RU2575795C2 - Method to measure relative longitudinal deformation of surface and extensometer for its realisation - Google Patents

Abstract

FIELD: instrumentation.

SUBSTANCE: extensometer comprises two reference bodies in the form of sharpened indenters, at the same time one indenter is rigidly connected to the instrument body, and other one is installed as capable of movement, and also a system of transmission of these movements. In the instrument body there is additionally a laser with an optical system of radiation collimation, a focusing lens, the focus of which matches the controlled surface, a light-dividing mirror, a lens, a coordinate-sensitive photoelectric converter and an arreter for a pendulum. The movable indenter is made with an optical reference element, the centre of curvature of which is matched with the tip of the indenter and with the controlled surface, and is rigidly installed on the pendulum hingedly suspended in the upper part of the body. An electromagnet is suspended between the movable and fixed indenters. Substance: the distance "A" between sharp edges of two indenters is measured before installation onto the surface. The pendulum is stopped, a source of light is connected, the perceived focus of beam of which is matched with the tip of the movable indenter, at the same time the image of the focal point of the laser beam reflected from the spherical mirror, with the optical magnification "K" is focused in the position corresponding to the medium position of the light spot on the coordinate-sensitive photoelectric transducer, and they record the conventionally zero coordinate "B" of the energy centre of the light spot. In the stopped position they install an extensometer onto the deformed surface, and the pendulum is uncaged, then the pendulum is pressed with the movable indenter and spherical mirror to the deformed surface by means of the electromagnet. The coordinate of the energy centre of the light spot "C" is recorded, the deformable beam is loaded, and the coordinate of the energy centre of the light spot "D" is recorded. The relative longitudinal deformation is calculated using the formula.

EFFECT: increased extent of accuracy of detection of coordinates of selected base points, accuracy of measurement of distance between sharp edges of indenters and their mutual movements due to deformation of surface, also with account of structural non-uniformity of the deformed material.

2 cl, 3 dwg

Description

The invention relates to means for measuring relative longitudinal deformation on the surface of material bodies.

The most acute question about the method of high-precision measurement of relative deformation arises in connection with the need to calibrate various strain gauges. For this purpose, specialized stands are created that reproduce, with varying degrees of accuracy, the required level of relative longitudinal deformation of the surface. If it is assumed that metrological organizations should have the standard means providing metrologically confirmed accuracy of determining the relative deformation at the level of 10 ^-4 , then with a “clean bend” of a beam 40 mm thick (state standard of relative deformation [1]) the length of the measuring base should not exceed the value 40-50 mm. At the maximum value of the relative deformation within the elasticity range of 0.003, the absolute value of the deformation will be ± 150 μm, while the measurement error of the increments should be no more than ± 0.015 μm. In order to recreate the calibrated value of the relative tensile-compression strain, stands are used to obtain the so-called “pure bend” standard. In this case, pure bending is understood to mean such a bending of a rod (beam) of constant cross section, in which for a limited section of such a beam the bending moment has a constant value in the absence of longitudinal and transverse forces, as well as torque.

An analysis of the world's exemplary stands shows that none of the known stands guarantees high accuracy in ensuring clean bending. One of the reasons for this is the lack of precision instruments for measuring relative deformation, which could confirm the presence of a constant along the length of the working segment of the beam and determined with the necessary accuracy of the relative deformation. But even in the case of the supposedly rigorous methodological support of pure bending, determining the true value of the relative deformation of the beam surface is impossible due to the lack of appropriate measuring tools.

Deformation measuring instruments can be divided into the following main groups:

- strain gauges of various types;

- strain gauges;

- fiber optic strain gauges based on the effect of selective reflection of light from Bragg gratings;

- laser instruments for measuring strain based on the use of the kinetics of the “speckle” structure of the reflected light flux;

- devices that record the mutual movements of specially created on the surface of the reference "signs", which are obtained by gluing to this surface (reflecting strips of film material) illuminated with a laser;

- extensometers, devices that are brought into mechanical contact with a deformable object by means of support pointed knives (indenters), which do not mechanically load the object, registering mutual micromotion of given points.

