is secured to measuring device, connect the body cavity to the pressure source, apply pressure inside the test unit without disassembling it, observe the change in the shape of the domes of both samples and measure after a given interval of pressure increase, the total thickness of the test unit, then the received information about the change in the shape of the domes depending on the type of deformation is processed. 2. The method according to claim 1, characterized in that for elastic materials the mechanical characteristics of the samples are evaluated by the values of the elastic modulus, calculated by the formula

where E _i is the current modulus of elasticity of the sample material;
N is a matching coefficient;
p _i - uniformly distributed pressure inside the test unit;
D is the diameter of the working part of the sample;
ν is the Poisson's ratio of the material;
H _i - the thickness of the test unit at a pressure p _i ;
N _a - the total thickness of the package assembled test site;
H ₀ is the initial thickness of the test unit before testing;
d is the transverse size of the load body. 3. The method according to claim 1, characterized in that for materials with plastic deformations, the mechanical characteristics of the samples are evaluated by the conditional elastic modulus according to the formula

,
where E _sr - conditional elastic modulus of the sample material;
A is a parameter characteristic of a particular material, taking into account a possible change in the structure of the material during loading of the sample;
k is the coefficient characteristic of the material of this sample (0≤k≤1);
e _i is the strain intensity. 4. The method according to claim 1, characterized in that the dimensions of the load body are selected taking into account the level of specified voltages:

,
where d is the diameter of the load body;
N _A is the total thickness of the package of the assembled test unit (case thickness together with the thickness of the samples and sealing gaskets);
h is the initial thickness of the test sample;
D is the diameter of the working part of the sample;
σ is the specified voltage;
E ₀ is the initial modulus of elasticity of the sample material. 5. A device for implementing the method of testing thin-walled samples under voltage, containing a source of the working medium, a line for supplying a working medium for exerting one-way pressure on the sample, a pressure measuring device, a measuring system for measuring the geometric parameters of the sample, characterized in that the samples are arranged in a single test a unit consisting of a casing in the form of a disk with a central hole in which the load body is placed, on both sides of the casing one sample is sealed casing gaskets, clamping rings are located on the outside of the samples, which have fixing connections, in the case there are channels for connecting the cavity of the test unit with a pressure source. 6. The device according to claim 5, characterized in that the load body has a spherical shape of the appropriate size.

Images