RU2507514C1 - Method to assess damage of structure material - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: assessment of material damage (at the stage of accumulation of scattered microdamages) of operated elements is based on determination of a criterion of extent of metal damage in elements and determination of time dependence on the moment of control to probable damage of equipment element. At the same time they measure delay of surface, shift and longitudinal waves of ultrasonic oscillations on the surface of the metal of the new element, in the zone of emergency damage of element metal and on the surface of the metal in the controlled zone of the element, which is in the process of operation.

EFFECT: higher validity of control of structural material.

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Description

The invention relates to the field of non-destructive testing (ND) using ultrasonic (ultrasound) (elastic) waves and can be used to control accumulated damage to metal structures by measuring the propagation time of ultrasonic waves in a controlled section of the structure.

One of the main tasks of the development of modern engineering is to ensure the safe operation of structures as a whole and of individual elements. A long service life leads to the manifestation at different periods of operation of various mechanisms of degradation of the material, the incubation periods of which proceed secretly. Moreover, in the surface layer of the metal with a depth of the order of the grain size, the process of damage accumulation proceeds more rapidly compared to the volume [1]. The limiting state of the material of machines and structures corresponding to the initiation of macrocracks depends on the accumulated damage both in the surface layer and in the thickness of the metal.

There are objects for which the presence of a macrocrack is unacceptable; accordingly, information is needed on the state of the material at the stage preceding the initiation of a macrocrack [2]. Measurement of the degree of metal damage during operation directly at the facility is a rather complex and time-consuming task. The question of the quantitative assessment of material damage in real structures remains open.

Assessment of the degree of damage, and therefore, ensuring the safe operation of numerous operating facilities, can be solved by diagnostic tools using non-destructive testing methods. Acoustic methods should be recognized as one of the promising methods of non-destructive testing in diagnostic problems. This is due to the fact that ultrasonic waves react to all processes occurring in a controlled environment, and perform the functions of a universal sensor that provides information about a controlled environment. This is quite natural, since the parameters of ultrasonic waves depend on the density, elastic moduli, size and geometry of structural inhomogeneities, and other characteristics of the medium under study. The analyzed information is averaged over the area determined by the acoustic contact zone.

In practice, the speed of ultrasonic waves is used as a sensitive element, to measure which it is necessary to measure the delay time (propagation) of ultrasonic waves and thickness (path length).

The closest, selected as a prototype, should be considered the method of non-destructive testing of the degree of damage to metals of the operated elements of heat and power equipment, based on measuring the delay of the surface ultrasonic wave on the metal surface before operation (intact element), the delay of the surface ultrasonic wave in the zone of emergency destruction of the metal of the element and delays of a surface wave on a metal surface in a controlled zone of an element in operation [3]. Further, to assess the degree of damage, a comparison is made of the change in the delay of the surface wave in the controlled area with the maximum possible range of changes in the delay of the destroyed and undamaged material. Thus, the criterion for the degree of damage to the surface of the metal of an element is determined in relative units, allows you to judge the quantitative value of damage to the surface layer and stop the operation of heat and power equipment to replace the controlled element, based on the accepted level of safety. The range of damage parameter variation is from 0 to 1. In particular, for objects of power equipment made of 12Kh1MF steel under normal operating conditions, the maximum possible change in the damage parameter taking into account the safety factor is 0.7-0.9. Accounting for the average operating time of an element of a certain metal before its destruction allows you to determine the time to failure in hours.

The disadvantages of this method of ultrasonic damage control include the fact that a surface layer with a thickness of the order of the wavelength is controlled. For a frequency of about 3 MHz, the thickness of the controlled layer is about 1 mm, respectively, in the thickness of the metal is not controlled, which reduces the reliability of the control results. The disadvantages also include the need to take into account the influence of other external factors on the state of the surface layer, for example, roughness, machining, corrosion, which also affect the speed of surface waves, over a long period of operation, the state of the surface layer can change significantly even if special protection conditions are created.

These shortcomings are eliminated by the proposed solution.

The task is to increase the reliability of control.

EFFECT: increased accuracy of control by determining accumulated damage both in the surface layer and in the thickness of the controlled assembly.