Known extensometer "Epsilon" [2], selected for the prototype as a device, and also as a device that implements a method of measuring the relative longitudinal deformation of the surface by means of reference bodies. The indenters included in its composition are those reference bodies whose micro displacements with some accuracy reflect the micro displacements of the selected material points on the deformable surface, which determine the length of the base segment. Two pointed indenters are applied to the deformable surface with some effort, the deformation being defined as a change in the distance between these points.

At the same time, the main problems of the Epsilon extensometer are that the mutual displacements of the tip of the indenters are transmitted using levers subject to various kinds of deformations, and also that it is not possible to determine the coordinates of the selected points on the deformable surface with the necessary accuracy, in that due to structural heterogeneity of the deformed material [3].

Thus, the main disadvantage of the Epsilon extensometer is the fundamental impossibility of achieving the technical result indicated below.

The objective of the invention is to provide a measuring tool that eliminates the influence of kinematic errors and allows you to achieve a high degree of reference "signs" (bodies, surfaces) with respect to the selected base point on the controlled surface, for which the distance to another base point on this surface is known exactly.

The technical result achieved in the proposed group of inventions is to increase the degree of accuracy of determining the coordinates of the selected base points, the accuracy of measuring the distance between the sharp edges of the indenters (reference bodies) and their mutual displacements due to surface deformation, including taking into account the structural heterogeneity of the deformable material .

The specified technical result in the implementation of the group of inventions by the method is achieved by the fact that in the method for measuring the relative longitudinal deformation of a tensile-compressive surface, two reference bodies in the form of pointed indenters are fixed on the deformable surface, and the distance “A” between the sharp edges of the two indenters is measured to installation on the surface, one of the indenters is movable, and the other is rigidly connected to the device body, apply the specified deformation force, using coordinate measuring The means measure the relative displacement of the reference bodies and calculate the relative longitudinal deformation of the surface, while according to the invention, the movable indenter is made with an optical reference element, for example, in the form of a spherical mirror, the center of curvature of which is aligned with the indenter tip, they are rigidly mounted on a pendulum with one degree freedom in the plane of strain measurement, the pendulum is arrested, the light source is connected in the form of a laser, the imaginary focus of the beam of which is also combined with the tip of the moving ndentor, while the image of the focal point of the laser beam reflected from the spherical mirror, with an optical magnification of "K" is focused in the form of a light spot in the position corresponding to the average position of the light spot, for example, on a photodiode ruler, and conditionally zero coordinate "B" of the energy the center of the light spot, after which an extensometer is installed in a caged state with some effort excluding slipping, with pointed indenters on the deformable surface, the pendulum, then the pendulum is pushed with a movable indenter and a spherical mirror to the deformable surface by means of an electromagnet suspended on a frame with a flat spring between the movable and stationary indenters, which ensures a predetermined pressing force, after which the coordinate of the energy center of the light spot “B” is recorded, and the deformable beam is loaded and after loading register the coordinate of the energy center of the light spot "G", and the relative longitudinal deformation is calculated by the formula:

ε = (G-B) / K (A + (B-B) / K).

The combination of the center of curvature of the spherical mirror with the indenter tip and their rigid installation on the pendulum make the system of reference bodies insensitive to the bending deformations of the pendulum.

The installation of a pendulum with one degree of freedom provides mobility of the reference body in a given measurement plane.

The use of a laser allows the use of the so-called “optical lever”, which eliminates kinematic errors.

The use of an electromagnet suspended on a flat spring ensures a constant effort of pressing the indenters to the controlled surface.

The arresting of the pendulum with reference bodies allows transferring (comparing) the size of the base segment “A” from a precision meter of this value to the deformable surface.

