RU2648309C1 - Method of determination of the contribution of plastic deformation to the amount of acoustic anisotropy in measuring of details of machines and elements of the design - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: using to determine the contribution of plastic deformation to the value of acoustic anisotropy in measuring of details of machines and elements of design. Summary of the Invention is that carried out ultrasonic measurement of acoustic anisotropy, that makes it possible to determine the contribution of plastic deformation to the value of acoustic anisotropy by comparing the values of acoustic anisotropy, measured at the control point of a detail or an element before and after grinding its surface to a depth of at least half the characteristic grain size of the metal, and the grinding cycles and the subsequent measurement of the acoustic anisotropy on the ground surface at the reference point are continued, until the relative difference in the values of acoustic anisotropy in the two neighboring cycles would not exceeding 10 %.

EFFECT: providing an opportunity to assess the degree of structural damage during operation without mechanical unloading of structures with a high degree of reliability.

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Description

The invention relates to the field of non-destructive methods of diagnostics of materials and structures and can be used to determine the magnitude of mechanical stresses and plastic deformations in machine parts and structural elements during construction, installation and operation of objects in various industries and transport (thermal and nuclear energy, mechanical engineering, chemical industry, etc.), as well as in the examination of industrial safety, investigation and prevention of the causes of accidents and emergencies.

Known methods for determining the magnitude of plastic deformations, assessing the degree of damage, the proximity of the material to failure, the magnitude of mechanical stresses based on the measurement of various characteristics of ultrasonic vibrations caused in metals by special external pathogens. The measured parameters are substituted into the patented formulas for calculating the value of mechanical stresses, criteria for the degree of damage, degree of failure, residual life, and estimation of the magnitude of plastic deformation.

In the method of A.S. SU 1805289, G01B 15/06, G01N 29/00, publ. 03/30/1993, BI No. 12, it is proposed to measure the propagation time of an acoustic shear wave in a metal before and after plastic deformation, while plastic deformations and mechanical stresses are found when solving a system of three nonlinear equations in which the nonlinear dependence of time delays on stresses and deformations is not a set of three empirical influence coefficients is taken into account and used, the method of determination of which is not indicated. In reality, nonlinearities have a significant effect, and the entire system of equations loses its meaning.

In the method of A.S. SU 1802301, G01L 1/00, publ. 03.15.1993, BI No. 10, it is proposed to evaluate the deformation by changing the distance between concentric ring risks. The disadvantage is that this method cannot be used in real constructions, since unprotected surfaces undergo corrosion damage, and painting does not allow measuring the distance between risks with a sufficient degree of accuracy.

In the method of A.S. SU 892203, G01B 7/24, publ. 12/23/1981, BI No. 47, it is proposed to measure the magnetic characteristics of the metal and to judge plastic deformations by their change, but the method description itself indicates that the coefficients in the patented formulas vary several times depending on the method of heat treatment of the sample.

In the method of US Pat. RU 2107288, G01N 29/00, publ. 03/20/1998, it is proposed to calculate the deformation and stress in the bolts and studs, measuring the time delays between pulses before and after tightening the threaded connection. In the patented formulas, only the geometric change in the size of the samples is taken into account, although it is known that the effort also changes the speed of sound itself, which will significantly affect the calculation results using the patented formulas, especially in the presence of plastic deformations.

In the method of US Pat. RU 2107907, G01N 29/00, publ. 03/27/1998, the amplitudes of the harmonics (first, second and third) contained in the reflected ultrasonic wave are measured, and the magnitude of the voltages in the connecting bolts is determined using the formulas with empirical coefficients by the ratio of the amplitudes of the second and third harmonics with the amplitude of the first harmonic. The strength criterion is the proximity of these stresses to critical. The coefficients are determined on the basis of a preliminary study of a batch of bolts.

The main disadvantages of these methods are:

- A significant error in the formulas used, which does not allow them to be used for medium and large plastic deformations.

- The need to use in most methods the empirical values of the coefficients in patented formulas to calculate strains, stresses and damage criteria. To determine these coefficients, a number of witness samples are required with a known amount of plastic deformation, damage, or a known residual resource, which is feasible only in the case of testing large-scale products and, as a rule, leads to large errors in the range of medium and large plastic deformations (deformations from 3 to thirty%).

