The results of a study of the mechanism and kinetics of metallic material fracture (dual phase steels, binary Zr-2,5Nb and multicomponental Zr-1,2Sn-1Nb-0,3Fe alloys, multifilamentary Nb3Sn and Nb-Ti based superconductors and HTSC-based composition wires) are given. The quantity fracture analysis is based on measurements of acoustic impulse peak (maximum) amplitudes by non-resonance sensors for linear measurement of acoustic shifts and crack parameters measurement. The developed methods of absolute calibration of AE equipment were checked by testing various types of materials and crack parameter measurements in laps and fractures. The calibration dependencies for quantity measurements are shown. The possibilities of AE for quality analysis and for characterizing of materials in the process of various mechanical tests and in the process of pressure processing with the help of a developed experimental computerized AE system are demonstrated.
KEYWORDS: quantitative AE-measurements, AE materials quality monitoring, computerized AE system, deformation and fracture, materials testing, pressure processing
At the Moscow State Institute of Steel and Alloys (MSISA) the basis of the program for developing AE quantitative methods is formed by procedures and instruments that are based on the notion of the linear relation between the maximum peak amplitude of the acoustic field and the elastic energy of the AE source at the rate of the AE source evolution close to the sound velocity.
The instruments and techniques of measurements developed and employed in this work are based on non-resonance damped wide-band sensors. This allowed linear measurements of acoustic displacements and, using the latter, one can assess the dimensions of radiation sources, in other words, cracks.
There are two ways of practical implementation of AE-method in MSISA. The first way is monitoring of quality of materials with the help of AE measurements during laboratory research, which is called "Quality monitoring of materials," that allows the evaluation of ductility margin, fracture resistance, resistance to SCC, etc.
The second is AE-monitoring of processing of materials and AE-monitoring of work conditions of technological equipment, which is called "Technological monitoring."
In the framework of "Quality monitoring of materials" and "Technological monitoring" various methods of quantitative analysis of processes and materials with the help of AE were developed.
In the framework of this research specific methods and equipment were developed for quantitative AE analysis. In combination with other methods of investigation, such as metallography, electron microscopy, fractographical analysis, AE measurements will allow analysis in real scale of time various processes of deformation and fracture of tested materials.
The developed equipment and methods for AE-measurements are used for carrying out a wide range of practical problems: development of cryogenic and cold-resistant dual phase steels , development of technology for processing Zr-based tubes for nuclear power reactors , development of technology for manufacturing of multifilamentary superconductors for large scale magnetic systems.
AE-Devices for "Monitoring of Materials" and "Technological Monitoring"
The principles stated above are fixed in the basis for development of the following AE equipment:
Small Size Systems for AE-Measurements -The objectives of laboratory AE-measurements of fracture are comparisons of events in structure with generated AE signals. The peculiarity of an AE signal, generated by a microcrack, is a low level of a signal - which can be compared with the noise level of receiving amplifying devices.
- Small size laboratory equipment for AE-measurements of tested materials.
- Microprocessor detectors of AE signals - the devices with built-in calculating machines for automatic detection, storage and processing of information about AE signals during mechanical tests of materials.
- Information-measuring system on the base of IBM PC.
High sensitivity of AE-method is achieved by implementation of resonance sensors [1-4]. A multireflected acoustic wave passing through a piezoelectric element, increases the ratio "signal/noise" of the output electric signal to 102-103. But a continuous reverberation makes it difficult the identify an output signal and doesnot allow to detect a single stage of the fracture process.
In the program for developing AE quantitative measurements it was intended to use another method, based on implementation of nonresonance damped piezoelectric elements . In that case the sensor from the opposite side to the tested sample is damped by acoustic trap, which absorbs an acoustic wave, passing through the piezoelectric element. This leads to increase of time resolution of devices and allows the detection of the formation of inside cracks with the help of AE, to calibrate equipment for crack size measurements and to make quantitative analysis of kinetics of damage accumulation in deformed material.
