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·Methods and Instrumentation
Radiation - acoustic method for diagnostics of metals and alloysA.P. Mamontov, B.V. Chakhlov
We made the installation for registration of AE signals, arising in metals and alloys under various type of loading. This installation is shown in Fig.1.
Fig 1: The schema of experimental installation for study of acoustic emission in metals and alloys during irradiation by X-rays:|
1 - X-rays; 2 - sample; 3 - piezoelectric sensor ; 4 - preamplifier; 5 - main amplifier; 6 - multichannel analyzer of impulse; 7 - digital frequency meter; 8 - oscillograph.
The elastic waves, arising in metals and alloys (2) during irradiation by X-rays, propagate to the surface. The signals of acoustic emission are exhibited as surface oscillations of a material and detected by a piezoelectric sensor (3), which converse mechanical oscillation in electrical one. The electrical signals after preamplifier (4) go through main amplifier (5) to the oscillograph (8), multichannel analyzer of impulse (6) and digital frequency meter (7).
The AE signals were measured during irradiation by X-rays (1). The preamplifier (4) was made on the basis of K284UD1 chip. It has a high input resistance, low noise level during operation with a high-resistance sensor and it has also possibility of scale regulating of an amplification factor in the bandwidth of the frequency.
The frequency band of preamplifier is set by exterior components of the circuit. The level of natural noise of this preamplifier did not exceed 10 mkV. We used the frequency selective microvoltmeter SMV-11 as the main amplifier. We can set frequency of an explored signal from 0.01 up to 20 MHz (the bandwidth of the frequency is 9 kHz) with the help of this device. Digital frequency meter H3-33 detected the explored signals.
There are some types of waves depending on a direction of oscillations of particles in a sample. If the particles of metal or alloy oscillate along a sample, then in sample arise the compressive- tensile strains and will take place a longitudinal wave. If the particles oscillate perpendicularly to sample, then will arise the shear deformations and will take place cross and shift waves. The longitudinal waves arise in all mediums, whereas transverse waves - only in samples have a shift elasticity.
The velocity of propagation of longitudinal C1 and cross C2 of waves depends on the module of a longitudinal strain E (module of Young) and Poisson's ratio Vp.
The travelling wave, which originates in metal or alloy, will be decreasing with distance because of its attenuation. The decreasing of the plane wave with initial amplitude A0, which arises in a distance r, may be written by the formula:
where k - attenuation factor, which is determined by an absorption constant kn and scattering one kp, i.e. k = kn + kp.
The sound energy at absorption has been transformed in thermal one because of the effects of interior friction, imperfect elastic properties of metal or alloy and other factors. At scattering on irregularities of a material the sound energy go away from a direction of wave propagation.
The energy of sound oscillations was detected by sensor, which was made from a piezoelectric ceramics CTS-19. This ceramics was made as a disk (polarized along thickness), diameter d=20 mm and thickness h=5 mm. The frequency of the basic resonance fres for longitudinal oscillations of the disk was determined from formula:
where r - densities of a piezoelectric ceramics, h - thickness of a piezoelectric sample.
|Fig 2: Dependence of the number of AE impulses alloy VCo-8 from time of irradiation by X-rays:1 - initial samples;2 - samples with treatment by fluid nitrogen Tinit.=200oC, ttret.= 15 min.|
We studied the acoustic emission during irradiation by X-rays of a tungsten and hard-facing alloys (WCo-8; T15Co6), which are used for manufacture carbide-insert drill bits and hard-alloy-tipped drill bits for drilling mountain breeds. The acoustic emission was explored for two types of samples: initial samples (without treatment) and samples, which were treated by fluid nitrogen (start temperature Tinit. = 200C; time of cryogenic handling ttret = 15 minutes). After cryogenic treatment both samples (with treatment and initial) were placed in installation for measuring of AE signals, where them irradiated with the help of a radiological apparatus RUP-150-10. The results of measuring of an acoustic emission in initial samples (curve 1) and in samples with treatment by fluid nitrogen (curve 2) are shown in Fig. 2.
This curves, which describe dependence of the number of AE impulses from time of an exposure by X-rays of a hard-facing alloy, have two peaks of an acoustic emission in both cases. It testifies that in a hard-facing alloy take place both point (the first peak), and linear defects (the second peak). The first peaks for both samples have the same position, but the second peak for samples with cryogenic treatment, is displaced in area of smaller time of an exposure by X-rays.
It can be explain that at cryogenic treatment of a hard-facing alloy take place structural - phase change because of thermal action on linear defects.
The position and shape of curves of an acoustic emissions obtained at various conditions of cryogenic treatment, allows to estimate character of this processes. So, the position of peaks of an acoustic emission does not depend on conditions of cryogenic treatment. However the number of AE impulses for various conditions of treatment have big difference. The smaller number of AE impulses is detected for more essential treatment (multiple treatment by fluid nitrogen). In this case the bigger concentration of point and linear defects will be take place in the samples, and this defects dissipate the ultrasonic waves and consequently they do not reach a piezoelectric sensor.
The results of research of an acoustic emission are completely agree with the data of strength tests of WC8 and T15Co6 alloys, which were treatment in fluid nitrogen. We studied following strength characteristics of this alloys: a breaking strain, bending strength, compression strength, rigidity, Rockwell hardness, microhardness. The obtained data are shown in the table. In this table also is shown the number of AE impulses.
|The number of AE impulses||Breaking strain, P, kgf||Bending strength, kgf/cm||Deformation, mm||Rigidity, kgf/mm||Rockwell hardness, N||Micro-hardness, kgf/mm2|
The results have shown that takes place a complete correlation between number of AE impulses and mechanical properties of the alloys. Besides we can see a complete correlation between number of AE impulses and time-life of carbide-insert drill bit and mechanical velocity of drilling by this carbide-insert tools.
The AE generation is stimulated by annihilation of interstitial atoms with vacancies during an X-ray irradiation of this nonequilibrium materials. There is a thermal spike (Fig.3) in the area of defect annihilation by release accumulated energy in a crystal, its value is 10 eV . The release of energy cause sharp increase of temperature in a small volume, then temperature practically instantaneous decreases up to environmental. As a result of this processes arise an impulse of pressure, which one is generated an acoustic wave, and its wave is scattered on the point and linear defects, passing along metals and alloys. Number of AE impulses is determined by defect concentration in a material. The materials with pure structure have less centers scattering acoustic waves, and so more AE impulses is detected. Most heavily this process take place in fields of enclosing dislocation and on the boundaries of grains, reducing amount and sizes dislocations, and also lowering temperature of phase transition. The annihilation areas in this case are points of bifurcations, defining self-organizing of materials at an irradiation by X-rays .
|Fig 3: The distribution of temperature (T) in alloy, stipulated by annihilation of interstitial atom with vacancy during irradiation by X-rays, in time (t) and space (x).|
The AE impulses are observed during 30 s, as during an irradiation the character of interaction of a X-rays with a material varies. The most probable reason of this phenomena was an interference of X-rays with acoustic waves.
The application of irradiation by X-rays with simultaneous detecting of ultrasonic waves allows effectively to predict a resource fatigue life of metals and alloys at various type of handling.
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