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Section 11. Transport
Kurbanov Janibek Fayzullayevich, PhD., head of the research laboratory "Alarm, centralization, blocking and communication", Tashkent Institute of Railway Transport Engineers E-mail: [email protected], Kolesnikov I. K., assistant professor, Ortikov M. S., assistant
APPLICATION OF ULTRASONIC WAVES FOR RAIL FLAW DETECTION
Abstract: The purpose of this article is to identify the possibilities of excitation and reception of ultrasonic waves and the effect of defects in rail lashes on the processes of propagation.
Keywords: ultrasonic inspection, waves, rail, piezoelectric transducer, polarization, generator.
The main of all methods of ultrasonic flaw detection is the excitation of elastic mechanical vibrations propagating in a controlled environment [1, 7-8].
Quantifying changes in the processes of propagation of ultrasonic waves provides the necessary information about the detected defect. For the development of technical means of flaw detection and analysis of the results, it is necessary to know the peculiarities of the propagation of elastic waves in continuous homogeneous and inhomogeneous media. Such environments are: rail steel without internal defects and with defects; the materials from which the source of excitation of mechanical vibrations is made, the plexiglas-constructive medium between the source of vibrations and rail steel [2, 76-77].
In addition, the medium may be air, water, alcohol, special substances that provide acoustic contact between the source and the rail.
Flaw detectors usually operate in the frequency range of ultrasonic vibrations from 20 kHz to 106 kHz. In general, the operating frequency of ultrasonic flaw detectors is 2.5 MHz, and recently, the working range has been extended in the direction of low frequencies to 0.2 MHz, which increases the possibilities of flaw detection.
The differential wave equation can be represented for an elastic wave:
a d2 d2 d2
where A = —- +--- +--2 - is the Laplass operator.
dx dy dz
S - is the displacement of the particle from the equilibrium position.
For the detection of defects, an important characteristic is the length A of the elastic wave:
(1)
v
For rail steel v = 5900 m/s; v = 2.5 MHz, then the wavelength A = 2,36 mm. This is the length of the longitudinal wave in the metal of the rail at the operating frequency of the flaw detector.
Waves propagate in the medium; longitudinal, shear or transverse; superficial; normal and other waves.
The transverse elastic wave is a propagating elastic deformation of the form (shear, bending, torsion) only in solids.
The longitudinal elastic wave is the propagation of the elastic deformation of the volume (tension, compression) -in solid, liquid and gaseous substances. Elastic Wave Speed:
(2)
where p is the density of a substance.
When propagating longitudinal waves, the modulus M is the Yunga modulus E, which characterizes the elasticity of the volume.
For liquids, M is the modulus of bulk elasticity to:
k =1, (3)
B v 7
where B is the bulk compression modulus of a fluid.
For rail metal v = 5900 m/s, for water v = 1450 m/s, for plexiglass v = 2657 m/s. When a transverse wave propagates,
Section 11. Transport
the particles of the medium oscillate perpendicularly to the direction of the wave.
The speed of its spread in the metal is:
= 0,55a
longitudinal
(4)
In rail flaw detection, the excitation and reception of ultrasonic vibrations is carried out using piezoelectric or electromagnetic acoustic transducers.
Piezoelectric transducers are based on dielectric polarization.
The polarization of dielectrics is characterized by the polarization vector P, which is equal to the sum of the electric dipole moments of all molecules per unit volume of matter. For isotropic dielectrics:
P = s0k E, (5)
where x is the dielectric susceptibility of the substance; s0 is the electric constant dielectric constant of the substance: S= 1 + x . (6)
The dielectric constant characterizes the weakening of the external electric field in a dielectric by polarization charges. The value of s depends on the frequency of oscillations of the electric current, since the polarization does not occur instantaneously, but it takes time. In ionic crystals, t ~ 10-13 C, in polar dielectrics, t ~ 10 6 -f10 8 C. With increasing frequency, polarization decreases and tends to the value close to unity (electrons do not have time to oscillate after the field).
Polarization can occur in some dielectrics, not only under the action of an electric field, but also as a result of mechanical stresses arising in it. Such dielectrics are called piezoelectrics.