Closest to the proposed invention is the "Method for assessing the damage to the material of structures", patent RU No. 2507514, G01N 29/04, authors Khlybov Alexander Anatolyevich, Uglov Alexander Leonidovich, priority 24.07.2012, publ. 02/20/2014.

The method consists in determining the delay times of the pulse of ultrasonic waves (longitudinal and two mutually perpendicular shear) on the surface of a new element, in the fracture zone of the same but already destroyed element, and in the controlled zone similar to the first two in shape, size and nature of loading external forces of the operated element. Then determine the criterion of the degree of destruction by the formula

,

,

where Ψ _A is the damage to the metal in relative units, (Ψ _A = 0 is the metal without damage; Ψ _A = 1 is in the fracture zone of the structure),

t _R - pulse delay of the longitudinal wave, in which:

the index t corresponds to the exploited element,

index 0 - to a new element,

index * - to the destroyed element in the destruction zone,

the index R is the momentum of the longitudinal wave,

α _D , α _R are the coefficients determined experimentally by studying a number of similar elements with varying degrees of destruction or examining one element with an increasing load up to its destruction,

coefficient D takes the following values:

ge t ₁ - shear wave delay, the polarization vector is directed along the applied load,

t ₂ - shear wave delay (perpendicular polarization),

t ₃ - longitudinal wave delay.

The decision on the diagnostic results is made on the basis of the Ψ _A criterion: the probability of destruction is maximum if Ψ _A = 0.7-0.9 (according to the text of the patent).

This method contains two main disadvantages:

1. The need, along with the metal sample or the metal structure under study, to have at least three other parts of the same shape and size: the initial unloaded sample, the destroyed sample and the loaded sample that does not contain a critical amount of damage (for assessing α _D , α _R ) .

According to [Hirao M., Pao Y. N. Dependence of acoustoelastic birefringence on plastic strains in a beam // The Journal of the Acoustical Society of America. 1985. V. 77. No. 5. P. 1659-1664] these samples should be cut from one sheet of rolled metal in one direction relative to the movement of the rolls, have the same internal defects, which reduces the practical significance of the method for conducting technical diagnostics.

2. The method uses the above time delays of the reflected acoustic signal, which carry information about stresses and plastic deformation, as well as damage accumulation. Moreover, plastic deformation and damage separately give a comparable, and sometimes significantly larger than mechanical stresses, contribution to the measured time intervals and all the results of acoustic measurements derived from them. In this method, the separation of information is carried out only on the basis of empirical experience in the examination of similar samples - witnesses under the assumption that the delays are monotonically dependent on the degree of plastic deformation or the degree of fracture.

In the work (Belyaev A.K., Lobachev AM, Modestov BC, Pivkov A.V., Polyansky V.A., Semenov A.S., Tretyakov D.A., Shtukin L.V. Assessment of plastic deformations using acoustic anisotropy // Solid State Mechanics No. 5, ⋅ 2016, pp. 124-131) it was established by experimental studies that even with uniform uniaxial loading in the region of medium and large (from 3% to 30%) plastic deformations, a strong nonmonotonicity of the dependence of the value of temporary delays and all derivatives of acoustic signal parameters from it. In this regard, any predetermined linear coefficients do not allow calculating the objective value of plastic deformation and assessing damage, with the exception of some special cases when large batches of serial parts from the same material are examined and compared.

The problem solved by the invention is the creation of a method for determining the contribution of plastic deformation to the value of acoustic anisotropy when measuring parts of machines and structural elements, which makes it possible to carry out measurements directly during operation without mechanical unloading of parts or structures and without the use of numerous samples - witnesses, the use of empirical coefficients is excluded. Thus, the accuracy, independence and reliability of the damage assessment is significantly increased. There is an opportunity without preliminary expensive experiments with witnesses to conduct a practical assessment on the existing loaded structure.

The problem is achieved in that in a method involving ultrasonic measurement of acoustic anisotropy, the contribution of plastic deformation to the value of acoustic anisotropy is determined by comparing the values of acoustic anisotropy measured at a control point before and after grinding the surface of the structural member to a depth of at least half the characteristic grain size of the metal

where A _p is the magnitude of acoustic anisotropy due to plastic deformation of the material,

A ₀ - the value of acoustic anisotropy, measured before grinding the surface,

A _n is the value of acoustic anisotropy measured after grinding the surface.

