- International Symposium on NDT Contribution to the Infrastructure Safety Systems, 1999 NOV 22-26 Torres, published by UFSM, Santa Maria, RS, Brazil

TABLE OF CONTENTS

Abstract The acoustic emission technology

Abstract

The acoustic-emission technology starts to be applied more widely in different industries to a non-destructive inspection and technical diagnostic of industrial objects: pipelines and pressure vessels, tanks, heat exchangers, bridges, cranes and other metallical structures.
The modern acoustic-emission systems are multichannel and multifunction systems built on the basis of personal computers.
If the intensive exploitation of a system in field or industrial conditions is supposed, for its moving to object of tests, failure-free operation and the non-admissions of diagnostic data loss are indispensable:
• Compactness and small weight of instrumentation,
• Small consumed power and capability of battery backup,
• Presence of the built-in uninterruptible power supply,
• The expanded temperature range of operation of instrumentation,
• Hardening of instrumentation from effect of shocks, moisture and dust.

From 1997 our corporation produces new model of the device "Locus - D", all digital, having the expanded set of an measured parameters, and also lot of new functions. Principled difference of the new architecture of the device is the all digital processing of a signal realised on powerful FPGA (fully programmed gate arrays).

The model provides conventional qualities of devices of developing of the "Eltest" corporation - minimum overall dimensions and weight (no more than 7,5 kgs for the 16-channel device), the maximum immunity to exposures - dust, moisture, temperature, vibration and shocks (up to IP66), and also lower consumed power at a feed from accumulators, that in the total provides maximum productivity of the control at absence of loss of the data.
The software of the device is made in style of a Windows 95. It allows individually to set all parameters of each channel, provides an autotuning, floating threshold, powerful data filtering on any parameters, miscellaneous, including three-dimensional, graphical representation of the data.

The acoustic emission technology

The acoustic emission technology starts to be applied more widely in different industries to a non-destructive inspection and technical diagnostics of industrial objects: pipe lines and pressure vessels during hydrotest, pneumotest, including in an operation mode, without a stop of the product transfer; tanks; heat exchangers; bridges; cranes; winchs and other metallical structures. In particular, the employees of our corporation ELTEST successfully apply acoustic emission technology more than 15 years.
It is known also, that the phenomenon of an acoustic emission has a broad spectrum of applications covering not only a non-destructive testing, but also control of manufacturing processes, and so on down to the control of loads on the osteomuscular apparatus in sporting medicine. Because of this a many specialized AE systems always will be in use, but the consideration by which one leaves for frameworks of the present article. Here will be reviewed only AE systems intended for diagnostics of a defects in an industrial objects (we shall call it AE control systems).
The modern AE control systems is a composite multifunction system built on the basis of the personal computer. As the objects of the control are characterized by the considerable dimensions and/or by composite structure the essential requirement is the presence of many AE channels. The AE systems used now have number of channels from 4 up to 64, and large number of channels have, as a rule, fixed systems. In advertising there is a mention of a 1024-channel system. However, because of problems with the laying of cables and also because of its limited length, for the control of pipelines 6-8-channel systems are, as a rule, applied and for the control of pressure vessels the number of channels usually does not exceed 32.
To the fundamental requirements, with which one the AE control systems should fit, it is possible to attribute its authenticity and productivity.
If the intensive exploitation of a system in field or industrial conditions is supposed, for its moving to object of tests, failure-free operation and the non-admissions of diagnostic data loss are indispensable:
• Compactness and small weight of instrumentation,
• Small consumed power and capability of battery backup,
• Presence of the built-in uninterruptible power supply,
• The expanded temperature range of operation of instrumentation,
• Hardening of instrumentation from effect of shocks, moisture and dust.
• Reliability, ease and convenience in usage.