One of the ways to control the critical parts of mechanisms, machines, rail lashes is ultrasonic flaw detection. The process of finding defects is carried out with the help of an ultrasonic flaw detector - ultrasound (Figure 1.)
Generation
I
I
Piezoelectric transducer
AJ
D
Near zone
^ Far
\ zone
\
Homogeneous environment
Figurel. Block diagram of the flaw detector operation
The main element of the device is a quartz plate. When a sound wave reflected by a defect (a crack in the rails, wear of the base of the rails) falls on it, the quartz is compressed and stretched at the oscillation frequency of the sound wave (v = 2.5 MHz) and an alternating electrical voltage appears on its faces. From the generator, a high-frequency signal 2 enters the quartz plate. The quartz plate begins to oscillate and radiates ultrasonic waves into the volume of the rail rail being tested [3, 112-117].
Reflecting from a defect (rail crack), ultrasound returns to the plate, turning into an electrical signal 3, which goes to an oscilloscope 5. From the distance between the direct and reflected pulses, you can determine the depth of the defect 4 [3, 35-41].
The principle of operation of a flaw detector is that, under the action of a mechanical stress, an electric charge arises on the surface of quartz and some other dielectrics as a result of the polarization of the dielectric [5, 171-175].
In the construction of piezoelectric transducers rail flaw detectors are most often made of lead titanite zirconate of the
PZT-19 brand. The piezoplast resonance occurs when the frequency of its natural oscillations coincides with the frequency of the supplied alternating voltage.
The oscillation frequency of the piezoplates is determined
u / \
by: v= ' (7)
where v is the oscillation frequency of the plate; v-oscillation velocity, which for the plate: v = 3300 m/s.
Of these conditions, the thickness of the piezoplates is found:
u
b = — = 0.7 mm. 2b
(8)
In resonant mode, the transducer emits mechanical oscillations of the greatest amplitude. The piezoplastin should be located at a noticeable distance of z0 = 15 mm from the rail track.
In the far zone, with the distance from the radiator, the amplitude of the ultrasonic oscillations decreases exponentially. The attenuation coefficient is determined by:
5=5a , (9)
where Sa - is the absorption coefficient; Ss - is the scattering coefficient.
The emission zones of the piezoelectric transducer are shown in (fig. 2).
In rail steel, the attenuation at the operating frequency of 2.5 MHz is not large, about:
1
1-8 —. m
Ultrasonic vibrations spread over a distance of 5-6 m. The attenuation coefficient increases with increasing frequency. With a decrease in the operating frequency of the
Figure 2. Zones of radiation of the piezoelectric transducer
flaw detector, ultrasonic vibrations can spread over the entire length of the rail.
Based on the above, we can draw the following conclusions:
1. The physical basis of all methods of ultrasonic flaw detection is the excitation of elastic mechanical vibrations propagating in a controlled environment.
2. The attenuation of ultrasonic vibrations decreases with a decrease in the working position of the flaw detector.
References:
1. Хорбенко И. Г. Звук, ультразвук, инфразвук. - М.: Знание, 1990.- 158 с (Наука и прогресс).
2. Колесников И. К., Халиков А. А., Каримов Р. К. Электромагнитные поля и волны. Ташкент «Янги аср авлоди» 2008.- 217 с.
3. Kurbanov J. F. The control system of a single unit of the spatial field // European science review.-Vienna, 2016. - No. 7-8.-P. 112-117.
4. Колесников И. К., Курбанов Ж. Ф., Саитов А. А., Джурабаева Ф. Б. Размагничивание рельсовых плетей в рельсос-варочном производстве с помощью единого пространственного поля // Проблемы энерго- и ресурсо-сбереже-ния,-Ташкент, 2016. - № 3-4.-С. 35-41.
5. Kolesnikov I. K., Kurbanov J. F. The control system and the hardware implementation of a single unit of the spatial field // International Conference "Perspectives for the development of information technologies". - Tashkent, 2015. 4-5 November, Tashkent university of information technologies (TUIT). - P. 171-175.
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