The specified technical result in the implementation of the group of inventions on the device is achieved by the fact that the extensometer for measuring the relative longitudinal deformation of the tensile-compression surface contains two reference bodies in the form of pointed indenters mounted on a controlled surface, while one indenter is rigidly connected to the device’s body, another is installed with the possibility of movement relative to the housing, as well as a transmission system for these movements in the form of coordinate measuring means, while but according to the invention, a laser is additionally installed in the device’s body as a light source with an optical radiation collimation system, a focusing lens whose focus coincides with the surface being monitored, a beam splitting mirror, for example, a photodiode array with a control unit, while the movable indenter is made with an optical reference element in the form a spherical mirror, the center of curvature of which is aligned with the tip of the indenter and with the controlled surface, and is rigidly mounted on a hinged in the upper part to rpusa pendulum between the movable and the fixed indenters by flat springs, in the housing frame fastened thereon is suspended electromagnet is also mounted for pendulum detent as the movable abutment and the presser spring.

The device is characterized in that the optical reference element is made in the form of a cylindrical mirror.

The presence of a laser provides a collimated beam of light radiation and eliminates the kinematic errors of the device.

The presence of an optical reference element in the form of a spherical or cylindrical mirror makes it possible to manufacture the device with almost any necessary accuracy of the reflecting surface, which allows us to reduce the kinematic errors to an almost arbitrarily small value.

The combination of the center of curvature of the optical reference element with the tip of the movable indenter and their rigid installation on the pendulum make the system insensitive to bending deformations of the pendulum.

The presence of a pendulum solves one of the most important tasks in protecting the measuring system from the influence of almost any mechanical deformation of structural elements while ensuring the mobility of the reference body in the measurement plane.

The presence of an electromagnet with a frame mounted on a flat spring ensures fixation of the extensometer body in a plane perpendicular to the plane of symmetry.

The presence of a flat spring provides a constant effort of pressing indenters to a controlled surface.

The presence of an arrestor allows accurate transfer (comparing) of the size of the base segment "A" from a precision meter of this value to the deformable surface.

The analysis of the prior art by the applicant, including a search by patent and scientific and technical sources of information, and the identification of sources containing information about analogues of the inventions, allowed to establish that the applicant did not find analogues characterized by signs identical to all the essential features of the method and device. The choice of the prototype allowed us to identify a set of essential distinguishing features of the device, not known from the prior art and not arising for a specialist in an explicit manner from the prior art. The applicant believes that the claimed invention meets the conditions of "novelty" and "inventive step", as well as "industrial applicability".

The invention is presented in the following drawings.

Figure 1 - General layout of the extensometer.

Figure 2 is a variant of the structure of the measuring means for precision control of the base segment A.

Figure 3 is a diagram explaining the effect of heterogeneity of the microstructure on the measurement accuracy.

The proposed extensometer (figure 1) consists of the following main parts and elements. In the housing 1 there is a light source in the form of a laser 2 with a collimating lens included in its structure, a pendulum 3, in the lower part of which a spherical mirror 4 is rigidly fixed, the geometric center of which coincides with the tip of the indenter 5, also rigidly connected with the pendulum 3, and the upper the part of the pendulum 3 is pivotally suspended on the housing 1. The pendulum 3 has a second fulcrum at its end opposite the indenter 5, with respect to which it has one degree of freedom in the plane of strain measurement. In the housing 1, a focusing lens 6 is rigidly mounted, the focus of which coincides with the surface 7, as well as a beam splitting mirror 8, lens 9 and a coordinate-sensitive photoelectric converter 10 in the form of a photodiode array with a control unit. The second indenter 11, which is fixed, is also rigidly fixed on the housing 1. An electromagnet 12 is mounted between the movable and stationary indenters, suspended on the frame 13 by means of a flat spring 14 with the upper end of the frame pinched on it, so that the tangential forces of interaction between the electromagnet and indenters do not occur. In the housing 1 is also installed an arrestor for the pendulum 3, consisting of a movable stop 15 and a clamping spring 16.

The extensometer considered above works as follows.

An extensometer is installed with the distance between the indenters “A” preliminarily measured using, for example, an additional precision instrument and with the cradled pendulum 3 on the reference beam, and movable 5 and fixed 11 indenters are fixed on its surface 7. Turn on the laser 2, in the caged state of the pendulum 3 fix the image of the focal point of the laser beam on the photodiode array 10 and conditionally register the zero coordinate "B". Next, the pendulum 3 is opened and pressed to the deformable surface 7 by means of an electromagnet 12. The obtained coordinate of the energy center of the light spot “B” is recorded. Load the beam, i.e. deform it according to the type of “pure bending”, and thereby create tension or compression on its surface, register the coordinate of the light spot “G” obtained after deformation, and calculate the absolute and relative deformations.