In a specially developed wide band piezoelectric converter for broadening the band of linear transformation the AE piezoelectric element is damped by a conic acoustic trap. Amplitude-frequency characteristic of a sensor is linear on the level of 3 dB in frequency band not less than 20 MHz. The design of the sensor allows to the registration of a signal directly from the working part of the tested sample. The acoustic sensor contacts the sample through the oil layer.
The developed small size device for AE-signals registration in dynamic range (Fig. 1) consists of a set of wide band piezosensors; a set of small-size amplifiers (Df = 0,01¸15 MHz; 32 dB); preamplifier, connected directly to the AE sensor; a unit for signal processing, which consists of a power unit of preamplifier, passive RC-filter additional amplifier with step change of amplification factor (0,8; 14; 20; 40 dB) and a high speed (50 V/ms) peak detector.
Fig 1: Schematic of acoustic emission facility and test schemes.
The reservation of dynamic range of amplitudes during the laboratory registration of AE signals, is insured by recording of a detected signal with the help of a multichannel high speed recorder, or directly in the memory of the PC. The dynamic range of peak amplitude registration is not less than 72 dB. The AE impulses of 0,1 ms is linearly processed.
Time resolution of impulses is determined by the frequency limit of a detector, which for a high speed recorder is fmax = 150 Hz.
Peak (maximum) AE impulse amplitude Up is expressed in dB (Vp = 20×lg(Up/Un)) in relation to average noise impulse amplitude (Un), which is determined for each sample after the second loading, the first stage of test, in a zone of Kaizer effect.
Microprocessor Detectors of AE-Signal -Microprocessor detectors of AE-signals (MDS) were developed for registration, transformation, preliminary processing and recording of results of quantitative analysis of deformation fracture and process kinetics.
The built-in calculating machine allows to one make a direct superposition of deformation diagrams of mechanical tests and AE-diagrams when less than 107 are cracks formed in one test, and processing according to one of the programs in the software packet.
The device transmits digital information by communication channels for a distance of up to 100 m; indicates mechanical and AE parameters; registration of input information arrays; up monitoring of the work condition of the device.
Multichannel Computerized System "Test AE" Type - The"Test AE" type system, developed by scientists at MSISA, allows measuring, processing and archiving of AE signals . Such an information measuring system is designed by a modules principle. AE-signals are detected by a processor. Preamplifying and the filtration of signals is carried out by analogue, connected to a multichannel module of AE signals numbering. This module is on an interface board, which is mounted in an IBM PC. Data input is fulfilled by a 16-channel digital module. Numbering of signals is fulfilled by a 12-digit analogue-to-digital converter. Maximum beat frequency is up to 20 MHz; for preliminary digital processing of signals a RISK-processor is mounted in the interface board.
Interface of a system for processing of signals is based on technology of virtual (image) devices, such as Laview software, which is used for realization of algorithms of information processing and data archiving.
AE Devices for Monitoring of Pressure Processing of Materials -High sensitivity of AE radiation allows the implementation AE method in conditions of high background noise for monitoring pressure processing of metals, for example, drawing of a wire made of composite materials.
The following problems were solved when the set of devices was developed:
- Detection of acoustic signals directly in a zone of minimum distance from deformation zone, in conditions of high deformation rate, high temperature and vapor of technological liquids.
- Extraction of legitimate acoustic signals from the deformation zone of the background of technological noise of the loading unit and industrial noise. Block-schema of AE device is shown in Fig. 2.
Fig 2: Information and measurement unit for drawing AE-control of composite superconductors.
The information measuring device has 16 channels. The AE-detecting block consists of a measuring cell with 2-12 piezoelectric sensors (3), connected to them are small size preamplifiers (4) and electronic devices of preliminary processing of AE signals (5).
The amplification is 60 dB in a frequency band of 0,1 to 20 MHz with time resolution 10-4 s. The complex and measurements are controlled by 12 channels with the help of a "Test-AE" computerized system.