This technical result is achieved by the fact that in the method for assessing damage to the material of structures, which consists in determining the delay time of an ultrasonic surface wave on the surface of a new element, in the zone of destruction of the element and in the controlled area of the operated element, then determine the criterion of the degree of damage, additionally measure the shear wave delay time in two mutually perpendicular directions and the longitudinal wave and establish the degree of damage at the stage of accumulation of races eyannyh microdamage until microcracks from the relation:

$${\Psi }_A={\alpha }_D\frac{D_t-D_0}{D^{*}-D_0}+{\alpha }_R\frac{t_R^t-t_R^0}{t_{{}^R}^{*}-t_{{}_R}^0}$$

where Ψ _A - metal damage in relative units, Ψ _A = 0 - metal without damage; Ψ _A = 1 - in the zone of destruction of the structure,

index t, 0, * - corresponds to the current state at the moment of measurement, the initial and limiting in the fracture zone, respectively, α _D , α _R , are the coefficients determined experimentally,

coefficient D takes the following values:

D = D1 + D2

$$D\mathrm{one}=\left(\frac{t_{\mathrm{one}}}{t_3}-\mathrm{one}\right)*\mathrm{one}{0}^{\mathrm{four}}$$

$$D2=\left(\frac{t_2}{t_3}-\mathrm{one}\right)*\mathrm{one}{0}^{\mathrm{four}}$$

where t _R is the pulse delay of the longitudinal wave,

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 probability of destruction is maximum if Ψ _A = 0.7-0.9.

The essence of the invention is to determine the criterion of the degree of damage to the metal of long-running structures. For this, the delay of surface, longitudinal and shear waves in the metal control zone of the new element is measured, the delay of surface, longitudinal and shear waves in the zone of emergency destruction of the metal of the element and the delay of surface, longitudinal and shear waves on the metal surface in the controlled zone of the element in operation .

In this criterion, the delay time of ultrasonic waves is measured, this expression does not depend on the thickness of the material and, therefore, it is possible to control according to the results of direct measurements: change in the propagation time of ultrasonic waves. The criterion excludes the measurement of metal thickness, since in most control cases it is not possible to measure the thickness.

The physical meaning of the criterion is the ratio of changes in current acoustic parameters to the absolute range of variation of these parameters. When choosing such a criterion for assessing the damage to a material, it is possible to control a thin surface layer and the entire thickness of the metal at each measurement point, and Ψ _A can be determined with an accuracy to constants. Frequency-dependent attenuation is commonly used in measuring structural heterogeneities, for example, grain size, porosity. When measuring the attenuation of ultrasonic waves α (ω), the measurement error can reach more than 10%. Such low accuracy largely depends on the design of the experiment: installation of sensors, contact fluid, surface roughness, etc. To determine Ψ _A , the delay speed of ultrasonic waves is used, since in this case greater accuracy, reliability, and repeatability of the results are ensured. The error in measuring the velocity of body waves in laboratory samples is 0.05%. The change in speed in the considered problems of damage accumulation is about 1%. Additional measurements of the delay time of ultrasonic waves allows you to build an algorithm for determining damage, using only measurements of the delay time of shear waves (t ₁ and t ₂ ), polarized along and perpendicular to the direction of load, longitudinal (t ₃ ) and surface (t _R ) waves.

For the practical implementation of the proposed damage control method, the ASTRON measuring and computing complex was used, based on measuring the speed (time) of ultrasonic wave delays. The basis of the operation of the hardware of the system is a method for detailed registration of the entire series of reflected acoustic pulses for their subsequent processing by the software of the system. In the processing part of the system (NOTEBOOK-type computer), the oscillograms of reflected pulses are sequentially converted with a certain discrediting step from the moment of sensing the metal until the nth reflected pulse arrives.

By the value of the criterion of the degree of damage to the metal surface, it is possible to establish the time to the probable moment of destruction of the metal of the element, as well as a rational period of time until the next control measurement, or to determine quite accurately the time of replacement of the element of equipment before its emergency destruction. Thus, the proposed method for the first time allows not only to quantify the degree of damage to the surface of the metal element during its operation, but also to relate it to the time of possible (permissible) operation.

An example implementation of the method.

To test the operability of the proposed control method, the column body of the deformed glass for synthesizing iron pentacarbonyl was studied after 15 years of continuous operation. Deformation of the column is unacceptable. The column is built of steel 12X18H10T, 12 m high, 1250 mm in diameter, 16 mm wall thickness. Operating conditions: temperature 200 ° C, pressure no more than 250 atm, working medium 95% carbon monoxide.

At the factory, a damaged column was replaced. In operation were several more similar columns with no signs of deformation. It was necessary to evaluate the technical condition of the columns and decide on their further operation.