The proposed method for measuring the relative longitudinal deformation of the surface is as follows.

In order to achieve high accuracy in measuring relative strain, it is necessary to know the exact value of the “A” value in advance, which requires a measuring tool that makes it possible to measure the distance between the sharp edges of two indenters with high accuracy.

The diagram (figure 2) shows a variant of the structure of an additional measuring tool for precision control of the base segment “A”: a laser 19 with a collimating lens, a movable 5 and a fixed 11 indenters of a calibrated extensometer, two photodiodes 20 and 20a, an electrical unit 21, which serves to indicate difference of photocurrents of said photodiodes. The better the light beam is focused on the tip of the indenter, whose edges mirror the light flux, the greater the sensitivity of such a system to the position of the tip of the indenter along the XI axis. The distance between indenters can be most accurately measured using a laser interferometer 22 mounted on the base 23 with guides 24 and an extensometer moving mechanism 25, as well as a photoelectric microscope (not shown). Measure the base segment "A" between the tips of the indenters 5 and 11 in the caged state of the pendulum 3.

Thus obtained in advance, the result of measuring the distance "A" between the tips of the indenters is entered into the memory of the computer as part of the control unit. An extensometer is mounted on the controlled surface 7 of the reference beam so that both indenters are located on one straight line lying in the plane of the upcoming bend. In the case when the non-magnetic material of the beam is controlled, it is necessary to fix the steel plate on the surface 7 in one way or another. The presence of a flat spring 14 in this case ensures a constant pressing force of the indenters 5 and 11.

A collimated (parallel) laser light beam of laser 2 is directed through a beam splitter mirror 8 and focused by a lens 6 on a controlled surface 7. An optical reference element in the form of a mirror 4 (spherical or cylindrical), rigidly connected to an indenter 5 and a pendulum 3, is fixed using an arrestor 15 -16, while the light beam reflected from the optical reference element and from the beam splitter mirror 8 is focused with a lens 9 in the middle of the photodiode array 10, the photoelectric signal of which is converted to the coordinate of the energy center of the light spot in the coordinate system of the X2 line using the electronic unit included in the structure of the photodiode line 10. Using the computer included in the control unit of the photodiode line, conditionally zero coordinate “B” of the energy center of the light spot is recorded.

Any structural material has a granular structure, especially well observed under a microscope, provided that the surface is prepared by grinding, polishing and etching. For steels, such an important parameter as microhardness can vary by an order of magnitude from grain to grain (perlite - cementite). This circumstance may be a source of additional error due to the fact that for small contact forces between the indenters and the material, when the contact area of the indenter is less than the grain size, the tip of the indenter can shift toward the grain with less hardness, as a result of which the distance between the indenters “A” changes (figure 3).

Numerous studies have shown that universal hardness has an unstable, spasmodic nature of change, especially in the region of low loads. An example of this is the data presented in the information source [3], where the universal hardness is not only spasmodic in the region of low loads, but also the opposite nature of the change in hardness with increasing load depending on the type of indenter (cone, pyramid or ball). When carbide, diamond, and Vickers pyramids were used as indenters, hardness was high at low loads.

After registration of the coordinates of the light spot “B”, the pendulum is snapped and the electromagnet 12 is turned on, fixing the extensometer on the controlled magnetically conductive surface 7. As a result, both indenters are pressed against the beam surface with a certain force. In this case, due to the microinhomogeneity of the beam material, there is a probability of a change in the length of the base segment “A”: as a result of the possible displacement of both indenters, the light spot on the photodetector will shift by a certain amount ΔX2. After that, a new coordinate “B” of the light spot on the photodetector is determined, which allows you to calculate the difference between the second and first coordinate (BB). When a controlled longitudinal deformation of the surface occurs in pure bending mode, the moving indenter 5 with the mirror 4 is shifted by ΔX1, and the energy center of the light spot on the photodetector will acquire the coordinate “G”, which allows calculating the movement of the moving indenter as a result of the load (G-B) / K ( figure 3).