Results and Discussion
AE Monitoring of Materials
The development of the AE method to quantitative monitor materials is implemented by us in two main directions. The first direction is the use of the AE method as an indicator of the initiation end or intensity of deformation and rupture processes that proceed during material testing. In this case AE measurements were carried out directly in the process of tensile testing materials using various schemes (tension, bend, torsion, etc.). AE and loading diagrams were analyzed together with the
results of the metallographic and fractographic studies into changes that take place in the structure or fracture. This makes it possible to set up unambiguous correspondence of the acoustic signal to the event that gave birth to it. This complex approach allowed the analysis of the fracture processes in various materials and the acquisition of the information that could not be obtained if the indicated methods were used individually.
The second direction in the evolution of the quantitative AE analysis is a direct measurement of the dimensions of the fractured areas using AE signals.
Some results of material monitoring are given below.
Assessment of Ductility Margin of Materials
The AE method was widely used by the authors to assess the ductility margin in mechanical tests for tension and bend of high manganese steels [10, 11] and Zr alloys (Zr-1,3Sn-1Nb-0,4Fe; Zr-2,5Nb) in different structure conditions [2,9,10].
Fig 3: Diagrams of strain and AE upon tension Zr-1.3Sn-1Nb-.4Fe samples containing fine particle (a); and aggregates of coarse particles (b).|
The joint analysis of strain and AE diagrams demonstrates the interrelationship between the ductility determining route of plastic flow stability loss and the mode of AE. Figures 3 and 4 illustrate the AE upon the plastic flow stability loss due to a "geometric loss of strength" by the steels and Zr-alloy where they are of the uniform plastic strain and the level of AE does not exceed the level of a noise.
Fig 4: Diagrams of strain and AE upon tension of hydrogenated Zr-2.5Nb samples containing fine (a) and coarse (b) hydrides.|
The figure illustrates the earlier loss of flow stability due to the formation of an "internal" neck induced by microcracks imitating structure defects (aggregates of coarse particles, brittle secondary phase, etc.). Here, before a load drop into the to test several strong AE impulses are recorded due to microcrack openings which are corroborated by the metallographic and fractographic analyses of samples.
Thus, the structure determined differences in the way the flow stability and the deformability of materials are lost are unambiguously revealed in the joint analysis of the deformation and AE diagrams. This allows the application of AE measurements for monitoring the alloy quality in the standard mechanical tests.
AE Analysis of Material Damageability in Operations Composite Superconductor Twisting -The technological ductility of composite superconductors is limited by a crack formation
at the drawhole-die interface. The studies into the kinetics of a damage accumulation upon twisting complex composites, i.e., multifilamentary superconductors containing up to 50 000 filaments became possible only after special instruments and techniques of AE measurements were created. The AE measurement implemented together with electron microscope and fractographic analyses revealed the main stage of deformation and crack formation as well as the degree of damageability of superconductors at different stages of twisting. This allowed the assessment of the ductility margin of superconductors having various designs and the control of their damageability in the process of twisting.
The AE diagrams of twisting superconductors of different designs reveal three distinctive stages that differ in the acoustic radiation power (Fig. 5). The first stage is distinguished by an increase in the AE power upon transition to the plastic deformation of a superconductor. The second stage corresponds to a uniform deformation and is described by AE with a small amplitude of signals. The duration of this stage depends on the ductility margin of a superconductor. Upon going to the third stage one observes a raise in the power and a broadening of the amplitude spectrum of acoustic signals that accompany the formation and evolution of defects, i.e., cracks.
Fig 5: Acoustic emission of twisted composite conductor.|
Crack Measurement by Acoustic Emission
A brittle crack is opened up at the rate of the order of the sound velocity. At the stress s and elastic modules E an increment of elastic energy is U = s/2E, a crack with diameter D releases the energy W = U´d3 during the time t = d/s and at little variable stress one obtains the proportionality between the peak displacement in an AE impulse and a crack area F @ d2:
The linear detection with recording the extreme values of the impulse amplitudes in a wide dynamic range is promising because the scope of a fracture may be seen. Dependence (2) was checked by measuring the displacement Up with a linear detector (from the electric signal Vp for internal cracks upon tension round samples of high manganese dual phase steels  and of hydrided Zr-2.5Nb alloy .