To determine the mechanical and diagnostic characteristics of the destroyed column, flat tensile standard samples were made from the bottom, middle and upper parts of the column glass. Tests have shown that the yield strength, strength, hardness in steel increases, but at the same time, the plastic properties and impact strength decrease. The results are presented in table 1

To develop a control technique, we studied the change in the propagation time of ultrasonic waves: shear, longitudinal, and surface. The measurements were carried out using the ASTRON IVK. The accuracy of measuring the propagation time is 2 ns. The thickness of the samples was measured with an accuracy of 0.01 mm. From the results of measurements of the delay time and thickness of the samples, the propagation velocity of ultrasonic waves was determined. The measurements were carried out at a frequency of 8.5 MHz. Shear waves measured the delay in two mutually perpendicular directions. Acoustic characteristics were measured at no less than 10 points within a single sample. These data were averaged, and the delay value was determined for each type of ultrasonic waves used. Based on the results of acoustic studies, elastic moduli were determined.

Analysis of the results of acoustic measurements showed that during the operation of the material the speed of ultrasonic waves in it increases. For comparison of acoustic characteristics the speed value is given.

Analysis of the research results shows a high sensitivity of the acoustic method. The reason for the change in acoustic characteristics is embrittlement of steel during long-term operation. This is evidenced by the results of additional studies: microstructural and x-ray.

The degree of damage to the material at the time of diagnosing the metal was determined by the above expression for Ψ _A based on the measurement of the propagation time of elastic waves at different times the material was used. The results of the analysis are presented in table 2.

As a result of the analysis of the experiments, it was found that at Ψ _A > 0.7 the metal is in a state close to brittle fracture and reaches its ultimate state. Ψ _A = 1 in the zone of structural destruction, Ψ _A = 0 - metal without damage.

The obtained criterion was tested in the study of columns similar to brittle fracture. Measurements on the remaining columns gave a diagnostic criterion Ψ _A <0.6 in the most dangerous areas of the column. Acoustic measurement data gave a good agreement with the results of metallographic studies conducted on thin sections from metal cuttings.

Thus, the developed acoustic criterion of the limiting state can be used in assessing the state of long-term working metal of technical devices of hazardous production facilities.

Information sources

1. Non-destructive testing: Handbook / Ed. V.V. Klyueva. M. Engineering, 2004, V.4, - 736 p.

2. Methods of substantiation of the resource of nuclear power plants. / F.M. Mitenkov [et al.]. - M.: Mechanical Engineering, 2007 .-- 448 p.

3. RF patent No. 2231057 G01N 29/00, publ. 02/10/2004.

Table 1  Mechanical properties of samples, steel 12X18H10T Sample Number σ _T , MPa σ _V , MPa Hardness, HB δ,% Ψ,% 0-0 (bottom) 540 548 175 270 240 1-1 (middle) 554.6 727 180 33 40 2-2 (mid) 602.5 752 395 33 thirty 3-3 (top) 561.6 671.5 170 26 23

table 2 Acoustic Modulus Results Sample E, GPa G, GPa ν V ₃ , m / s V _1, m / s Ψ Source 198 77 0.29 5768 3419 0.00 one 235 97 0.2. 5940 3600 0.81 2 230 94 0.25 5790 3528 1.00 3 228 95 0.22 5769 3500 0.84 four 220 87 0.26 5852 3450 0.44 5 215 85 0.25 5800 3475 0.39

Claims (1)

A method for assessing damage to the material of structures, which consists in determining the delay time of an ultrasonic surface wave on the surface of a new element, in the zone of destruction of the element and in the controlled area of the operated element, then determining the criterion for the degree of damage, characterized in that it additionally measures the delay time of the shear wave in two mutually perpendicular directions and a longitudinal wave and establish the degree of damage at the stage of accumulation of scattered microdamages to n manifestations of microcracks from the ratio:
$${\Psi }_A={\alpha }_D\frac{D_t-D_0}{D^{*}-D_0}+{\alpha }_R\frac{t_R^t-t_R^0}{t_{{}^R}^{*}-t_{{}_R}^0},$$

where Ψ _A is the damage to the metal in relative units, Ψ _A = 0 metal without damage; Ψ _A = 1 in the zone of structural destruction,
index t, 0, * - corresponds to the current at the moment of measurement, initial and ultimate, in the fracture zone state, respectively, α _D , α _R , are the coefficients determined experimentally,
coefficient D takes the following values:
D = D1 + D2;
$$D\mathrm{one}=\left(\frac{t_{\mathrm{one}}}{t_3}-\mathrm{one}\right)\cdot \mathrm{one}{0}^{\mathrm{four}},$$

$$D2=\left(\frac{t_2}{t_3}-\mathrm{one}\right)\cdot \mathrm{one}{0}^{\mathrm{four}},$$

where t _R is the pulse delay of the longitudinal wave,
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 probability of destruction is maximum if Ψ _A = 0.7-0.9.