In addition, it is possible to cyclically refine the measurement results. In this case, the grinding and subsequent measurement of acoustic anisotropy on the polished surface at the control point continues until the cycle in which the relation

,

,

where n is the serial number of the grinding cycle, A _n-1 is the value of acoustic anisotropy measured immediately before the last grinding cycle of the surface.

however, the total grinding depth should not exceed 5-7 characteristic dimensions of the grain of the metal.

When providing access to the opposite control point of the surface of the tested machine parts or structural elements, it is possible to perform double-sided grinding at the control point, while the contribution of plastic deformation to the value of acoustic anisotropy is determined from the expression

.

.

The invention is illustrated by drawings, where in FIG. 1 shows a test sample with control points; FIG. 2 shows graphs of changes in acoustic anisotropy at control points depending on the grinding cycle.

The proposed method is as follows.

To determine the magnitude of acoustic anisotropy using the formula:

where t ₁ and t ₂ - the propagation time of transverse shear waves polarized in mutually perpendicular directions.

This value according to the well-known formula from [Hirao M., Pao Y. N. Dependence of acoustoelastic birefringence on plastic strains in a beam // The Journal of the Acoustical Society of America. 1985. V. 77. No. 5. P. 1659-1664] can be represented as the sum of three terms

where A _T is the initial anisotropy, determined by the properties of the initial unloaded material (texturing),

A _y - anisotropy associated with elastic deformation,

A _P is the anisotropy associated with plastic deformation and damage.

The value of A _T + A _y can be considered constant during the measurement process.

Indeed, the initial anisotropy A _T and the anisotropy caused by mechanical stresses A _y (maybe they have different values at different measurement points) depend on the initial state of the metal and on the external mechanical load and do not change without changing the load.

We proceed to determine the value of A _P :

1. Conduct a standard determination of the value of acoustic anisotropy A ₁ for the studied metal section at a control point on the surface of the structural element, for which t ₁ and t ₂ are measured — the delay time of the reflected pulse of shear waves polarized in mutually perpendicular directions (or the propagation time of the direct and reflected pulse transverse waves in the material), and then A ₀ is calculated from them according to the formula (4).

2. Grind the surface at this control point to a depth d of at least half of the characteristic grain size of the metal. As a rule, this value is about 50 microns.

3. After grinding, similarly to the first stage, the acoustic anisotropy value A _{1 is} re-determined. Grinding is possible both manually and with the help of mechanisms, the only necessary condition is the absence of heating of the grinding surface by more than 100 ° C. If both sides of the structural element are accessible (the side from which the acoustic signal is induced and from which the acoustic signal is reflected), grinding can be carried out symmetrically from both sides.

4. A comparison is made of the acoustic anisotropy values A ₀ and A ₁ obtained in steps 1 and 3.

If the change differs by less than 10%, i.e. | A ₁ -A ₀ | / | A ₀ | <10%, then the measurements can be stopped. In this case, the value of A ₁ measured at the last step can be considered as cleared of the term A _p either completely in the case of two-sided grinding (if the part side opposite the control point is accessible for grinding), or half if one-sided. Thus, we determine the contribution of plastic deformation to the value of acoustic anisotropy during one-sided grinding

With double-sided grinding

If the change in the value of acoustic anisotropy after grinding exceeds 10% (| A ₁ -A ₀ | / | A ₀ |> 10%), then the cycle of grinding and measurements after grinding at the control point must be repeated.

5. The grinding and measurement cycles are repeated until the relative difference in the values of acoustic anisotropy in two adjacent cycles reaches a value not exceeding 10%. That is, measurements are stopped on a cycle with serial number n, in which | A _n -A _n-1 | / | A _n-1 | <10%, where n is the cycle number.

In this case, the contribution of plastic deformation to the value of acoustic anisotropy during unilateral grinding

With double-sided grinding

6. The experience of applying the measuring procedure to completely destroyed samples shows that the maximum grinding depth required for measurements does not exceed 500-700 microns (no more than 10-14 iterations) and decreases monotonically with a decrease in plastic strain to about half of the grain size (about 50 μm). Therefore, in the case of large plastic strains (more than 10%), it is possible to reduce the number of measurements due to the greater grinding depth at each cycle or by approximating the experimental dependence of the anisotropy on the grinding depth, followed by extrapolation to a depth of 500-700 μm.