To analyze the fracture processes by the AE signals are to be quantitatively compared to the events that generated them. For this purpose one is to provide and calibrate the single-valued linear relation "crack increment-acoustic signal amplitude-amplitude of recorded electric signal." The single-hyphen valued relation between the amplitudes of acoustic field and electric signal is given by linearization of sensor by mechanical damping of a piezoelement. In this case a sensitivity drop is unavailable, but, e.g., in our instrument  the signal is at the noise level at a microcrack diameter of d @ 10 mm.
But the simple summation of the impulse numbers ("counting rate," "total count") may give the process kinetics even if all the recorded acts of fracture are of the same scale. It is not acceptable when impulse amplitudes vary by an order and more when single acts are replaced by "cooperative" ones.
The numerical investigation of the problem on a single source immersed in a semispace  shows that with the unchanged released energy W and the depth of a source location r0 the peak (maximum) value of linear surface displacement Up above the source depends on the time of its action t:
In the hydrogenated Zr-alloy there were available platelet hydrides of the axial orientation; the fracture shows internal cracks along hydrides crossing the cup.
For 138 cracks in high manganese steels a linear dependence was found lgVp(lgF) with correlation coefficient of r = 0.92 (Fig. 6a) . For 15 cracks in the Zr alloy the amplitude-area relationship is also linear lgVp(lg F) with r = 0.91 (Fig. 6b) .
The proper reproducibility of the peak amplitudes of single sources makes it possible by going from one sample to another to calibrate the instrument against an acoustic impulse and to retain once directly constructed graduation for crack dimensions.
Fig 6: Structural calibration of AE-equipment at measurement of crack sizes. Internal cracks: manganese dual phase steels (à); Zr - 2,5Nb alloy containing hydrides (b)|
The linear "amplitude-crack size" correspondence is to be expected at the invariable rate of propagation of all the cracks. The possibility is not excluded of an incomparable graduation for a brittle and "slow" tough crack. But the event of the coincidence of "the amplitude - crack size"graduation for tough and brittle cracks (Fig. 6) points to a higher rate of a band breakage in a group of pores near a tough crack.
Thus, the AE measurements of a crack increment is feasible under the following conditions:
- Discrete mode of process. The time between crack ramps is more than the constant of the instrument time.
- Relatively high elastic energy of release in a single fracture event and high rate of opening which allows the use linear detection for cracks with sizes of 10 m
m and higher.
- Wide dynamic range of instruments that allows measurement of large cracks without losing small ones.
- The feasibility of direct graduation by fractographic measurements of cracks under identical conditions.
The quality level of materials can be determined only by complex analysis of AE measurements data and comparison of these data with standard mechanical tests.
At the MSISA the basis of the program for developing AE quantitative methods is formed by procedures and instruments that are based on the notion of the linear relation between the maximum peak amplitude of the acoustic field and the elastic energy of the AE source at the rate of the AE source evolution close to the sound velocity.
The developed system of AE quality monitoring of materials united all the experience which was obtained for the period of 10 years in testing of materials in various technological conditions with the help of the AE method. The system allows the conductance of a complex analysis of mechanical characteristics and the results of AE measurements. This allows a unique scientific and technological information to be obtzined.
The quantity fracture analysis is based on measurements of acoustic impulse peak (maximum) amplitudes by non-resonance sensors for linear measurement of acoustic shifts and crack parameter measurements. The developed methods of absolute calibration of AE equipment were checked by testing various types of materials and crack parameter measurements in laps and fractures. The calibration dependencies for quantity measurements are shown. The possibilities of AE for quality analysis and for characterizing of materials in the process of various mechanical tests and in the process of pressure processing with the help of developed experimental computerized Test-AE system are demonstrated.
The high sensitivity of the test-AE system provides a new level of quality monitoring of materials.
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