The scale factor "K" of the ratio of the displacement ΔX2 along the sensitivity axis of the photoelectric converter X2 to the displacement ΔX1 of the indenter along the X1 axis is defined as K = ΔX2 / ΔX1.

The value of the relative strain "ε" in the first approximation has always been calculated as ε = ΔX1 / A.

In the present invention, the value of the relative longitudinal deformation of the tensile-compression surface can be calculated with an increased degree of accuracy in determining the coordinates of the selected base points, with high accuracy in measuring the distance between the sharp edges of the indenters (reference bodies) and with high accuracy in their mutual displacements due to surface deformation, including taking into account the structural heterogeneity of the deformable material according to the formula proposed by the author: ε = (G-B) / K (A + (B-B) / K), which allows us to solve the Adachi and achieve the said technical result.

Information sources

1.http: //standartgost.ru/%D0%93%D0%9E%D0%A1%D0%A2%208.543-86

2. http://www.scan-group.ru/devices/testing_devices/Epsilon_extensometers/

3. The Scientific Library of CyberLenink: http://cyberleninka.ru/article/n/istoriya-sovremennye-dostizheniya-i-perspektivy-razvitiya-tverdometrii#ixzz2H0aSoJhn

Claims (2)

1. Extensometer for measuring the relative longitudinal deformation of a tensile-compressive surface, containing two reference bodies in the form of pointed indenters mounted on a controlled surface, while one indenter is rigidly connected to the device body, the other is mounted for movement relative to the body, and a transmission system for these movements in the form of coordinate measuring means, characterized in that a laser is additionally installed in the device’s body as a light source with an optical system the topic of radiation collimation, a focusing lens whose focus coincides with the surface being monitored, a beam splitter, a lens and a coordinate-sensitive photoelectric converter in the form of a photodiode array with a control unit, while the movable indenter is made with an optical reference element in the form of a spherical mirror, the center of curvature of which is aligned with the tip of the indenter and with a controlled surface, and is rigidly mounted on a pendulum pivotally suspended in the upper part of the housing, between the movable and the fixed An indenter is suspended by means of a flat spring with a frame fixed to it; an electromagnet is suspended in the housing; a cradle for the pendulum is also installed in the form of a movable stop and a clamping spring. 2. A method for measuring the relative longitudinal deformation of a tensile-compressive surface, which consists in fixing two reference bodies in the form of pointed indenters on a controlled surface, the distance “A” between the sharp edges of two indenters being measured before being installed on the surface, one of of indenters — movable, and the other — rigidly connected to the device’s body, apply a predetermined deformation force, measure the relative displacement of the reference bodies with the help of coordinate measuring means, and calculate about relative longitudinal deformation of the surface, characterized in that the movable indenter is performed with an optical reference element in the form of a spherical mirror, the center of curvature of which is combined with the indenter tip, they are rigidly mounted on a pendulum having one degree of freedom in the plane of strain measurement, the pendulum is arrested, the light source is connected, and the light source is connected in the form of a laser, the imaginary focus of the beam of which is also combined with the tip of the moving indenter, while the image of the focal point of the laser beam reflected from a spherical mirror feces, with an optical magnification of "K" focus in the form of a light spot in the position corresponding to the average position of the light spot on a coordinate-sensitive photoelectric converter in the form of a photodiode array, and conditionally zero coordinate "B" of the energy center of the light spot is recorded, and then in a caged state an extensometer is installed with some effort, excluding slipping by pointed indenters on the deformable surface, the pendulum is opened, then the pendulum is pressed with by a moving indenter and a spherical mirror to the deformable surface by means of an electromagnet suspended on a frame with a flat spring between the movable and fixed indenters, which ensures a given pressing force, then register the coordinate of the energy center of the light spot "B", load the deformable beam, and after loading register the coordinate the energy center of the light spot "G", and the relative longitudinal deformation is calculated by the formula:
ε = (G-B) / K (A + (B-B) / K).

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