The authenticity of the control depends both on instrumentation, and in not of a smaller degree from a procedure of its realization. The instrumentation should provide reliable fixing of presence and arrival time of AE pulses, and also measurement of their characteristics. Among last now are usually used amplitude, duration and rise time of AE pulse, and also area under its envelope as an energy parameter. The total number of AE pulses is necessarily recorded. A averaged level of a signal and signal from external parameter (more often pressure in object) is frequently measured. This list could be considerably prolonged, however it is necessary to take into account, that in usually used methods of an AE signal analytical treatment will be used only 1-2 of the pointed parameters. The fixing of presence of a pulse depends on threshold of the instrument, which one at competent designing is coupled to a noise level of instrumentation. Last makes, as a rule, 5-10 µV(rms) in reduction to an input of the preamplifier, that is comparable to a sensor noise level. The threshold of AE pulses discrimination is usually adjusted through 1 dB separately on each channel. Sometimes it can be established floating (dependent from an noise level on the channel), that can reduce activity of the operator on set-up, but at the same time requires his constant attention for preservation of a threshold value, reasonable on a procedure.
The relative AE pulses arrival time should be measured with accuracy approximately 2 - 4 µS, thus the error at the expense of it of a defect place determination will matter about diameter of the sensor (1 - 2 sm). It is understandable, that higher accuracy of measurement of pointed parameter will have physical meaning only in case of applying sensors of a new construction with the considerably diminished acoustical contact area sizes (for example, optical).
The instrument should also must to process sufficient number of pulses (usually about 1000 pulses per channel in one second), that will allow to not lose information signals in conditions of intensive defect growth.
The productivity of the control also strongly depends on combination of the technical and ergonomical characteristics of the instrument. The number of channels, permissible length of a signal cable, simple and convenient software here have a great importance.. The reliable cables and connectors, rugged construction, resistance to power supply failures (that frequently results in loss of already accumulated information) are relevant also.
The typical sceleton diagram of a modern AE system you may see in a fig. 1.

Fig 1: Skeleton diagram of LOCUS AE system(8 or 16 channels)

The sensors are piezoelectric, they put on contact lubrication and are placed to object more often with the help of special magnetic holders.
The preamplifiers are intended for amplification (usually on 40 dB) small signals from sensors, and also for a signal transmission through lengthy (up to 300m) cable from preamplifier to the instrument.
On a data aquisition board the main amplifiers (linear or logarithmic, characteristic gain 60dB, for linear adjustable through 1 dB), further bandpass filters limiting bandwidth of the instrument for decreasing of noises, then circuit of an amplitude discriminator, digitization of signals and calculus of parameters of each pulse are located.

The notebook computer executes mapping and recording of pulses and their parameters, and also calculation of defect position. In a post-processing mode all necessary graphs and tables can be printed.
Let's tell also some words about AE instrumentation software features.
As in a data aquisition mode the AE system works in a real time mode and should fast process and image the obtained signals, for it the real time operating system, for example, QNX would be better. However all similar systems are extremely expensive, and in practice will be used or MS DOS, that is more preferential, as boosts response, reliability and reduces the requirements to the hardware, or different versions of WINDOWS.
Particular to the AE instrumentation software are the functions of defect position calculation in a real time and their mapping on the object chart in real coordinates. The example of defect position mapping is showed on a fig. 2.

Fig 2: AE events on industrial vessel

Spot of the greater dimension in a fig. 2 image clusters, i.e. area of a congestion of large number of places of the localized pulses. The small squares with numbers image sensors.
In a post-processing mode the AE instrumentation software should allow to get sufficient quantity of different graphs and tables for drawing up of the representative report about results of diagnostics.

The Eltest corporation designs and produces different modifications of AE instrumentation during eight years. This one successfully have passed check in field and industrial conditions at diagnostics of real industrial objects - pipe lines, pressure vessels etc. Only during last four years (1996-1999) our corporation personnel test more than 700 km of pipelines, 300 dangerous crossings of pipelines with the highways and railroads, 270 pressure vessels, where we find more than 10 thousands of different defects, including 314 very dangerous defects, which must be immediately removed to prevent a failure.
From year 1997 our corporation produces new model of the device "Locus - D", all digital, having the expanded set of an measured parameters, and also lot of new functions, including waveform registration, power saving etc. Principled difference of the new architecture of the device is the all digital processing of a signal realised on powerful FPGA (fully programmed gate arrays). A software reconfiguration of the instrument now is possible with the purpose of addition of new parameters and functions.
The model provides conventional qualities of devices of developing of the "Eltest" corporation - minimum overall dimensions and weight (no more than 7,5 kgs for the 16-channel device), the maximum immunity to exposures - dust, moisture, temperature, vibration and shocks (up to IP66), and also lower consumed power at a feed from accumulators, that in the total provides maximum productivity of the control at absence of loss of the data.
The software of the device is made in style of a Windows 95. It supports activity with the instrument up to 80 channels with autodetection of their number (in a plug and play mode) and allows to set individually all parameters of each channel, provides an autotuning, floating threshold, powerful data filtering on any parameters, miscellaneous, including three-dimensional, graphical representation of the data.
In summary it is necessary to note, that the rapid progress of microchips causes immediate appearance new, more compact, convenient and informational - capacious AE instruments.

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