Confirmation of the operability of the method:

The proposed method was implemented on a sample made for strength testing from an AMC alloy plate with a thickness of 20 mm. The sample had a standard dumbbell shape, FIG. one.

The wavy line shows the line of fracture of the specimen under tension. The icons show the points at which the cyclic measurements were made.

To perform measurements of acoustic anisotropy and longitudinal wave velocity, the IN5101A instrument was used. The initial anisotropy A _T was measured before the mechanical loading of the sample and amounted to A _T = (0.5 ± 0.05)%.

Strength tests were conducted. The sample was stretched in an INSTRON 50 tensile testing machine until it was completely destroyed.

Grind the measurement zone on the destroyed sample manually, in four stages, at each of which the acoustic anisotropy was measured. At each stage, an average of 125 microns of the surface layer was removed on both sides of the sample. As the study area, points were randomly selected on the axial line of the sample, at a distance of 13 mm from each other (these points are shown in Fig. 1). The magnitude of the plastic deformation of the sample material at all points at which measurements were carried out was different. The measurement results are summarized in table 1.

Based on table 1, graphs of changes in acoustic anisotropy A _n are plotted depending on the grinding cycle number n for various points on the axial line of the sample (Fig. 2). The abscissa axis shows the grinding cycle number n. On the ordinate axis, the values of A _{n are} plotted as a percentage. Each line corresponds to a specific measurement point. The distance from the measurement point to the boundary of the working part of the sample is indicated in the captions to the graph for each curve in mm.

It can be seen from the graph that, at each subsequent grinding step, the acoustic anisotropy at all the studied points tends from its initial value obtained immediately after deformation and fracture of the sample — A ₀ , to the level that was measured on the initial sample before tensile testing (A _T = (0.5 ± 0.05)), and at 500 microns polished from each surface (grinding was performed on both sides of the sample), acoustic anisotropy completely reaches its initial level. The value of anisotropy associated with plastic deformations is estimated by the formula (9). The calculation results are shown in table 2.

Thus, the contribution of plastic deformation to acoustic anisotropy was determined directly without using empirical coefficients in the basic formulas without witness samples and test load - unloading of the test sample. The limiting value A _n obtained as a result of the cyclic process (n = 4 is shown on the right) can be used in the procedure for determining mechanical stresses in accordance with the corresponding GOST.

The obtained value of A _P is an independent diagnostic parameter and characterizes the magnitude of plastic deformation, if it is many times higher than A _n , it can be concluded that there are very large plastic deformations in the metal under study.

Claims (12)

1. A method for determining the contribution of plastic deformation to the value of acoustic anisotropy when measured in machine parts and structural elements, including ultrasonic measurement of acoustic anisotropy, characterized in that the contribution of plastic deformation to the value of acoustic anisotropy is determined by comparing the values of acoustic anisotropy measured at the control point of the part or element before and after grinding its surface to a depth of not less than half the characteristic grain size of the metal A _p = 2 (A ₀ -A _n ), where A _p is the magnitude of acoustic anisotropy due to plastic deformation of the material, A ₀ - the value of acoustic anisotropy, measured before grinding the surface, A _n is the value of acoustic anisotropy measured after grinding the surface. 2. The method according to p. 1, characterized in that the cycles of grinding and subsequent measurement of acoustic anisotropy on the ground surface at the control point continue until the cycle with serial number n, in which the relation | A _n -A _n-1 | / | A _n-1 | <10%, where n is the sequence number of the grinding cycle, A _n-1 is the value of acoustic anisotropy measured on the n-1 surface grinding cycle, however, the total grinding depth should not exceed 5-7 characteristic dimensions of the grain of the metal. 3. The method according to PP. 1, 2, characterized in that, provided that access to the opposite control point of the surface of the tested machine parts or structural elements is achieved, two-sided grinding is performed at the control point, while the contribution of plastic deformation to the value of acoustic anisotropy is determined from the expression A _p = A ₀ -A _n .

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